cranking pressure on this combo?

[Tony W]

I am working up the figures on my next motor. It'll be a 360(+.030") with a stroker 4" crank. Pistons I am looking at have a compression heigth of 1.457"(and 5 cc valve relief). If I deck the block .020", this will give me a zero deck heigth with these pistons/4" crank and standard rod length. Heads will be Edelbrock aluminum with the closed 65cc chamber. With a .040" gasket, I have a quench setup with a 11.58 static compression. Now I am looking at the Ultra Dyne NF60 cam with a gross lift of .570", net lift of .554", duration at .050" is 251, 108 lsa and a 104 intake centerline. Can anyone guess what the ballpark cranking pressure will be with this combo?

[HOLESHOT]

LOOKED AT THE FIGURES AND ACCORDING TO THE PROGRAM I HAVE AT .040 GASKET THICKNESS IT SAYS YOULL END UP AT 10:5 AND AT .020 GASKET YOULL GET 11:02.THINK THIS CAM IS TOO LARGE AT .050 BUT DEPENDS ON GEARS AND WIEGHT OF SETUP,TRANS ETC.CRANKING PRESSURE ? NOT SURE

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[Billydelrio]

I plugged the numbers into PTEA and got:
Static Compression: 11.67 to 1
Dynamic Compression: 7.6 to 1
Cranking Compression: 190 psi

I assumed a gasket diameter of 4.1 inches.
Billy

[This message has been edited by Billydelrio (edited December 24, 2000).]

[Billydelrio]

a70duster, you are correct that cranking compression is a direct reading from a pressure tester. However, you can't multiply the calculated physical(static) compression ratio by atmospheric pressure to get the cranking compression because of the valve movements. The intake valve is open during the first part of the compression stroke.

Advancing the cam timing 8 degrees can increase the cranking compression by 50 psi while not affecting the static compression ratio.

A more radical cam profile will have a lower dynamic compression ratio, thus the need to raise the static compression ratio in order to get the cranking compression desired.

Also, when air is compressed, it heats up. If engine is warm, heat is added and pressure gained. If engie is cold, heat is absorbed and pressure lost.

Way too complicated for me to calculate exactly, so I let a computer program estimate it for me.

The 190 psi in my earlier post came from a computer engine dyno that made an estimate.
Billy