effect of exhaust leaks on engine power

[jstoezel]

Hi:

I've been struggling with lack of engine power, mainly in the top end. Replaced the rings as I measured high leak down numbers, fixed a leak on the intake manifold too. Still not much power, even more on the top end.
I've noticed a leak at the collector, I run a Beaumia coated exhaust with 4-2-1 collector. The leak was easy to identify before I replaced the rings, it would smoke quite a bit from there. Now that the rings are fixed I can still see the leak because the male header doesn't mate very well with the female end of the collector. Not sure how to explain this, there is a white deposit present in about half of the wall surface, going out through the connection. In short it looks like the leak is about half the contact surface of the connection.

I wonder how much this leak is impacting power.

I believe part of the leak is due to a wider collector port for this specific header. Tried to fix this by hammering (tapping) the collector opening to get it tighter. Also I think part of it is due to the header lengths. See picture below. They do not have the same length going into the collector.

The top ones are roughly the same length, while the bottom ones are shorter. I believe this was designed this way so the collector is easier to install. You first slide the top ones (in the picture, they are actually the left headers on the car), then align the bottom ones.
The one header that is leaking is the one at the bottom right of the picture. It is the shortest of all and does not bottom in the collector.

I have been thinking of shortening the other pipes by 1/4" or so, so the shorter header can get deeper in the collector (and provide a better seal).

Could this detune the exhaust (assuming it's properly tuned now?). I was once told all headers should have the same lengths, and for a FV engine, they make the most power if they are multiple of 2". Where is this typically measured? From the end to end of the headers, or is it where the headers inside the collector, at the merging tip?

Jean


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[brian]

Exhaust leaks in theory can effect hp. A leak at the head will have more effect than one further down at the collector. I doubt that if you have lost top end, exhaust leaks are the culprit. Top end power is by far the most difficult to achieve, especially without killing low end grunt. Without knowing what components are in your engine, like ring pack, cam timing, head design etc., It will be hard to help you with your issue. Make sure you have maximized everything you have like ig. timing, fuel and even carb opening and see what happens. If the engine is running really well and just not making hp, then I'm afraid it may be time for the wallet. Pro built motor, heaven forbid, a new manifold or some other investment might have to be made.

[brp]

Since your pipes are within 1/4 inch of each other, just cut off the long ones so they are all at the same point before going into the collector. The were not built so the top pipes were longer than the lower to make them easier to install. The pipes move around after being used a while....you know a little bump here, another bump there.

The pipes also take a set after they are used a few times. The new pipes are sometimes a little work to get into the collector the first time. We instruct the owner to leave the nuts loose on the heads until after the collector is on part way. I usually put a block of 2/4 up against the megaphone and tap the collector onto the pipes for the first time.

Mike Beaumia

[SR Racing]

Brian is correct. It would take a massive leak or blockage to significantly hurt power. The ac/vw is a poorly designed induction/exhaust system. The exhaust ports actually flow better than the intakes which is backwards from a properly designed engine. That being, it would take a significant exhaust issue to give you a noticeable loss in power. Probably from the worst FV headers to the best 2+ HP would be involved.
You certainly want a good exhaust system and every 1/2 HP should be chased. But if you have noticable low HP, you have another problem.

The induction system is far more important (CFM and a/f ratio). Good heads are important, but still not to the level of carb and intake and of course the required racing engine specs. (Timing (ign. and cam), compression ratio (driven by deck height), valve lift, leak down, etc. )

If you are not experiencing a miss at RPM and it is just pulling slow compared to other race engines, your problem lies in one of the above areas.

Mike's (Beaumia) collectors are fine and will put you with the best of them. All primaries should be the same length (to each's merge point) But under an inch will not make any real difference. Primary headers typically are anywhere from 38" to 42" in length. The length (and the ID) will determine where that engine makes its peak HP and the breadth of that HP curve. (There is no 2" multiple that means anything.) In any case it will only vary by 100 to 300 RPM at most. Most people want that HP peak to be just before the passing zone of the track you race at.

You may already know this but the point of the primary length is so that when one cylinder makes its exhaust stroke those gasses fly by the next cylinder at the collector, creating a lower pressure there and scavaging that cylinder better. This allows for much better volumetric efficiency on the cylinder that is on its intake stroke. Most all primaries and collectors do this pretty well.

Long story, but I doubt an exhaust issue like you describe will cause you a seat of the pants loss of power.

[jstoezel]

Thanks for all the replies.

Make sure you have maximized everything you have like ig. timing, fuel and even carb opening and see what happens. If the engine is running really well and just not making hp, then I'm afraid it may be time for the wallet. Pro built motor, heaven forbid, a new manifold or some other investment might have to be made.

This is a pro built engine that came back from the shop last April. Only comments were that of the 4 WSCC/WCMA engines the same shop had rebuilt for this season, mine wasn't necessarily the best but it wasn't the worst either.

Since your pipes are within 1/4 inch of each other, just cut off the long ones so they are all at the same point before going into the collector. The were not built so the top pipes were longer than the lower to make them easier to install. The pipes move around after being used a while....you know a little bump here, another bump there.

The pipes also take a set after they are used a few times. The new pipes are sometimes a little work to get into the collector the first time. We instruct the owner to leave the nuts loose on the heads until after the collector is on part way. I usually put a block of 2/4 up against the megaphone and tap the collector onto the pipes for the first time.

Mike Beaumia

Thanks for the reply Mike. Always thrilling to get a reply from the designer! Will trim the headers over the week-end. There's numbers engraved here and there on the headers and collector. Are these just serial numbers or do they mean anything? Do you still build exhaust systems?

The induction system is far more important (CFM and a/f ratio). Good heads are important, but still not to the level of carb and intake and of course the required racing engine specs. (Timing (ign. and cam), compression ratio (driven by deck height), valve lift, leak down, etc. )

I just upgraded to better heads, I think. Though the old set of heads still holds the track record from last year (different car though). Carb-wise I think it's a good one. A few years back the engine builder recognized the venturi as one of his design.

I think it dawns on me that maybe the intake manifold is the culprit. Upgraded to this one last season, it was sold as a National level from 10-15 years ago. Don't laugh, around here (WCMA) we are typically good at being proactive in terms of rule changes (raised minimum weight a few years ago, running lower cost tires, etc) but our current rules don't protect anybody from showing up with a blown manifold. So far it's just been that there's a handshake agreement that we run "regular" manifolds (not blown). So a guy showing up with a 10 year old manifold- or just one of the last non blown intakes- would certainly have the best one.

There's only one last race this season, 3 weeks from now. If anybody has a good 10 years old manifold he doesn't know what to do with, I'll pay shipping back and forth, and some more to use it for 9 sessions (typical 2 day race week end). As long as it's not blown. And well if it works well, there's always the possibility I could buy it from you.

There is no 2" multiple that means anything.

Interesting, the engine builder was quite specific about this. We had a discussion about this as I was running a random exhaust last year. He always said there was a minimum length that worked great but couldn't be achieved in FV. From there the length had little effect on HP, as long as the lengths were multiples of 2. Dyno tested/proven apparently.

[Dietmar]

Jean:

The easiest test on the manifold is on a flow bench ( best if the carb is attached too) but it would give you some idea of what you have- and it would cost less than the shipping from Canada and it takes just a few minutes.
Even a 10 year old "national manifold" should perform but the truth is on the flow bench.
I have several manifolds here if there is any interest- you know the number.
Dietmar
http://www.quixoteracing.com

[SR Racing]

Interesting, the engine builder was quite specific about this. We had a discussion about this as I was running a random exhaust last year. He always said there was a minimum length that worked great but couldn't be achieved in FV. From there the length had little effect on HP, as long as the lengths were multiples of 2. Dyno tested/proven apparently.

Not on any dyno I have seen.. The multiple of 2 doesn't mean a thing. Also, the length (and ID) are all that there is to HP with headers. The speed of the gasses and the time they reach the collector merge point IS the HP factor (with headers). Depending upon cam timing, and a few other things, the headers will determine how well the cylinders are scavenged. They can be ANY length but that will determine the highest point of volumetric efficiency and thus peak torque and HP points. 39 inches (or 40.235) will just change that point. I think your builder might have been trying to say something else (but not sure what.)

BTW, I am sure someone your way has a chassis dyno? Typically for $60 or so, you can find out what you are getting to the rear wheels (and a/f ratios). Your problem could certainly be transaxle area losses. It's easy to lose a couple HP in the drive train. If you have decent HP at the rear wheels it takes all the guess work out of the equation. If you do find and use a chassis dyno, you want them to run it in "inertial mode" and normally third gear. You can also test in 2nd and 4th. The HP will vary slightly across the gears, but will be close. Using 2nd gear causes some thrust loads and inertial issues that MAY decrease observed HP a little. And 4th gear takes forever in a Vee

[jstoezel]

Hi Dietmar:

I guess the dyno guy needs special adapter for the VW manifold? No even sure where to find a flowbench in the area.

Jean

Jean:

The easiest test on the manifold is on a flow bench ( best if the carb is attached too) but it would give you some idea of what you have- and it would cost less than the shipping from Canada and it takes just a few minutes.
Even a 10 year old "national manifold" should perform but the truth is on the flow bench.
I have several manifolds here if there is any interest- you know the number.
Dietmar
http://www.quixoteracing.com

[Dietmar]

Jean:

The person who delivers your mail can direct you to a flow bench - in other words, send it south by mail ( insured) and it will get taken care of.

Dietmar
http://www.quixoteracing.com

[jstoezel]

A follow up on this. recently I acquired a soda blaster gun. So I disassembled both heads to clean up the combustion chambers. I noticed a funny shape on piston #3, by the way this cylinder always scored bad leak down numbers, even with new rings (the cylinder seems a bit oversize too). I was texting with a fellow competitor, joking about what does it mean when there's the shape of a butt with a crack in the middle on top of your piston. Well upon closer look indeed there's a crack under there!

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Removed the piston, soda blasted the top and bottom of it. The crack goes through. I wonder how much this leaks, could this be the cause of the low power issue? Any opinions?

By the way is it typical to see that tan/black colour under the piston? Or is this typical of a crack?

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So it looks like I will be replacing this piston. A fellow competitor said he has a few I can chose from (new and used). What is it that I need to look for? How to prepare these pistons? Precautions while installing back etc.
For instance, how do you end gap that spacer under the Dykes rings? And how do you even get it out / onto a piston?!

How much do prepared piston sell for? The on I have in hand seem to have been machined in and out, possibly balanced even?


Thanks,
Jean

[Dave]

The old Lawnboy mowers all had a hole in the top of the piston, they called it a sonic decompression release. Maybe that's what your competitors call it to.

Dave

[SR Racing]

Jean, The biggest thing you have to do is make sure the replacement piston matches the weights of the others. (within a gram)

The spacer doesn't have to be end gapped. You remove it with a couple pin pricks.

[jstoezel]

Hi Jim:

I went to a shop that has a stock of brand new FV pistons/cylinders (Noble stock from 10 years ago). As well they have pistons taken from expired engines.
My piston is much heavier than the Noble stock, by about 30g. This is funny because the wrist pin I use is also much heavier than the ones that came with Noble's.

Total weight for my piston with wrist pin and rings is 412g...

Anyhow we matched by 2g my piston with a used one. However, the 2 pistons are quite different. Mine has a very short skirt. Also the crown on mine is much shorter.

Pretty sure this wouldn't work due to deck height requirements. Apart from the crack on top my piston is not damaged and could be used as a template. Is this something typically done, send a template to a builder to match it up? What's the typical solution in such a case?

Jean

Jean, The biggest thing you have to do is make sure the replacement piston matches the weights of the others. (within a gram)

The spacer doesn't have to be end gapped. You remove it with a couple pin pricks.

[SR Racing]

Jean,

Yes. Send it to one of us engine builders and we should will be able to match it up for size and weight. We would need the piston pin and ring set also.

[jstoezel]

Got in touch with a few engine builders to try and source a replacement piston. Bill Vallis was able to locate an exact match to what I had, within 2g (even better, it's heavier) and pretty much right on in terms of wrist pin to top crown distance. He shipped it yesterday, I got it today. Might be installed in the engine tonight.

Bill Vallis seems to always amaze me, he is a legend

Thanks to all who offered to help.

Jean

[jstoezel]

Hi:

I've replaced the damaged piston and all cylinders and heads are back on the engine. I've installed the intake manifold and built a tool to check proper mating with the heads.

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The tool pressurizes the intake manifold and the heads (intake side). This only works if you apply low pressure (slightly higher than atmospheric pressure but low enough it doesn't push open the intake valves) and remove both rocker arm assemblies, so all intake valves are closed. Initially my intent was to check for leaks on the intake manifold by spraying soapy stuff and looking for bubbles. There's a repair that's visible on the right in the picture, on the elbow, just below the throttle cable. I didn't find any leaks on the repair or where the manifold connects to the heads, however I detected a type of leak that I wasn't really expecting. This type wouldn't have shown in leak down tests. I noticed cylinder #3 intake valve was leaking quite a bit through the valve guide. Actually it's leaking quite a lot.

For reference this is the cylinder that always scored bad marks with leak down tests. Initially I thought it was related to rings (which I replaced), then I found the cracked piston, and while both heads where off I did lap the valves.
I can stop the leak from the valve guide by rocking a bit the valve and the spring. All other intake guides do not leak, or it is not noticeable.

How critical is a valve guide leak? I wouldn't expect this would have shown in the leak down test, unless the intake valve was also not closing properly. Should I take corrective action on this?

Jean

[SR Racing]

If it is not detected in a leak down test it says the valves are sealing and your rings etc are ok.

However two things can happen with a loose valve guide:

1. Dynamically the valve may not seat at one or more RPM points. The leak down test is only a static condition.
2. On the intake stroke the guide will suck oil down into the cylinder. This MAY introduce excessive oil and even foul a plug. In any case you are sucking some air from the crankcase that is not the proper a/f ratio. You want all that intake stroke to bring in a/f from the carb.

Most ALL race prepped VW engines are a bit loose at the guides and suck some oil. It's a gray area as to how much is too much.

About the only way (other than just a eyeball) is to take an EGT at that cylinder. Not something that typically is done except on the dyno. I know some people run a couple EGT's, on the car but never seen one running 4.

[jstoezel]

Hi Jim:

Thanks for the reply. Hum I guess I'll leave it like this for the remainder of the season, I only have one race week-end left and won't have time to integrate an EGT sensor.

As a side note, I might have spoken too quick. Using the same testing setup I noticed leaks on both head ports (where the manifold mates to). I tried 2 types of copper seals: crush-able type and solid ones. It seems the leak is worst with the solid seals. However it doesn't totally disappear with the crush-able seals.

I guess here too it's a matter of dealing with the leak, and using an EGT/WBO2 to jet properly?

Thanks,
Jean

If it is not detected in a leak down test it says the valves are sealing and your rings etc are ok.

However two things can happen with a loose valve guide:

1. Dynamically the valve may not seat at one or more RPM points. The leak down test is only a static condition.
2. On the intake stroke the guide will suck oil down into the cylinder. This MAY introduce excessive oil and even foul a plug. In any case you are sucking some air from the crankcase that is not the proper a/f ratio. You want all that intake stroke to bring in a/f from the carb.

Most ALL race prepped VW engines are a bit loose at the guides and suck some oil. It's a gray area as to how much is too much.

About the only way (other than just a eyeball) is to take an EGT at that cylinder. Not something that typically is done except on the dyno. I know some people run a couple EGT's, on the car but never seen one running 4.

[FV80]

Jean,
FYI - I have NEVER had any of the solid gaskets seal properly by themselves. I always add a coat of red RTV to the surface at the head - install the gasket and add another light coat to the top of the gasket. Make it as smooth as possible and let it dry before installing the manifold. NO LEAKS.

The crushables, in my experience have sealed pretty well.. but not very reusable.

Steve, FV80

[Dietmar]

Jean:

You might try a teflon gasket on the head-manifold sealing area.

Secondly, you could find a flat surface and check the ends of the manifold- you'll probably see a gap. This can be corrected too.

Dietmar

[sharplikestump]

Jean,
Just happened across this, and to add to good advice you've already received, You might consider this:
While the cracked piston must be replaced, from the looks of it, I doubt that it is responsible for the power loss. Could be completely wrong, but I suggest you show those pipes, along with your plugs to Billy. They look awful lean on my screen. Might be as simple as a jet change. A bad leakdown is much more detrimental in the lower revs. If it was me, I would also be checking cam lift. Using the wrong oils today w/o adding some ZDDP eats our cams, which destroys power. Jim is right on the pipes. 1/4 inch is plenty close. You get some moisture up there....any rust inside the intake manifold? If the intake manifold is leaking, you'll spot it at idle. Once you are in the revs, vacuum falls off, and the volume multiplies. Both of these make a small leak insignificant. Mikes systems make good power, but I don't believe he does stainless. Any rust inside?
I have no idea what "multiple of 2" means.
Say hi to Billy for me. Your read on that man is spot on.
Mike Palermo Jr.

[sharplikestump]

Just as a side-note....You might consider replacing that old style fuel pump. The pressed in brass tubes can pop out. If the outlet one does at speed, you and your car are toast. I don't believe SCCA even allows them anymore. I had one pop on the dyno, and it gets real exciting, real fast.
M.P.

[jstoezel]

Hi Mike:

The pictures where taken after a week-end where it seems I had a leaky intake manifold. My intake was repaired with brazing and the weld was leaking. This is why, I think it shows so lean. I fixed that and also found the cracked piston.

Either way only piston 3 out of the 4 had bad leak down readings.

As for oil I have been using the Mobil 1 10W50, this is the type that supposedly still has plenty of Zinc (according to datasheet on Mobil 1's website).

No rust inside the intake manifold.

Funny how you mention the possibility of a fire... I finished the season earlier after then engine caught on fire. I think the carb bowl got stuck or something. I don't know. The (now black) engine is still in the car, waiting to be shipped to Vallis, probably in the New Year. We will see what he has to say about the whole thing.

Jean

Jean,
Just happened across this, and to add to good advice you've already received, You might consider this:
While the cracked piston must be replaced, from the looks of it, I doubt that it is responsible for the power loss. Could be completely wrong, but I suggest you show those pipes, along with your plugs to Billy. They look awful lean on my screen. Might be as simple as a jet change. A bad leakdown is much more detrimental in the lower revs. If it was me, I would also be checking cam lift. Using the wrong oils today w/o adding some ZDDP eats our cams, which destroys power. Jim is right on the pipes. 1/4 inch is plenty close. You get some moisture up there....any rust inside the intake manifold? If the intake manifold is leaking, you'll spot it at idle. Once you are in the revs, vacuum falls off, and the volume multiplies. Both of these make a small leak insignificant. Mikes systems make good power, but I don't believe he does stainless. Any rust inside?
I have no idea what "multiple of 2" means.
Say hi to Billy for me. Your read on that man is spot on.
Mike Palermo Jr.