I have a dyno program with a CR calculator. I enter what I think is the correct info, but the CR is different than what manufaturers say it will be. Here is what I'm entering for a 400 (406) engine I'm building

4.155 bore
3.75 stroke (stock)
64cc head volume
0cc dome
22cc dished pistons
.049 deck clearance at TDC
4.160 head gasket bore
.040 head gasket compressed thickness

Scat says their rotating assembly with 22cc pistons is 9.8:1, but the figure I get says 8.88:1. They don't give figures for the gasket thickness or the deck clearance, but that isn't going to account for an entire point difference.

Any ideas what I'm doing wrong?

 

i used a calculator off the internet and mine used the head gasket thickness as a point but they don't say what cc head they use to get that compression do they ?

 

Yes, they do. They give CR figures for (IIRC) 58, 64, and 76cc combustion chambers.

 

well you have the head for that compression i would try a different calculator online and see what you come up with good luck

 

Been there, done that.

That's kind of why I think I must be entering something wrong.

 

yeah i tried it whats your rod length im NOT an engine builder lol but you probably def have a number off somewhere

 

5.7 is what I plan on using and it's what's in the rotating assembly I'm comparing my numbers to

 

Did it ask for the compression height of the pistons?

 

Nope. Just what I have listed.

This is my first build (from the ground up), but would that matter if the pistons matched the rods? (ie. 5.7 rods and 5.7 pistons)

 

Sounds wrong to me. I'd check into that spec, it should be around .020 or maybe less.

Anyway, with the specs you gave its 8.88:1
With a zero deck, its 9.78:1 so... yes that will make up almost a whole point.
I used my own program to calculate that.

 

I think you are right.

I was thinking of the quench area, which includes the head gasket, but I only entered the quench area I wanted.

Thanks. That'll make buying pistons a little easier now that the numbers work.

 

You should verify the piston height, see where the piston is going to end up. The piston specs may very well assume a zero deck, whereas most chevy blocks are in the .020-.025 range so I'd try to find out what they assumed. If I had to guess, I'd say they figured zero deck or a .040 quench or something along those lines. You can correct for that if need be with your block or an appropriate gasket.

 

I haven't bought any of this stuff yet because I was going to try to do it all at once. I'm starting with a bare virgin 400 block and crank, and building from there. I already have two cranks, so I don't think I'll buy a complete rotating assembly. I think I'll order everything but the head gaskets and wait to measure what it comes out to. This will make the compression ratio exactly what I want it to be, as soon as I figure out what that is. I'm under the impression that 9.8:1 on an iron headed motor is just a touch too much, even with the Vortec heads I plan on using.


Here's another question. If I bring the compression ratio down by using a thicker head gasket (after pistons are bought and installed), will it be more prone to detonation because the quench wouldn't be perfect anymore even though the CR is down a hair?