A question about compression ratios
 

I'm sure that one of you gearheads can help me with a calculation. I'm currently running a 588ci quad rotor setup that has 8.5 to 1 with big chief heads (80cc chamber). If I wanted to put on some chevy-style heads, what would the ratio drop to with a bigger chamber and what size chambers are available? Is this do-able or would I have to change the pistons?

Thanks in advance,
Craig

Re: A question about compression ratios
 

Re: A question about compression ratios
 

Without knowing the variables, it's going to be hard to tell. I have no idea what the deck, head gasket thickness and piston net volume are. All things being equal, it appears that going from an 80cc chamber to a 112 will drop it fron 8.5 to 7.2. :(

Re: A question about compression ratios
 

If you know what your current setup is (8.8) you can work backwards to fill in the blanks, just play with the numbers.

I'll play around a little and see what I find.

Re: A question about compression ratios
 

That's what I did- it's currently 8.5 to 1

Bore=4.56
Stroke=4.5
Head cc's=80

Re: A question about compression ratios
 

Valve angle on Big Chief heads is different than standard BBC heads, so the valve pockets are different. I'm not sure if it would work or not, but if you try it, be sure to check your valve/piston clearance very carefully. I think you'd be better off changing over to the correct pistons. You will also need different rockers and possibly lifters if you're running offset lifters. Intake will change too. Cam profile, valves, and springs will all be different as well. Why do you want to swap heads? Most of us would love to have a Big Chief motor, especially one topped with a quad rotor!

:D

Re: A question about compression ratios
 

I'm just not one that likes to work on my boats and do 60 hour rebuilds. Speed really doesn't matter to me. These engines were in the boat when I bought it and I got a good deal on a great boat. What I would like to have in it is some 598ci/750hp's.

Here is a link to the classified if you really want them! :p :D :drink: :drink:

http://www.offshoreonly.com/esvon/page-6134.html

Re: A question about compression ratios
 

Craig,
Its going to depend mainly on where the piston sits in the cylinder and if your c/r was obtained by the head or the piston. If your pistons are dished and the block has been 0 decked, then in theory an 80 cc is 80 cc. The only thing that will change with the new head is the valvetrain geometry. If the new heads you are looking at have a higher CC then they can be milled as well. As long as the pistons arent coming out of the hole at TDC you can achieve what ever you need:)

Re: A question about compression ratios
 

I think you'll find that most standard configuration BBC heads that start out at 119cc (pretty common for Dart, Brodix, AFR, etc) can only be angle milled down to around 100cc. Also, chances are those pistons are deeply dished to get compression down with the small chamber pro-stock heads. That, and they're probably further down in the hole than they should be for optimum quench. If you're changing heads you really should change pistons and reconfigure the rest of the engine to match.

I'd love to buy them but they're a little $$$ for my wallet!

:D

Re: A question about compression ratios
 

exactly right, if they are deep dished or sitting way down, then it may be best to get new ones as a ratio that low wouldnt be good fo an n/a motor.