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Old 06-18-2008, 11:04 PM
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heads for compression ratio iron or aluminum

hey guys i have a 355 sbc with 12.5:1 domes , lunati 300 cam w/ 515/515 lift, stock heads 76cc 194/150 165cc intake runners. performer RPM highrise intake and a holley 600cfm carb. I'm tryin to choose the right heads w/out raising my comp to high for pump gas. i have some 64cc/210cc aluminum

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Old 06-18-2008, 11:09 PM
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heads

sorry i didn't get to finish, 64cc/210cc aluminum Procomp heads or have access to some iron dart 76cc/170cc heads, both have 202/160 valves. i want the most HP i can get with out using racing fuel. I'm worried the 64's will take my compression to high for 93 octane kinda stumped here can anybody help solve this puzzle.
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Old 06-18-2008, 11:28 PM
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12.5:1 according to what? Are you saying that you HAVE 12.5:1 with 76cc heads? I don't get this CR piston rating at all. how can they say they are 12.5:1 pistons when they dont have a clue what they will be used in?
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Old 06-18-2008, 11:45 PM
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Heads

I have heard it's better to go with iron.
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Old 06-18-2008, 11:57 PM
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Quote:
Originally Posted by eatonde
sorry i didn't get to finish, 64cc/210cc aluminum Procomp heads or have access to some iron dart 76cc/170cc heads, both have 202/160 valves. i want the most HP i can get with out using racing fuel. I'm worried the 64's will take my compression to high for 93 octane kinda stumped here can anybody help solve this puzzle.
Since you didn't give/have all the Info needed I'll fudge some of it.
4.03"Bore X 3.48"Stroke, -8cc .500" Dome Piston, Block Deck Height .025, , Gasket Thickness .040, , Quench .065"
With 76cc Chamber Head CR=9.92 to 1 /////72cc Head CR=10.38 /////////// 64cc Head, CR=11.45 to 1 ////////// 58cc Head CR=12.44 to 1

Those Dome pistons probably wont work with a heart/kidney shaped combustion chamber.

Last edited by SSedan64; 06-19-2008 at 12:18 AM. Reason: correction
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Old 06-19-2008, 03:31 AM
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12.5:1 and pump gas are rarely two things you can say in one sentence without choking. You're typically into 110 octane territory there... especially with huge domes like that. I can see running alcohol in a race engine with that type of setup, but domes kill flame front speeds requiring more ignition lead... all of which reduces detonation tolerance big time.

I would reconsider your combo. What vehicle, what is its weight, rear gears, transmission, tire size, any suspension modifications, how will the vehicle (realistically) be used?
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Old 06-19-2008, 08:22 AM
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Compression will lower depending on what cam he uses. depending on intake closing point ABDC will determine his final compression ratio.

Go to the knoledge base, technical data, and find the link to the Keith Blake site. There you can find out your final comp ratio with the cam, you might be lower then you think.
With out knowing your deck height will play a big roll in this too.
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Old 06-19-2008, 08:31 AM
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12.5:1 rating is most likely for 64cc heads, which is too high for pump gas.

Assuming the above is true, a 76cc head would make 10.7:1 cr which would work nicely with 93 octane and your cam.

You will need a 4.11 gear and 3,000 rpm stall for that cam.

A 600 carb is too small. Get a 750

It would make more power with the 210cc heads. If you can, change pistons and use the bigger heads. You may be able to have the machine shop cut most of the domes off and rebalance the pistons.
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Old 06-19-2008, 02:32 PM
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Quote:
Originally Posted by eatonde
hey guys i have a 355 sbc with 12.5:1 domes , lunati 300 cam w/ 515/515 lift, stock heads 76cc 194/150 165cc intake runners. performer RPM highrise intake and a holley 600cfm carb. I'm tryin to choose the right heads w/out raising my comp to high for pump gas. i have some 64cc/210cc aluminum
How about a part number and manufacturer for the pistons. When advertised to have 12.5 to 1 compression, that's against a set of ground rules that say the deck clearance is X of an inch, the gasket thickness is X of an inch, and chamber volume is X cubic centimeters. Variations from those Xed dimensions cause the static compression ratio to change.

Generally, aluminum heads will tolerate more compression compared to cast iron. The reason is that aluminum transfers heat to the coolant faster than iron so the cylinder doesn't see the temperature spikes common to iron heads, therefore, aluminum heads will tolerate more compression before temperature induced detonation sets in.

Bogie
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