m20 2.8 stroker cylinder psi

[e30rapidic]

Any of you stroker guys ever put a cylinder compression gauge on your motors? I am running the M52 crank, eta rods and b25 piston setup and looking at about 170psi. Runs like a top....zero pinging with performance chip, 19# injectors and M30 AFM.

Just curious if anyone else has tested theirs? I know a stock M20 is closer to 150 psi.

[Mike]

I thought stock was about 160-170 on a fresh 2.5? Don't quote me though. My 2.8 runs:
-81mm eta cast crank
-85mm Ross flat top 9.75/1 supposed cr (they pop up slightly)
-135mm aftermarket rods
-272 cam (set at stock timing mark- never degreed)

I get about 210-220psi !

What cam do you run, and are you running it retard/advanced?

[hotballs]

I'm going to throw in a "how to" compression test question.

Do you crank the motor the same amount of times per cylinder? or do you just let the engine crank until the gauge maxes out and then record that PSI per cylinder?

[whodwho]

Have a strong fully charged battery, remove all plugs, open the throttle when cranking and they should top out at similar number of revs.

[Mike]

I usually:
-stick the throttle open
-remove all plugs
-verify all valves are .010 cold
-verify battery is +12v
-insert trusty bump starting tool into pins 11+14 on multipin connector
-initiate test with #1
-while cranking, listen and count the compression stroke while watching guage peake
-try cranking each other cylinder the same time while testing
-record results for each cylinder

Note: don't forget to make sure your spark plug seats are debri free before testing. Diag for results on specific cylinder low values can be pinpointed with leakdown test. Low across the board, add a bit of oil first, then repeat.

[jlevie]

Have a strong fully charged battery, remove all plugs, open the throttle when cranking and they should top out at similar number of revs.

Correct, but pull the fuel pump relay or fuse to avoid washing the cylinders with raw gas.

[Mike]

Correct, but pull the fuel pump relay or fuse to avoid washing the cylinders with raw gas.

This is true, I forgot to put that down. Might get fuel in your eye if you forget this step.

[Sirotto]

Any of you stroker guys ever put a cylinder compression gauge on your motors? I am running the M52 crank, eta rods and b25 piston setup and looking at about 170psi. Runs like a top....zero pinging with performance chip, 19# injectors and M30 AFM.

Just curious if anyone else has tested theirs? I know a stock M20 is closer to 150 psi.

170 +/- 8 I think, is correct.

[digger]

more than 210 is too much you need more cam in it, or timing is too far advanced.

[Mike]

more than 210 is too much you need more cam in it, or timing is too far advanced.

Ive felt like moving my cam around, to see what the motor likes best. I have a nuke cam gear, dont have the slightest clue about what kind of valve to piston clearance I have to work with. I was under the impression that maybe I am running on the retarded side, and the cam needs to be advanced. What do you think digger? Its tough to get feedback on this scenario....

[digger]

Ive felt like moving my cam around, to see what the motor likes best. I have a nuke cam gear, dont have the slightest clue about what kind of valve to piston clearance I have to work with. I was under the impression that maybe I am running on the retarded side, and the cam needs to be advanced. What do you think digger? Its tough to get feedback on this scenario....

advancing will generate higher cylinder cranking pressure because the intake valve closes sooner "trapping" more mixture at low rpms.

Unless you check valve clearances then i wouldn't adjust anything.

You can check in place (i have done this) but you need a good lever to compress the valve springs otherwise you can install a soft set of valve springs to make it easier to compress and it could be done without tearing the engine apart. Needs to be done with zero lash as well.

Look to get some tuning via WAR and ditch the splitsecond.

[e30rapidic]

170 +/- 8 I think, is correct.

I was going with the increase in displacement was fairly equal to the increase in cylinder compression so it makes sense. Thank you sir for the confirmation!