compression test number go up when engine is reved

[724doorE]

is this normal? I was checking #'s 4,5,and 6. Was just turning it over with the starter 10-15 times and got 85# and 135# on 6 and 5. then decided to start the car when I did 4. At first it was about 85 so I reved it to 2500-3000 and tyhe comp went up to 135 or so. I wonder if this could be do to rings not yet seated, or valves being out of adjustment tight.

[911pcars]

If you insist on doing a compression test in this manner, please be consistent for all cylinders. Otherwise, do it the old fashion way and:

- warm up the engine, then engine OFF
- remove all spark plugs
- one by one, take a compression test of each cylinder. Crank the engine consistently for each cylinder. 4-8 compression strokes is fine. Do not run the engine during a compression test.
- record your result, then repeat for the rest of the cylinders.
- If a cylinder is low (more than 20% difference), squirt some oil into the spark plug opening, then repeat the test. If the compression is now higher, the rings are leaking compression. If it's still low, it's something else (head gasket, valve, hole in piston).

Tell us your results and the board will continue from there.

Sherwood Lee
http://members.rennlist.org/911pcars

[Tyson Schmidt]

You should never do a compression test this way. It will mask a bad valve.

That is probably why you see 85 at cranking and 135 at 2500. I'd adjust the valves and retest.

Ive done compression tests on cars with valves that had just barely kissed the piston when a timing belt broke. It ran on 4 with a new belt, but a leakdown revealed the slightly bent valves. The compression wasn't down much either.

If it can't reach full compression while cranking, then something is wrong.

You should recheck the low cylinder at cranking speed, and see if it improved. If not, then you need to do a leakdown to determine the problem.

[Wayne 962]

Yup, thumbs down on purposely running the engine on 5 cylinders and spraying unburnt fuel into your cylinder...

-Wayne

[jluetjen]

I agree that given the standard instrumentation I doubt that this test will tell you anything.

At first it was about 85 so I reved it to 2500-3000 and tyhe comp went up to 135 or so.

From an intellectual perspective, I think what you are seeing is the difference between Static Compression and Dynamic Compression. As an engine "comes on cam" the harmonics in the intake and exhaust systems will start to increase the amount of air in the cylinders.

A hypothetical example; if your engine is a 3.0, each cylinder burns .5 liters of air/fuel mixture (at ambiant pressure). At low rev's the volumetric efficiency is pretty low so each cyclinder may only be getting .1 liters of air (in reality it still takes up .5 liters of space, but the pressure is 1/5 or 20% of ambiant prior to the compression stroke) because the throttles are closed. At this point the engine is making about 20% of it's theoritical HP.

As you rev the motor up towards the peak torque engine speed (4200 RPM in an SC), everything starts working together. The throttles are open so that very little is impeding the passage of air into the cylinders. The intake and exhaust harmonics hit a sweet spot so that more then .5 liters of mixture are crammed into the cylinders, let's say .505 or .51 liters of mixture, so the volumetric efficiency has gone over 1. Since the volume of the cylinder is still the same (.5 liters), the pressure inside has to go up 110% or 120% of ambiant prior to the cylinder even compressing the mixture. At this point the cylinder is burning more mixture then you would expect from it's .5 liter capacity and so the HP has gone above the theoretical HP for this capacity.

This is why engines which have cams with a lot of overlap (like a 906) can make so much torque and HP per liter compared to an SC or an industrial motor. In the case of your motor it looks like there was a 58% change in the efficiency of your motor.

BTW - when you did the initial 80 PSI test, were the throttles open?

[724doorE]

Think I will check my butterflies and idle jets on thsoe cylinders, set all the valves and see whats up. The engine is less than 10,000 miles on rebuild, but its been laid up for about 10 years with infrequent running till I reaquired it a few years ago. MFI out of adustment and valves have not been adjusted recently. Thanks, you answered my question concerning the testing procedure. BTW, it was not my intention to test the cylinders differently and then compare them, I know that makes no sense. I just wanted your thoughts on why the comp went up when the engine was running. Thanks guys

[SC-targa]

Maybe I'm missing something here. No one else said anything about it, but don't you need WOT for a representative compression test result? If the throttle's not wide open you put a restriction on the air intake and you lower your basic compression test numbers

If you had the engine running, you can't hold the throttle wide open without damaging the engine.

[Doug Zielke]

John...thanks for that detailed explanation!

[lendaddy]

Yep, I did the comp test both ways. WOT and closed that is. My numbers were identical. made no difference at all.

[john walker's workshop]

carbed and MFI engines need wide open throttle because they have individual intake stacks, and with the throttle closed, you basically create a vacuum, and them compress it, for a low reading. engines with a common plenum, like CIS or carrera style intakes will draw air from the plenum, and really show no difference WOT, or closed.

[Wayne 962]

There will probably be a slight difference with a close throttle, but not as noticeable as with engines with individual throttle bodies. I recommend doing the test with wide open throttle (WOT) for consistency...

-Wayne

[724doorE]

Didn't have the WOT, and it is of course mfi, so that may be why I had the low reading. Hopefully this weekend I'll get the chance to work on it some if the Fing cubs don't waste to much of my time....

Thanks John W.