tmdog

Older SBC bored 60 over w/ Vortec heads and Comp XM 270 cam. Comp readings are 208 on the high and a low of 190 on the cylinders. These numbers seem very high to me. Is this normal or should I get worried. Comp tester has been used on other engines and is reasonably correct. For test, engine was given 5 revs and open throttle. Thanks.

1BIGJIM

iTrader: (1)

Sounds HIGH to this guy

tmdog

What would cause the comp to be high? Should I be concerned? What negative effect on engine?

hillbilly24

That's way high, water in the clyinders will do that. pull the plugs and crank the motor and watch for water to come out of the clyinders.

tmdog

Prior to the test all plugs were removed. When checking out the first cylinder, wouldn't water be removed from the other cylinders while checking the first? I hope this makes sense.

JJONES

Cyclinders Have To Be 10% Of Each Other To Be Ok,if Lower Then You Have A Problem,190 Range Is Not To High Depending On How The Heads Were Cut,how Thick The Head Gasket Is And What Type Of Pistons Your Running,190 Is High For A Stock Motor Though,are The Heads Open Champer Or Closed Chamber?this Also Will Change Comp Readings,the Higher The Comp-the Higher The Octane Needed To Eliminate Pinnging.you Might Want To Check Your Pulgs For Correct Jetting Aswell.

tmdog

These are closed chamber heads (Vortec). When I pulled plugs, 5 plugs looked extra clean, like out of the box. The other 3 had slight color. Very slight. Changed jets on the 4150 to 78-86, 4 sizes up from comp. check. will read tomorrow. Thanks for the ideas. Need more input. Feel free to chine in.

1BIGJIM

iTrader: (1)

You better be running very good GAS or you are going to have a problem. There are alot of variables that go into compression PSI. That cam is nothing special. Either you have pistons with big domes or something. My opinion 170 should be an target range. A stock 330hp will produce 140-150 all day long. Good luck

rws

iTrader: (3)

I believe that Vortec heads have a small combustion chamber like 58cc or 64cc with a flat top 350 piston 64cc will be about 10-1 compression ratio.I ran a 10.3 to1 406cu. with a jumbo solid roller cam that had similar compression #'s .I had a hard time keeping head gaskets on it.Make sure that U run premium.

Rob

NOBODY

Cranking compression and actual compression ratio are two different things. Just by advancing and retarding the timing makes the cranking compression change. If you advance the cam you add cranking compression and if you retard the cam you lower cranking compression to a certain extent. You might want to put a degree wheel on it to verify the cam degree and see what the cam manufacture wants the degree to be. You can have a low static compression ratio and have a high cranking compression with camshaft selection and where the intake valve closes. Any thing over 180 pounds cranking compression will be hard to tame on pump fuel.