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Discussion Starter #1
I was thinking about milling the set of y303s (aluminum gt40s) so that im about 10.5 to 1. Is this worth doing? It will be with whats in my signiture plus a performer intake and 65 throttle body. Are ther any drawbacks to doing this on a car besides typical stuff like high octane and no blower/turbo? Im hoping that the additional compression will bolster the power band a bit maybe like 10-15 hp and tq. What do you guys think?
 

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If you can mill them that far--which is another question altogether--it will potentially help you out more than you're hoping. I'd be inclined to retard the cam a couple of degrees at the same time to decrease the chance of detonation with the stock cam/long runner/higher comp combo.
 

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Discussion Starter #4
Thanks, I really didnt consider if it was possible to mill them that much but thats a good point. Does anyone know if you can mill thes heads that much and if not what can you get them down to?I was considering retarding the cam 4 degrees, does this sound apropriate? Thanks, Dave.
 

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The general answer is that each additional 1 point of compression is worth approximately a 3-4% increase in overall power production. The nice part that I’ve noticed from experience is an increase in torque and throttle response, kind of a good “seat of the pants” feel.
 

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slowhatch said:
I was thinking about milling the set of y303s (aluminum gt40s) so that im about 10.5 to 1. Is this worth doing? It will be with whats in my signiture plus a performer intake and 65 throttle body. Are ther any drawbacks to doing this on a car besides typical stuff like high octane and no blower/turbo? Im hoping that the additional compression will bolster the power band a bit maybe like 10-15 hp and tq. What do you guys think?
Each .5 point (i.e. 9.0 - 9.5) gives ~15HP as a rule of thumb. Actual results will vary (disclaimer). :D
 

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Discussion Starter #7
I like what im hearing. So by raising the comp. from about 9.0 to 10-10.5 i could possibly see up to 30 extra hp? That sounds like the route im going to take with these heads. A friend of mine is running 107 with the iron gt40s, im hoping that i can pull that or maybe a bit better. Thanks guys.
 

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Re: Re: How much hp will raising the comp. 1.5 points gain?

tmoss said:
Each .5 point (i.e. 9.0 - 9.5) gives ~15HP as a rule of thumb. Actual results will vary (disclaimer). :D
I think that might be a little generous, though we can be certain that an additional 1.5 points will have a significant effect. It’s probably safer to use a percentage than an actual horsepower number since the effect of an increase in compression will have a different effect on a stock engine and a race engine, or a 302 vs. a 347. Just sharing thoughts.

Regardless, bump the compression.
 

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Yes, 3-4 % is the best way. When I milled the head on my Briggs Straton lawn mower engine .060, I didnt get any where near 30 HP :)
 

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I'd have to say to raise the comp. that much you'll going to have to mill the hell out of those heads, and then its not going to line up with the intake. If I remember correctly you can mill up to .030 without running into problems with the intake lining up and thats not going to get you any where near 10.5 to 1
 

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If your block and heads already have good mating surfaces it might be less hassle just to buy new pistons to get the extra comp, Although then u will probly need to get it bored out and may still need to get the block decked to zero or better for quench. Just a thot.
 

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I don't have money for aluminum heads right now, so when I do my carb swap I'm gonna use the heads of my '69 351W . Maybe get a little work done on them, and I wanted to have them shaved down to give my 302 about 10:1. Question is, can I run 10:1 on pump gas with iron heads ?
 

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its a lot easier to do with iron than aluminum!!!
 

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Do you think the 351 heads are going to be worth my time fooling with them ? Surely with those heads at 10:1, a Performer RPM intake and a 600 carb I can gain some power over what it is now.
 

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Discussion Starter #16
Thanks for the input guys, the heads are going to be installed on a stock shortblock. I am prepared to mill the intake to fit properly so thats not a problem. Am i going to have a problem with the quench area after milling these heads?
 

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slowhatch said:
Thanks for the input guys, the heads are going to be installed on a stock shortblock. I am prepared to mill the intake to fit properly so thats not a problem. Am i going to have a problem with the quench area after milling these heads?
Milling the heads has no effect on quench.

Compressed gasket thickness and where the piston is in relation to the block do however.

Example:

My pistons reside .005 out of the whole at TDC. Combine that with a .039 compressed head gasket and you get a quench distance of .034.

Greg
 

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mustanggreg said:
Milling the heads has no effect on quench.

Compressed gasket thickness and where the piston is in relation to the block do however.

Example:

My pistons reside .005 out of the whole at TDC. Combine that with a .039 compressed head gasket and you get a quench distance of .034.

Greg
So your saying that quench is measured by the distance from the top of the piston to the bottom of the combustion chamber. Rather than the top of the piston to the top of the combustion chamber? And therefore the hieght of the combustion chamber itself (which IS affected by milling the heads) has no effect on quench?
 

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Discussion Starter #19
mustanggreg said:
Milling the heads has no effect on quench.

Compressed gasket thickness and where the piston is in relation to the block do however.

Example:

My pistons reside .005 out of the whole at TDC. Combine that with a .039 compressed head gasket and you get a quench distance of .034.

Greg
I dont understand how milling the heads has no effect on the quench area. When you mill the heads it effectivly reduces the cumbustion chamber size which increases the total surface area not occupied by the chamber. Wouldnt this be increasing the quench area? Also does anyone know what the typical piston height in relation to the deck is of a stock short block?
 

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correct. Quench is measured outside the combustion chamber. Quench is the clearance between the piston top and the flat portion of the head.

Ideal quench will vary depending on the operating RPM of the engine. The slower your going to spin it, and expect power, the tighter the piston has to get to the head.

The higher the RPM, and the higher up the curve your looking for operating the vehicle the looser it can be and still be effective.
 
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