Figuring compression ratio ....

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Brooks James

Brooks James

VET, CPT, Huey Medevac Pilot
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Whats the formula to use for Figuring compression ratio ?
360 magnum
Stock bore
Stock dished Pistons down in the hole 50 thousandths
60cc chambers
80 thousandths head gasket
 
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I think Karl has a how to article on that.
 
Without knowing the size of the piston dish, I couldn't even give a wildassed guess....
But ill give one anyway.... low eights.
Edit: ran numbers on Wallace site with the 11.2cc dish of a sealed power replacement piston, and your numbers. Got 8.48, might be a little less than that. 87 octane engine.
 
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Formula is ; total space above piston at TDC, plus swept volume, divided by total space above piston.
Rather than trying to do conversions from cc's to inches, and all the calcs yourself, it's easier to use a CR calculator. I like the Wallace racing one.
Bottom line, NO calculator can be accurate without actual measurements of the actual components involved. Some info provided by manufacturers can be believed, some can't. Mopar compression ratios are notorious for being less than claimed.
 
33IMP said:
Without knowing the size of the piston dish, I couldn't even give a wildassed guess....
But ill give one anyway.... low eights.
Edit: ran numbers on Wallace site with the 11.2cc dish of a sealed power replacement piston, and your numbers. Got 8.48, might be a little less than that. 87 octane engine.
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Thank you, after I posted, I did more research and ran into the need to know the dish cc. Was unable to find it.
 
MOPAROFFICIAL said:
Why .080?
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Great question. Depending on configuration , I have enough nitrous Equipment to deliver a 100-300 hp shot. Using the formula 2 degrees retard per 50 hp, A 300 shot would be 18 degree retard. A MSD 8680 dash controlled retard module has a IIRC has a 0-15. Degree .range. With a 200-300 shot cylinder pressure may be enough to cause head gasket sealing issues. The copper ..80 thou gasket is specifically designed for magnums. So...Im hoping to find a happy medium between dumping a lot of timing and/ or keeping compression lower.
Ideas ??
 
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Brooks James said:
Great question. Depending on configuration , I have enough nitrous Equipment to deliver a 100-300 hp shot. Using the formula 2 degrees retard per 50 hp, A 300 shot would be 18 degree retard. A MSD 8680 dash controlled retard module has a IIRC has a 0-15. Degree .range. With a 200-300 shot cylinder pressure may be enough to cause head gasket sealing issues. The copper ..80 thou gasket is specifically designed for magnums. So...Im hoping to find a happy medium between dumping a lot of timing and/ or keeping compression lower.
Ideas ??
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The heads in question end up 66cc after the chamber work to access the valve job.
66cc or less with milling is what you have to play with IF you decide to go with them.
Figure that in.
 
Brooks James said:
Great question. Depending on configuration , I have enough nitrous Equipment to deliver a 100-300 hp shot. Using the formula 2 degrees retard per 50 hp, A 300 shot would be 18 degree retard. A MSD 8680 dash controlled retard module has a IIRC has a 0-15. Degree .range. With a 200-300 shot cylinder pressure may be enough to cause head gasket sealing issues. The copper ..80 thou gasket is specifically designed for magnums. So...Im hoping to find a happy medium between dumping a lot of timing and/ or keeping compression lower.
Ideas ??
Click to expand...
Uh, 300 shot, 12°? Still, when EM blew up their 305 with a 300 shot, Monte Smith said it would have survived if they had taken more timing out.....
If you're willing to run good gas along with the nitrous hit, more compression, meaning more n/a power, will result in more power from a given hit.
Much harder to tune a decent sized shot on pump, than on good gas.
 
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