Postby Hasbeen » 04 Jul 2009 23:36
Don, You don't know if your head has been shaved, before you got the
car, or what your current ratio is, & although the best way to raise
compression is with pistons, I see your point, about a recent
rebuild. You could quite safely shave enough off the head to bring
it up to 9.5 to 1, so what I'd do is this.
With the engine out, with spark plugs removed, set the engine up,
with the spark plug holes the highest point of the combustion
chamber, in a way which allows you to turn the crankshaft.
Run a bit of oil down No1 cylinder ports, to help seal the valves, &
through the plug hole to seal the rings. Bring the thing to top dead
centre on No 1.
Get a pipette, filled with light oil, or water is OK, & fill No1
until it is full to the bottom of the plug hole. The amount you put
in is your combustion chamber volume. Best to measure it in cc with
our cars.
Divide your swept volume, [499.5 cc] by the combustion chamber
volume. For our purpose, this is your compression ratio. With this
ratio, you, or any GOOD machine shop can figure out how much to
shave off to give you 9.5 to one.
This has been done to most US spec 7s in OZ, with out problems with
heads, or gaskets.
You may have to replace your timing chain, & tensioner, if they are
old, & you may need a vernier cam sproket to get the valve timing
correct.
I would probably do a trial assembly with putty on the pistons, to
check the valves will not hit the pistons, but many have not
bothered. They may have been lucky.
Have fun with it.
Hasbeen
PS. you can just measure the volume of the head, on the bench, &
gestimate the volume of the area in the compressed head gasked. Add
the two for combustion chamcer volume, to come up with a "close
enough" figure.
H
