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Author Topic: Let's talk about cams.  (Read 13932 times)

ace.cafe

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Let's talk about cams.
« on: January 03, 2009, 03:15:00 PM »
After a post on another thread by Jon Applegate about cams, I thought it might be good to take a look at cams for the Bullet.

First, let's find out what cams do.
Cams are actuators for the valves. They translate rotary motion into linear motion. So, as a cam is turned by the crankshaft gear, it has a "cam lobe" which is ground eccentric to the centerline, and  moves the valve lifter up and down, according to the designer's intentions for the expected application of the engine.
This is the basic function of cams in an engine.

In the world of increasing performance in an engine, cams are often discussed as a central theme. After all, the cams control the valves, and the valves control the breathing, and the breathing controls the power. So, it stands to reason that cams are a big part of making an engine produce more power.

How do cams control the breathing?
Well, this is simple. They are in control of when the valve opens and when the valve closes. The longer the valve can remain open, the more flammable mixture can enter the cylinder for burning, and the more power can be produced. And the higher the valve can be lifted off its seat, the more mixture can come into the engine, as well.
So, this defines our 2 main areas of concern with cams. "Duration" and "Lift". These combine to regulate our breathing in the engine. For performance purposes we want  long duration to keep that valve open as long as possible, and high lift to open that valve as far as we can. Doing both things to try to maximize the "gulp of air" that we are pulling down the port.

So, what's the big deal, let's just open them up wide open and leave them open forever!
Well we can't do that, because the engine has cycles for intake, compression, power, and exhaust, so they need to be closed sometimes, and mechanical restrictions in the engine design will dictate how far they can be opened.

And this is where the "art and science" of cam design comes into play. 
How much can we do with our particular engine design, for our particular purposes?
That's the big question.

First we need to look at the effects that come from longer duration.
Many people may think that valves just open at TDC(top-dead-center) on the intake stroke, and close at BDC(bottom-dead-center) on the intake stroke. It's easy to assume that the piston works like a hypodermic syringe, and only fills when the piston is descending. And, for a long time, that's what people thought, and how they designed intake systems. TDC to BDC breathing cycle.
Then as engineering became more sophisticated, better methods were developed.
For example, we can't open the valves instantly from fully closed to fully open. They require some "ramping", so that we don't lose control of them from insane acceleration rates, which the valve springs can't handle. So, they need  some time to get open, and some time to close down. So, we open them a bit before TDC, and close them a bit after BDC. This way, they are open pretty much fully for the entire TDC to BDC intake stroke. That's a good start.
So, what happens when we try to have them open even more than that?
Well, then we have some interactions going on that we need to consider.
The first one is that we need to understand that the air coming into the engine has a mass and a flow rate curve. When we draw the piston down, the air in the port doesn't immediately accelerate to full velocity. It begins to speed up, and there is a bit of a time lag before it's flowing at full velocity. And on the other end, even after the piston stops going down, the air still is moving into the cylinder because it has some inertial energy of its motion that causes it to still keep coming into the cylinder, because of this time lag of the mixture velocity curve. So, some of the filling of the cylinder is actually accomplished after BDC, when the piston is no longer pulling it down. After BDC, the air is coming in by virtue of it's own inertia, on the tail end of the flow curve, which is lagged behind the motion curve of the piston movement.
So, we can utilize this additional fill ing potential by NOT closing the valve at BDC. We want to keep it open longer, so that we can capture the incoming mixture that is still flowing in by inertia. How much later can we close it? Well that depends on alot of things, but we may get to that later.
Ok, now that we know that we can close the valve later than we thought, to attain more performance, what about opening it earlier?
Well we can do that too. But how?
If we open the valve before TDC, isn't the ascending piston going to push air back up the intake port? Well, yes it could, but we have a way around that.
The piston slows down as it approaches TDC and BDC, and has what is known as a "Dwell period" where very little vertical motion occurs as it passes the top and bottom of the stroke. This is due to the circular motion of the crank, and at top and bottom of the stroke, the crank is moving horizontally(primarily) and not vertically, so the piston is hardly even moving at all for about 20 degrees on either side of TDC and BDC. So, this is an area where we can work in.
But, how do we get anything coming in, when the piston isn't pulling down like a syringe?
The answer to this involves the exhaust. On the exhaust stroke, which is going on just prior to the intake stroke, the exhaust valve is open, and there is a rush of exhaust gas flowing out the exhaust port. If the exhaust flow speed is fast enough, it causes a low pressure zone around the intake valve, and if we open the intake valve at the right point, we can "hitch a ride" on this exhaust flow to start pulling some of our intake mixture into the cylinder while both valves are open around TDC. This is called "cam overlap", where both valves are open at the same time.  Yes, some intake mixture goes out the exhaust port, but this cool mixture helps to cool the exhaust valve a little bit, and the main benefit is to start the intake flow curve happening a bit earlier than it normally would. Of course there is a limit to this, and if you open the intake valve too early on the exhaust stroke, you will have exhaust backing out the intake port. Timing is important on the opening and the closing valve events, so that you can get what you want, and avoid what you don't want.

So,we've looked at ways we can open the intake valve eariler, and close it later, to get addtional mixture into the engine. And we've briefly looked at cam overlap, and what it does.

I'm continuing to write this, but I'm posting it up in sections as I go along. More to come in a few minutes.
« Last Edit: January 03, 2009, 03:40:35 PM by ace.cafe »
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ace.cafe

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Re: Let's talk about cams.
« Reply #1 on: January 03, 2009, 03:45:22 PM »
Ok, now to continue, and discuss about how this applies to the Bullet.

The Bullet is a long stroke, long rod, engine design.
This type of engine has a pretty large amoung of "dwell period" around TDC and BDC. We can work with this to our advantage.
By using a later intake valve closing timing, we can use the inertia of the mixture after BDC to really assist our cylinder filling, because the piston isn't starting to come back up right away on the compression stroke. In fact we can leave it open for nearly  75 degrees after BDC, if we can use that properly.
2 things need to be attended-to, in order for this to work.
We need to have sufficient intake air-speed inertia, so that the air will still be rushing in, or else there is no point to leaving the valve open. So that means that our intake port cannot be too big, so as to cause a slowing of airspeed in the port. We NEED to have high port speeds for this, or it doesn't work. This is why it is so important to not enlarge the port too big. Enlarging increases potential mass flow, but kills the air velocity in the port, and we need that for late intake valve closing timing to work right.
The second thing is that with late intake valve closing timing, the piston is ascending, and trying to build compression. If the intake mixture speed that is still trying to fill the last bit of mixture into the cylinder is overcome by the attempt of the piston to build compression, the mixture starts to get pushed back up the intake port, and we get reversion, lower compression, and is generally counterproductive.
So, the ideal is to close the valve at the exact point where the intake mixture flow is just still slightly higher than the building of compression. Then close it down tight to get the maximum fill. In practice, some building of compression will be lost in this process, so that's why it is critical to use a higher-compression piston with cam timing like this. We need a higher compression piston to help us get back any compression that may have been lost with the late valve timing, in the remaining part of the compression stroke.

Ok, now let's look at the earlier intake valve opening timing for the Bullet.
The earlier intake valve opening event relies on rapid exhaust speeds to start pulling intake mixture in, as the intake valve is opened at the tail end of the exhaust cycle, during the overlap period.
Unfortunately, the Bullet exhaust port and exhaust pipe is oversize in stock form, and does not provide us with much exhaust speed to speak of. And there's not much we can do about it, because it's already too big to start with.
So, early intake valve opening timing does not do us much good with the Bullet engine.
It works ok at higher rpms, and that's where wide valve overlaps are at their best, but the problem is that our Bullets are low-revving engines which are rpm limited by piston speeds and strength of the crank and rod in the bottom end. So, we can't really access these higher rpms unless we are racing, or have the expensive crank and rod package that most of us don't have or can't afford.

However, the cam designers are designing race cams. They expect high-rpm use with their cams. So, they build in alot of early intake valve opening timing for wide overlap, with the idea that we will all be riding around at 7000 rpms, which is not the case for 99% of us.
The result of this is that we get the very "soft and spongy" feel of the engine in the low and midrange rpms, that comes along with too much cam overlap timing, and we can't access the top rpms where these cams really work well at.
So we get a spongy-feeling engine with power losses down low, and only can access about 1000-1500 rpms of the area where the cams will be working right.
This translates into "less than desirable" road application, and it explains why the cams are not real popular with Bullet owners who don't race.
All of the current performance or racing cams for the Bullet today, no matter where you get them, or who makes them, suffer from too much overlap timing for a good road application. They sacrifice too much of the low and midrange torque, for a small slice of power that we can access at the top end of the rpm scale with a standard Bullet bottom end.
That's the long and short of it, and none of the current cam designers have decided to really address this matter for normal Bullet road riders with standard-issue bottom ends. Because you see, what sells race cams is max hp and max rpm figures. They don't ever even discuss what happens to your midrange, because top end power is all they are after.

What's needed for the Bullet is a cam set with a late intake valve closing timing, and a more moderate intake valve opening timing, along with an exhaust cam with earlier exhaust valve closing time. This will shorten the overlap period on both ends, improve the cylinder trap, and provide good top-rpm power in the ranges where we can access it, and not lose any of the low and midrange torque that we need for our road riding. Use a cam set like this with the hi-compression piston and a properly ported head with a multi-angle valve job, free-flow exhaust, and good carburetion and free-flow intake filter,with about 4 degrees retarded ignition timing, and it should put you over 30hp at the rear wheel, and still allow use of the standard bottom end. And there would be no losses anywhere in the rpm range, compared to what you have now. It would be all gain, and no sacrifice at any rpm.. It's a cam set to produce torque and power in the ranges that a standard Bullet bottom-end can access.

That's about it for now.

Thread is open for comments and questions.
 ;D
« Last Edit: January 03, 2009, 04:54:06 PM by ace.cafe »
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Vince

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Re: Let's talk about cams.
« Reply #2 on: January 03, 2009, 05:07:29 PM »
     Ace, I would appreciate it if you would go into the flow or wave dynamics of exhaust technology, particularly as it applies optimizing combustion chamber filling. Most Enfield owners go to a free flowing exhaust. However,  more modern systems are using 2-cycle expansion chamber technology and flow formulas for the exhaust systems to aid scavenging as well as well as intake.
     In high-revving engines  this type of exhaust allows for wilder cam timing yet still manage to produce  a relatively wide power band. Taken to extremes these exhaust systems even have variable valving in the system itself to change resonance frequencies at different RPM.
     How would any of this technology benefit a low revving engine such as the Enfield? Would a tuned expansion chamber allow for more effective use of existing cams and port sizes?

Cabo Cruz

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Re: Let's talk about cams.
« Reply #3 on: January 03, 2009, 05:23:09 PM »
Vince, I join you -- and othres --  in anticipation of the next installment by The Doctor, er, our dear colleague, Ace.  And, Jim (HRAB) forgive me for usurping the moniker of your favorite racer, THE DOCTOR, or #46!
Long live the Bullets and those who ride them!

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ace.cafe

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Re: Let's talk about cams.
« Reply #4 on: January 03, 2009, 06:07:54 PM »
     Ace, I would appreciate it if you would go into the flow or wave dynamics of exhaust technology, particularly as it applies optimizing combustion chamber filling. Most Enfield owners go to a free flowing exhaust. However,  more modern systems are using 2-cycle expansion chamber technology and flow formulas for the exhaust systems to aid scavenging as well as well as intake.
     In high-revving engines  this type of exhaust allows for wilder cam timing yet still manage to produce  a relatively wide power band. Taken to extremes these exhaust systems even have variable valving in the system itself to change resonance frequencies at different RPM.
     How would any of this technology benefit a low revving engine such as the Enfield? Would a tuned expansion chamber allow for more effective use of existing cams and port sizes?

Hi Vince,
Wave tuning is something that is generally thought of as very technical and mysterious, but it's really a very simple principle.
It is all just the timing of the sound waves as they move down the exhaust port and reflect back up. That's really all there is to it.
You make the pipe a certain length which is the right length to time the reflected sound wave to come back into the combustion chamber at the point when the valve is open for the effect you want it to have.

From a road riding view, this is not alot of use to us, because the pipe length tuning is for a specific rpm, and the harmonic intervals of that specific rpm.
So, you could tune your pipe for max hp rpm, but that's pretty much the only place it's going to work, or maybe half that rpm also, as the next lower harmonic. Beyond that, it's not going to really do anything. It pretty much primarily works best on an open pipe without a muffler. This is good for racers, but it's of dubious benefit for road work, and many/most of the road exhausts have wave-breakers in the form of slant tips or baffles, or even anti-reversion cones to break these effects on street machines. Sometimes you see the reversion effects as a "stand-off" of gas mist outside the carburetor throat, as the wave reversion from the exhaust passes thru the open exhaust valve and thru the open intake valve during the overlap period at idle, and blows this mixture out the carb, where it stands-off for a short period and is then sucked back into the intake.

An expansion chamber works in a similar way to the end of an open exhaust pipe.
When the exhaust "sees" a big change in air pressure, such as an opening of a pipe or an wide-diameter expansion chamber, there is an "impedance mismatch" which reflects the sound wave back up the pipe. With the expansion chamber, this can be placed fairly close to the exhaust port, without having a super short overall exhaust pipe length. This causes rapid reflection behavior which should be timed to the engine to give best results at a certain rpm range. This is what's commonly called being "on the pipe" in 2-stroke vernacular. As you noted, this would generally be productive for higher revving machines, since they need quick reflections at those high rpms. We don't have that. We need longer pipes to get our results, because we are operating at slower rpms, and thus the reflections don't want to be that quick.
In any case, these kinds of mods are very "tuned" to specific rpm ranges of the engine, and in a Bullet where wide torque band is the hallmark of our engine's ridabiity characteristics, we would be better served with a normal exhaust system that doesn't get "peaky" at any particular range, but instead gives a broad and flat torque curve. At least, for road riding that would be preferred.

Variable valve timing is a whole other can of worms that has alot of good benefits, but we'll never see them in our Bullets. The main function of VVT is to reduce overlap at lower throttle openings for best torque in the lower and midrange rpms, and widen overlap at higher throttle openings to take advantage of where the overlap can give benefits. It overcomes the issues seen in fixed valve timing at different rpm and load conditions, and provides improved efficiency, fuel economy, and power, with enhanced ridability throughout the range of rpms and throttle settings. And yes, since the valve timing will be varied, it will give varied result in the wave behavior in the exhaust and intake systems as well. Some engines, such as the MV Agusta, even vary the length of the intake tracts via motorized velocity stack systems, to optimize the intake wave tunings for various engine requirements.
This is all computerized stuff, and is way beyond anything we could dream about with the Bullet.

Our engines would benefit more from what you see in the Harley-Davidson category, which is closer that what our engine design is like. In fact, our Bullet is rather like "half" of the old Sportster engine, with hemi chamber, 2-valve, pushrod, long stroke, aircooled design. Really quite similar.

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Vince

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Re: Let's talk about cams.
« Reply #5 on: January 03, 2009, 06:41:49 PM »
     Ace, when I asked about variable valves I meant that some modern bike have a valving mechanism actually in the pipe after and in addition to the exhaust valve. It does not completely block the port but rather is a relatively small restriction in the port. This restriction changes with RPM and throttle setting with more restriction at low RPM and less at high RPM.
     Two stroke engines drive this with gears driven by a Sprag clutch. Four stroke engines are usually computer/servo controlled. Less refined systems could be throttle cable controlled.
     Would this be of help or even be worth the trouble?

ace.cafe

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Re: Let's talk about cams.
« Reply #6 on: January 03, 2009, 07:12:11 PM »
     Ace, when I asked about variable valves I meant that some modern bike have a valving mechanism actually in the pipe after and in addition to the exhaust valve. It does not completely block the port but rather is a relatively small restriction in the port. This restriction changes with RPM and throttle setting with more restriction at low RPM and less at high RPM.
     Two stroke engines drive this with gears driven by a Sprag clutch. Four stroke engines are usually computer/servo controlled. Less refined systems could be throttle cable controlled.
     Would this be of help or even be worth the trouble?

Ohhhh!
You mean something like the Yamaha EXUP system. I get it now.

Yes, that is sort of like a carburetor butterfly valve for the exhaust system.
It can work, and I think Yamaha had some good results with that, and I believe that the patent has recently expired on it.

I never experimented with something like that myself.
I suppose if you could rig it up at the right spot, and make a suitable control mechanism for it, you might be able to get something out of it.

In the end, it basically ends up being a "variable diameter exhaust pipe" to a certain degree.
The problem that I see with it is that it happens only over a short part of the exhaust system so speeds are dramatically changed over a short distance, and I'm not sure how great that is for overall results.
But heck, I never tried it, so maybe I'm all wet about that. It might be worth a try.
If these other companies are using it, then there's probably something useful about it, or they wouldn't put it on there.
« Last Edit: January 03, 2009, 07:17:06 PM by ace.cafe »
Home of the ACE Fireball 535 Bullet,  Ace GP Hi-Lift Roller Rocker Head . Pistons, cams, etc. Highest performance Bullet engine mods available .  AVL mods. Redditch 700/750 Twin mods. UCE kit soon.

Please visit my new website:
http://autos.groups.yahoo.com/group/AcePerformanceBullets/

Vince

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Re: Let's talk about cams.
« Reply #7 on: January 03, 2009, 07:36:34 PM »
     Thanks Ace! Very interesting  and well expressed!

redcat

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Re: Let's talk about cams.
« Reply #8 on: January 03, 2009, 09:23:06 PM »
Wow! That was the best piece I have read on this site! Thanx Ace....Tony
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PhilJ

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Re: Let's talk about cams.
« Reply #9 on: January 03, 2009, 11:14:42 PM »
Learn - learn - learn - Now my head hurts. ::)

Cabo Cruz

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Re: Let's talk about cams.
« Reply #10 on: January 03, 2009, 11:17:46 PM »
Amen, Phil...   :o
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jonapplegate

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Re: Let's talk about cams.
« Reply #11 on: January 05, 2009, 02:06:33 AM »
  I like to think I know a little about the mysteries of the internal combustion engine and try to keep on top of the tech.
  I stand corrected.
   I stand in awe of both Vince and Ace.
    I gots sum learnin' to do.....
       
  SO, I was wondering about slightly enhanced cams instead of "performance" and "full race" options. Designing a new cam is a little more involved than just adjusting the Duration, Overlap, Lift etc. Would it be easier (less expensive) to design and produce a higher ratio rocker? I don't know the ratio of the RE rockers but say something modest like going from 1.5 to 1.6? And if someone was were going to do this, how about a roller?

ace.cafe

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Re: Let's talk about cams.
« Reply #12 on: January 05, 2009, 03:56:52 AM »
 
       
  SO, I was wondering about slightly enhanced cams instead of "performance" and "full race" options. Designing a new cam is a little more involved than just adjusting the Duration, Overlap, Lift etc. Would it be easier (less expensive) to design and produce a higher ratio rocker? I don't know the ratio of the RE rockers but say something modest like going from 1.5 to 1.6? And if someone was were going to do this, how about a roller?

Hi Jon,
Well, you may be interested to know that I've been discussing this with some other engine builders and Bullet owners that I know, on some private technical forums, for about 3 years.  We have worked on trying to do design improvements in these areas, and every way you look, there are limiting factors. These people in this group I've worked with are some of the most knowledgeable people you are going to come across, and have worked with people like Yoshimura and Jerry Branch.

Let's take a look at the option of increasing the rocker ratio. It's possible to get just a little bit of change, before you angle the pushrods too much. We've looked at that, and at an entire re-design of the rocker system. Even gone so far as to CAD/CAM some designs, and run them thru computer modeling. It's not impossible, but very difficult and costly to get anywhere near where we want to go.
And even if we could get some benefit, the lift amounts needed to get appropriate lift to achieve a true hi-lift valve system that can achieve the cone swirl around the valve head in this hemi chamber is on the order of .650" lift, which is never going to happen in the Bullet. We are stuck with a low-lift system, and even the max attainable is something around .425" lift, which is hell on the valve train and still doesn't get into hi-lift territory.

So, basically I can tell you that this territory has all been covered with a fine tooth comb. Certain things "could" be done, but nobody is about to fork over the amount of money necessary to do them, because there's a very small number of people interested in purchasing such items. and the costs to have them tooled up and made are very high. These ambitious projects are just DOA because of that.

Now, that is not to say that some things can't be done, and I have some answers for you.
The answers are primarily involved with knowing how to maximize what we have to work with, and making the best of it.

These answers revolve around making the best of what we can fit into the Bullet package without heroic efforts that cost beyond what is recoverable, and work in the ranges that a normal Bullet bottom-end can withstand, or even a bit higher with the billet crank and steel rod and alpha bearing big end.
We need to  maximize our low-lift package, and we have duration room to work with.

First of all, we need to be realistic about our goals. We are not going to be a Molnar short-stroke Manx with 150mph capability on the track. We want good streetable performance.

For cams, the best option for street use, while retaining good low and midrange torque is a re-phase of the stock cams, like I mentioned on the other thread. This activity is not for the casual user. This is for somebody who knows how to work an engine.
The stock cam timing, as fitted, are not the same as the original Redditch cams. They have been adjusted to suit the larger port size in the Indian-made head, and give larger burnt-gas-fraction(BGF) so as to be better suited for emission control. They are more suited for TDC to BDC cylinder fill, and thus have reduced power levels compared to the original Redditch engine, and the hp figures bear this out.
It turns out that the Indian-made intake cam is further advanced in it's timing than the original Redditch cam. It's advanced enough that it is on the order of about one tooth difference, about 18 degrees. So, if we retard just the intake cam by one tooth, and leave everything else alone, we can approximately restore the old 1950s Redditch cam timing, and get back up to the Redditch 25hp figure of the old British Bullets.
Now, we have to be careful, because the Indian-made Bullets have cams which seem to vary in timing from one to the next. So, we have to use a degree wheel and dial indicator to check the cam timing with what we've got, so that we can be sure what we are doing is going to work out as we intend. "Assuming" that our cam timing is as expected, we can then retard the intake cam by one tooth, and leave the exhaust cam alone, and restore the original cam timing for more power. BUT, we don't have the small ports of the Redditch Bullet in our head, so we need to install a 8.5:1 hi-compression piston to make this work right with the bigger ports in the Indian head, due to lower port speeds in the larger head, so we can recover the lost early compression building involved with the later cam timing.
Ok, that sounds pretty good, but we can go further.
If we use the CMW adjustable cam timing pinion, we can then advance both the intake and exhaust cams by 4.5 degrees, and make the intake cam closing timing a little bit better for the big Indian-made head port, and also close the exhaust cam a little earlier to limit the overlap on the exhaust side for better cylinder trap, since we can't really use all that overlap with our oversize ports.
These mods can get us up into the 30hp and 30ft/lbs torque area at the rear wheel, assuming we have already done the freeflow intake and exhaust system changes and re-jetted.
In fact, this cam timing change can get us within 1hp of the Performance Cams sold by CMW and Hitchcocks, without the serious midrange losses that those cams give.
This is your answer to streetable cams for the Bullet that are better than stock, but not as "lumpy" as the Performance Cams or Race Cams.
What we are doing here is re-phasing the stock cams to move the intake valve closing timing of the intake cam later in the cycle, where we can use it for better power, while moving the opening timing of the intake valve later, because our  overlap performance is poor enough that it isn't doing us much good at all. We can sacrifice early opening to get later closing, and that effectively extends our useful duration by 14.5 degrees where it will do us some good. We don't really lose anything by opening the intake valve 14.5 degrees later. In fact we gain something in power, and the reduced overlap makes it so we don't lose our bottom end torque. The earlier closing timing on the exhaust also helps reduce overlap, and helps keep bottom end torque. Since we are not revving up real high, the shortened overlap period is not hurting us.
It's the answer to hot road cams for the Bullet.

Now, you mentioned roller cams. Well, we really can't design and have these roller cams made for any affordable sum. But we can get some of their effects in another way.
This is pretty complex, and I hope I'm not losing anybody.
The hemi head in the Bullet is very good at low lift breathing capability. The hemi chamber is not shrouded by chamber walls like bathtub chambers are. This is a big breathing advantage over most chamber types. And we have low lift cams, and an engine design which mechanically limits us from getting into the hi-lift neighborhood.
So, we can try to maximize the low-lift flow performance of our system, to get more air in there in the low-lift area that we can access.
If you look at the valve and valve seat, they use a 45 degree seat angle, like most engines do. The valve needs to lift off the seat at least .040" before the valve is high enough to unshroud the valve curtain and allow relatively unimpeded flow into the cylinder, because the seat angle creates this condition. Normally, in bathtub chamber heads, this is not an issue because the chamber walls are partially shrouding the valve anyway, and the 45 degree seat directs the incoming air up along the chamber wall. But, in a hemi head, we don't have chamber wall shrouding, so we can work with the seat angle.
David Vizard, head porting guru, has done alot of work with Mopar Hemi heads, and has found that they can benefit from shallower seat angles. A hemi is a hemi, so the Bullet can also benefit from shallower seat angles for the same reasons.
If we can't get alot higher lift, we can start our flow earlier and get more out of our low-lift area, where most engine builders just ignore.
By cutting in a 30 degree valve seat angle on the seat and the valve, we can unshroud the valve curtain earlier, and begin flow at lower lift than the 45 degree seat would allow.
In practice, this does the same effect as a roller cam. It has been shown possible to get 15-25% more power out of a hemi head by using a 30 degree seat angle on the intake valve.
Now, with the Bullet, we have to go to the bigger intake valve to get the 30 degree seat angle, and in the process we get more valve area to flow, but we also lose a bit of mixture speed at the valve transition area. However, this would seem to be a good trade, as the benefits can outweigh the losses. We have to be very careful to flow the area around the valve seat to have minimum impedance to the airflow, because we've expanded the area there, and we can't afford to slow the flow any more with bad flow shape. We want to keep as much  flow speed as we can, so we can use that late valve closing timing to best effect.
If we do it right, we can expect about 15-25% greater performance than using a 45 degree valve seat angle.
This could get us into the 35hp neighborhood at the rear wheel, if all goes well.
And it will all run on pump gas.

This stuff is highly critical in nature, to make sure that it all comes together right. There is not alot of room for making mistakes. It's for the experienced builder. But, it provides a way to get good power and torque in the Bullet, without much or any sacrifices, and doesn't cost a huge sum of money, but takes alot of time and care.
Perhaps even if you don't get everything exact, you can still get a good fraction of the performance, and wind up with 30hp or maybe 32hp.

Since we're operating now with higher compression and better breathing, we need to retard the ignition timing to suit. About 30 degrees of max advance would be about right, or maybe even a bit less. Failure to address the ignition timing issues will cost you a burnt piston.

Truthfully, this is about on the ragged edge that a stock bottom end can take, and it could push some "less than perfect" stock bottom ends over the edge, and take out the big end or the main bearings, or even the rod. And the stock clutch won't handle this either.
Best bet would be to use the billet crank and steel rod setup, and the expensive racing  clutch.
With that setup, you'll be able to wind it to 6000rpm without concern of failure, and it will really kick ass for a street Bullet.

Again, this is not for just "throwing together a bunch of parts". This is to be done with the same care as if you were building a racing engine, which ispretty much what you are doing.



« Last Edit: January 05, 2009, 04:13:26 AM by ace.cafe »
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Blltrdr

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Re: Let's talk about cams.
« Reply #13 on: January 05, 2009, 04:30:19 AM »
 Ace, wow-wow-wow! That's some great technical writing and explained in a way that most will grasp.  Will the installation of the adjustable cam spindles and the re-phasing of the cams bring the HP up to Redditch specs on a stock motor or is it in conjunction with the high comp piston? Also, are you planning on installing a set of these spindles yourself? This has been a very entertaining thread, keep up the good work!   Blltrdr
« Last Edit: January 05, 2009, 04:33:09 AM by Blltrdr »
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ace.cafe

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Re: Let's talk about cams.
« Reply #14 on: January 05, 2009, 05:00:13 AM »
Ace, wow-wow-wow! That's some great technical writing and explained in a way that most will grasp.  Will the installation of the adjustable cam spindles and the re-phasing of the cams bring the HP up to Redditch specs on a stock motor or is it in conjunction with the high comp piston? Also, are you planning on installing a set of these spindles yourself? This has been a very entertaining thread, keep up the good work!   Blltrdr

Blltrdr,
Just to clarify, I'm discussing the adjustable timing PINION, not adjustable cam spindles.
The adjustable timing pinion is a replacement  for the standard crankshaft pinion gear which drives the cams. The adjustable one offers 2 extra keyway slots on it, so that it can provide 4.5 degrees of change in cam timng, either advanced or retarded, depending which keyway you line up on the crank key.

Yes, you absolutely do need to use the higher compression piston to get the benefit from this rephase of cams, when changing by one tooth.
I haven't done this on my road Bullet yet, because I haven't purchased the hi-compression piston for it yet.
But, I know at least 3 people who have done it on their Bullets with fine success, and one is racing with this cam option. It has been proven to work as stated.
I will be re-phasing my cams as soon as I can afford to get the piston necessary to complete the job. Without the hi-compression piston, it's not appropriate to do the cam change.
Home of the ACE Fireball 535 Bullet,  Ace GP Hi-Lift Roller Rocker Head . Pistons, cams, etc. Highest performance Bullet engine mods available .  AVL mods. Redditch 700/750 Twin mods. UCE kit soon.

Please visit my new website:
http://autos.groups.yahoo.com/group/AcePerformanceBullets/