Hi Kyle,
1) Well, the easiest way is with a thicker headgasket. I am told they
exist in copper, (Was it Stan maybe?) I have never tried one, and
frankly would be sort of leery about it. Maybe Stan can relay his
experiences. I wouldn't mess with two head gaskets.
2) The next easiest way is to run a dished piston or take off material
from the top of flattop. You need to lose about 3.5 cc's of material to
get down to about 9:1. (with a perfect head)
Of the top off MY head that's about .090 off the pistons which is sort
of alot considering they are wimpy as is. I think .045 would be safe,
.060 prob ok (Since you are lowering the compression detonation
will be less of an issue)
3) Toughest approach is machine the head, but it may be your choice since you
indicated you didn't want to tear into the bottom.
I would cc all 4 chambers and find out which one is the biggest.
Calculate the CR with that volume and see how close it gets you to where
you want to be. If close enough, then hand machine the others to match
it by losing material around the valve area, cleaning away the rough/sharp
areas etc. Then re-cc etc. Time consuming work.
If you need to lose more material in the head, flycutting/milling is the way
to do it. You use a gasket as a template so that you AVOID the gasket
sealing areas. Then re-cc, and play some more. You want to distribute your
work throughout the chamber, unshroud the valves, and round off sharp edges.
Have fun,
Jim
My 92 300Z is fussy about 93 octane at 10.5:1 WITH all the fancy electronics.
A low octane tank of gas is a miserable experience ping, ping, clatter,clatter.
IMHO, I sure wouldn't build a street motor over 10:1 period.
Kyle Hagemann wrote:
>
> There's been a lot of information circulating re: higher compression...
> but what about lower? Suppose someone wanted to get his U20 down to
> 87 octane territory - with the SU cam.... what CR would that be? I'm
> guessing about 9.0:1 or so? What is the stock CR, 9.5, maybe more?
>
>
> TIA for any suggestions!
>
> Kyle
> '69 2000 (??:1)
> '71 521 (9.5:1, more head work to come [ping-a-ling!])
> '72 240Z (8:1, runs on anything;-)
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