How do i figure compression ratio?
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From: Walla Walla, Wa
How do i figure compression ratio?
I am looking at this shortblock, and i want to know some numbers. With 61CC heads the motor had a compression ratio of 10.4:1. Which is to high for me to run with my Supercharger. It is a 347 CID Stroker motor. I am curious how i would find my compression ratio if i used the current heads i have, which are a 65.6CC combustion chamber. I really want this motor, as its a good rotating assembly, i just got off the phone with the machine shop, he was really impressed with the quality of components the guy used. I have looked around, the stroker kit he used is about $1500 by its self. He bought a complete Eagle rotating assembly. Here is from Jegs website, which lists the kit for 1100 pluss shipping.
According this, i would be right about 9.8:1, is this going to be to high for a blown motor? I will be changing my pulley to the 6lb pulley instead of 8 pounds.
Fits Ford 302 5.0L. Includes 5140 SIR I-beam connecting rods, ''ESP'' std/std cast steel crank, Keith Black hypereutectic 4.030'' bore pistons, Clevite rod and main bearings, file fit plasma moly ductile iron ring sets, and new harmonic balancer and flywheel or flexplate. Stroke: 3.400'', Rod: 5.400''
1981-95 SB-Ford 347ci Street & Strip Rotating Assembly for Manual Trans, .030'' Overbore
Bore x Stroke: 4.030'' x 3.400''
Rod Length: 5.400''
Pistons: KB -6.5cc Flat Top
<LI>Comp Ratio: 10.7:1 w/58cc, 10.2:1 w/62cc, 9.8:1 w/66cc
Bore x Stroke: 4.030'' x 3.400''
Rod Length: 5.400''
Pistons: KB -6.5cc Flat Top
<LI>Comp Ratio: 10.7:1 w/58cc, 10.2:1 w/62cc, 9.8:1 w/66cc
Postmaster
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From: Spokane WA
Hotshot
Joined: Dec 2003
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From: iowa
sorry guess I forgot the link here is the one I was talking about.
http://raceabilene.com/kelly/hotrod/compression.html
http://raceabilene.com/kelly/hotrod/compression.html
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Posting Guru
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From: Walla Walla, Wa
Well, i used robs caclulator, according to the numbers i have
Bore 4.03
Stroke 3.4
Combustion Chamber 65.6CC's
Headgasket thickness(from frod racing, for the gaskets i will use) 0.040
Piston to deck(from PO) .086
Dome volume -6.5CC
I geta CR of 9.3, which i can live with.
Bore 4.03
Stroke 3.4
Combustion Chamber 65.6CC's
Headgasket thickness(from frod racing, for the gaskets i will use) 0.040
Piston to deck(from PO) .086
Dome volume -6.5CC
I geta CR of 9.3, which i can live with.
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Joined: Dec 2003
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From: iowa
You can live pretty well with that, and you might even be able to run it with a little more boost if you have a boost timing computor.
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From: Walla Walla, Wa
What I will do is when I get it tuned on the dyno, I will do it with the 8lb pulley. Then I will run around with the 6lb pulley for everyday driving, and throw the 8lb on when I wanna race or something.
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From: Spokane, WA
You really need to get a boost timing computer on there. 9.3 compression with 6lbs of boost is still a 13:1 final compression. Even running high octane you're gonna be on the edge of ugly in terms of detonation. I wouldn't even think about running the 8lb pulley without bling bling race fuel. Is there any way you can get a piston with a slightly larger dish? I'd rather run lower compression and more boost to compensate than to risk detonation.
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Joined: Dec 2003
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From: iowa
you could do a little massaging on the combustion chambers, and gain a little there if you want to reduce the compression (work around the valves to unshroud them some even if not really needed with a blower) but getting the boost timing computor, and will take care of most of it.
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