Compression ratio Question with GT-40 heads

95CobraMike

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#1

• Jun 29, 2005

• #1

What sized air pockets to the iron GT-40 heads have in them?
do the pockets effect the comp. ratio?

what sort of Comp ratio should i shoot for with a blower? (considering im looking for a new shortblock)


thanks!

 

95CobraMike

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#2

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no one?

 

CManT1914

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#3

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If by air pockets you mean combustion chambers, I would imagine they are around 64cc or so. Generally, if you're running boost, you want low compression, 9.0:1 or less. I see a lot of guys with blowers running 8.5:1 or so.

EDIT: Oh yea, combustion chamber size does effect compression ratio. The smaller the chamber, the smaller the space the piston has to compress the air/fuel into, and so the more pressure it makes.

 

95CobraMike

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so what if im running a blower with like 9.5:1 ?

 

95CobraMike

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or even 10:1?

 

CManT1914

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I personally wouldn't want 10:1 compression on a blown setup. But if you're keeping the boost low (9lbs or less) then you should be fine. On a stock motor, most guys don't run any over 9lbs. I didn't see anything about forged pistons or the sort in your sig, so I imagine you're still running the stock internals. I wouldn't run 10:1 compression, maybe 9.5:1, but I'd rather have it around 9:1. Just me though. Bump for ya

 

B

BULLITTSTANG95

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#7

• Jul 4, 2005

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i believe gt40s are 9.5:1 but not completely sure and i believe he is correct on the 64cc.

 

I

igotyofire

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#8

• Jul 4, 2005

• #8

BULLITTSTANG95 said:

i believe gt40s are 9.5:1 but not completely sure and i believe he is correct on the 64cc.

Click to expand...

what do u mean by gt40s. do u mean 302 block with stock gt40s? if so the compression is still 9.0:1, 94-95 cobras have the same compression as the GT, Id say keep compression around the same if not higher with forged internals as with a centrifugal u wouldn't want to possibly loose some low end off boost power for cruisin around town

 

95CobraMike

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im sorry, it has stock (re-cut) rods and stock crank... but the pistons are forged TRW's and i have apr main and rod bolts

 

95CobraMike

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#10

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what are the pros and cons of a 9.5 or 10:1 compression ratio on a supercharged engine rather than a 8.5 or 9:1?

 

CManT1914

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#11

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igotyofire said:

what do u mean by gt40s. do u mean 302 block with stock gt40s? if so the compression is still 9.0:1, 94-95 cobras have the same compression as the GT, Id say keep compression around the same if not higher with forged internals as with a centrifugal u wouldn't want to possibly loose some low end off boost power for cruisin around town

Click to expand...

I guess I should clear up what I meant in my last post. With stock internals, I would most definitely not want higher than stock compression (9:1 I think right?). If I had forged internals (pistons are the main thing) then I would feel a lot safer running 10:1 or so, but since you have stock internals (or so your sig says) then I would not want high compression if I were you.

 

95CobraMike

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i forgot to put in my sig that i have forged TRW pistons oops

 

CManT1914

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95CobraMike said:

i forgot to put in my sig that i have forged TRW pistons oops

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Well then you should be safe with a little more compression, but I don't know how much. I've never had a blown car, or a car with forged internals. So all I know is what I've been told by others. Good luck!
-Chris

 

W

wytstang

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Jul 4, 2005

#14

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• #14

The lower the CR the higher the psi's you can run from my understanding.

 

4Jenna

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#15

• Jul 4, 2005

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10.1 is idiotic for a mild street car with a blower that runs on pump gas.
I would rec 9.0~1 if u plan to boost 8-10psi, and if you plan to make big power and boost it more you'd wanna be 8.8 or less.
10.1 would be a n/a car or a nitrous car.

 

Labora

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#16

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• #16

wytstang said:

The lower the CR the higher the psi's you can run from my understanding.

Click to expand...

The higher PSI you can run and still use Pump gas!

If you are going to be building a boosted engine I would suggest reading Maximum Boost by Corky Bell it goes over in detail how compression ratio effects the everything aswell as what kind of pump gas you can use and how much Boost you should run for your setup etc.. It will give a little bit better detail then people here are IMO.

 

95CobraMike

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#17

• Jul 5, 2005

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i'm pretty sure im at approx. 9:1