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Compression ratio/head swaps

Baja71

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Oct 16, 2004
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I'm thinking with a SBF 302/5.0 that every head chamber cc equals a change of one tenth of the compression ratio.

Just as an example: If 53cc head happens to give 10:1 then a 63cc head would be 9:1. sound correct as far as the difference between the two? (Assuming same pistons and everything.)

My head swap is from a 65cc head, which yields a 9:1 compression ratio on MY particular 5.0 shortblock, according to Ford Racing.....to a stock C9TE 58cc head which should yield a 9.7:1 ratio. Correct?

Head gurus?
 

Viperwolf1

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Aug 23, 2007
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I'm thinking with a SBF 302/5.0 that every head chamber cc equals a change of one tenth of the compression ratio.

Just as an example: If 53cc head happens to give 10:1 then a 63cc head would be 9:1. sound correct as far as the difference between the two? (Assuming same pistons and everything.)

My head swap is from a 65cc head, which yields a 9:1 compression ratio on MY particular 5.0 shortblock, according to Ford Racing.....to a stock C9TE 58cc head which should yield a 9.7:1 ratio. Correct?

Head gurus?

9.8 according to my calcs. This is assuming the same head gasket.
 
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Baja71

Baja71

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If this is a street driver with a 4bbl carb, stock EB manifolds and a 2:1 exhaust, how beneficial/detrimental is a 9.8:1 ratio? It's a little higher than I was shooting for, but I suppose I could put headers into that mix to improve performance some. I want to burn Regular gas.
 

Viperwolf1

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I think you're asking too much. Actual cyl pressure will depend on the cam used. Aluminum heads would be more tolerant of high compression ratios because they transfer heat out better. Eliminating any sharp edges in the combustion chambers would help too.

You would gain more power by changing to better breathing heads and keeping the cr at 9.0. Is this for a restoration where you want a stock appearance? If so you can always get some better breathing iron heads.
 
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Baja71

Baja71

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I think you're asking too much. Actual cyl pressure will depend on the cam used. Aluminum heads would be more tolerant of high compression ratios because they transfer heat out better. Eliminating any sharp edges in the combustion chambers would help too.

Explain "asking too much". Do you mean too much compression for Regular gas? The compression ratio given was taking into consideration the roller cam specs which is installed in the shortblock. I got these numbers from Ford Racing, so I'm sure they're good.

As I recall, the HIPO 289s and such ran 10:1 with cast heads, a 4bbl, and cast (albeit better flowing) exhaust manifolds. Not sure if they came with factory duals or not though....
 
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Baja71

Baja71

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You would gain more power by changing to better breathing heads and keeping the cr at 9.0. Is this for a restoration where you want a stock appearance? If so you can always get some better breathing iron heads.

Yes, a stock appearing restoration. I was considering the cast GT40 heads, but these came with the new block, rebuilt and pre-installed.
 
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Viperwolf1

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Yes, too much compression for regular gas. You have to keep in mind that octane ratings were a lot higher back in the day, mostly due to leaded fuel. The cam comes into play because it's all really about pressure built in the cylinder, not just the static compression ratio.
 

needabronco

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Baja, I'm in a similar situation, and I've done all the math for my situation. I have 58cc heads, and want to swap in my GT40 heads that are 63cc. Right now I'm at 9.54:1 and figure using a standard thickness head gasket (.039 or .041) I'll lose about .5:1 in compression. Now if I spent more money on special racing gaskets that are .020 thick that number will almost be split. I've gone around and around with this one and asked machine shops and pro engine builders and they all seem to agree that we're only talking about 5 or 6 horsepower in loss, but in my situation the better heads would add power and torque anyway. They all state that super low torque monster big blocks are all low compression engines and they all usually last forever...

All said I can only assume you'll gain 5 or 6 horsepower, so I'd say just run it...
 
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Baja71

Baja71

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Baja, I'm in a similar situation, and I've done all the math for my situation. I have 58cc heads, and want to swap in my GT40 heads that are 63cc. Right now I'm at 9.54:1 and figure using a standard thickness head gasket (.039 or .041) I'll lose about .5:1 in compression. Now if I spent more money on special racing gaskets that are .020 thick that number will almost be split. I've gone around and around with this one and asked machine shops and pro engine builders and they all seem to agree that we're only talking about 5 or 6 horsepower in loss, but in my situation the better heads would add power and torque anyway. They all state that super low torque monster big blocks are all low compression engines and they all usually last forever...

All said I can only assume you'll gain 5 or 6 horsepower, so I'd say just run it...

Will the stock ex. manifolds bolt up to the regular (non P) GT40 heads? In your case, it seems like you'd want to run the GT40s and the regular mil head gasket, or are you trying to keep up your compression?
 

broncnaz

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May 22, 2003
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Yes stock manifolds will bolt to standard gt40 heads. You should be ok with the 9.8 comp ratio but as was said you might have to run premium gas. As far as power outputs what I've seen in testing is that with all things equal and only a comp ratio change only yeilds a max of 10 hp so theres no real benifit from higher compression.
Personnally I'd go with some better heads than the old C9 castings since you'll need to upgrade the valve springs for the roller cam anyways you might as well get some heads with better chambers and maybe bigger valves also. The C9 castings should also have some hardened exhuast seats installed to run todays gas.
Also I would recommend running headers they will help improve power you have improved the cam and intake but are choking the improvements with stock manifolds. SBF's need a lot more help on the exhuast side of the engine than on the intake side.
 
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Baja71

Baja71

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Oct 16, 2004
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Yes stock manifolds will bolt to standard gt40 heads. You should be ok with the 9.8 comp ratio but as was said you might have to run premium gas. As far as power outputs what I've seen in testing is that with all things equal and only a comp ratio change only yeilds a max of 10 hp so theres no real benifit from higher compression.
Personnally I'd go with some better heads than the old C9 castings since you'll need to upgrade the valve springs for the roller cam anyways you might as well get some heads with better chambers and maybe bigger valves also. The C9 castings should also have some hardened exhuast seats installed to run todays gas.
Also I would recommend running headers they will help improve power you have improved the cam and intake but are choking the improvements with stock manifolds. SBF's need a lot more help on the exhuast side of the engine than on the intake side.

Yes, I agree with that. When I got this new 5.0 block, the C9 heads were already installed and set up for the roller cam. I was OK with that as I was going for a stock look. The reason for the stock exhaust manifolds is purely to give an original appearance, and I would be able to use my original air cleaner with thermal (heat riser) choke assy, etc., etc. That, and I made them look awesome. ;D I have always considered going to headers as I know how they can seriously wake up a SBF, but with the C9 heads it didn't seem as critical. If I go with GT40s, I'm pretty much going to have to, as you say, or never unleash the potential of this 5.0.

My research shows that the GT40s will give me 60-63cc, or 63-66cc, depending on the part number. Those would be better breathing, and give me a more usable compression ratio for the intended use. I checked with a local motor builder and he said he could supply me a set, rebuild them for my roller block for $650. It sounds a little high, but 'unknown' GT40 heads seem to be going for $400+.
 

Broncobowsher

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Jun 4, 2002
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Correctly built and tuned a high 9:1 compression engine will run on pump gas. I have my RV cammed 351 running on 91 octane and it has 11:1 compression. I did smooth all the sharp edges on the heads and pistons. Not a huge cam either, I have a little over 200 PSI cranking compression. I also have a very tight quench area between the head and piston as well. Trying to lower compression with a thicker head gasket often results in poor performance and the need for MORE octane as the quench isn't working as it should.

I would do 9.8:1 for a build. If 87 octane isn't enough, they do have higher grades. There is no reason for 9.8:1 to need more octane then what you can get at a local pump. Compression is a great thing. It makes more power off the same drop of fuel.
 
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