Formula78

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!

Wally

iTrader: (1)

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

robyw1

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

BAD-HABIT

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

traviss

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

Formula78

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.

BAD-HABIT

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

traviss

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

Formula78

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?

rmbuilder

Mahle pistons....very good pieces!
The black is Grafal friction coating.
The 350 flat tops from Mahle have a -3.53cc dome volume.
Bob