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Lowering Compression
Double gaskets and other short stories
Squish Band
This is defined as the area in the combustion chamber where the piston (at TDC)
comes extremely close to the head. It's usually around the perimeter of the
piston, and no mixture is expected to burn there. Physical contact is the only
factor determining the 'height' of the squish band, so practically no mixture
will be expected to be there as the flame front moves from the spark plug
outwards.
Here's how it works: the upcoming piston squishes out that mixture, forcing it
to blend with the rest around the plug. This action creates extra turbulence and
homogenizes the resultant mixture, which means that it burns cleaner and
quicker, requiring less ignition advance. As a bonus, the outmost edges of the
combustion chamber are closer to the plug tip, further reducing the need for
extravagant ignition advance. All this leads to efficiency, especially in
off-boost situations, where the mixture is not well homogenized (there are rich
and lean spots within it)
Under boost, the role of squish takes the backseat. The compressed air is
already agitated enough for the squish turbulence to make any major difference.
Therefore race engines designed to run mainly on-boost don't have to pay much
attention to it.
Thicker gaskets?
If someone fits a gasket thicker by 1mm (or steel plate to that effect) the new,
deformed chamber will be sacrificing 4.16% of the volume with not much squish
effect
These are static figures, would be accurate at revs while cranking the engine by
hand.
In real-life rpm there's a lot of inertia involved, and the stock squish can
easily be below 1% while the 'deformed' one over 5%. The static CR would be
about 8.3:1 in this scenario.
Is that bad then?
Yes, because it takes a fixed number of milliseconds at those revs for the flame
front to propagate from the spark plug to the edge of the charge. (It also
depends on the strength of the mixture, effective scavenging, etc. Check out the
ignition section)
The whole piston crown is NOT the bottom of the combustion chamber. The mixture
is squeezed into a homogenous blob around the plug tip just before TDC
Fitting a thick gasket DEFORMS the combustion chamber shape, creating an extra
'donut' a couple of mms tall and around 8mms thick. Most of the mixture in this
space will NOT be burnt - it's too far from the plug tip.
Some of it will indeed burn, but too late, after the power stroke (not good)
In the process you've lost quality of the rest of the charge that DID burn,
because it wasn't stirred up as thoroughly as the designer intended it.
That's why running double (or more) the stock boost with stock CR is not how the
manufacturer would have done it. There are complicated formulas to calculate
*how* much the CR should be (it's not linear!) but whatever the figure, one has
to make sure that the combustion chamber is not being deformed in the process.
Combustion chamber efficiency is the engine's most powerful tool against
detonation.
How do we reduce the Compression Ratio then?
The DIY methods of lowering the CR range from botches to acceptable. The
majority is in the first category - thicker gaskets, double gaskets, steel
plates, machined pistons.
The only acceptable one is to use a lower-compression piston that retains the
squish band incorporated by the OEM design
Call the piston manufacturer and ask them - if they don't know what you're
talking about, look elsewhere. People have been known to skim metal out of stock
pistons to reduce compression. This is a serious botch, it weakens a piston
that's not strong enough to begin with!
Of course there's always the cheap'n'cheerful way of altering the cam timing to
achieve a similar result, as I describe in the 'cams' section.
...So is thicker gasket a no-no?
Lets not get paranoid here.
We're talking about a drop of efficiency of a few percentage points.
It could have been a lot worse, the squish area is already relatively small.
If you're hell-bent on running 2 bar on stock pistons, then you'll have to fit a
2-3mm gasket because you've got no other choice (intercooling isn't enough at
this stage). In this scenario, retaining squish is a bit of a luxury. Off-boost
efficiency will be sacrificed for on-boost reliability.
If, however, you're only doing up to 20psi overboost, and no more than one bar
is sustained, then it might not be worth upsetting the delicate balance of a
well-designed combustion chamber. For a reliable high-boost setup that is
expected to survive prolonged full-throttle, a proper set of pistons will bring
the static CR down to 8:1 or thereabouts.
In-cylinder cooling provided by such means as water injection or small shots of
nitrous oxide can also work wonders in keeping the engine together in such
conditions.
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