Bullet inlet tract extension

[ace.cafe]

Hi guys!

I just completed and tested my latest mod, which is an inlet tract extension on my carburetor.
It essentially performs a similar function as a velocity stack, but perhaps a little less perfectly. However, it allows me to use my existing K&N air filter on the end, because it does not have a flared bellmouth on it.
So, I get the inlet tract length increase that I wanted, and still get to use my air filter.
I felt that I'd get enough benefit from the constant-diameter tube to suit my needs, and wanted to keep the filter on the end, so that was the basis for the decision making process.

There are a couple main targets for the inlet tract increase.

One is very simply to permit a larger amount of air volume on the "engine side" of the filter barrier. The stock position of the K&N filter is right on the carb mouth, and the only volume inside the filter barrier is basically the volume of the carb throat and the inlet port. This does not amount to even 500cc, which would be the minimum volume needed for "one big gulp" of air when opening the throttle quickly. Having more air volume inside the filter barrier can provide better throttle-response because it doesn't have to draw air thru the filter for the first "gulp". It's common practice to provide greater air volume than the engine displacement inside the filter barrier for this purpose.
So, it accomplshed that goal at the very least. The "pod type" filters such as the K&N are less than ideal in their normal mounting position on the carb, because of this factor.

Another target is to achieve a "ramcharging" effect, by the inertia of this moving air inside the inlet tract. Once this "cylinder of air" in the inlet tract gets moving, it continues to move from inertia, even when the piston stops drawing air in. This assists the filling of the engine cylinder by using all the available time that the inlet valve is open, to get the most air into the engine.

And lastly, it is for "resonance wave tuning", which uses the reflection of sound waves that reflect up and down the intake tract during running. By tuning the wave behavior in the inlet tract by using certain lengths, the waves can be used to assist the flow of the air into the cylinder, in a very simlar way that exhaust length tuning helps to extract the exhaust out of the engine. Just the other side of the same coin, so to speak. The longer tract can bring the reflection frequency down lower in the rpm band, where it can be better utilized by the Bullet's slow-revving engine design. The short inlet tract in the stock engine is not optimized in any way but for fitting on the motorcycle. It's too short to provide any real wave-tuning benefits in the low rpms that we can access. Short inlet tracts serve higher rpms better, and we can't even run our engines at rpms that would be good for an inlet tract length that is as short as the stock Bullet has.
Lengthening this inlet tract can work out as beneficial for us in the low rpms that our engine can use.

So, for a very cheap cost, some benefits can be achieved for the actual running improvements of the bike. It increases torque and hp at certain rpm points, and we strive to time this activity to coincide with the rpm of our engine's torque peak.
Actually, the length that I am using is still a little shorter than optimal, because I ran out of room to put the extension, without it hanging out too far off the side of the bike. So, I  selected a 6" length, which would be good for rpms slightly higher than the torque peak, but not too much higher, and still useful, while fitting into the available space and visual appearance parameters.

For those interested, I used the stock rubber bellows that usually connects the carb throat to the stock air filter housing, to connect the stack to the carb. I used a rubber o-ring as the "clamp" to hold it on the carb.
The tube is nothing more than a 1.75" I.D. tube, cut to 6" length.
The K&N just fits on the end, because the tube is the same diameter as the carb throat body, and it just clamps right on.
I used a cable tie to secure the "flying end" to the frame, so that it didn't sag and fall off, or over-stress the intake rubber manifold part from the added weight and distance. Gives the whole thing more support and stops it from falling off due to vibrations. You could use a mounting clamp or bracket for this purpose too. I just used a cable tie, and it seems to work ok.

Now, I realize that this may not be a pretty as Chumma7's stack, nor probably quite as ideal in function. Chumma's stack is really nice. But I wanted to have the K&N on the end, so I adjusted the concept to fit my need.

Here's a photo of it on my bike.

[geoffbaker]

looks good ace.

general thought... on my diesel and the scoop, I can build in some sort of extension (though it might have to turn some corners). What difference do you think there would be between using flexible tubing (with its spiraling shape) vs a smooth bore tubing of some kind?

[Chasfield]

Interesting write-up, Ace. It accords with my abortive attempt to install that kind of filter close up on the carb. Even though I had gone up a couple of jet sizes, I found I was getting dreadful just-off-idle to 1/4 throttle response - the mother of all flat spots - so I went back to the stock air filter, reluctantly because I wanted the tool box back for putting tools in!

I might fashion a similar tract extension and see if there is an improvement.

Chas

[ace.cafe]

looks good ace.

general thought... on my diesel and the scoop, I can build in some sort of extension (though it might have to turn some corners). What difference do you think there would be between using flexible tubing (with its spiraling shape) vs a smooth bore tubing of some kind?

General rules would be to provide the widest sweeping bends possible in the application, and use smooth-bore tubing.
Any type of tubing that won't collapse from the intake vacuum, and won't be adversely affected by the fuel will be fine. It doesn't have to be metal.
The spiral type tubing which looks like vacuum-cleaner hose, has the effect of reducing the effective working diameter of the hose, because the spiral ribbing inside creates eddys and vortices at the walls of the tube, and forcing the main airflow to concentrate more in the center. This causes the spiral ribbed tube to effectively act as a smaller I.D. tube, dynamically.

If you want to use a scoop, make a wide sweeping "U" shaped inlet tube that is long, and ends up pointing it's mouth forward. Put your K&N or whatever filter pod you have on there, with a coffee can around it, also pointing forward, with the air filter inside and the hose thru a hole in the middle of the bottom of the coffee can, and you have your "scoop". This creates a high-pressure zone inside the coffee can when you are riding, and gives you the "ram air" effect, and you get the extended inlet tract benefits along with it, and you can use your filter with it too.
Mount the coffee can to your front frame downtube with a bracket, for cold intake air,  paint it black, and you're on your way for a few bucks.

[ace.cafe]

Interesting write-up, Ace. It accords with my abortive attempt to install that kind of filter close up on the carb. Even though I had gone up a couple of jet sizes, I found I was getting dreadful just-off-idle to 1/4 throttle response - the mother of all flat spots - so I went back to the stock air filter, reluctantly because I wanted the tool box back for putting tools in!

I might fashion a similar tract extension and see if there is an improvement.

Chas

Chasfield,
Interesting!
Because I too, had a flat spot like that after installing the K&N on the carburetor.
This mod eliminated that problem, and it was the first thing that I monitored to see what effects it would have on that. Problem solved by this, in my case.

[The Garbone]

Um, call me a skeptic but I have a few doubts.

I don't really think that the inertia/momentum of a 6" column of air would offset the inductive effect of extending the filter into more into the airstream on your setup.  I am thinking of inline inductors used to dewater spaces on ships or such.  I think even with your leg out in front of the filter this would be an issue.

Also I I tend to think the breathing issue is not really helped all that much by increasing the volume on the engine side of the filter when we are talking about such a low volume.  Your will still have to replace the displaced air through the filter medium and taking into account the actual permeable surface area of the KN or some such filter I believe this would also be a negligible improvement.  I was under the impression that most of the stock airflow restrictions were created by the box/multiple boxes employed in the system.  

I think it would be better to get some of flexible header pipe (available at Napa / Autozone and such) and bend it around 180 degrees into the airstream  with the addition of a scoop to create a true ram air effect. Of course this adds the issue of ingesting large volumes of water when it rains etc.  

My 2 pennies...

[ace.cafe]

Um, call me a skeptic but I have a few doubts.

I don't really think that the inertia/momentum of a 6" column of air would offset the inductive effect of extending the filter into more into the airstream on your setup.  I am thinking of inline inductors used to dewater spaces on ships or such.  I think even with your leg out in front of the filter this would be an issue.

Also I I tend to think the breathing issue is not really helped all that much by increasing the volume on the engine side of the filter when we are talking about such a low volume.  Your will still have to replace the displaced air through the filter medium and taking into account the actual permeable surface area of the KN or some such filter I believe this would also be a negligible improvement.  I was under the impression that most of the stock airflow restrictions were created by the box/multiple boxes employed in the system. 

I think it would be better to get some of flexible header pipe (available at Napa / Autozone and such) and bend it around 180 degrees into the airstream  with the addition of a scoop to create a true ram air effect. Of course this adds the issue of ingesting large volumes of water when it rains etc. 

My 2 pennies...

Interesting points.

First, it's hard to see from the perspective, but it is located behind my leg as you surmised in your comments. As such it is in an area of turbulent air anyway, so as to minimize any "drawing-out" induction effects from the airstream. I'm not totally discounting that possibility though, and I'm not seeking "ram air" in the sense of pressurized intake from the external airstream, in any case. I'm simply seeking the known effects of added intake tract length, which is standard engineering practice.
I offer "Exhibit A"


Also, it is standard engineering practice to have airbox volume inside the filter barrier which exceeds engine displacement, often by a factor of 2, for the very purpose I described. You'll notice on virtually any factory-built vehicle, a volume greater than displacement on the engine side of the filter. Even on sportbikes, where space is at a premium.

So, while I have no dyno, or other way to fully determine the effects, I feel comfortable in relying on proven engineering practice in the automotive/motorsports field for my modifications.

[geoffbaker]

General rules would be to provide the widest sweeping bends possible in the application, and use smooth-bore tubing.
Any type of tubing that won't collapse from the intake vacuum, and won't be adversely affected by the fuel will be fine. It doesn't have to be metal.
The spiral type tubing which looks like vacuum-cleaner hose, has the effect of reducing the effective working diameter of the hose, because the spiral ribbing inside creates eddys and vortices at the walls of the tube, and forcing the main airflow to concentrate more in the center. This causes the spiral ribbed tube to effectively act as a smaller I.D. tube, dynamically.

If you want to use a scoop, make a wide sweeping "U" shaped inlet tube that is long, and ends up pointing it's mouth forward. Put your K&N or whatever filter pod you have on there, with a coffee can around it, also pointing forward, with the air filter inside and the hose thru a hole in the middle of the bottom of the coffee can, and you have your "scoop". This creates a high-pressure zone inside the coffee can when you are riding, and gives you the "ram air" effect, and you get the extended inlet tract benefits along with it, and you can use your filter with it too.
Mount the coffee can to your front frame downtube with a bracket, for cold intake air,  paint it black, and you're on your way for a few bucks.

So if I use a larger diameter spiral hose I can offset the spiral wall issues...
I'm just thinking that with the space restrictions it may be impossible (cost effectively that is) to build anything without hose that turns easily in the tightest corners. Rubber tubing would require many sections and turns and clamps to achieve the same thing.

COFFEE CAN? On my Enfield??
I think not. Perhaps a fat piece of PVC pipe would be more classy

[ace.cafe]

General rules would be to provide the widest sweeping bends possible in the application, and use smooth-bore tubing.
Any type of tubing that won't collapse from the intake vacuum, and won't be adversely affected by the fuel will be fine. It doesn't have to be metal.
The spiral type tubing which looks like vacuum-cleaner hose, has the effect of reducing the effective working diameter of the hose, because the spiral ribbing inside creates eddys and vortices at the walls of the tube, and forcing the main airflow to concentrate more in the center. This causes the spiral ribbed tube to effectively act as a smaller I.D. tube, dynamically.

If you want to use a scoop, make a wide sweeping "U" shaped inlet tube that is long, and ends up pointing it's mouth forward. Put your K&N or whatever filter pod you have on there, with a coffee can around it, also pointing forward, with the air filter inside and the hose thru a hole in the middle of the bottom of the coffee can, and you have your "scoop". This creates a high-pressure zone inside the coffee can when you are riding, and gives you the "ram air" effect, and you get the extended inlet tract benefits along with it, and you can use your filter with it too.
Mount the coffee can to your front frame downtube with a bracket, for cold intake air,  paint it black, and you're on your way for a few bucks.

So if I use a larger diameter spiral hose I can offset the spiral wall issues...
I'm just thinking that with the space restrictions it may be impossible (cost effectively that is) to build anything without hose that turns easily in the tightest corners. Rubber tubing would require many sections and turns and clamps to achieve the same thing.

COFFEE CAN? On my Enfield??
I think not. Perhaps a fat piece of PVC pipe would be more classy

Theoretically a larger spiral hose would flow sufficiently, but I really don't know what the total diffraction effects of the spiral on the sound waves would do.
I have doubts.

Check Lowe's Hardware. I know they have very flexible hose of different types, and large diameters, that they will custom cut to lengths you desire,  Not expensive.

[taildraggin]

You might want to try some ducting from Aircraft Spruce.  The CEET tubing might work well - it's double wall, smooth inside.  You can cut to length to experiment with best length.

http://www.aircraftspruce.com/menus/ap/ducting.html

[dogbone]

Ace I tried the same mod last year, used sewer pipe pvc, and heated the end to form fit the carb base. It worked well, I started at 12" and finally ended up with 3 3/4 ",cutting off 1/2 "at a time. My K&N was a much larger filter, and I was able to flare a bell on the end kinda like the bell on a stack.
I will try moving the carb back from the head next,spacing the manifold.
I am trying for torque, not hi rpm power 

[ace.cafe]

Ace I tried the same mod last year, used sewer pipe pvc, and heated the end to form fit the carb base. It worked well, I started at 12" and finally ended up with 3 3/4 ",cutting off 1/2 "at a time. My K&N was a much larger filter, and I was able to flare a bell on the end kinda like the bell on a stack.
I will try moving the carb back from the head next,spacing the manifold.
I am trying for torque, not hi rpm power 

Dogbone,
I like the idea of moving the carb out, but the damn tank is in the way of the top of the carb where the cable comes in.
If you find a way around that, please post it.

[geoffbaker]

The tube is nothing more than a 1.75" I.D. tube, cut to 6" length.

ace, where's a good place to get this kind of mild steel pipe in these diameters?

I'd like to find some with a tight right angle bend in it for that matter.

thx

[ace.cafe]

Maybe you can find some large diamter conduit pipe for electrical purposes, locally.
If not, Ebay usually has almost anything.
Or McMaster-Carr has practically anything too.

Related to right-angle bends, that is not good for sonic waveguide purposes..
They don't like right angle bends. A "sweeping turn" is better, and straight with no bends is best.
But, if you have no other option, it might be better than nothing.
In any case, it can get you the ram air you were wanting, even if the sonic activity gets thwarted with sharp bends.

[geoffbaker]

Maybe you can find some large diamter conduit pipe for electrical purposes, locally.
If not, Ebay usually has almost anything.
Or McMaster-Carr has practically anything too.

Related to right-angle bends, that is not good for sonic waveguide purposes..
They don't like right angle bends. A "sweeping turn" is better, and straight with no bends is best.
But, if you have no other option, it might be better than nothing.
In any case, it can get you the ram air you were wanting, even if the sonic activity gets thwarted with sharp bends.

I need to build an intake pipe to mount to the squarish shaped engine air intake. That iwll have to turn 90 degrees backward, and the turn should occur in less than 3 inches. I'll make a flange to fit the intake, and weld the pipe to it.  After that, I have already built a scoop box which I think will work, I just need to take some pipe and mount a filter in the box. The pipe will enter at the front of the box and turn down so the filter will mount beneath it inside the box.
Once this is done I just need to tie the system together with some rubber hose and a carburetor and bob's your uncle.
I'll post pictures when complete.

[ace.cafe]

Maybe you can find some large diamter conduit pipe for electrical purposes, locally.
If not, Ebay usually has almost anything.
Or McMaster-Carr has practically anything too.

Related to right-angle bends, that is not good for sonic waveguide purposes..
They don't like right angle bends. A "sweeping turn" is better, and straight with no bends is best.
But, if you have no other option, it might be better than nothing.
In any case, it can get you the ram air you were wanting, even if the sonic activity gets thwarted with sharp bends.

I need to build an intake pipe to mount to the squarish shaped engine air intake. That iwll have to turn 90 degrees backward, and the turn should occur in less than 3 inches. I'll make a flange to fit the intake, and weld the pipe to it.  After that, I have already built a scoop box which I think will work, I just need to take some pipe and mount a filter in the box. The pipe will enter at the front of the box and turn down so the filter will mount beneath it inside the box.
Once this is done I just need to tie the system together with some rubber hose and a carburetor and bob's your uncle.
I'll post pictures when complete.

Cool!
That's all we can do.
Every application has its own difficulties to overcome, and there are often compromises involved to fit the application.
As long as we know what the ideal is, and then make our compromises to least adversely affect the goal, then we are doing all we can do.
Every engineering problem has these kinds of issues come up.. It's all in the hands of the engineer to get as close to his goal as he can, while still working within the structure that he has to fit into. That's the "art" of engineering.

[geoffbaker]

a search through mcmaster-carr isn't showing any useful tubing - it's all thick steel or it doesn't have a 1.75 ID.

their medium walled electric conduit shows an ID of 1.7, that may be as close as I can come. Where did you get your pipe?

[ace.cafe]

Well, first of all, I selected the 1.75" size based on the size of my inlet of the carb I'm using. If you have a different-sized inlet on your throttle body for your injected engine, then you should use that size. It may be the same, I don't know, but the size is determined by what your inlet throat is, so they match.

I got my tube off Ebay. It's aluminum. $12 plus shipping.
But mine is straight. Elbows might be harder to find.

In fact, I just happened to look at the picture of where your present air-filter is, and you do have a particularly difficult application. I can see now why you need that tight turn.

[Rick Sperko]

Interesting write-up, Ace. It accords with my abortive attempt to install that kind of filter close up on the carb. Even though I had gone up a couple of jet sizes, I found I was getting dreadful just-off-idle to 1/4 throttle response - the mother of all flat spots - so I went back to the stock air filter, reluctantly because I wanted the tool box back for putting tools in!

I might fashion a similar tract extension and see if there is an improvement.

Chas

Chasfield,
Interesting!
Because I too, had a flat spot like that after installing the K&N on the carburetor.
This mod eliminated that problem, and it was the first thing that I monitored to see what effects it would have on that. Problem solved by this, in my case.

This flat spot has me concerned. I was hoping to put a flatside carb and cone filter on before spring, but I would hate to spend the money to find the bike responds poorly. Is there much risk of that?

Thanks,
-Rick

[ace.cafe]

Interesting write-up, Ace. It accords with my abortive attempt to install that kind of filter close up on the carb. Even though I had gone up a couple of jet sizes, I found I was getting dreadful just-off-idle to 1/4 throttle response - the mother of all flat spots - so I went back to the stock air filter, reluctantly because I wanted the tool box back for putting tools in!

I might fashion a similar tract extension and see if there is an improvement.

Chas

Chasfield,
Interesting!
Because I too, had a flat spot like that after installing the K&N on the carburetor.
This mod eliminated that problem, and it was the first thing that I monitored to see what effects it would have on that. Problem solved by this, in my case.

This flat spot has me concerned. I was hoping to put a flatside carb and cone filter on before spring, but I would hate to spend the money to find the bike responds poorly. Is there much risk of that?

Thanks,
-Rick

Rick,
No, not a big deal.
With a different carburetor like the flat slide, you might not even get any flat spot.
The stock Mikarb comes set up lean from the factory. Many times there is no problem just up-jetting with satisfactory results. But in some circumstances, there is this nagging flat spot right at the transition throttle-position from the pilot jet to the needle.
That spot is around the 1/4-throttle position, and is governed by the throttle slide cutaway.
So, of course I ordered the richer throttle slide, but that was in early August, and it's been back-ordered since then, and I have no idea when it's coming in. So I've been limping along. If it takes much longer, I'm just going to buy a 32mm Mikuni  flatslide and put it on, and tune that one in instead..

[Chasfield]

Regarding possible raw material for inlet extension:

The outside diameter of my Bullet's exhaust front pipe is 1.75 inches. If that was going to be near enough then cut six inches out of an old one (plenty of those about, I reckon) and you would have a nice chromed induction tract. A curved section might offer even more possibilities but the pipe seems to be waisted in wherever it bends.

Also, best put a wire brush down the inside to clean out the grunge.

[Rick Sperko]

Wouldn't a long round K&N (6-10 inches) provide the same functionality? It seems like there could be even more volume and quicker recharge because it is all filter.

-Rick

[ace.cafe]

Wouldn't a long round K&N (6-10 inches) provide the same functionality? It seems like there could be even more volume and quicker recharge because it is all filter.

-Rick

It would provide the necessary volume of air inside the filter barrier. Yes.

But it wouldn't do anything for ramcharging effect on intake mixture speeds or wave tuning at all.

[Blltrdr]

 Ace, your mod is very inexpensive and could be fun to tinker with for a while. But is there any way to test these tubes on the bike to generate some data to see if the effort is worthy? Maybe hook up a vacuum gauge and take some readings? Maybe add a fan to simulate a forced air condition, then compare it to stock figures and so on? Having data to prove the value of something is half the fun of the chase to improve something.

[ace.cafe]

Ace, your mod is very inexpensive and could be fun to tinker with for a while. But is there any way to test these tubes on the bike to generate some data to see if the effort is worthy? Maybe hook up a vacuum gauge and take some readings? Maybe add a fan to simulate a forced air condition, then compare it to stock figures and so on? Having data to prove the value of something is half the fun of the chase to improve something.

Well, we start off by doing some calculations.
Here are some of the basic formula calcs
Pulse in these calcs is the same as reflected wave.

Intake Tract Tuning for RPM = (1100 x Half Intake Cam Duration x 0.960)/Length = 2nd Pulse
(1100 x Half Intake Cam Duration x 0.705)/Length = 3rd Pulse
(1100 x Half Intake Cam Duration x 0.538)/Length = 4th Pulse

Intake Tract Tuning for Length = (1100 x Half Intake Cam Duration x 0.960)/RPM = 2nd Pulse
(1100 x Half Intake Cam Duration x 0.705)/RPM = 3rd Pulse
(1100 x Half Intake Cam Duration x 0.538)/RPM = 4th Pulse

The earlier pulses like 2nd pulse are strongest, and then get progressively weaker as they increase to 3rd pulse, 4th pulse, and so on.
Nascar tunes to 3rd pulse.

This gets you some intake tract lengths to look at for starting points, and the challenge is to try to find where you are going to put all that intake length once you figure out how long it's going to be. And it IS gonna be long. the lower the rpm that you target, the longer the tube must be. And they get real long at the low rpms that we're targeting.
So, then we just probably try to get 4th pulse if we can, and there's realy not enough room for anything that looks relatively "normal" for that length either, so we "could" just try for as long as we can fit. That's what I did.

I do have a full-length 4th pulse tube here that I'm going to strap on the bike and see what it gets me. Just to see. Just so you know what it is for a 4th pulse on the Bullet at 3500 rpms, the inlet tract(this length includes everything from the  back of the valve head all the way to the tube air inlet opening) is 22.3" for the half of 264 duration calc,

Anyway, our goal for the Bullet would be to tune the pipe for the torque peak, which is somwhere between 3250 rpm and 4000 rpm, depending on what the mods on your motor are. For a Bullet equipped with performance exhaust and 28mm Mikarb, it's probably going to be around 3500 rpms for torque peak. Larger carb will have higher torque peak.
The other kind of goal is to tune at higher rpm for the horsepower peak. That's a shorter pipe. But it's only good at your rpm at peak hp.
So, you make the call. I think boosting torque peak is better for us, because with higher torque peak, all the hp figures above torque peak also get boosted. Downside is a real long tube.

Cam duration for the intake cam on a Bullet is 264 degrees at .012" lash, or about 230 degrees at .040" lash(which is where the valve curtain is unshrouded). Some people design one way, and other people use the other figure. It's up to you.

So, that gets you somewhere in the neighborhood.
You can't use a flow bench for this, because we're not increasing flow. We're making wave tuning modifications to extend flow longer after BDC which doesn't show up on a static flow bench.
And unless you have a dyno, you have to use the "seat of the pants dyno".
If it's working, you'll know it, beause you'll have more torque, and everything above the torque peak(hp) will be stronger.

A fan won't help with anything. It's all based on engine filling during the intake period.
Vacuum gauge won't help anything either.

If you really want hard numbers, you have to do it on a dyno.


This is well-proven technology. All serious racers do it.
Most racers use high-revving engines, so their tubes don't need to be that long.
Our difficulties come from our low target rpms, and not having alot of room to put the tube. It can be curved gracefully with shallow radius curves, if that helps any.
I think we need to make a compromise between what we can get for length that fits into the appearance of the bike.
But, it's fun to experiment!

Any inlet length increase is going to be picking up some wave reflection at some rpm. It just depends on what it happens to be. 7" inlet tract, which is what we have from vavle to carb mouth, is too short for any wave pulse  to be strong enough to really do anything for us at our low rpm range. It's just too short. So, anything "could" possibly be a help at some rpm. The longer, the better. You'll run out of room to put the tube before you hit the optimal lengths. But it's worth a try to get something.
A BSA Gold Star racer I know uses a 15" length pipe on his Gold Star carb. With a big fat K&N filter on the end. He has a support bracket for it coming off his rear shock mount. It goes that far back!

[Blltrdr]

 Ace, can you use a cone shaped tract to get more volume instead of a long cylinder? Or is it necessary to have a tube like your using?

[ace.cafe]

Ace, can you use a cone shaped tract to get more volume instead of a long cylinder? Or is it necessary to have a tube like your using?

Yes, for certain purposes you could do that. That would handle the needed volume on the engine side of the filter barrier. That is a different matter.
But, not for the purpose that we're talking about here.
For wave functions, the length is the key. The reason is that we're timing that wave reflection to coincide with our air intake charge. The length directly determines the timing of that sound wave, and we can't get around that. So, we need to have the correct length if we want to get this effect.

[Blltrdr]

 I have the PWK. If you think about it there is plenty of room, look at the spacing from the head to the carb on a stock bike. Shouldn't be a problem with the PWK.

[ace.cafe]

I have the PWK. If you think about it there is plenty of room, look at the spacing from the head to the carb on a stock bike. Shouldn't be a problem with the PWK.

I can't think of any downsides to having it, then.

[ace.cafe]

Ace, your mod is very inexpensive and could be fun to tinker with for a while. But is there any way to test these tubes on the bike to generate some data to see if the effort is worthy? Maybe hook up a vacuum gauge and take some readings? Maybe add a fan to simulate a forced air condition, then compare it to stock figures and so on? Having data to prove the value of something is half the fun of the chase to improve something.

Well, we start off by doing some calculations.
Here are some of the basic formula calcs
Pulse in these calcs is the same as reflected wave.

Intake Tract Tuning for RPM = (1100 x Half Intake Cam Duration x 0.960)/Length = 2nd Pulse
(1100 x Half Intake Cam Duration x 0.705)/Length = 3rd Pulse
(1100 x Half Intake Cam Duration x 0.538)/Length = 4th Pulse

Intake Tract Tuning for Length = (1100 x Half Intake Cam Duration x 0.960)/RPM = 2nd Pulse
(1100 x Half Intake Cam Duration x 0.705)/RPM = 3rd Pulse
(1100 x Half Intake Cam Duration x 0.538)/RPM = 4th Pulse

The earlier pulses like 2nd pulse are strongest, and then get progressively weaker as they increase to 3rd pulse, 4th pulse, and so on.
Nascar tunes to 3rd pulse.

This gets you some intake tract lengths to look at for starting points, and the challenge is to try to find where you are going to put all that intake length once you figure out how long it's going to be. And it IS gonna be long. the lower the rpm that you target, the longer the tube must be. And they get real long at the low rpms that we're targeting.
So, then we just probably try to get 4th pulse if we can, and there's realy not enough room for anything that looks relatively "normal" for that length either, so we "could" just try for as long as we can fit. That's what I did.

I do have a full-length 4th pulse tube here that I'm going to strap on the bike and see what it gets me. Just to see. Just so you know what it is for a 4th pulse on the Bullet at 3500 rpms, the inlet tract(this length includes everything from the  back of the valve head all the way to the tube air inlet opening) is 22.3" for the half of 264 duration calc,

Anyway, our goal for the Bullet would be to tune the pipe for the torque peak, which is somwhere between 3250 rpm and 4000 rpm, depending on what the mods on your motor are. For a Bullet equipped with performance exhaust and 28mm Mikarb, it's probably going to be around 3500 rpms for torque peak. Larger carb will have higher torque peak.
The other kind of goal is to tune at higher rpm for the horsepower peak. That's a shorter pipe. But it's only good at your rpm at peak hp.
So, you make the call. I think boosting torque peak is better for us, because with higher torque peak, all the hp figures above torque peak also get boosted. Downside is a real long tube.

Cam duration for the intake cam on a Bullet is 264 degrees at .012" lash, or about 230 degrees at .040" lash(which is where the valve curtain is unshrouded). Some people design one way, and other people use the other figure. It's up to you.

So, that gets you somewhere in the neighborhood.
You can't use a flow bench for this, because we're not increasing flow. We're making wave tuning modifications to extend flow longer after BDC which doesn't show up on a static flow bench.
And unless you have a dyno, you have to use the "seat of the pants dyno".
If it's working, you'll know it, beause you'll have more torque, and everything above the torque peak(hp) will be stronger.

A fan won't help with anything. It's all based on engine filling during the intake period.
Vacuum gauge won't help anything either.

If you really want hard numbers, you have to do it on a dyno.


This is well-proven technology. All serious racers do it.
Most racers use high-revving engines, so their tubes don't need to be that long.
Our difficulties come from our low target rpms, and not having alot of room to put the tube. It can be curved gracefully with shallow radius curves, if that helps any.
I think we need to make a compromise between what we can get for length that fits into the appearance of the bike.
But, it's fun to experiment!

Any inlet length increase is going to be picking up some wave reflection at some rpm. It just depends on what it happens to be. 7" inlet tract, which is what we have from vavle to carb mouth, is too short for any wave pulse  to be strong enough to really do anything for us at our low rpm range. It's just too short. So, anything "could" possibly be a help at some rpm. The longer, the better. You'll run out of room to put the tube before you hit the optimal lengths. But it's worth a try to get something.
A BSA Gold Star racer I know uses a 15" length pipe on his Gold Star carb. With a big fat K&N filter on the end. He has a support bracket for it coming off his rear shock mount. It goes that far back!

Well, I tried the real long inlet tract extension today.
The full 4th pulse length of about 23", and I had to snake it around the side of the bike and it went back to the rear shock mounting bolt. Very long.

Took it out for a spin, and it apparently began to work at a slightly lower rpm than predicted, but not too far off. Around 3300 rpm.

And here's how I knew where it began to work.
I hit a spot where I couldn't accelerate any further. It got to 3300rpm(approx) and then it started coughing and sputtering because it was going lean.
Couldn't get it to go any faster.
So, then I put on the enrichment lever, and I was able to get a few hundred more rpms before sputtering again.

So, I need to re-jet richer in the main jet.
The effects of this mod are changing the air going into the engine sufficiently to require jetting-up richer on the main jet.
I'm going to order some bigger jets to test this thing further.

[Blltrdr]

 Keep us all informed of your testing. It is very interesting! What is the target RPM's you are trying to achieve with your mod?

[ace.cafe]

Keep us all informed of your testing. It is very interesting! What is the target RPM's you are trying to achieve with your mod?

With this long extension, I worked it out for 3500rpm target at the estimated torque peak..
It is always an approximation, because of variations in temperature, and the fact that I didn't measure my inlet tract to the exact millimeter and stuff like that.
So, it was actually pretty close to what I targeted, by getting some results around 3300 rpm.
I'm figuring my torque peak to be in that neighborhood of 3300-3700 rpms somewhere.

Without a dyno, it's all estimation. So, I'm just playing around with it, and seeing what I can do to tune it in by observing what it's doing and making changes from there.

The long extension pipe is pretty unusual looking, for sure. I'm not sure if I can get used to something like this, from an appearance viewpoint.

But, whether I end up using it permanently or not, I'm having fun with the learning experience and experimentation. I like doing this kind of stuff.

[Blltrdr]

Keep us all informed of your testing. It is very interesting! What is the target RPM's you are trying to achieve with your mod?

With this long extension, I worked it out for 3500rpm target at the estimated torque peak..
It is always an approximation, because of variations in temperature, and the fact that I didn't measure my inlet tract to the exact millimeter and stuff like that.
So, it was actually pretty close to what I targeted, by getting some results around 3300 rpm.
I'm figuring my torque peak to be in that neighborhood of 3300-3700 rpms somewhere.

Without a dyno, it's all estimation. So, I'm just playing around with it, and seeing what I can do to tune it in by observing what it's doing and making changes from there.

The long extension pipe is pretty unusual looking, for sure. I'm not sure if I can get used to something like this, from an appearance viewpoint.

But, whether I end up using it permanently or not, I'm having fun with the learning experience and experimentation. I like doing this kind of stuff.

 I was thinking yesterday you could run your extension out then down between the gearbox and swingarm. Maybe configure some kind of scoop. I'm not sure how the bends in the tube would effect your tuning? It would give you some length and some stealthiness! I was picturing the end of a auto carpet cleaning attachment as the scoop. Maybe with a gauze face plate. Just an idea.

[ace.cafe]

Keep us all informed of your testing. It is very interesting! What is the target RPM's you are trying to achieve with your mod?

With this long extension, I worked it out for 3500rpm target at the estimated torque peak..
It is always an approximation, because of variations in temperature, and the fact that I didn't measure my inlet tract to the exact millimeter and stuff like that.
So, it was actually pretty close to what I targeted, by getting some results around 3300 rpm.
I'm figuring my torque peak to be in that neighborhood of 3300-3700 rpms somewhere.

Without a dyno, it's all estimation. So, I'm just playing around with it, and seeing what I can do to tune it in by observing what it's doing and making changes from there.

The long extension pipe is pretty unusual looking, for sure. I'm not sure if I can get used to something like this, from an appearance viewpoint.

But, whether I end up using it permanently or not, I'm having fun with the learning experience and experimentation. I like doing this kind of stuff.

 I was thinking yesterday you could run your extension out then down between the gearbox and swingarm. Maybe configure some kind of scoop. I'm not sure how the bends in the tube would effect your tuning? It would give you some length and some stealthiness! I was picturing the end of a auto carpet cleaning attachment as the scoop. Maybe with a gauze face plate. Just an idea.

Yes, I had the same thought!
And it could just barely fit down there. I tried it.
But after looking at it, I sort of liked it better going toward the back of the bike.
It looks odd anywhere you put it.
Nobody is accustomed to seeing an 18 inch long pipe coming off the carb, with a K&N on the ehd,  No matter which direction you run it.

[Jon]

A friend of my son rigged up a fan from a computer between the filter and the
carb on one of those BMW/Rotax singles and claimed beneficial results wonder
if that might work?

[ace.cafe]

A friend of my son rigged up a fan from a computer between the filter and the
carb on one of those BMW/Rotax singles and claimed beneficial results wonder
if that might work?

I really can't see something like that working, Jon.
The air pumping ability of the engine itself far exceeds any of those fans.
As soon as the intake valve opened, the fan wouldn't be able to keep up with demand, and it would act as an impediment to the flow under those circumstances.
It sounds like it should work, but the peak demands of the engine would over-run the ability of the fan to keep up.

[PhilJ]

Ace,

All this about increasing air flow, now let me date myself. years ago it was desirable to use "dead air space" with a large volume to allow the engine to draw what it needed in the manner it was needed.

I'd be interested in hearing (reading) your thoughts or commentson "normal" aspiration.

[ace.cafe]

Ace,

All this about increasing air flow, now let me date myself. years ago it was desirable to use "dead air space" with a large volume to allow the engine to draw what it needed in the manner it was needed.

I'd be interested in hearing (reading) your thoughts or commentson "normal" aspiration.

Good question, Phil.

The dead air space is basically a requirement for throttle responsiveness.

With current technology, this dead air space requirement has been pared-down to approximately double the engine displacement. In a 500cc engine, then we'd be looking for about a liter of dead air space on the engine side of the filter barrier. This includes the air in the port and in the carb, as well as inside the airbox and  filter body itself.
In some cases, this size of dead air space is partially compromised for space to fit it on the bike. Such as Ducati is currently using about 1.3 liters of dead air space in their 999. But, a bit more than that is desired when space permits.

Now, the dead air space, as I understand it, is primarily to allow a big gulp of air on initial throttle openings, for rapid throttle response.
I don't think that it really has much to do with turbulent air in the vicinity of the intake throat.
And there is little(if any) benefit to provide more dead air space than is actually needed for the purpose, Because the dead air space will replenish itself thru the filter by natural movement of the air. It's when the dead air space becomes less than the engine displacement where it can show itself as a flow problem, primarily as lag in throttle response time..

So, current practices would be to use always more than engine displacement volume in the inlet tract on the engine side of the filter barrier. And preferred to provide double the displacement volume if it is practical to do.

Is that sufficient? Or do you want discussion about how to optimize inlet flow in  normally aspirated engines? That's quite a big discussion, in fact there's whole books written just on that alone. But, the wave tuning stuff that I've been discussing on this thread is part of that.

[PhilJ]

That's good, but it does bring up another question. If the dead air space covers the initial big gulp need for throttle response, and the wave tuning for torque (in our application), it doesn't seem practical, space being the limiting factor, to incorporate both methods. So what, in your opinion, would be more important?

[ace.cafe]

That's good, but it does bring up another question. If the dead air space covers the initial big gulp need for throttle response, and the wave tuning for torque (in our application), it doesn't seem practical, space being the limiting factor, to incorporate both methods. So what, in your opinion, would be more important?

The avaliable dead air space on the engine side of the filter is the more practical, and the more important for normal street riding.

I think the wave tuning is more suitable for racers, or maybe hot-rodders. The wave tuning works well, but I think it is too long and unusual looking for most street Bullet applications.

Actually the stock Bullet airbox and inlet system has sufficient dead air space in it as it is.
But when we take that off, and put the short K&N pod filters on the carb, then we change that.

A few inches of tube extension between the K&N and the carb inlet opening should do the  trick to restore enough dead space.
But, it is important to put a support bracket of some type on, to support the carb and filter assembly, or else it is going to hang too much on the rubber inlet hose and make its life even shorter than normal.

[PhilJ]

Thanks Ace.Cafe,

I thought that may be your answer. I'm glad to know that for my application, it's as good as it gets. 

[BigDon]

That's good, but it does bring up another question. If the dead air space covers the initial big gulp need for throttle response, and the wave tuning for torque (in our application), it doesn't seem practical, space being the limiting factor, to incorporate both methods. So what, in your opinion, would be more important?

The available dead air space on the engine side of the filter is the more practical, and the more important for normal street riding.

I think the wave tuning is more suitable for racers, or maybe hot-rodders. The wave tuning works well, but I think it is too long and unusual looking for most street Bullet applications.

Actually the stock Bullet airbox and inlet system has sufficient dead air space in it as it is.
But when we take that off, and put the short K&N pod filters on the carb, then we change that.

A few inches of tube extension between the K&N and the carb inlet opening should do the  trick to restore enough dead space.
But, it is important to put a support bracket of some type on, to support the carb and filter assembly, or else it is going to hang too much on the rubber inlet hose and make its life even shorter than normal.

Ace, that was my thinking after reading all the other posts on the subject and the info from tech articles on "air box volume" and why I opted for the K&N in  the air box route to my bullet mods.

[Chuck D]

Just dusting off some interesting old threads. Enjoy!