[Autoengineer]

My paperwork from my engine builder states 9.8:1 compression. Going from a .050 head gasket to a .043 a few years ago. I then thought I was at around 10:1.

But....I had a suspicion that my compression was lower based on the 150psi pressure I had measured with a compression tester (back before my broken timing chain fiasco). My old Patriot heads are off the car now because of bent valves so I measured my combustion chambers. I greased 2 of the valves, slid them onto the heads, inserted a spark plug, and filled the chamber with alcohol. I definitely have the 74cc heads as I was able to easily pour 70cc without spilling a drop and there was room to spare. With my dished pistons (+12cc) that puts me at only about 9.3:1 !! ....AND thats assuming 0 deck clearance (Which I don't think is the case, but I haven't gotten a good measurement on that yet.)

For my new heads, I was going to go 65cc. That should put me no more than 10.15:1.

Are my calculations sound? I have a .040 overbore.

Thanks/

 

[Autoengineer]

Let me clarify...the 150psi cranking compression test didn't have any input into my calculations. I only mentioned it to explain why I bothered to look into my compression into the first place. It was the reason I was suspicous about my supposed 10:1. Thanks

 

[Autoengineer]

OK, so using a dial indicator, I measured how far the edge of piston 1 is below the deck at TDC. I got .032"

Plugging that into the calculator, this knocks my compression even lower. About 8.7:1 w/the old heads and 9.5:1 with the new. I definitely made the right choice with the 65cc heads. I think the 9.5:1 is a better # for my cam.

I know why the builder probably did it. I originally had a very mild GM ZZ4 cam and he probably felt that the lower compression fit that cam, its just irritating that he didn't fix the paperwork to match that. Also would have helped if Patriot had marked their heads with the size!

 

[Autoengineer]

If you want to verify things and find how much 'in the hole' you are then do this: with your dial indicator, set it up over the piston, rotate crank so piston goes toward TDC, at the highest reading the dial indicator makes, STOP. Put a straight edge over the deck of the piston, place a feeler guage under the piston and straight edge and see what you get. This will give you piston to deck height. If you have a caliper, you can even double check things with that using its depth stick.

What I did was set the dial indicator on the edge of the deck. Zeroed it out. Moved it over a few mm onto the surface of the piston and rotated the engine and looked for TDC, Took the difference. The magnetic base was fastened to the top of the deck, so at least theoretically, moving the indicator side to side, it should have stayed parallel to the deck.

I can try to verify with a straight edge and a feeler gauge.

 

[Autoengineer]

In my world, regardless of cam and compression, if your cranking compression is 150, your engine is lazy, it needs more compression- see what I mean for using the cranking compression as a tool.

The idea that 9.5 compression is just right makes no sense if your running around in an engine that can only produce 150 psi cranking compression, you either need to raise compression or get a different cam.

You can easily run 87-91 octane on an engine with 185 psi cranking compression and I see this kinda of number as minimum.

Agreed on the 150psi being too low.....Let me clarify though. The 150psi cranking pressure was with the old 74cc combustion chamber heads. I would expect it to be more with the new 65cc heads. The 9.5:1 compression was calculated with the 65cc heads.