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Old 04-22-2012, 03:01 PM
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compression ratios

can anyone tell me If I would notice any difference / what hp gains I can expect to see stepping up the compression ratio on my mild 350 chevy from 8.8.1 to 9.6.1CR ?

cheers

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Old 04-22-2012, 04:44 PM
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The gain you get from a 1 (one whole) number compression ratio increase
is not linear at all compression ratios. You gain more going from 6:1 to 7:1 cr than you see when going from 12:1 to 13:1.

But in the 8 to 10:1 zone you get around 3-4% (theoretical) power gain for every 1 cr increase. You feel the torque increase more than the horsepower increase.
You may have to run a better fuel octane.

If you are changing the cylinder head to get the cr increase you have to
consider any +/- port air flow effect or change in combustion chamber shape
that will effect efficiency +/-.
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Old 04-22-2012, 04:56 PM
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Quote:
Originally Posted by swompz
can anyone tell me If I would notice any difference / what hp gains I can expect to see stepping up the compression ratio on my mild 350 chevy from 8.8.1 to 9.6.1CR ?

cheers
If this is to be run on pump fuel you would be wise to compute the dynamic compression ratio using the new CR to be sure the octane requirements aren't excessive.
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Old 04-22-2012, 05:01 PM
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I was going to use rhs pro torker 67cc heads 12402,s so stock/ mild performance which would give me the 9.6.1 , but ive been offered a set of stock 76cc heads for. Third of the money, so weighing up best bang per buck option, still trying to keep in 300hp zone on. 350+40 bore with performer manifold & cam/ headers etc.
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Old 04-22-2012, 05:23 PM
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best bang for the buck low buck head swap to get a compression ratio increase is home ported 305HO heads installed with a thin head gasket felpro 1094. Ported #4416 or #601's w 1.94 valves and a new valve job.
I have done this many many times. Its very cost effective.

You want to home port them before installing or you miss half the benefit of this effective head swap.

Will not work on 87 octane gas. 92+ octane is best.

The ignition curve will need to be recurved. Both the mechanical and the vacuum advance curve will need tweeking.

Vortec heads are very very effective swap too.
Bigger power increase, but higher cost. but well worth it.

The gasket you use must be considered.
Why bother with a stock 76cc head?

good 305 heads can be bought often for less than 76cc heads.
Not all the 305 heads are desirable for this.
Often you can buy the whole running 305 motor to get the heads off of.
check the head casting number before buying.

A few other better cams for this specific swap.

Lunati #301A3LUN
Comp #12-208-2
Isky #201264271-12

Last edited by F-BIRD'88; 04-22-2012 at 05:36 PM.
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Old 04-22-2012, 06:00 PM
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Quote:
Originally Posted by F-BIRD'88
best bang for the buck low buck head swap to get a compression ratio increase is home ported 305HO heads installed with a thin head gasket felpro 1094. Ported #4416 or #601's w 1.94 valves and a new valve job.
I have done this many many times. Its very cost effective.

You want to home port them before installing or you miss half the benefit of this effective head swap.

Will not work on 87 octane gas. 92+ octane is best.

The ignition curve will need to be recurved. Both the mechanical and the vacuum advance curve will need tweeking.

Vortec heads are very very effective swap too.
Bigger power increase, but higher cost. but well worth it.

The gasket you use must be considered.
Why bother with a stock 76cc head?

good 305 heads can be bought often for less than 76cc heads.
Not all the 305 heads are desirable for this.
Often you can buy the whole running 305 motor to get the heads off of.
check the head casting number before buying.

A few other better cams for this specific swap.

Lunati #301A3LUN
Comp #12-208-2
Isky #201264271-12
use a Performer RPM hi rise intake manifold. It is just a much better intake manifold in every way over the lowly performer, even on milder motors.
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Old 04-23-2012, 12:28 AM
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It's a case of whats available to me hear in UK at sensible money, Which is slim pickings compared to you guys.
Mostly it's stock later model heads or a thin selection of performance heads
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Old 04-23-2012, 01:07 AM
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Quote:
Originally Posted by swompz
It's a case of whats available to me hear in UK at sensible money, Which is slim pickings compared to you guys.
Mostly it's stock later model heads or a thin selection of performance heads
What brand name aftermarket cylinder heads can you buy locally there?
Edelbrock, Brodix, AFR ???

Some stock GM heads to look for if you don;t mind reworking them.

4416 305 HO
601 305HO
113 L98 corvette and camaro TPI 5.7 L98 aluminum
083 Camaro firebird TPI L98 Cast iron.

GM 96 to 2000 L-31 vortec 062 or 906

Summit Racing will ship to your door in the UK with low hassle.

The RHS pro torkers can work well with some deck milling to dial in the chamber volume to 64ish cc and respond very well to some hand porting to wake them up. The RHS 906 vortec heads work well too.
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