ok got a rebuilt 69 block 350 about 8 years ago.was going to put some new heads with a better intake!...so im going to check the compression with a tester. whats the least psi i should have? i hear someone say over 100 is good! but then i hear it should be more? i dont want to put this much money into it and maybe hurt something unless it will be ok to do so!....thanks.........john

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72ss t-10/ 373 posi. 350 /30 over/ 300+ hp msd 6AL.. heli core wires. new holley 650 dp ...mech/secondaries. mild crane cam ..headers/ 3 inch flowmasters... red with black ss stripes

you defenetly want over 100... about 130 to 160 seems to be the norm i think.. someone else can get alittle more deeper into it...

It's not really so much of a how much PSI as it is a how even each cyl. is thing

all 8 cyl. low just means you dont have alot of compression and/or a really big cam.

When doing a compression test your looking for trouble spots, and that shows up as a cyl. way down in PSI from other/rest of the cyl.

You want no more than a 10% diff on all 8

As far as how much PSI, well it's hard to say cause if you drop a 300+ duration cam in a 7.5:1 engine your cyl. pressure is gonna be way down.

I would say around 130 min. 190-200 psi max if running on pump fuel with iron heads.

I have around 218 psi, non of the cyl are below 216 non is above 218. That shows the engine is in good cond. as far as ring seal, valve sealing, etc

My numbers are high cause of my compression (10.18:1) and a smaller cam (216/228* @ .050")

going to buy a tester tomorrow! so if maybe i get readings around 100 or less..i should rebuild?

If they are all even but low you may have the wrong cam for your engine. It is bleeding off too much low speed pressure. That is the reason matching compression to camshafts is so important. Other wise you will be building no lowend power cause you are bleeding all your compression off. If you are worried about it being worn our why not to a wet and dry test to determine the quality of your ring seal. A dry test is just screw in the tester with the plugs out and the carb held WOT. Pump it till the gauge comes up to max (about 5 rotations). Then squirt a little oil down the cylinder through the spark plug hole. rotate it a couple of times to spread out the oil. Then do the test again. If there is a significant difference in PSI your rings are shot.

Cam timing varies the cranking compression a lot. You want at least 140 psi, no more than 190.

My 355 10:02:1 motor put out 214PSI with a 228/234@.050 cam. Medium humidity, warm day, warm motor.

How this for putting some questions to bed...
or creating some new ones

From a 1972 Chiltons manual.
1969 Chevy Camaro, Chevelle, Nova:

350 cu in., 255 HP advertised compression ratio 9:1, Intake opens 28° BBDC, cold cranking pressure should be 160 on a new motor

350 cu in, 300 HP, advertised compression ratio 10.25:1, intake opend 28° BBDC, cold cranking pressure 160psi.


1969 Corvette:
350 cu in, 350HP, advertised compression ratio 11:1, intake opens 52° BBDC, cold cranking pressure 160psi.

The only 350 listed with more cold cranking pressure is the 1970 370HP with 11:1 compression and a mechanical cam cranking at 190psi. And the only thing with less is the 1972 330HP with 9:1 compression cranking 150psi.

So there gives you some ideas what a new, fresh motor with rather docile valve timing 'should' have. Typically a rebuilt will have lower compression due to using a thick rebuilders head gasket, adding readily available large chamber smogger heads and substituting flat top pistons or dished pistons to make the motor more friendly to low grade pump fuel. Add the typical over-caming and the cranking psi can get low in a hurry. They will run with anything over 90-100psi, motors with anything over 180 to 190psi and iron heads typically get real detonation prone on todays fuels.

If you really want to get a good idea of the condition of the motor, forgo the compression test and do a leak down. This is going to tell you the cylinders sealing ability, and listening to where the leak is going (exhaust, intake, or into the crankcase)is going to give you much better info than a comression test. If leak is more than 7-8% the motor is quite 'tired' and freshening things up 'should' be part of the process if your dumping any $'s in to it at all.
A compression test is like when your mom stuck her hand on your forehead to see if you had a fever. It will only tell you if 'maybe' something is sick inside. A leakdown is more like the lab test the Dr ran,, it will tell you which end is sick, and just how serious it is.

ok, how do you do a leak down test?

Please go over to Pat Kelley's great, used to be called, "Dynamic Compression Ratio Calculator" site!!

Pat calls it "Effective Compression Ratio Calculator" now due to some wording confusion...

Read it over at least 4 to 5 times and it will open up a whole new meaning regarding matched motor combinations!!

I also use the "ROSS Racing Pistons" site's compression ratio calculator and then type in "Boost Calculator" on yahoo and use the second one in the list to find dynamic/effective compression ratio's!!

I try to use Pat's, but I have trouble loading and saving it is the only reason I don't use his site.... (I have to admit, I'm no computer whiz is all here)...

AND you have to use advertised duration numbers to load into it..

I think that Pat has a way of converting duration at .050" spec's over to advertised duration spec's but it is still not dead-on, but Pat thinks it's usable..

pdq67