Tech Talk: Should I, or Shouldn't I "Port" my Bullet's Cylinder Head?

[ace.cafe]

Okay, I thought I'd approach a technical subject here, which often comes up early in conversations about modifying the performance of the Classic Iron-Barrel Bullet engine.
This pertains to 500cc Bullets, and not the 350(which is different).

Alot of people get to thinking about doing performance mods to the Bullet to "perk it up" a bit more. Of course the exhausts and carb re-jet and filter come first. but many then wonder about getting "a little more" and think about pistons, cams, and "porting".

What is "porting"?
"Porting" in terms of cylinder head modifications is simply the concept of changing the shape of the inlet port and/or the exhaust port, in an effort to allow more mixture into the engine, so it can burn more, and thus increase power. That's the concept.

The "traditional" way of thinking about porting is to make the ports larger. After all, it stands to reason that if the ports are bigger, they can let more air in. And in many cases that is true, and this is done as common practice in many engine modification procedures for performance.

However, we need to look at all the things surrounding this "porting", to determine how much, if any, work needs to be done in this area. And it totally depends on the exact design of the particular engine that is undergoing modifications, as well as the goals for the modifications.

So, you say, "I want it to go faster". Sounds simple enough.

Ok, let's look at our Bullet engine design parameters and see what we have to work with.
First, we can look at the limiting factors which will define our boundaries for our goals.
This is a basic set of goals, based on the idea that you still want to ride your Bullet on the road, and not just be strictly a track bike.
We have the displacement to feed, which is defined by our bore and stroke for the cylinder that we want to fill.
We have a stroke length which is very long, in comparison to most modern engines.
We have a goal of good street powerband with increased power over a fairly wide range of rpms.
We want to maintain decent reliability, and not have to rebuild the engine very often.

Now, for optimal filling of the cylinder, knowing the basic cylinder displacement already, we need to look at how many rpms we are going to be seeing with this engine. And in our case, it is generally about 5000 rpms, with a possibility of going to 5500 rpms on occasion, and certainly never going over 6000 rpms. This is so we don't blow up the engine bottom-end from revving beyond its capability.
So, that's a limit. And now we need to feed that 500cc's, from 0-5k rpm ,or maybe up to 6k rpm.

Now, when does the port get into this? Right now.
Ports are a transport mechanism for our air volume to enter the engine.
The air flows in with certain air mass, and certain air speed. If we want to maximize our fill, we want to get as much air mass into the cylinder as we can, in the time allotted by the intake valve's open time.
So, now comes the "garden hose analogy".
If we are watering the lawn, and we have a hose of a normal size, we get a certain flow rate of a certain mass and a certain speed, and this adds up to the volume that comes out in an allotted time. If we make the hose bigger, but don't change the water tap setting at the faucet, then what happens? The water comes out slower, doesn't it?
But, even if it comes out slower, if more comes out over the time period, we get gains because the diameter was previously restrictive. And if less comes out, we get losses because diameter wasn't restrictive, but flow speed dropped. Similarly, if we make the hose narrower, the water comes out much faster. And if it comes out faster, and still can flow more water over the time period, we get gains.  But, if it comes out faster, but is restricted to the point that even though it's moving faster, we get less overall water coming out over the time period, then we get losses from the restriction.
So, there are boundaries in both things. Faster flow can help, and larger size can help. But,  only to the points where they create lesser volume flow by either restriction or insufficient speed.

And this is why ports have a certain size. They don't just cast a hole in the head and hope it works.

So, where does this leave us with the Bullet?
Well, the Bullet ports are not very well designed for performance purposes. They are mostly "backwards" in design, because they get wider from the manifold joint to the valve head. A performance port would get slightly narrower from the manifold joint to the valve head, because the ideal would be to force the volume of the larger opening at the manifold to speed up just as it enters the cylinder at the valve, this getting the volume we want at the highest speed.
Unfortunately, we cannot do this with the Bullet head, and this is primarily why porting the Bullet doesn't work as people expect it to.

Engineering practices have shown that air speeds around 300ft/sec is a good flow rate for an intake port to keep flow going into the cylinder real well. The Bullet in stock form with an Indian-made cylinder head can do that, but it requires a very high rpm to do it. 32mm port is big for this engine, at the rpms it can generate. It's actually about as big as it can get, without causing weak areas in the casting, so we can't even really enlarge it much, even if we wanted to. So, we'll never get the ideal port shape, within the flow rate boundaries necessary, because we're already too big to start with for low rpm benefits that we can use with this engine. And casting sizes preclude us from really ever getting the ideal shape anyway.
All this really means, is that we can't enlarge the Bullet ports for beneficial effects at the rpms that we can run it on the streets. Enlarging is not the answer for the Bullet.

So, what can we do?
We try as best we can to improve flow rate marginally, because we already have enough mass capability for our needs. And we can't idealize the shape, really, because of the previously mentioned limitations.  So, we work on general smoothing and improvement of the shape of the port at the downward turn to the valve, and the area just prior to the valve seat.
We can't "change the size of the hose", but we can make sure that the hose is real smooth inside, and has least ill-effects from flowing out the nozzle by optimizing the shape of the nozzle. So to speak.
Our "hose"(port) has alot of rough casting flaws and marks inside it that can impede flow speed, by causing turbulence in the airstream. So, we smooth them down as smooth as we can, without doing any significant enlargement. We just sand down the protrusions, and smooth the surfaces, but don't try to smooth out all the little pits and pock-marks. They don't really matter much. It's the things sticking up into the port that we want to knock-down, and smooth into the general shape of the port.
No "high polish" is needed, nor wanted in the intake port. 220-grit sandpaper finish is just fine in the intake.
That's about all we need to do in the general "tubular area" of the intake port.

Then we come to the bend in the port, after the valve guide location. This is where we can do some good. The casting usually gets pretty rough around this bend. Sand off all the big  casting flaws that stick up here too. And make a nice smooth shape around the outside of the valve guide support hump(not the valve guide itself. leave that alone).

Now we come to the "transition area" where the port changes shape into the "bowl" around the valve guide, and you'll notice that on the floor of the port, there's a pretty sharp turn down to the valve. This is an important area. DO NOT LOWER THE FLOOR OF THE PORT AT THE TURN! Just lightly radius any sharp edge at the peak of that turn, VERY LIGHTLY, so it's not a "table edge", but instead is a a very small radius "bullnose" edge. JUST A LITTLE!

Then work the bowl area with your 220 sandpaper, to smooth the bowl into a nice bowl shape, and try to smooth out the crags and craters as best you can, and blend it very nicely to the joint where the port meets the valve seat insert. If there is a mismatch at the valve seat insert where the bowl is narrower than the seat insert, blend that to match the seat insert, so the flow is smooth from the bowl to the seat insert.

When doing this, make sure that you don't scratch or sand the valve seat insert unnecessarily, and certainly do not even touch the actual valve seating angle cut onto the insert.

Then, take the head to your automotive machine shop, and have them do a 5-angle valve job on the valve seat. This is where most of your gains will come. A good 5-angle valve job, blended to the port bowl, will give more result than all the other stuff you did.

So, here's what we accomplished.
We used the already large diameter port size to keep our air mass levels, without enlarging it any, so as to not have a negative impact on our port speeds.
We helped the port speeds to reach their maximum capability, within the framework that we could work in, to remove obstructions and smooth the pathway into the engine. We rounded any sharp "table edges" on the short-radius turn to help the air hang on to the port as it goes around the bend on the floor. And we shaped the bowl and transition to the valve seat insert to remove obstructions, and generally smoothed the shape. And we got the right kind of valve job to assist the air to hang onto the turn at the valve seat into the cylinder.

This is the best you can do with an Indian-made Bullet cylinder head.
You don't need to do jack-shit with the exhaust port, because it's already so big that we actually want the restrictions in there. You can actually leave the exhaust port just as it is, or if you want to knock down any big mountains or ridges left by the casting process, you can do that.

If you do all this correctly, it can be good for maybe up to 15% improvement in power.

And this is why, if you buy the "ported head", it is done just like this, except they put in big valves(which aren't needed for a street Bullet), and do a real nice beadblasted finish, and use top-quality valves and valve guides..
And don't get me wrong, the Stage 1 and Stage 2 heads are really well-done. And they are very good for those who don't want to do the work themselves. But, they are done very similar to what I just described. They are not "hogged out" giant ports.

If you "hog out" the ports oversize, you will KILL the flow in this head, for the engine rpms that we will use on a street Bullet.  Do NOT enlarge the ports.

I hope this has been helpful to those of you seeking more power levels out of your Bullet Iron-Barrel 500 engine.
Please feel free to ask specific questions about this procedure here, and I can help you with those, or clarify things that I perhaps didn't describe as well as needed to be fully understood.

[Cabo Cruz]

Ace, what a great lesson!  And, you explained the process so well that even an old Cuban like me understood every single phase!  WOWZAA!!!

[geoffbaker]

Ace,

Good analysis.

I talked to several machine shops about my diesel engine, and I ended up doing the following mods to improve performance. (I should add that my goal was NOT speed, but efficiency; but in truth there is not much difference - more efficiency can translate into either higher speeds or more torque or more mpg, depending on other factors including your driving habits).

The diesel head was ported much the way you describe, with the intent not of enlarging any particular surface, but to SMOOTH out the surfaces to reduce turbulence. 

Additionally the head was coated with a ceramic coating which disperses heat. The theory behind this is that the better the engine throws off heat the less energy it has to expend mechanically removing it via the exhaust chamber, or by the oil cooling system. Therefore the entire internal area of the exhaust port was coated to "throw off" heat back into the exhaust itself, rather than absorbing it into the head for the cooling system to remove. Additionally, the coatings made both intake and exhaust ports micro-smooth.

Thirdly, the piston, bearings, crankshaft and flywheel were all balanced to witin a couple of grams. Again, any eccentricity in weight distribution not only damages the engine over time (by ovalling the bearings and machine surfaces themselves) but also is energy that is transferred into unnecessary vibration, instead of being channeled into the power output itself.

I can't comment on the result in any way except to say that the engine seems to run exceptionally cool (max temp so far is about 170 degrees, but I'm not pushing it hard) and, for a diesel, very smooth. Although noisier than the gas engine, it does not offer significantly more vibration.

Ace, you may want to comment more on these areas; I'm no expert, merely following the opinions of a couple of machine shop pros.

[ace.cafe]

Ace,

Good analysis.

I talked to several machine shops about my diesel engine, and I ended up doing the following mods to improve performance. (I should add that my goal was NOT speed, but efficiency; but in truth there is not much difference - more efficiency can translate into either higher speeds or more torque or more mpg, depending on other factors including your driving habits).

The diesel head was ported much the way you describe, with the intent not of enlarging any particular surface, but to SMOOTH out the surfaces to reduce turbulence. 

Additionally the head was coated with a ceramic coating which disperses heat. The theory behind this is that the better the engine throws off heat the less energy it has to expend mechanically removing it via the exhaust chamber, or by the oil cooling system. Therefore the entire internal area of the exhaust port was coated to "throw off" heat back into the exhaust itself, rather than absorbing it into the head for the cooling system to remove. Additionally, the coatings made both intake and exhaust ports micro-smooth.

Thirdly, the piston, bearings, crankshaft and flywheel were all balanced to witin a couple of grams. Again, any eccentricity in weight distribution not only damages the engine over time (by ovalling the bearings and machine surfaces themselves) but also is energy that is transferred into unnecessary vibration, instead of being channeled into the power output itself.

I can't comment on the result in any way except to say that the engine seems to run exceptionally cool (max temp so far is about 170 degrees, but I'm not pushing it hard) and, for a diesel, very smooth. Although noisier than the gas engine, it does not offer significantly more vibration.

Ace, you may want to comment more on these areas; I'm no expert, merely following the opinions of a couple of machine shop pros.

Geoff,

Yes, the street Bullet would have similar goals for porting as your diesel, with both requiring to maintain or increase lower and midrange torque, along with any hp increases. And port speeds are directly related to torque production.
The coatings in the ports, especially in the exhaust port, are helpful for the reasons you mentioned.That coating in the exhaust port will keep engine temps lower, and assist in a faster exhaust speed by keeping the heat in the exhaust gases, and not transmitted into the head. The other coatings also help, but IMO the exhaust port is where you are getting the most benefit from those coatings.

Certainly a properly dimensioned and balanced bottom-end is crucial for best engine life, and least vibration. No question about it. That's a maxim.

[REpozer]

Ace, thanks for all your input, I enjoy the reading . Even a lean- burn guy can benefit. .

[Rick Sperko]

Last night I took off my intake manifold and saw how rough it was just inside. I was thinking about the turbulence. I was trying to come up with a way to smooth it without taking the engine apart. I guess I will learn how to take it apart and find a good machine shop. Thank goodness for the manuals (Snidal, Service, & Parts).

Thank you,
-Rick

[ace.cafe]

Last night I took off my intake manifold and saw how rough it was just inside. I was thinking about the turbulence. I was trying to come up with a way to smooth it without taking the engine apart. I guess I will learn how to take it apart and find a good machine shop. Thank goodness for the manuals (Snidal, Service, & Parts).

Thank you,
-Rick

Rick,
One of the quick and easy things to do is to match the manifold gasket to the manifold.
Very often, the hole in the manifold gasket does not perfectly align concentrically to the hole in the manifold, when it's installed on the engine. Sometimes the hole is a little too small to match, or it is off-set in some way, which forms a partial barrier to flow going thru from the manifold into the port. Sure, the manifold bore itself is smaller than the port hole, but this is not the issue right now. I'm just discussing making sure that the manifold gasket isn't making the manifold hole any smaller than it's supposed to be for that carb.

With the carb off the bike, stick your finger into the manifold, and see if you can feel the gasket itself sticking out somewhere in the hole. If you can, this is a restrictor plate, acting to choke-down your airflow.

Very simple to remove the manifold, get a new gasket, and temporarily put the manifold on with the gasket(don't tighten it down hard), and determine where the intrusion of the gasket is occurring.
Then remove it, and use a small round file to shape the hole in the gasket to not intrude into the port hole.
Many times the bolt holes have a lot of slop, and this allows the gasket to move its position when you install it. Sometimes all it takes is to use your finger to hold the manifold gasket in the correct position when you bolt the manifold on, so it doesn't slide on its slop enough to interfere with the port hole entry. Other times it needs a little filing to match it to the hole.

In either case, making sure tha manifold gasket isn't acting as a restrictor plate is always beneficial.
Smoothing the manifold internally wouldn't hurt anything either.

[Rick Sperko]

I meant that I noticed the roughness in my inlet port. I like the suggestion about cleaning up the gasket too. I was trying to come up with a way to smooth the inlet port without taking the engine apart. It is a black box to me right now. But then many other things were too.

Thank you for your technical input ace. I enjoy reading your posts.

-Rick

[ace.cafe]

I meant that I noticed the roughness in my inlet port. I like the suggestion about cleaning up the gasket too. I was trying to come up with a way to smooth the inlet port without taking the engine apart. It is a black box to me right now. But then many other things were too.

Thank you for your technical input ace. I enjoy reading your posts.

-Rick

Glad to help, Rick.
Ya gotta take the head off and the valves out, to do port work.

[Vince]

     Ace, I always enjoy your analytical discourses. I do have a question about this. It is my understanding that on the intake side one does not want a mirror finish. At low  RPM that mirror finish will not provide enough turbulence to keep the fuel in suspension. This will cause puddles of fuel, an inconsistent mixture, and poor throttle response at low RPM. Certainly, as you mentioned, any large flashing or sharp angle that will disrupt flow should be removed. You mentioned using 220 grit paper. I guess my question is: Can you quantify the right amount of "rough" finish needed in the intake tract? Would your answer be Enfield specific or of general application?
     On the other (exhaust) side it is my understanding that a mirror finish will aid exhaust flow and minimize carbon build up. How much does this apply to the Enfield specifically, or for general application?

[ace.cafe]

     Ace, I always enjoy your analytical discourses. I do have a question about this. It is my understanding that on the intake side one does not want a mirror finish. At low  RPM that mirror finish will not provide enough turbulence to keep the fuel in suspension. This will cause puddles of fuel, an inconsistent mixture, and poor throttle response at low RPM. Certainly, as you mentioned, any large flashing or sharp angle that will disrupt flow should be removed. You mentioned using 220 grit paper. I guess my question is: Can you quantify the right amount of "rough" finish needed in the intake tract? Would your answer be Enfield specific or of general application?
     On the other (exhaust) side it is my understanding that a mirror finish will aid exhaust flow and minimize carbon build up. How much does this apply to the Enfield specifically, or for general application?

Thanks Vince!

Regarding the finish of the intake port.
From a perspective of drag on the incoming air, it has been found that the slightly rougher surface, about 220-400 grit finish, is best. You could bead-blast to this grit also, if you want. It's not to make it "rough",in that sense, but just enough to create the desired effect. Tests have shown that this finish has the effect of forming a slight aerodynamic boundary layer over the port walls, which while very minimally reducing effective port diameter(only a couple thousandths of an inch), actually provides less drag on the incoming mixture than the mirror-polished port wall would have. It effectively eliminates the "skin-drag" that happens when the air runs right on the port wall.
As far as fuel puddles from insufficient port turbulence, I haven't heard about that, but I suppose it's possible. However, the sandpapered finish isn't to provide general port  turbulence, but conversely, it is to reduce it. But, the boundary layer  is a form of very thin  turbulent layer that is only along the walls, and will keep the fuel off the walls, so it might be less likely to condense on the walls. Perhaps that's what they meant?

As for the exhaust port, you are correct that the mirrored finish is good for that, in a general sense. The Bullet being an exception, only because to get a mirror finish on the Bullet exhaust port would require opening it up wider to get that finish on it, and would be counterproductive for us, because our exhaust port is too big already.
However, in general, mirror finish is good in exhaust ports because it reflects the heat well, and reduces the amount of heat transferred into the head from the exhaust. This keeps the heat in the exhaust, keeping the exhaust gases at maximum expansion, and thereby speeding exhaust flow, which assists scavenging effect during cam overlap periods. And it also helps to keep the head from getting too hot. And the mirror finish is less prone to pick-up and hold carbon deposits on it. So yes, in general the mirror finish in the exhaust port is a good idea.
For the Bullet, it would probably be better to use the Ceramic-Metallic reflective thermal-barrier coating in the exhaust port, because we could narrow the exhaust port a few thousandths with that, and also provide the same effects on the exhaust gases.
I'm currently doing an experiment with a torque-cone, like they use on Harleys, to see if I can get a low-rpm torque boost out of the Bullet with it, and still not lose any of the top end because the Bullet revs so low anyway. We'll see how that goes. I might get around to that this week-end, if the weather permits.
The Bullet is woefully lacking in exhaust gas speeds, and it really needs a smaller port and a narrower header pipe. I'm hoping I can get partway there with this torque cone project.

[Vince]

     Thank you, Ace! Nicely put- clear and concise. A pleasure to discuss these things with you.

[geoffbaker]

I would imagine, ace, that the ceramic thermal coating in the exhaust would be particularly effective on the Enfield head, as it tends to run pretty hot. As I said, it is keeping my diesel ridiculously cool...

[ace.cafe]

I would imagine, ace, that the ceramic thermal coating in the exhaust would be particularly effective on the Enfield head, as it tends to run pretty hot. As I said, it is keeping my diesel ridiculously cool...

Yes Geoff, I agree.

[sewerman]

Something I noticed when I ported my 04 is the rubber flange connecting the carb to the manifold.  Inside is a internal flange that protrudes into the air flow (its there for the carb and manifold to mate up to).. I used my 60 grit "flapper" wheel to turn it down to match the complete intake diameter.  Seems a shame to match all ports and leave this sticking out into the intake trac.  Just my two cents.