buying a head, comp. ratios, etc.
11-10-2002, 05:06 AM
#1
Burning Brakes
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buying a head, comp. ratios, etc.
I am looking at buying a new head for my NA, and having it redone, and then swapping for the one on my car now. This way I'd have less downtime, plus I would have a backup, and I could take my time to get it done right.
My car is an 85.5, the head I'm looking at is an '87. Looking at FAQs, it seems like the 87 engine had a higher comp. ratio. (10.2:1, vs. 9.5:1) Was this done by shaving the head, or changing the pistons? I wanted to know what I had to work with, if I could shave the one I'm buying, or if I should leave it as is, to be safe. I'd like to know as much as possible about my options going in before I buy it, vs. getting stuck with something that I can't really use. If the head as-is is going to get me to 10.2:1, maybe I should just leave it alone, instead of pushing my luck with what is a street car, for now.
Is $250 a pretty decent price for a good shape used head that's going to be reworked? Seems pretty reasonable to me.
Thanks!
My car is an 85.5, the head I'm looking at is an '87. Looking at FAQs, it seems like the 87 engine had a higher comp. ratio. (10.2:1, vs. 9.5:1) Was this done by shaving the head, or changing the pistons? I wanted to know what I had to work with, if I could shave the one I'm buying, or if I should leave it as is, to be safe. I'd like to know as much as possible about my options going in before I buy it, vs. getting stuck with something that I can't really use. If the head as-is is going to get me to 10.2:1, maybe I should just leave it alone, instead of pushing my luck with what is a street car, for now.
Is $250 a pretty decent price for a good shape used head that's going to be reworked? Seems pretty reasonable to me.
Thanks!
11-10-2002, 10:38 AM
#2
Three Wheelin'
The compression ratio in the later engines was raised by decreasing the size of the dish in the piston top.
You can increase the compression ratio by milling the head. Removing .005 inch will decrease the size of the combustion chamber by about 1cc.
I am running the later 10.2 to 1 pistons and have my head milled .035 inch. This is as far as I could go and still maintain proper piston to valve clearance with the aftermarket cam I am using.
Dave Fracolli
85.5 944
You can increase the compression ratio by milling the head. Removing .005 inch will decrease the size of the combustion chamber by about 1cc.
I am running the later 10.2 to 1 pistons and have my head milled .035 inch. This is as far as I could go and still maintain proper piston to valve clearance with the aftermarket cam I am using.
Dave Fracolli
85.5 944
11-10-2002, 04:12 PM
#3
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[quote]Originally posted by DangerIsland:
<strong>Looking at FAQs, it seems like the 87 engine had a higher comp. ratio. (10.2:1, vs. 9.5:1)</strong><hr></blockquote>
The FAQ is wrong - the (1988) M44/09 is the 10.2:1 engine (by way of pistons, as pointed out).
I would check with PartsHeaven and 944 Ecology to see if they could best that price. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
<strong>Looking at FAQs, it seems like the 87 engine had a higher comp. ratio. (10.2:1, vs. 9.5:1)</strong><hr></blockquote>
The FAQ is wrong - the (1988) M44/09 is the 10.2:1 engine (by way of pistons, as pointed out).
I would check with PartsHeaven and 944 Ecology to see if they could best that price. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
11-10-2002, 09:26 PM
#4
Race Car
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Post 85.5 8 valve NA engines have a slightly reduced compression ratio (not that you would notice). But have a longer opening exhaust valve timing.
You could use an early head but wouldn't get the longer valve timing.
You could use an early head but wouldn't get the longer valve timing.