My question concerns my compression check. With an average compression of only 140 psi, does this indicate a lower comp ratio?
This engine has less than 1000 miles on it. No oil consumption.
I have 113 heads that are milled, with the biggest valves we could get in them. Valves came from a 400ci Pontiac. .040 pistons were flat tops, cast and oversize .040

I did not check to see how far down the hole they were.

The engine is a 312. In my Ford manual it shows comp of 160psi and police 150psi, due to lower comp ratio.

Just curious, as the engine runs great! Impresses me and my friends.

Jim

  Jim Rowe Elkhorn, NE

I think the cam can affect it and the timing, straight up, advanced, or retarded. Is that stock?

My old engine and new engine both indicated about 150psi, with a repop blower cam. Both engines are significantly different and those differences include at least 1 point of compression increase. The cam/lifters, block, and timing set are the only things shared between both engines. When I checked the new engine, it was relatively new. Maybe the compression will be higher now, with over 10K miles on it.


Lawrenceville, GA

I'm writing to agree with Charlie.

Cranking compression pressure may not correlate with an engine's mathematically calculated static compression ratio.

Camshaft intake and exhaust opening and closing timing, valve event overlap, and camshaft installation (advanced or retarded) will affect cranking compression pressure readings which may in some fashion relate to the engine's dynamic compression ratio.

If you are using an aftermarket replacement camshaft, published original equipment cylinder cranking compression data is no longer meaningful. The value of the cranking compression pressure test is reduced to telling you if the cylinders are performing equally. Significant variations in cylinder pressure readings may indicate piston ring, valve, or gasket problems.

With an aftermarket performance cam, an engine with a low cranking compression pressure readings may breath better at higher engine r.p.m.s than an engine that has high cranking compression pressure readings. This gets into the business of low speed and high speed engine performance, engine power band location, and other stuff I try to understand with the help of others on this Forum.

Hope this helps some...

NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Thanks Guys,

Yes it does have a special grind ISKY cam that was spec' by Ted Eaton.

The Block was decked as well, don't know how much.

The motor will pull well over 6 grand... Usually shift at 5600 when I'm playing with it.

Thanks again for helping me understand a little more about cranking compression.

Regards,
Jim

  Jim Rowe Elkhorn, NE