[Ecotec] Compression after head shave

[VSSECO]

Can anyone tell me how much higher my compression ratio will be after a 15thou shave off of my heads? I've tried to use online compression calculators but didn't have all the necessary measurements to get a definitive answer.
Any rough ideas? It doesn't sound like much of a shave...

 

[BowTie]

Compression tester will happily tell you.

Not trying to be a smart ass as im genuinely curious myself.

 

[Matchport]

CC your chambers and calculate

 

[VSSECO]

Thats the problem... I didn't CC my chambers before putting the heads back on. On the compression calculators i've tried I was getting about 9.8:1 which sounds like too much of an increase over stock for just a 15 tho shave

 

[383 hatch]

You won't be able to work out the comp ratio without the CC of the chambers.

 

[VSSECO]

I know it wont be accurate but I was hoping that someone who has been there and done that could give me a rough guestimate. Just some kind of ballpark figure.

To me, 15 thousanths of an inch seems like sweet FA of a difference considering there is like 4 inches of piston movement and a fairly large head volume... hell the gaskets alone are 60tho thick, but the head builder didnt want to take anymore than that off. I asked for a much as possible but he didn't want to do more than 15thou.

 

[Matchport]

If you take more of the heads, your inlet may not fit in the valley, what are you after??

 

[acarmody]

If you want more compression then shave the block too. Just be careful, you don't want the tops of the pistons protruding from the block.

 

[maldotcom2]

You can apparently get to 10:1 by shaving the block and the heads.

 

[bozz73]

Hi just a few calcs
If we take 633cc as stroke volume and 9.4 as your starting CR you end up with about 75.35cc Compression volume for a std. normally aspirated ECOTEC (Vstroke+Vcomp /Vcomp = CR)

If we assumed the volume that was taken off the compression chamber was a cylinder:
15 thou = 0.381mm and say bore is 96.5 (or therabouts) volume would be around 2.785 cc

Deduct the 2.785cc from what we originally calculated the compression volume was (75.35cc) and re calculate CR with above formula and your new CR is 9.72:1

Since the shape of the combustion chamber is not a cylindrical shape you can safely assume that the volume shaved off the heads is less and your CR will be somewhere between the original value and the calculated value.

Cheers,
Elwin