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Compression Test Results

roys33

By roys33,
in Engines & Forced Induction

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The Dan Grand Master

The Dan

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What if your engine has between 180 and 190 psi??

We've got a Snap-On gauge and an ABW gauge. I'll give you $50 if you can show me 190psi on your engine on either one.

*sub clause* as long as it is an unopened engine haha

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The Dan
The Dan

When you get a compression test, the results are taken statically. That is why you will hear a lot of people refer to them as static compression results. Three guys get a compression test done, Guy

roys33 Apprentice

roys33

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so next question apperantly my motor is supposed to rebuilt 10000klms ago so id take a uneducated guess and say it wasnt obviously but since they are so close wouldnt that lessen the chance of me having bad blowby?

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The Dan Grand Master

The Dan

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so next question apperantly my motor is supposed to rebuilt 10000klms ago so id take a uneducated guess and say it wasnt obviously but since they are so close wouldnt that lessen the chance of me having bad blowby?

It doesn't mean it's not built. Forged engines can have lower static compression due to the forging being made from a different material than the factory piston. They can expand more when at temp so the piston to bore clearance is more than the factory cast piston when they are cold and even when they are hot. Just not as much.

I would say get a second test done at a different place and compare the result. Definitely make sure the engine is up to temp before pulling it apart.

Also get them to do a wet test and a leak down test. This will help you determine where the compression loss is occurring.

You could have valve train losses rather than ring problems and no, that won't give you excessive blowby

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AngryRB Grand Master

AngryRB

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Oh and I forgot to add that 50psi is nothing compared to the 1500-2000psi your combustion chamber sees at peak power and cylinder pressure.

Engine A might have 2000psi whereas Engine B might have 1850-1950psi due to his lower static result. Both cars will make within 5-10 HP of one another and potentially live just as long as one another.

thanks for explaining all that..

Im not sure i quite get my head around some of that , if the motor were down 50psi at 400rpm i would of thought thats a 30% loss of gas.. so at peak power it would be 30% still? so thats more like 1400psi instead of 2000psi which would be more than 5hp loss?

comparing my E85result on 16psi to some others on 25psi, seems like my compression could be the difference to me.. thats what i though anyway.

Edited by SliverS2
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The Dan Grand Master

The Dan

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thanks for explaining all that..

Im not sure i quite get my head around some of that , if the motor were down 50psi at 400rpm i would of thought thats a 30% loss of gas.. so at peak power it would be 30% still? so thats more like 1400psi instead of 2000psi which would be more than 5hp loss?

comparing my E85result on 16psi to some others on 25psi, seems like my compression could be the difference to me.. thats what i though anyway.

A lot of people think that but in actual fact it's not the case.

Once the gases passing the ring/ring gap reach supersonic speed, none more can pass no matter how much more pressure is added. Gasses reach supersonic at fairly low engine RPM

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The Dan Grand Master

The Dan

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There are a couple of different types of electronic tests. Pulse test and cranking speed test.

The pulse test measures variations in manifold pressure I am fairly sure. The pulses will usually be off if the cylinder is down on compression. The cranking speed test is done by monitoring cranking speed. A cylinder with reduced compression will have less resistance and therefore the engine will crank faster on that part of the cycle. I am fairly sure it's like a timing light with a scope type rpm counter. I have used neither of these methods and have only ever heard about them. From what I was told, they are pretty accurate also. I don't know about the pulse test as measuring the cylinder on the intake stroke is not really going to be terribly accurate as most of the rings made for modern engines are torsional so they act differently on the downward stroke. I don't know enough about it to argue with someone about its accuracy though lol

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The Dan Grand Master

The Dan

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We use cranking speed fairly regularly when we suspect a dead engine. We don't use a tester though. All you do is unplug the CAS and crank it over. Listen to the note of each stroke. If you are down on one cylinder, you will hear it quite easily.

Healthy engine will be: da da da da da da

Unhealthy will be: da da DA da da da f**k!

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dkbcleaner Proficient

dkbcleaner

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We've got a Snap-On gauge and an ABW gauge. I'll give you $50 if you can show me 190psi on your engine on either one.

*sub clause* as long as it is an unopened engine haha

Hey mate Ive had 3 compression tests done at the same place with all results showing 180 to 190. Not sure what gauge they have used but I understand what your saying. It is a built 2.8 though. So does that make a difference?

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The Dan Grand Master

The Dan

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Hey mate Ive had 3 compression tests done at the same place with all results showing 180 to 190. Not sure what gauge they have used but I understand what your saying. It is a built 2.8 though. So does that make a difference?

Yeah if you have had the engine built and it's higher comp ratio, that will improve the normal results usually. You would only need to go up to 9.5:1 at most to see figures like that.

Do you run standard head gasket or a spacer multi layer?

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defari Grand Master

defari

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There are a couple of different types of electronic tests. Pulse test and cranking speed test.

The pulse test measures variations in manifold pressure I am fairly sure. The pulses will usually be off if the cylinder is down on compression. The cranking speed test is done by monitoring cranking speed. A cylinder with reduced compression will have less resistance and therefore the engine will crank faster on that part of the cycle. I am fairly sure it's like a timing light with a scope type rpm counter. I have used neither of these methods and have only ever heard about them. From what I was told, they are pretty accurate also. I don't know about the pulse test as measuring the cylinder on the intake stroke is not really going to be terribly accurate as most of the rings made for modern engines are torsional so they act differently on the downward stroke. I don't know enough about it to argue with someone about its accuracy though lol

cheers for the info.. the reason i asked is with RACQ they do an electronic compression test.. when i bought my 33 it showed 5 cylinders were 95% and 1 cylinder at 90%.. it was hard to find any info on how it was calculated.. but i assumed the engine was still quite healthy.. 3 years on and its still boosts fine, doesnt drink oil etc..

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The Dan Grand Master

The Dan

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Yep bore scope.

We have one at the shop for just such occasions. It really is surprising how many people sell cars under false advertising.

We can also tell if you have forged rods and ARP studs inside by looking up the sump plug hole

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SilverECR33 Veteran

SilverECR33

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Yeah if you have had the engine built and it's higher comp ratio, that will improve the normal results usually. You would only need to go up to 9.5:1 at most to see figures like that.

Do you run standard head gasket or a spacer multi layer?

Dan,

Can you tell me if you believe using a multilayer metal head gasket say 1.3mm would impact on the results and how so?

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tranman Enthusiast

tranman

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Posted (edited)

For anyone interested in how my car has from I believe 'worn' over time - was never tuned for a couple of years.

I didn't know the test was meant to be done at temp so results are cold:

Cyl./psi

DATE: 27/05/2010

1/155

2/150

3/150

4/150

5/155

6/150

DATE: 29/09/2012

1/155

2/152

3/ 145

4/145

5/151

6/150? (forgot to jot down but was fine)

Not sure what may have happened to cyl. 3&4 but was certainly lower. No matter how many times I tested it wishing it was a bad read.

Was certainly giving the motor a hard time untuned though.

I feel a lot better now though after reading this thread regarding the 'dynamic' vs static result of how the engine will actually perform. The spread

Cyl. 1 and 3/4 seems high?

Edited by tranman
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