On Mon., Michael D. Porter replied to my Spit blower message . . . . .
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[ My neighbor thinks it is a "Roots" blower (he has about 3 other types in
his
attic - calm down you guys, they're for American V-8s), but perhaps
Allison or some other company made this thing. All I know is that I AM
going to put it on the Spit, somehow, someway. Gawd this is gonna be so
fun!!]
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I don't want to seem mean-spirited, but there are some things to consider
when using a supercharger of this type
first, the head gasket wasn't designed for much boost. In the last few
years, some racers have been using a gas-filled metal o-ring on each
cylinder top as a means of increasing head gasket pressure. The downside
of this practice is that the block face must be machined to accept the
o-ring, which can weaken the cylinder-to-block wall webs.
Second, given the valves and guides used in this engine and the lack of stem
seals, small amounts of wear will cause considerable additional blow-by.
The correction
there might be to increase the size of the road draft tube (and frequently
check that it's
clean and unrestricted).
Third, except at low speeds, the boost produced is more or less linear.
Engine goes faster, more boost. There are several important considerations,
then. One
is roughly the equivalent of thermal runaway in a reaction. If the boost
goes high enough,
the throttle response may be much quicker than one can react to, and at high
rpms, that
might be enough to over-rev and destroy the engine, particularly in the
lower gears. Next,
since the blower sucks mixture into it, rather than pushing air into a
pressurized
carburetor, there's no vent for excess pressure (venting hot mixture into
the engine
compartment sounds as if it would cause a dandy explosion, and would be
almost as much fun to vent it into a hot exhaust stream), and that's why
there's no equivalent of a turbo waste gate on this blower (more accurately,
none I can see in the picture). Next, there seems to be no way to provide
any controls against catastrophic detonation. This seems to be the usual
mechanism of failure. The only way to minimize this, since one can't
control boost, would be the addition of an electronic ignition which had
input for a knock sensor, and hopefully get enough spark retard out of such
a system to minimize detonation. An additional difficulty is in the noise
of the blower--often, the noise of the engine and the blower combined make
it more difficult to hear detonation beginning, so one often doesn't have
time to get off the throttle quickly enough.
Fourth, even if one gets even 5 psi gauge from the unit, that means the
engine is working at roughly 135-140% of its normal maximum volumetric
efficiency. And that's 35-40% more heat to get rid of. This really
suggests if not a larger radiator, then at
least an oil cooler, and stronger, forged pistons, and better rods. If not
better rods, then at
least have the stock ones shot-peened.
Fifth, since it's an old unit, and one presumably doesn't have rebuild specs
available, it's a matter of faith that the clearances in the unit are
correct. If
there's a bad bearing, or a worn gear, this can cause the rotors to kiss
each other, and
they generally don't last long after that--and then all the pieces go into
the intake.
Last, indifferent timing can cause backfires into the blower. Because it's
sucking in mixture, rather than air, and heating it through compression,
that can
sometimes cause explosions in the blower, and the results are dramatic. I
once saw an
Allison V-12, with a couple of 6-71 blowers installed (unlimited
hydroplane), hiccup and blow pieces of one of the blowers about a hundred
yards in the air.
Now, after trying to alarm you, I will say that this particular blower may
be
very inefficient, and may not produce enough boost to be a problem.
However, to
be safe, and to save an engine which you've undoubtedly got plenty of money
invested, see if you can fit a larger pulley to the blower's input shaft,
along with a longer belt to match and start with that. That will reduce the
amount of boost, and with a manifold
pressure gauge installed, you can very carefully determine if detonation is
going to be a
problem. One last consideration might be that to help control detonation,
you may have to install plugs which are colder than is normally necessary,
and this can affect idle and
low-speed operation.
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Jesus, Michael, could you possibly be any more pessimistic about this whole
blower thing! (Actually I am grateful for all of the input, some of which I
had already considered, some of which I had not). I do plan to fit a larger
pully and slightly colder plugs, plus I'll need to adjust fuel/air mixtures
accordingly, blah, blah, blah, ad naseum.
As a practical matter, I'll install it, tweak it a while, play with it a
bit, and then probably go back to the stock set up after a few months.
It'll be fun for a while, but this car isn't intended to be an
autocrosser/racer/high strung little beast and the blower really isn't going
to be that noticeable around town puttering around and enjoying the
beach/sunshine/etc.
My other thought was to forget the blower altogether, put a tiny little
nitrous bottle in the trunk, run a line along the frame rail to the carbs,
and put a tiny little fogger nozzle in each air cleaner. This actually
makes more sense from a performance and engine-longevity standpoint since it
would be used for VERY brief periods of acceleration, is easily regulated
and controlable and involves a hell of a lot less engineering, machining,
etc. It also has the advantage that I can easily return it all to stock
settings with minimal fuss.
Ahhh, the search for horsepower never ends . . . . . . .
Ross D. Vincenti
64 Spitfire 4 (undergoing surgery)
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