Cylinder Heads

[Dellb]

Hi All
This one is for all the SBC buffs. My 1980 C3 Vette is fitted with a 400 sbc. During the initial restoration, it was completely rebuilt etc. It runs fine, and after sticking to the excellent rocker setting sequence supplied by Ross is a lot quieter.
My winter project is to replace the pressed in rocker studs with screw in type.
As I was removing the drivers head, I took note of the casting number (3947041). Being keen to learn more, I checked this number out, and it appears to be a very early 350 head with 64cc chambers. I then did a quick calc, and to my horror this seems to give me (with my flat top pistons) a CR of nearly 12:1
I don’t seem to get any pinking etc, but to my old fashioned view of things this seems way too high, and I need heads with 74cc chambers. Am I being paranoid?

Delb

 

[Oneball]

Have you taken into account the deck height ie how far down the bore the pitons are at TDC?

 

[Dellb]

The deck height on average is 0.015”.

 

[Oneball]

As you say that comes out as 12:1. If you’ve got the standard starter motor and it turns over nicely I doubt it’s that high though.

What were the head gaskets?

Flat top pistons are pretty unusual on a SBC are there now valve cutouts or a dish at all?

Only way to be sure of the chamber cc is to measure it.

If it does turn out that the CR is too high you can use a different head gasket

 

[Dellb]

The pistons do have valve cutouts. She spins over easily on the starter.
Currently awaiting arrival of burette

 

[Oneball]

They’ll probably be 10cc at least. More if youve got the uni-directional ones with 4 cut outs.

That’ll bring your CR down to 10:1 by my calculation.

 

[Corvette]

We like cr it makes hp. my old LT1 was 11.1 with standard dome top pistons something like in the pic. you will notice high cr on the struggling starter.

 

[Roscobbc]

What cam do you have? - if its an 'old school' rumpity rumpity one it will probably bleed-off some of that compression - you could always knock back the total timing and tailor the advance curve to bring advance i early to prevent any risk of pinking - perhaps also use Shell VPower Super Unleaded or other 99/100 octane........

 

[Dellb]

The cam is a new lunati voodoo hydraulic roller set.
Current ignition is 12 degrees at idle working up to 34 degrees. Carb is a New holley 4130 street avenger (670cfm)

 

[Roscobbc]

The cam is a new lunati voodoo hydraulic roller set.
Current ignition is 12 degrees at idle working up to 34 degrees. Carb is a New holley 4130 street avenger (670cfm)

If it has plenty of overlap perhaps I does reduce effective compression?