m20 10:1 2.9L stroker compression test results

[zaq123]

Did the wire measure different with multimeter?

I didn't even bother with measuring its resistance to be honest. a) they are all different length in the harness so not exactly can compare to each other precisely. b) I wanted to check if my cylinders are balanced as a part of troubleshooting why it was idling a little rough. So I went straight to a scope to check it all at the same time, coil, wires, injectors, VR....all that. Spark duration time difference and its shape on the scope can also be a sign of messed up AFR in a cylinder (rich mix with lower resistance - less spark duration, lean - longer duration)...or subpar wire as in my case. I have cheapo Hantek 11008C with HT25 ignition probe, CC65 current clamp and some other stuff. for under $200 is a good setup for quick diagnostics. I would have done relative compression test with the scope too but I don't have CC650 high current clamp so just went the traditional route. Otherwise CC650 over batt cable, ignition pickup on a plug one and crank it with injectors disabled. 5 min job and scope will show you what cylinder is easy to push. Maybe I put high current clamp on my Christmas list lol

[digger]

Did the wire measure different with multimeter?

[zaq123]

haha, yes more cam and better cyl head for sure. Just a note that might help someone: what made me do compression test was a troubleshooting of my not exactly smooth idle. I scoped the spark duration at idle and one cyl was at 800ms vs 1.4ms on other 5. Plug harness was brand new Bremi and spark plug swap showed no difference. It actually ended up being one lead being defective on the brand new Bremi harness. Was not totally bad as the spark was firing fine but bad enough to mess things up a little.
Swapping wires on stock harness ito test on another plug isn't exactly a straight forward task and requires harness cover disassembly to reach another spark plug as they are spaced out pretty well on the harness, what I ended up doing to confirm that it was a bad wire as short spark duration has moved to another cyl when the wire was moved.

[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.....

[MrBurgundy]

Better than 90!

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

So sounds normal to me.

[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

[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.