Thread: Changing compression ona built motor

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Originally Posted by HotRodN

Hey all,,, I recently acquired a 383 stroker to go in my Hot rod and it is supposed to have 11.5:1 ratio compression,,,, from what I understand it's not a good idea to run pump gas with the compression that high on steel heads,,, it is a running motor and I don't want to tear it down unless I have to but I'm okay with doing that,,,
I'd like to lower the compression to 10:1 and run normal pump gas without octane booster.
So any input on how to build a 383 with the existing parts I have now and what would need to be changed is greatly appreciated,

Here's what I can tell you about the motor,,,, if you need more let me know and I'll do my best to get it.

Good ole 10/20 block,

Scat crank with I believe 3.75 stroke,
5.7 Scat rods,
Dart Iron Eagle 72 CC heads,
Roller tip 1.6 rocker arms,,, steel stamped
12-292 cam,,, 501 lift I think and 110 lobe separation,,,, need to look up more on this one.

Domed piston but I could not get the dome CC,

Holley 750 double pump vacuum secondary carb,
Not sure of the intake but it's definitely not a street intake,,, single plain high rise,,, probably Edelbrock,

Oaky that's all I can think of at the moment.

Since it's running, warm it to operating temperature and do a compression test on all cylinders.
Remove the air filter element.
Remove all spark plugs to help the starter turn the crank.
Disable the ignition system to prevent a fire.
Wire the primary throttle blades wide open so the motor can breathe.
Let the motor have at least 5 "chuffs" on the gauge for each cylinder. Max the needle and write it down.
https://www.youtube.com/watch?v=X_tbksFYhl4

Crane Cams engineers state that an internal combustion engine can safely be operated on pump gas up to about 165 psi cylinder pressure, although I have witnessed many motors that have more cylinder pressure than that and operate OK on pump gas. I suspect that it has to do with squish/quench.

I know you didn't ask for a lesson, but here's one anyway.
As the piston ascends in the bore and approaches top dead center, the area of the piston opposite the chamber gets friendly with the flat area of the cylinder head opposite the chamber and squishes out the air/fuel mixture that is between the piston and the head. This sudden close proximity jets the mixture across the chamber toward the spark plug and the turbulence created homogenizes the mixture so that all of it will burn, instead of having rich and lean pockets that may or may not burn.

The quench part of squish/quench occurs with the piston in very close proximity to the underside of the cylinder head, where heat is released from the piston crown into the relatively cooler cylinder head, cooling the piston. The tighter the squish/quench, the more power the motor will make, up to the point where you have a collision between the piston and the head. On a small block, David Vizard has gone as tight as, I believe 0.027" before he encountered a kiss. A kiss might be defined as seeing the piston part number imprinted on the underside of the head at the next tear-down.

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