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Discussion Starter · #1 ·
Curiosity kills the cat, but hopefully not my motor. I have a few questions about the title, but first I need some help on my compression ratio calculations. Setup below.

.030 over
3.75” Crank
5.70 Rod
-31cc pistons
Ported and I would think slightly milled heads(for level surface) 64cc
Not sure on gasket thickness yet.

From online calculations, I get anywhere from 9.15-9.3:1 compression. To tell you the truth, I’m not sure if its correct. Can anyone help out on this?

Now to the good(or hard for me) stuff. I’m curious on the ramifications of running this setup NA. Can a larger cam, help with CR? Will higher lift/duration numbers drastically make up for the lack of CR? Not sure if I’m putting it right, but would jumping from a 227/239 to something in the 250/250 plus range help, or does lift have more to do with cramming the air in? Or does it not matter because the piston determines how much air is sucked in. Man, I think I’m lost here. Trying to keep my SRP forged pistons and not have to buy some higher compression pistons since I might(read probably) will be going blown again in the future. If not, a decent shot of the bubbly for the track. Any help could ease my mind. Thanks,

Mav
 
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Discussion Starter · #2 ·
Mav,
If going with the 31cc dish pistons, you'd better plan on a blower REAL soon.
The calculator on www.smokemup.com gives me 8.4:1 using 64cc heads with the .026 gasket. Milling the heads to 54cc chambers gives 9.2:1, still not enough for a NA LT1.
Going with a longer duration cam will only aggravate the low rpm dog syndrome. The only way I see this engine with those pistons being somewhat acceptable for NA operation would be to angle mill the heads to get the smallest chambers possible, and cam it with a CC XE 501 (212/218/112), or better yet one of their 4x4 cams with similar duration but higher lift, to keep the low rpm cylinder pressure up.
 
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Discussion Starter · #3 ·
it'll run, but not as well as an 11:1 motor. Why are you using the iron heads?

The lower compression will hurt the ability to burn at higher RPM when there is less time to burn. It won't make as much power at any RPM, but I specifically mention high RPM because that is what a bigger cam wants the motor to do, Rev higher.

Like I said, it'll run. People have been building lower compression motors for years. Until the LT1 came around running 11:1 or so on pump gas was nearly unheard of.
 
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Discussion Starter · #4 ·
Mav,
After driving a few low comp motors (383 in a 4x4 van 8.8:1 CR and 9:1 351C in my 73 Mustang), I gotta say they are drivable but not optimal. The big thing you will notice is lack of crisp throttle response. Couple that with a big cam, the motor is sure looking like dog doo out of the hole. If you are certain to boost it, it will make a great motor for it.

If you're are set on keeping the pistons, I'd use a smaller cam to keep cylinder pressure high at lower RPMS and when you install the blower, throw in the cam of your choice.
 
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