Do these compression test results seem low to you?

Great68 · Aug 2, 2011

Hi Stangnet!

After a 2 year hiatus from my Mustang (Moved, got married, yada yada...) I'm finally back in the saddle again.

I figured that I would start off this summer by doing a compression test on my motor (I had never done one on my car before).

I did the test about an hour after going for a drive and then shutting off the car (I have no idea how you'd do the test right after running the car without getting 3rd degree burns on your hands & arms).

The results are as follows:

1: 127
2: 130
3: 125
4: 129
5: 130
6: 130
7: 132
8: 127

While it seems like all the cylinders are pretty even, I'm wondering if those numbers seem low? I've searched through previous threads on this forum and I'm seeing people typically get results around 160-180psi.

My motor doesn't burn any oil at all, but I definately don't feel like I'm getting near the power level I should be for the modifications I have. My Mazdaspeed 3 could rape my Mustang 6 ways from sunday in a race.

Edit: I guess I should add my motor details:

289, AFR165 61cc heads, Comp XE262 cam, Crane aluminum roller rockers (1.6), RPM Air-Gap, Speed Demon 650 Vac, Repro Tri-Y's, MSD Pro-Billet Distributor

Rmoore45 · Aug 2, 2011

A pair of leather welding gloves works great for getting to the hot areas of the engine. Compression should be anywhere from 130-170, and since you let the car set for an hour I would say that you are well within that range. Since you mentioned power losses you might want to check the valves, the exhaust valves could be set to tight and robbing power. A trick my dad told me was to get a dollar bill and hold it near the tailpipe with the engine running, if it sucks in the dollar bill then it might be a valve.

rbohm · Aug 2, 2011

the numbers look fine to me. remember that your cam is going affect the cranking compression numbers. and since they are quite even, i wouldnt worry about them.

67rcks · Aug 3, 2011

I get same figures on somewhat warm 302 with similar specs - 218/228 .471 lift cam, ported d2 heads, 650 cfm holley and 9.2:1 CR.
There is a noticeable power drop between 1600 - 2500 rpm + engine is louder
This makes it hard to keep up with modern cars.

65fastbackresto · Aug 3, 2011

Those numbers are almost identical to mine.

My motor is a fairly fresh rebuild (less then 500 miles on it) and runs great though. If your losing power I bet its somewhere else. My build is fairly close to yours too.

MitchGT · Aug 4, 2011

the numbers and parts look ok.. What is your compression ratio? I think 61 cc heads might have reduced it a bit from stock.
What tranny and rear gears do you have?

65ShelbyClone · Aug 4, 2011

Great68 said:
I figured that I would start off this summer by doing a compression test on my motor (I had never done one on my car before).

Did you ground the coil wire and hold the throttle open when doing the test?

Great68 · Aug 4, 2011

MitchGT said:
the numbers and parts look ok.. What is your compression ratio? I think 61 cc heads might have reduced it a bit from stock.
What tranny and rear gears do you have?

C4, 3.55's and powertrax locker.

My best time at the track so far was a 14.9@88

Great68 · Aug 4, 2011

65ShelbyClone said:
Did you ground the coil wire and hold the throttle open when doing the test?

I held the throttle open, didn't think to ground the coil though, I just pulled the lead off.

Do these compression test results seem low to you?

Great68

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