Leak Down

[Garry Sharp]

I decided to do a leakdown test in preparation for the upcoming season. 3 of the 4 cylinders had 0 loss at 90 PSI. However the 4th one was down 4 lbs. at 90 PSI with a noticeable hiss out of the exhaust pipe. The thing is the heads are basically brand new with 1 qualifying session an 1 race on them. Is it possible that the valves have not been run enough to seat/break in completely or do I have to get it fixed?

Thanks,
Garry

[Nardi]

Unless my math is wrong, that is only 4.5% loss. Not that much in my book, but maybe a ton for your application.

[BLS]

Make sure you have the valve lash set properly. If you haven't checked after running, you should now.

[budawe]

If the engine has been sitting since last season, it's likely a little rust has found it's way to an exhaust valve and or seat. I back off my rockers and seal up the motor if the engine is going to sit for a while. Check the leakdown after the first weekend. 4% is not to much to worry about.

[sracing]

As pointed out here: That percentage is nothing to worry about. Also no doubt it happened during the off season with that valve open. After a session on the track it will be fine.

[kea]

Zero percent on cylinders, I'd try another gauge or a higher line pressure.
All cylinders are going to leak some, unless you have managed to remove any piston ring gaps.

[Garry Sharp]

I'll check again, but I guess they are good enough even if they are off a little.

[kea]

Yes they are.

[budawe]

If your engine has dykes top rings the pressure from the tester will seal the top ring and mislead the reader. Valves, on the other hand, act independently from the rings.

[sracing]

He has a special set of Total Seal rings(not dykes.) When broke in they typically read about 2 to 5% leak down at 100psi.

[Diamond Level Motorsports]

He has a special set of Total Seal rings(not dykes.) When broke in they typically read about 2 to 5% leak down at 100psi.

Could I get a set of those rings for my Vee engine? I have never seen leak down numbers greater then 5% and most of the time they are between 5-15%

[sracing]

Could I get a set of those rings for my Vee engine? I have never seen leak down numbers greater then 5% and most of the time they are between 5-15%

Careful. There are some variances in leak down gauges. I don't think Total Seal is going to make any special ring sets for a 1192 engine. Not a lot of action there. However, they will make sets for you at a $5000 + order.

I have 2 Leak Down gauge sets. (Longacre and Profrom?) Both read about 5 to 2% on a decent engine. Actually as an engine ages the leak down changes very little. It seldom drops slowly with age. It usually drops like a rock when something is very wrong. 30% or more. A leak down gauge (or compression checker) isn't much of a diagnostic tool for HP decreases. (unless it is a catastrophic issue (bent/burnt valve, cracked piston etc.).
It is a decent tool for testing right after the build (or valve job) OR to diagnose something that is broke
.
The slow decrease in HP as an engine ages is more often valve shrouding, weak springs, etc. NOT compression. If your compression or leak down has changed enough to cause a HP drop and you can see it with the gauge,... you went long past your time for a rebuild.

BTW, if you see 15% or more leakage, the noise should be very obvious at the intake, tailpipe, case, or cylinder head seal.

[Diamond Level Motorsports]

The noise is always detectable. Most of the time from the rings. That's why I guess I need a different ring package........

[Albatross]

Interesting...Don't read where anybody mentions that the leak down to be most accurate should be done at: engine operating temperature! Start the engine, warm it up to 180-200f operating temp. Shut it down and set up your equipment and check each cylinder. I never had an engine show 0 psi loss-never. You'll always have some ring loss. Total seal claims the best ring seal, what they don't say is how much drag those rings may have over other competition type rings. The Jake Lamont book states: In general, a new rebuilt engine with properly gapped AND broken-in rings and fresh valve seats should read less than 2%-3% leakage.

[Garry Sharp]

OK, so it couldn't have been 0%. The needle must have moved a little, but 2% is
hardly perceptible on this gauge. Anyway, it's good enough.

[budawe]

Just to try to bring some VW specifics to this discussion. I do agree that leakdown testing is best with a warm engine but I'd like to point out that warming up a vee engine to 180 or more, will likely overheat the cylinders and heads and I would not advise it. Secondly, with an air cooled cylinder, perfect concentricity is almost impossible to achieve. That fact will always generate some leak down. Unlike a water cooled cylinder, which are jacketed all the way around, there are hot and cold spots on air cooled cylinders. As a result, low drag, reduced ring size and modified (backcut etc) rings are a extra challenge to perfect. Hence the smoking usually see during vee races. Just a fact of life that we all try to improve upon.