What kind of psi should you expect when doing a compression test?

[747mopar]

I've been trying to look up some info on the but the results are all over the place some say 200+ and others much less. My compression ratio figured out at 9.8:1 and since I've been having some tuning issues I thought I'd better check the compression while I'm at it. This is a street motor with a 268 duration cam. I only had time to check 2 so far but they are both at 150psi on the nose.

 

[Banzaiii67]

Was the engine warm, all the plugs removed and the throttle open? 150psi isn't bad, all you should be concerned about is if all the readings are within 15 psi

 

[Meep-Meep]

Cranking compression depends on the cam and the static ratio so can't go by the book unless the engine is bone stock. If you expect to run regular gas then 8.5:1 and 150 - 160 PSI should do well. If you indeed have 9.8:1 (tested by CC-ing) and 200 PSI that will most likely be trouble even on pump 91. I think you want to shoot for 8.5 - 9.5:1 and 165 PSI for pump gas. My 440 is 10.1:1 and 190 PSI and it isn't completely happy on pump 91. Screams on 91 with a splash of 110, which averaged out to around 96. My hemi with 220 PSI is very unhappy on pump gas.

 

[747mopar]

So basically I'm where I want to be for pump gas? No I had let it cool off before checking, all plugs were out and throttle closed (probably should hold the throttle open). I'll get the rest of them checked today. The only thing CC'd was the heads and the piston reliefs, the rest is just by using the formula's to calculate the compression.

 

[nm9stheham]

Which ought to be adequate for calculating CR as long as you also add in the gasket thickness to the computations. Cranking compression varies a lot with the cam duration and the actual cranking speed; having all the plugs out will allow a faster crank speed and can lower the comprsssion readings a bit, but having all the plugs out also gives more consistent cranking speed and thus more consistent reading cylinder to cylinder. If they are all consitent, that is the most important thing.

 

[747mopar]

update, I got them all read and after looking at the gauge I realized it's in 5 lb increments so they all tested at 155 lbs with one showing 160. Moving on to the next things, 7 lbs of fuel pressure, timing at 16 initial and 36 full at 2500 rpm and 10 lbs of vacuum in gear @ 750. Plugs all look evenly colored but all are very lean which probably explains the poor performance, I'm thinking of stepping up to a larger 6.5 power valve to richen the circuit sooner since all of my driving so far has been in town and at low rpms.

 

[Garys1969RR]

I just got a 65 Belvedere with a 451 in it. 9.5 to 1 C/R. Power output is not that great, even though it has has the Eddy Perf RPM heads. Also equipped with a very mild cam. Slightly improved the performance by changing the Team G to a Holley Street Dominator intake, and a 3310 carb. Now working on the distributor. The power output is nothing like my other 451 with a 509 cam and higher C/R.

 

[747mopar]

I just got a 65 Belvedere with a 451 in it. 9.5 to 1 C/R. Power output is not that great, even though it has has the Eddy Perf RPM heads. Also equipped with a very mild cam. Slightly improved the performance by changing the Team G to a Holley Street Dominator intake, and a 3310 carb. Now working on the distributor. The power output is nothing like my other 451 with a 509 cam and higher C/R.

Sorry, should have updated this post too. After a long day of test and tune this 456 runs like a scalded dog and I mean it's wicked now. If interested in what was found and done you can read either of my other threads (restomod charger in the projects forum or the other thread right here a little help diagnosing).