surfacing the head to increase compression
07-09-2004, 03:00 PM
#1
Hitsquad Ninja
Rennlist Member
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Thread Starter
surfacing the head to increase compression
hey guys...i was just wondering if anyone has shaved off some of the head to increase compression...but then you have to like reposition the cam or get an adjustable cam gear...anyone done this before?
jake
jake
07-09-2004, 03:20 PM
#4
Race Car
I have one anglemilled to the valve (way below min and if you cut more, you will be cutting steel off the valve) and has lots of torque. I will have to consult Danno to chip that thing for more umph at high rpms.
07-09-2004, 03:30 PM
#5
Race Director
Had heard of done on 944-spec cars since rules allow 11:1 compression. There is only so much to be machined before there is nothing left.
Gain is a little hp. How much is varried, but I have seen as little as Zero to as much a rumored 10 hp.
Draw backs are many. Firstly you will need higher octane gas. Stock compressrino 944-spec can run on pump gas 91 oct. (mine does) shaved head cars typically run race gas and some need it to prevent detonation and headgasket failures.
When I rebuilt my 944-spec race motor I had them clean the head to flat and not shave it for compression. My idea was that I'd be giving up very little hp as we can't run non-stock cam gears for a great risk potential and possibly not being able fix he head in the future. So I just had them shave enough to ensure it was flat.
My results.. 131 Rwhp which is 1 hp shy of another spec motor with close to 11:1 compression (stock was 9.5:1), a few track records and my share of race wins.
Gain is a little hp. How much is varried, but I have seen as little as Zero to as much a rumored 10 hp.
Draw backs are many. Firstly you will need higher octane gas. Stock compressrino 944-spec can run on pump gas 91 oct. (mine does) shaved head cars typically run race gas and some need it to prevent detonation and headgasket failures.
When I rebuilt my 944-spec race motor I had them clean the head to flat and not shave it for compression. My idea was that I'd be giving up very little hp as we can't run non-stock cam gears for a great risk potential and possibly not being able fix he head in the future. So I just had them shave enough to ensure it was flat.
My results.. 131 Rwhp which is 1 hp shy of another spec motor with close to 11:1 compression (stock was 9.5:1), a few track records and my share of race wins.
07-09-2004, 03:30 PM
#6
Addict
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Rennlist Member
If you're just resurfacing the head (even all the way to the min), the changes to the cam would be minimal at best, more in keeping with the manufacturing tolerances of the belt. If you get way down there, it might become an issue.
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07-09-2004, 05:50 PM
#8
Rennlist Member
I have been told that around 0.020" is about the limit without running into other problems.
I have heard of cars that have had up 40 thousandths taken off and the timing belt could not be tensioned tight enough. That was on older cars with the offset belt tenisoning roller. The newer cars with the spring adjustment assembly, have a little more movement and can probably take up the additional slack.
I have not done the math to check it, but also heard that each 10 thousandths shaved off the head will raise the compression a quarter point (ie 8.0:1 to 8.25:1).
I cant comment on N/A 944s, but I know from first hand experience that taking off 0.020" will definitely wake up the bottom end on a turbo.
I have heard of cars that have had up 40 thousandths taken off and the timing belt could not be tensioned tight enough. That was on older cars with the offset belt tenisoning roller. The newer cars with the spring adjustment assembly, have a little more movement and can probably take up the additional slack.
I have not done the math to check it, but also heard that each 10 thousandths shaved off the head will raise the compression a quarter point (ie 8.0:1 to 8.25:1).
I cant comment on N/A 944s, but I know from first hand experience that taking off 0.020" will definitely wake up the bottom end on a turbo.
07-09-2004, 05:59 PM
#9
Nordschleife Master
The cam's driven by a belt not a gear. There's no need to shave it every time you remove it, that's a waste of metal. Down the road you might have to shave it, and if there's not enough meat left you're sol.
