glen242

I am trying to spec out parts for a 383. Going with a Scat/Eagle cast crank, 6" rods (because I want an internal balance engine), and reusing my TF 64cc heads. Block will be decked to +/-.005 and I want to use hypereutic pistons. Quench should be .044.

The only pistons I can find are KB or Speed Pro. Using the 6" rods, my choices seem to be KB 7cc flat top = 10.78 static CR, 8.8 DCR, KB 19cc dished = 9.5 static CR, 7.8 DCR, or Speed Pro 5cc flat top = 10.9 static CR, 8.99 DCR.

The car will be driven 99% on the street, and I need to be able to use pump gas. Right now my 350 is about 9.00 static CR, 7.5 DCR, quench is about .064 with felpro 1010 head gasket, .025 in the hole and I can run 89 octane without a problem.

Can I run the higher CR with less quench and my aluminum heads or will I have to go with the 9.5 static CR? I am trying to spec the cam for IVC 61* ABDC. This is what my above numbers are based upon.

Present vehicle specs are in my profile.

Jvette73

I cant respond too much in terms of technical jargon. I can say this. My 350 with flatops has 64cc heads milled .012. The factory lists earlier 350's with flattops and 64cc at 10.25 to 1. With my milled heads I gotta be tipping higher than that. Maybe approaching 11 to 1. I run an aggressive ignition advance curve and high test pump gas and have never had a problem with spark knock or detonation of any kind. My cam is relitivly small too. Higher cam duration can allow more tollerance for higher comp ratio.

MotorHead

Don't get too hung up on those numbers go with the speedpro flat top and stay away from the coated skirts it just rubs off

marshrat99

I'm thinking that you'll be OK on 93 octane as long as you stay below 10:1CR with iron heads. You can possibly get maybe 11:1CR with aluminum heads. Above these combo's then it is going to be a crap shoot.

Twin_Turbo

Don't hang yourself up on CR, calculate dynamic CR, it's much more closely related to cylinder pressures at max VE (TQ) and therefore detonation conditions, CR is just a number, cam duration, separation, intake & exhuast efficiency and such determine cylinder pressures which is what you really need to be aware of.

glen242

I did calculate dynamic CR. It is the DCR number in my post. Read somewhere that DCR should be between 7.5 - 8.5.

Twin_Turbo

OH oops, my bad.

mandm1200

I am in a similiar situation except I already have the parts: Scat crank and 5.7" rods with Speed Pro flat tops. I figured 10.4 static CR with 7cc pistons. I will be using a Lunati cam and the DCR calcualation came to 8.2. I am going to be using iron Vortec heads. I believe I am going to be good to go with pump gas. Since you are using alum heads, you should be able to go another half point above the Vortecs.

78 Vette

I drive my vette everyday I run 92 with a compression of 10.3 to 1 With aluminum trick flow heads. no pinging no problem.