m20 10:1 2.9L stroker compression test results

[zaq123]

Ok to start of: the gauge is verified with an accurate pressure gauge and they line up perfectly after 50psi (comp. gauge is 300psi so less resolution than accurate test gauge). schreder valve is located at spark plug end so the hose is out of picture as it should be.

ITBS are fully open during the test. Hot engine.
2.9L 10:1 Mahle pistons. block was shaved just to get level. stock head gasket thickness, cylinder head is not shaved (new AMC). Cam is 284/272 Schrick 3 degrees retarded.

cyl1 -255psi (was the first in the test so maybe why a little higher), cyl2-6-250psi exactly. Dry test, injectors disabled all that. Compression rises in following order as the crank rotates, the same for all 6 cylinders: 0 --- 90 → 140 → 155 → 180 → 200 PSI (5 cranks) and then increments in 5ps for each crank to 250 total (about 10 more cranks to plateau at 250psi).

Why is my compression so high? I didn't test the gauge after 150psi as my accurate test gauge is 150psi max but I don't see it being too much off.

Any ideas? Did I over cranked it, I mean normally you crank till gauge stops. No crazy carbon buildup based on my borescope. There were some rich issues initially when the tune was being dialed in but it has been 10k km ago. I can actually read markings on piston now.

[AWDBOB]

This was my experience as well at 10.5:1. Was something like 250psi across the board

Edit: just looked at my records- 245psi

[MrBurgundy]

Better than 90!

I usually do 5 cranks for a comp test or until the large jumps stop.

So sounds normal to me.

[digger]

Back in 2007 when i did a compression test on my 3.1L supposedly 11:1 CR (didn't actually cc it for real or optimize deck height/squish etc) MM rally cam i got like 165 across board it had a crazy late IVC so bleeds off compression like a you know what giving like stock numbers .

Some years later maybe 2011/12 with different but still decent sized cam i got like 185 i think i had some advance built in. Then in 2024 tracing a non start with different gauge i got 110 iirc, found old faithful gauge and rechecked and was like 175-180.

I normally do like 5-6 pulses (same for each for comparison) when it starts to plateau but mine would never go another 50psi going from 5-15 pulses not that i tried .

Did you actually cc everything to get the static CR? did you ever checked the true IVC number and/or the ICL with dial indicator and degree wheel as that has a pretty large effect on the "dynamic" CR?

i would expect 200 after half a dozen pulses so i think its about right, maybe needs some more camshaft.....