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Old 02-25-2008, 11:18 PM
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2 part question compression and est HP

Hello, I have a two part question, first part is I have a 350 chevy with 76cc heads it has 8.75 to 1 compression I am switching to 58cc heads. Does anyone know what the compression will be after I change heads and will the added compression creat much of a horsepower increse?..........part 2 I have a 350 crate engine it was originally a 260 hp model I have changed to a comp cams 292-501 hyd cam, victor jr intake, holley 570 avenger it has 76cc heads it currently has 1.94 intake and 1.500 exh valves and 8.75 to 1 compression I am switching to 58cc heads with 2.02-1.60 valves. does anyone know about what horsepower to expect with my changes and do you have any tips, tricks, or suggestions that would help to further improve what I have. Thanks in advance.

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Old 02-26-2008, 02:39 PM
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Check Knowledge


Check the knowledge base on this bulliten board. In the section on Engines, you'll find several formulas and even some calculators that will help you determine CR. Consider there is a difference between static CR and dynamic CR. Bench racer's usually discuss static CR, but the dynamic CR is more important. Problem with dynamic CR, I've looked and cannot find any reference point for dynamic CR as it relates to performance, or octane ratings needed for fuel, etc.

As for how much power you'll be makiing, a while back (30 days?) there was a member on this bulletin board that just got some new engine simulation software and was asking for people to send him their specs so he could run the numbers. Use the search capability and search the engine forum. Hope you find it.
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Old 02-26-2008, 03:23 PM
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1 full point of compression (like going from 8.5 to 9.5) will get you about 3% more power, so maybe 8hp for your 260hp engine. Alone, its not a very big increase and you won't feel it unless you have an insanely light car. What's more important is to match the cam to the compression so your cylinder pressures remain right for the application. Big cam = big compression.

Another important thing is head flow. If your 58cc heads have the same ports, then just expect your 3% gain. If they're smogger 305 heads, expect performance to go in the toilet. If they're really good aftermarket heads, expect more than 3% gains.

The crate 260 hp engine probably has a flat tops to achieve 8.75:1. The smaller 58cc chambers put the valves awfully close to the piston. Usually flat tops aren't used with 58cc heads for that reason. Even if you have the octane to handle the high compression, the chances that the existing pistons (designed to work with large chambers) will work with 58cc chambers is slim.

You're going to need a whopping head upgrade to go with 244* duration cam. you're strictly into fully-race-ported stockers, or hefty aftermarket heads.

Last edited by curtis73; 02-26-2008 at 03:37 PM.
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Old 02-27-2008, 09:47 PM
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RE: two part question

I had'nt thought about the possible piston to valve clearence so that I will check for sure. your information seems solid and to be truthfull I am starting to think the change may not be worth it. the money is already spent but the car is still together if you dont think the difference will be very noticable then I may pass on it and go ahead with my plans for a big block which I am sure will be exactly what I need. thanks for the information
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