Displacement is determined by the bore and stroke of the engine and is
not altered simply by changing pistons. You would have to bore the
engine or increase the stroke to achieve higher displacement.
Compression is the ratio of cylinder volume @ bdc/volume @ tdc. Simply
boring the engine will usually yield a higher CR using the same shape
piston. A different piston shape will also alter compression ratio.
Higher CR can also be achieved by reducing the volume of the combustion
chamber -- usually achieved by skimming the head.
As for carburettors and valves, it isn't necessary to change them, though
that would yield even more power. Howeever, at higher compression
ratios, you usually need a high-output ignition system and a revised
advance curve. Colder plugs are usually needed too. You'll also be
running octane booster CONSTANTLY and the extra compression does stress
the crank, rods and bearings much more than stock.
My advice is as follows -- if you live in California, DON'T SCREW WITH
IT!!! Just rebuild it as stock or you'll never pass smog again in a
million years. If you live outside California, my advice is to put
together a well balanced system rather than focusing on high-performance
from one element of your engine. You'll be much better served by going
to somewhat less compression than 9:1, ie like the 8.8:1 in the older
1275 engines and a mild road cam. A Weber DGV downdraft and a street
(not race) header would help too. Unless you plan to race the car, don't
go overboard with trick parts. You'll wind up with an undrivable and
unreliable car that you spent lots of $ just to screw up.
Just my $.0000000002 billion.
"When I was a toddler, my dad wanted me to learn to program, but my mom
thought it would be better if I learned to read and write and spell."
----------------Original Message Follows----------------
I plan on having the engine and transmission rebuilt on my '76
Midget. I would like to go with a higher displacement (i.e. 9:1 pistons).
What I need to know is what is involved, if anything, in going to the
higher displacement pistons. Is the higher displacment ratio caused by
shape or by the size of the piston head itself. Also, are thier any other
special considerations I should take into account? I'd like to keep
everything as original as possible.
I have a Zenith-Stomberg carb- will this do, or will I have to go with a
different carb assembly? Will I have to have the lift of the valves
increased? I'm fairly new to all of this- but I LOVE MY CAR!! HELP!!
---------------- End of Original Message -----------------