06/27/2003

Knowing that air flow makes power - not displacement - and the compressor will
move the air not the pistons.

We have two choice's.  We can expand the volume of the chamber or we can
reduce the volume of displacement or some combination of both.

Assuming a reasonable and efficient baseline engine, with adequate strength,
cooling - particularly around the exhaust ports, and sufficient head bolts to
hold the head on.

You'll have to rethink everything through.  Largish low resistance ports that
would not be optimal work spiffy fine under boost. (example).  The engines
breathing is much less sensitive to the valve arrangement and rpm - but will
be limited by how well it can breath under boost.  A 1.5 inch valve will only
flow so much and the ability to go larger will mean more power.

Once you have a reasonable engine, closely examine chamber and piston design.
Very few recent engines have chambers that will reduce compression past about
a single ratio without showing loss of turbulence.

You will generally have to change the displacement to get full advantage.

Start with bore.  There is no reason to increase the bore, unless, a better
piston is available cheaply at a slightly increased bore. You need the
breathing better.  If an oversized piston is available and it clears and opens
up the breathing of the engine, and the new bore does not weaken the block -
that would be desirable in budget.

Piston selection will sometimes get you a deep dish with a broad outer squish
area that will maintain turbulence.  

To this point, you must work with the most important thing - getting a medium
to high turbulence chamber including the piston that will work well under
boost.

In order to get this chamber shape, many times, a higher compression piston
will give it to you off the shelf than the best "low compression" piston
available.  

The chamber shape ( if the volume is reasonable ) is more important than
actual ratio.  Again, if a small overbore can increase the turbulence nicely,
its important to do it.

You've developed the chamber shape and volume to good.  We are now going to
reduce the compression - not by gutting metal out - but by reducing the
stroke.  DESTROKING.  Either by offset grinding or a shorter throw crank or
both.

Since you want the piston to stay about where its at, it give you the
opportunity to increase your rod length and to sometime move the pin to a
better location.

You retain a decent chamber and turbulence, you lower your compression without
the turbulent losses, you raise your rpm upper limit, the shorter stroke is
more responsive to acceleration and other good thing happen.

To recover all the power lost to displacement, you simple increase your boost
appropriately.  Life will be bad when you have to put an oversized turbo and
way jack up the wastegate to keep from detonatating.

Most engineers can use low compression high boost - when you think it through.
DOHC, OHC, OHV, L-head - is less important than getting a good air flow thru
the valves and a good chamber.  Pick whatever you can get and use it.  Parts
is parts and turbos know less about what type a part it is than you do.

And when you get itchy about displacement reduction, remember that a 4000
pound sedan is driven speeds ridiculous with an economy slushbox six - 30+
cubic inches smaller than the first Chevy V-8 ( 265 ).