Compression ratio question
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Car: '91 Chevy 1500
Engine: 350
Transmission: 700R4
Axle/Gears: 3.73
Compression ratio question
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?
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?
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Car: '91 Chevy 1500
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Axle/Gears: 3.73
Re: Compression ratio question
Yes, they do. They give CR figures for (IIRC) 58, 64, and 76cc combustion chambers.
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Car: '91 Chevy 1500
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Re: Compression ratio question
Been there, done that.
That's kind of why I think I must be entering something wrong.
That's kind of why I think I must be entering something wrong.
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Car: '91 Chevy 1500
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Re: Compression ratio question
5.7 is what I plan on using and it's what's in the rotating assembly I'm comparing my numbers to
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Re: Compression ratio question
Did it ask for the compression height of the pistons?
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Re: Compression ratio question
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)
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)
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Re: Compression ratio question
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.
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Car: '91 Chevy 1500
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Re: Compression ratio question
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.
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.
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Re: Compression ratio question
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.
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Re: Compression ratio question
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?
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?
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