Engine running comp test.

[Tom68]

Seeing all the talk of dynamic IVC compression I'd always thought you may just as well calculate it rather than doing a cranking test.

So then I thought how about an engine running compression test to get a more realistic view of dynamic compression.

I'd never heard of doing it but of course there's plenty of people on youtube doing it, 100 odd psi at idle, 200 plus with a wide open blip, that was on rattly old shitters.

Maybe everybody's been doing it engine running ?

[HQM383]

Would it provide false readings (high or low?) without high pressure evacuating exhaust gasses not present to contribute to the cycles the way they do

[rebelrouser]

You check the cranking compression, and then start the engine, when you snap the throttle the snap reading should be 80% of the cranking number. Link to the proper way to do it. It is good to show up valvetrain issues, like broken springs, worn camshaft etc. The trend now is to do a running test with a scope hooked up to a transducer, and get a pattern of what is going on inside the cylinder. On some cars you have to take the intake off to remove the plugs, so they are using a transducer in the tailpipe to get a wave form, that is just as accurate as the in cylinder test.


https://www.alldata.com/us/en/support/c ... sion-tests

https://www.youtube.com/watch?v=ZsNtJ1zto64

[Tom68]

Would it provide false readings (high or low?) without high pressure evacuating exhaust gasses not present to contribute to the cycles the way they do

Yer exhaust could bugger it up.

[af2]

Seeing all the talk of dynamic IVC compression I'd always thought you may just as well calculate it rather than doing a cranking test.

So then I thought how about an engine running compression test to get a more realistic view of dynamic compression.

I'd never heard of doing it but of course there's plenty of people on youtube doing it, 100 odd psi at idle, 200 plus with a wide open blip, that was on rattly old shitters.

Maybe everybody's been doing it engine running ?

You will get 90 lbs give or take

[Geoff2]

I think the cranking test is more for a comparison between the cyls on a given engine, not to compare to other engines. Variations in cranking speed, battery condition, internal engine friction etc, I would expect to give different readings.

[jeff swisher]

I had one 350" engine I went with .028" piston to head clearance gasket included.
5cc valve reliefs and 54cc chamber coupled with a 224 @.050 cam 106 intake centerline 245 psi cranking pressure with throttle blades closed.

I did not check it with them open.
I checked that same engine with 58cc heads and more piston to head clearance and a 102 ICL 218 cam and it was 225 psi opened throttle blades and 210 closed throttle blades.

That 245 psi I decided to pull a spark plug and test idle compression.
It broke my compression tester.
I never tried to keep one hooked up to a running engine after that.

My cranking pressure was done with a fully charged battery and old fat GM starters.
The faster you spin one the more cranking pressure you will have.

If you do not believe that take an old weak battery that barely turns the engine over and do a test with it.

Friend of mine stuck a small body Vortec starter on his Chevelle and it gained him 15 psi over his old fat starter that worked just fine.

[Stan Weiss]

Unless your valve train is rock solid you will see a small change in DCR as you increase RPM. Unless there is no change in weather you will see a change in PSI.

The major change in psi you will see in a running engine as you increase the RPM is due to an increase in VE%

These graph is just a rough illustration.

Stan

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[learner1]

https://www.automotivetestsolutions.com ... 1b09d3.pdf

[F120]

If you use a gauge with a hose the reading won't be accurate because of the volume of the hose