The only way to make more power is to increase cylinder pressure and burn more
fuel. The main purpose of the supercharger is to supply the motor with a more
dense air charge, which allows for the ability to burn the additional fuel. By
adding a supercharger, additional air should no longer be a problem. Ensuring
that there will be enough additional fuel to maintain the proper air to fuel
ratio will be the key to using the maximum effective compression.
All motors have a static compression ratio. This is the amount that the air
inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC
to the volume at TDC. When a supercharger is added, additional air is forced
into the cylinder effectively raising the compression ratio. The result of this
is called effective compression. The formula for finding the effective
compression is very easy:
((boost psi / 14.7) + 1) x motor compression = effective compression.
The effective compression allows a supercharged motor to be compared to a
normally aspirated motor. For the most part, a supercharged motor with the same
effective compression as a (similar) normally aspirated motor with the same
static compression should have about the same overall power.
This may bring up the question that if the overall power should be about the
same, why go with a supercharger? The main advantage of the supercharger is that
it allows for a moderate compression level during normal driving while allowing
for very high compression levels when needed. Obviously a high compression motor
of about 14:1 makes a lot of power, but it would never survive daily driving. A
lower compression motor is great for daily driving, but greatly reduces the
potential for power. The supercharger allows for higher compression levels than
could be used without a supercharger, while still offering the benifits of a
standard compression motor. Many street supercharged systems will go beyond 18:1
effective compression under boost. Under race conditions, many supercharged race
motors will go well beyond 22:1 effective compression. Both of these levels are
far beyond what could be done reliably or cost effectively without a
supercharger.
This brings us back to the question of just how much boost or compression can be
run. Obviously there can't be a simple number that could be used for every
application. This is why it's so critical to chose the proper components. It's
not necessary to build a low compression motor to use a supercharger, but the
correct parts are still necessary. The biggest factors will be in things like
head bolts (or preferably studs), gaskets, and the strength of the other engine
components. It goes without saying that the incredible power that a supercharger
can add, can easily start breaking things. It is very important that as the
boost levels rise, the need for a stronger crank, rods, pistons, etc... becomes
very critical. Many people forget this as the motor itself is relatively mild,
while the supercharger pushes it well beyond the practical limits it was
intended for.
Now, back to the compression issue. Anyone who has looked into supercharging has
heard that you need a low (static) compression motor. This may have been true
once upon a time, when roots type (positive displacement) superchargers ruled
the land, but it's not so necessary now. The problem with a low compression
motor is that it relies heavily on the supercharger for its power. An 8:1 motor
is definitely not going to be a power house. Sure, you can throw 18 lbs of boost
on it and get some real power, but why? A higher compression motor of 9.5:1 will
have much more power without the blower. Then, with less boost you could easily
have the same overall power - only it would be much more usable. Both of the
motors (8:1 with 18 lbs boost and 9.5:1 with 12 lbs boost) will have almost the
same effective compression and about the same overall power. The big difference
will be where you see the power, and how much of a demand will be placed on the
supercharger. Obviously, the 9.5:1 motor is going to have far greater torque and
low end power as the boost is only starting to come in. It is also going to be
much easier to find a blower to survive only 12 lbs of boost -vs- one that would
have to put out 18 lbs. It is now very easy to see why a higher compression
motor with lower boost is becoming so popular.
Please understand that when I say higher compression and lower boost, there are
limits to each. Going over about 10:1 will make the amount of boost that is
usable drop quickly to the point that the supercharger is somewhat wasted. In my
opinion, anything less than 8 lbs of boost is a waste of a supercharger. Going
over 10:1 will also make daily driving with pump gas much more difficult. In
this same way, compression levels much under 9:1 will require substantial boost
levels to make massive power gains. This would require boost levels that are
very demanding of a supercharger. This is truly unnecessary. This isn't to say
that the lower compression / higher boost set-up doesn't have a slightly higher
potential for power, because it does. A lower compression motor has the ability
to contain more volume. This can be an advantage, but is such a minor one that
it's not necessarily worth the effort - unless it's for an all out race motor.
Even then there are limits for the same reasons as the street / strip motor.
Once again, the compression -vs- boost issue. For a car that will see the
streets (actually for most applications), the best thing to do is start with a
motor compression that is high enough to make the horsepower you want for normal
driving. Don't rely on your supercharger to make all your horsepower. With a
good motor compression, add as much boost as is safe for your particular
application. Decide on a final effective compression, and work your way back
through the formula to find your maximum boost level: ((effective compression /
motor compression) - 1) x 14.7 = boost. With the proper fuel system and related
engine components, an effective compression of 16:1 to 18:1 should be more than
workable. For heavily modified cars, effective compressions over 20:1 should be
very carefully considered. Remember, even Indy cars only run about 18 Lbs of
boost and reasonable static compression levels. Technology has come a long way
and modern day supercharging should take full advantage of this.
While these opinions are not exactly the most popular, they are based on facts
and real world performance. While there will always be those who continue with
tradition and stick with what was done in the past, it is those who reach for
something more that are winning races. Often times, some of the best advice can
be found from those who have done what you want to do. All too often it is those
who know the least that offer the most advice. After having been involved in
supercharging for many years, I have heard it all. Most of it was worthless. It
was often the least mentioned things and trail and error that have been the most
rewarding. Hopefully this information will help to explain some of the often
misunderstood aspects of supercharging.
