This is topic How Much HP increase going from 8 1/2.1 to 10.1 comp ratio? in forum Tech Talk at Northern California Ford Owners  .


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Posted by Glenn (Member # 520) on :
 
My car is at 8 1/2.1 I am thinking about milling my heads, how much HP increase would I see?
 

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Posted by JohnB (Member # 969) on :
 
You won't be able to mill them enough to make 10...MAYBE 9.
Keep in mind, when you mill the heads, you also bring the valves closer to the pistons..and you're on the edge right now what your current camshaft.

No need to tear the engine down...just add a supercharger!!
 

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Posted by shade- (Member # 298) on :
 
I thought the rule of thumb was ~ 10 HP for every full point at aroud our level (300) some cams really love compression though, so it could be a little more. I'd say as much as 20, in your case for another 1.5 full points.
 

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Posted by 66 AC COBRA of CA PERFORMANCE (Member # 904) on :
 
u mill the heads then u have to screw with intake alignment, have to get the heads milled some where, take the heads off ur car, spend money on gaskets, etc etc

or u can either toss a charger, turbo, or nitrous on, turbo being expensive, nitrous cheap from me , or the middle priced charger, either way is taken ur heads off to gave 15hp worth it, while there off my as well port them to, and port the intake
the car runs great right now i assume, so leave it alone and either behappy with it or a power adder is one or a few paychecks away

[ September 24, 2003, 12:59 PM: Message edited by: 66 AC COBRA of CA PERFORMANCE ]
 

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Posted by Jeff S (Member # 371) on :
 
Why did you build an 8.5:1 motor? Were you planning on a blower?
 

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Posted by Glenn (Member # 520) on :
 

quote:

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Originally posted by Jeff S:
Why did you build an 8.5:1 motor? Were you planning on a blower?

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quote:

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Originally posted by JohnB:
You won't be able to mill them enough to make 10...MAYBE 9.
Keep in mind, when you mill the heads, you also bring the valves closer to the pistons..and you're on the edge right now what your current camshaft.

No need to tear the engine down...just add a supercharger!!

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quote:

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Originally posted by Glenn:

quote:

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Originally posted by Jeff S:
Why did you build an 8.5:1 motor? Were you planning on a blower?

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yeah, but also heard that I can add bout 8psi to a 10:1, just trying to make as much HP as I can.

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quote:

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Originally posted by Glenn:

quote:

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Originally posted by Jeff S:
Why did you build an 8.5:1 motor? Were you planning on a blower?

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yeah, but also heard that I can add bout 8psi to a 10:1, just trying to make as much HP as I can.

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Originally posted by 91 5.0:

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Originally posted by Glenn:

quote:

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Originally posted by Jeff S:
Why did you build an 8.5:1 motor? Were you planning on a blower?

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yeah, but also heard that I can add bout 8psi to a 10:1, just trying to make as much HP as I can.

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Yeah, but now you can run more boost. You'll get way more power with the lower compression and increased boost. Stick with the lower compression. Just my .02 cents.

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quote:

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Originally posted by 93PONY:

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Originally posted by 91 5.0:

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Originally posted by Glenn:

quote:

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Originally posted by Jeff S:
Why did you build an 8.5:1 motor? Were you planning on a blower?

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yeah, but also heard that I can add bout 8psi to a 10:1, just trying to make as much HP as I can.

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Yeah, but now you can run more boost. You'll get way more power with the lower compression and increased boost. Stick with the lower compression. Just my .02 cents.

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To expand a little on this.....
The larger your combustion chamber, the more air you can pack in. If you plan on running a total of say 16 to 1 compression (calculated WITH boost), you'll get more power out of a larger chamber. A larger chamber volume at 8psi will make more power then a smaller chamber volume at 8psi....more then what upping the compression ratio will add.

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So your saying a car with lower compression with the same amount of boost will make more than a car with higher compression at the same exact psi?
 

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Posted by 93PONY (Member # 60) on :
 
Depends on the application....
Glen is looking at a 15-20RWHP increase N/A from going from 8.5 to 10.0 to one compression. First off he'd have to take out 14cc's somewhere to get this...this would mean milling off .084 inches off the heads...which isn't gonna happen. LOL
But, if he were to take out 14cc's from the combustion chamber this would give him 10 to 1 compression.
There is roughly 7cc's in the head gasket itself. Total chamber volume for 10 to 1 compression on a 306 is ~68cc's. With 8.5 to 1 compression it's 82cc's. That's a 17% difference.
Total compression being the same on both applications (total meaning with boost) you're looking at an increase in power of basically 17%.
But...it would take more boost to get the 8.5 to one compression motor to make the same 'effective compression' as the 10 to one motor.
By how much you say???
Well...lets see....

A 10 to 1 compression motor with 8psi has an effective compression of 15.5. Anything under ~16 is okay for use with pump gas.
An 8.5 to 1 compression motor needs 12psi boost to make the same 15.5 effective compression.

Now...that's 4psi more...to make the same 'effective compression' & due to the larger chamber volume this will make ~17% more power then the 10 to 1 compression motor.
But you may say that more boost = more heat! Ah yes...it does. But think of it this way..... Typically blowers will have intercoolers to cool the air before entering the motor. So, on one motor the air is compressed to 8psi, cooled a little, then compressed again to 10 to 1 by the pistons, which creates a LOT of heat & gives us 15.5 effective compression. The other motor is pushing 12psi that is cooled off before being compressed 8.5 to 1. This will also give 15.5 to one effective compression...but the air-charge will be a few degrees cooler due to more air being cooled by the intercooler & not being compressed by the pistons so much.

Ah...but what about 2 motor pushing the same boost one with 8.5 compression & one with 10. Well...it's a toss up. One is making 15.5 effective compression with a 68cc chamber, the other 13.2 effective compression (15% less) with an 82cc chamber (17% more). Basically it comes down to heat in the aircharge & compressor effeciency as to which combo would make more power. For sure, the higher compression motor would have snappier throttle response & rev faster when not in boost. But peak power output....it's a toss up IMO. But, I'd go for the cooler aircharge.

[ September 24, 2003, 05:17 PM: Message edited by: 93PONY ]
 

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Posted by 87 Saleen (Member # 1549) on :
 
Baaaahhhhhhhhhhh! Put it on the dyno.
 

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Posted by street_illegal_stang (Member # 1554) on :
 
save up for a turbo setup
 

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Posted by Glenn (Member # 520) on :
 
BOOST IT IS!!!
 

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Posted by 91 5.0 (Member # 1527) on :
 

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Originally posted by Glenn:
BOOST IT IS!!!

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Posted by Red97SVT (Member # 2811) on :
 
Less compression will allow you to run more timing with a given amount of boost or more boost with the same timing with out detonation. More timing or more boost = more power. Take your pick!
 

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