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What compression ratio should I aim for in a 350?

Old 01-29-2004, 01:56 PM
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Cool What compression ratio should I aim for in a 350?

Ok, I think I have my combo all picked out. 40 over 350 block, Sportsman 2 heads (72cc chambers), XM270HR cam, full roller valvetrain, Performer RPM intake. The engine is at the machine shop and I need to make a decision of rods and pistons today. I'm going with a house brand forged H-beam 6in rod, and forged JE pistons, I'm just not sure what Compression Ratio I should aim for? 9.0, 9.2? more, less? The motor is going into a raw water cooled 20ft Formula that will only see 93 octane.

Thanks guys!
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Old 01-29-2004, 02:11 PM
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If you know you will only use 93 octane and know its of good quality then you can push it as far as 10:1. It if were me id stay closer to the 9:1 area incase you are stuck trying to find gas and have no choice but to get the crapy 87 octane lake gas.....then you can back down the timing a bit and still be ok till you find the good stuff
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Old 01-29-2004, 02:11 PM
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It sounds like you're on the right track with 9.0 to 9.5 Make sure you have a healthy but not overkill exhaust.

Roby
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Old 01-29-2004, 05:14 PM
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Go with a reverse dome "D-Dish" piston and run 9.0-9.5compression....Also be sure to not have to much qench. Then just make sure you don't have to much timing and you should have a good set-up. Im running the same cam with Vortec Heads and a RPM Air-gap manifold. The Vortec heads let you run a little more compression than stock heads. Let us know how it turns out.

BH
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Old 01-29-2004, 05:18 PM
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Contact the cam manufacture and talk to them, see what comp ratio they recommend for that cam. i would stick around 9-9.5. I ran 9:9:1 on a small block and made a few pistons into art work I had used a lower grade gas before i knew i needed more octane. I also had ran 10.8:1 on 110 octane. Will be running 12.6:1 on 110 octane this year
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Old 01-30-2004, 12:35 AM
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Ok, so any guesses about what compression will be with a flat top piston, 72cc head and a FelPro gasket?

Oh, and I'm not doing anything fancy with the exhaust right now. Just GLM manifolds and risers. Hopefully it will not kill all performance, but I do expect it to suffer some.
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Old 01-30-2004, 03:50 AM
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Figure out which piston manufact. you want to go with and tell them what cc combustion chamber you have. Tell him you want "X" for a compression ration and want a reverse dome "D" Dish piston. They should have what you want. Once installed check your distance between the block and the top point of #1piston at TDC. Add the thickness of your gasket and you get your quench. VERY IMPORTANT STEP!!! You want to get a quench of between .035-.055 NO MORE! You want as close to .038 as possible.....

BH
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Old 01-30-2004, 06:48 AM
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I would think a flat top and 72 cc and a .039" felpro gasket the comp ratio would be around 10.2-10.4:1. I ran a flatop this past summer with 68cc chambers and had about 10.8:1

Most 350 chevy blocks have a deck height of 9.025". So standard pistons are "in the hole" .025". With that .025" and a normal felpro around .039" that gives you way to much quench. You need to deck the block or run a embossed steel shim gasket from felpro, with is .015. or another option is to have a set of pistons made that have a additional .025" pin height so the piston will be at zero for deck height, then run a composition ( spelling) gasket.

For head gaskets I am going be using COMETIC. They are a newer gasket that is supposed to stand up to high compression, supercharge, turboed. ect. A little pricey but well worth it.


If this is all confusing I am sorry I just got outta bed

Last edited by traviss; 01-30-2004 at 06:51 AM.
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Old 01-30-2004, 01:22 PM
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Ok, thats a lot of info! Ok, I guess flat tops are not the way to go. I am getting some high dollar piston I have never heard of. They are made by a german company and are suposed to be the best thing going. When I looked at them they look very different, they were black. I really trust my machine shop as they do a lot of race automotive engines as well as a lot of marine work. All the local yards use this shop. I have heard this quench term used before, what exactly is it? Is it the distance from the top of the piston to the head at tdc?
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Old 01-30-2004, 02:37 PM
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Mahle pistons....very good pieces!
The black is Grafal friction coating.
The 350 flat tops from Mahle have a -3.53cc dome volume.
Bob

Last edited by rmbuilder; 01-30-2004 at 02:45 PM.
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