i did a compression test on my motor and got 170 psi, (with throttle closed, and enough cranks to get a steady reading). it's a 356 with a 9.9 static compression and a 268 extreme energy cam, with 68cc afr heads. My experienced mechanic friend says 215-220psi is generally the limit for pump 93 gas, but i when i did my research for the engine parts it seemed 10:1 was about what i wanted. where did all my compression go? i have the cam on the zero advance tooth on the cloyes timing chain. the engine always has been a bit disappointing... where did all my compression go? what static compression should i shoot for to get the most out of 93 i can?
-thanks
Andy

Andy- I know the upper limit for 91 pump is around 185 PSI.....but I think 215-220 on 93 pump might be a little high. Most of this depends on your Dynamic Compression Ratio (DCR) and your camshaft timing. Here's a link to give you some more input on DCR:

http://members.uia.net/pkelley2/DynamicCR.html

I am sure the guys will be able to answer any questions you have concerning this.

I believe that your PSI measurements are on the money for the build you have described. I really dont think that you should be anywhere near 215-220 with the parts and camshaft you have.

Am I reading correctly that you performed the test with the throttle CLOSED? If so, you may want to go back and test again with the throttle blocked OPEN. The idea is to get as much air into the cylinders as possible to get a correct reading.

At least this is the way I have always heard that it is supposed to be done, and I have seen others perform it this way as well.

Throttle blades need to be all the way open or you won't get an accurate reading.

Chad

If you have a true 9.9-1 compression, you should have about 190-195 psi with the throttle blades open. My 355 w/9.4-1 and a xe268 cam generates 180 psi.

My specs are unknown as the motor was done before I bought it. But I read 240 psi and get by on Sunoco 94, barely. What ballpark compression ratio would that suggest? I know this is vague so I'm just looking for guideance.

Mine is 210 psi and I get by with 91.

Cranking compression numbers in the exact same motor will change depending on the cam timing

Two isuues:

1. The throttle needs to be blocked open when you do your testing. Having them closed will make it read low.

2. I do not think your static compression ratio is 10:1, with 68cc heads and flat top pistons your compression ratio would be about 9.5:1. Most stock engines actually have a dish piston which would lower your compression even more.

i will have to redo the compression test and see what i get, i did do it w/throttle shut. in my 9.9 calculation i took into account the valve reliefs in the pistons, deck height, gasket thickness, etc. I already understand why cam duration affects psi, i was just curious if i should consider a smaller cam, or getting the heads shaved, to increase compression over what i have... but first i obviously need to do a proper compression test-

hey sorry to be off topic but how much of a gain should you see when you open the throttle blades for the compression test??? i did mine without and was a bit dissapointed as i thought i would be up around 185 and wasn't, thanks
jake

Well just walked in from the garage... did the compression test with throttle open... made no change. i thought so, because when i did it the first time, i had pulled all the plugs first, then did it. therefore none of the other cyclinders were sucking air, and the carb's idle setting is enough to support the one cyclinder's intake. so... now i'm still back to wondering where my promised 190 psi went? it's a 4.04 bore, 3.48 stroke, .040 gasket, .005 deck, 3.5 cc for total valve relief (actually measured it)flattop pistons, cam installed on the zero advance setting and 68 cc heads. makes me wonder... only thing i didn't measure for sure is the heads. are they actually 68 cc?.... i'm not thinking so... :rolleyes:

frustrating... i think i need a yuengling... graemlins/beers.gif

imo, don't get overly concerned about cranking compression. too many variables in the mix. cam timing, speed the motor is cranking (battery strength),etc. just look for consistency in the numbers, and do a leakdown test if your worried about the state of the motor. those numbers don't sound bad to me, and as eric stated, your static compression could be lower than you think. my 406 w/ 11.6:1, and 250 at .050 duration on 108 lsa cam has a cranking compression of 220psi. heck, my buddies 632 i help wrench on has around 180-190 cranking compression, it dynoed at 1180hp. my point is, if it runs good, don't worry. just my .02 graemlins/thumbsup.gif

Is the motor brand new? maybe the rings have not seated yet.....