compression test??

sneakers O'toole · Post by **sneakers O'toole** » Tue Jun 29, 2010 6:12 pm

i did one on my car today and got 150 across all cylinders. now after looking around here for a bit, this seems really low. mag head is shaved .020 so my C.R. should be around 9.7:1 or so. is something wrong?

LowNSlow · Post by **LowNSlow** » Tue Jun 29, 2010 6:44 pm

If it's 150 across all four I wouldn't stress it.

OB · Post by OB » Tue Jun 29, 2010 7:30 pm

Compression should be 170-225, so you're low all around. Max variance is 25% between cylinders, but that doesn't apply to actual peak pressure, just the difference from one cyl to the next.

sneakers O'toole · Post by **sneakers O'toole** » Tue Jun 29, 2010 7:43 pm

OB wrote:Compression should be 170-225, so you're low all around.

that's what i thought too and that's why i'm concerned. car doesn't smoke or have any indications of a problem. runs good and pulls pretty good for what it is. i saw a non-r/t on here at 168/9ish and an acr at 205 across the board. 150 doesn't seem right.

although...the same guage read 125ish on two cylinders on a '92 maxima 3.0....i'm thinking about going out and buying a new one. the one i used was at work..i dunno. maybe i did it wrong? i had all plugs out and did it that way...wrong way??

LowNSlow · Post by **LowNSlow** » Tue Jun 29, 2010 8:11 pm

150 is low like said above but if they are ALL 150 and it's not running bad...continue to drive it and just keep up with it.

You should be fine.

sneakers O'toole · Post by **sneakers O'toole** » Wed Jun 30, 2010 12:17 am

ok so i checked my compression again with my brand new gauge and the first two i got to 180...i didn't get to the other two because of a spark plug tube issue i had to fix. but it's late and i have to do a head gasket on a cherokee and a tranny swap on some haggard ass saturn tomorrow, i'll do the test tomorrow after work. i'm going to bed..

Post by **occasional demons** » Wed Jun 30, 2010 12:37 am

Also, if you are not doing it, the throttle should be open for an accurate test. It doesn't need to be at WOT, a large screw driver stuck in there to hold the plate open is sufficient, and it is heavy/large enough, it won't get sucked in. (Good luck getting the handle past the plate anyways)

If it has not been run since your previous test, put a charger on the battery too.

darthroush · Post by **darthroush** » Wed Jun 30, 2010 1:31 am

LowNSlow wrote:150 is low like said above but if they are ALL 150 and it's not running bad...continue to drive it and just keep up with it.

You should be fine.

I'd rather have them all equal across the way and be a bit low than every cylinder having a different reading. Pretty decent IMO.

Another thing to do after you retest it, put a little oil in one of the cylinders through the spark plug hole and see if it bumps the compression up.

Harobmx477@aol.com · Post by **Harobmx477@aol.com** » Wed Jun 30, 2010 1:33 am

with my spun bearing i get 130 across the board. i thought that was good for these cars lol..

Post by **occasional demons** » Wed Jun 30, 2010 8:00 am

Well a spun bearing will have no effect on a compression test, if it is not keeping the starter from spinning at normal speed.

There are other factors that can influence the readings too. The cam profile can make a huge difference, but not too many stock/near stock neon engines have that problem. They make pretty good low rpm cylinder pressure.

sneakers O'toole · Post by **sneakers O'toole** » Wed Jun 30, 2010 11:49 am

well i didn't have the throttle open at all for any of the tests

..i'll be sure to do that after work today. like i said, before my "spark plug tube issue" last night, i got 180 in cyls 3 and 4 WITHOUT the throttle open at all.....then my battery died and i had to compression start it down my driveway because my battery tender/jumpstarter is M.I.A.

i have an '01 mag cam in the mag head that's milled .020 for very close to 9.8:1 CR..but i suppose that's still considered basically stock so i don't think that'd make much of a difference. either way i'll post my findings later today then.

:edit: harobmx, i see your in bangor? you should check out the P.A. ECN Meets once a month.

Post by **occasional demons** » Wed Jun 30, 2010 12:15 pm

Thinking about it, the IAC opens up while cranking, so there may be a built in throttle prop, if it is doing it's thing. But propping it open still can't hurt.

The R/T cam is still a zero overlap cam, so no loss of cylinder pressure there. IDK if the Crane 14 has enough overlap to really have an effect.

You definitely want the battery in good shape, to get a consistent cranking rpm between the first, and last cylinder.

sneakers O'toole · Post by **sneakers O'toole** » Wed Jun 30, 2010 6:03 pm

ok, so i did the test again a little bit ago with a warm engine, throttle about 1/4 open, with my battery hooked up to my charger (that someone saw fit to "place" in the shed under the seat of a busted ass lawn tractor) since i found it. i got 190 across all cylinders, i feel better now, haha.

LowNSlow · Post by **LowNSlow** » Wed Jun 30, 2010 6:07 pm

I do mine at WOT... everything I have read on the net said to do wot but I'd imagine that it wouldn't make much difference.

Nice numbers.

sneakers O'toole · Post by **sneakers O'toole** » Wed Jun 30, 2010 7:13 pm

thanks. maybe if i did it right the first time i wouldn't have been so worried, lol. i get a warm fuzzy feeling when i know all is well with the neon, haha.

FWIW, i did the test with the throttle closed first and it was @180, then 1/4 to 1/2 open is when i got 190, maybe if i do it at WOT i'll get 200...maybe i'll do it over the weekend..maybe i'm overthinking again..

ricker91878 · Post by **ricker91878** » Wed Jun 30, 2010 8:25 pm

I got a 1st gen r/t with the dohc, 180 on all 4 cylinders

Jamie · Post by **Jamie** » Thu Jul 01, 2010 10:21 pm

Your always going to get higher compression psi from a warm engine than a cold engine. We do it with airplanes all the time. If the compression is kinda low (not low low) the we will run it then check it again. Heat causes the rings to expand yada yada. so 150 cold isn't that bad. And like everyone saying, even numbers across the board is a good thing. It means all of the rings have set evenly.

If your not burning oil, your rings are fine. You have a healthy engine and I wouldn't sweat it.