Vincenti, Ross wrote:
>
> Okay, took a good long look at the blower yesterday. It's only about a foot
> long but it has a fairly long nose piece (about 6 to 8 inches - 15 to 20 cm
> for you folks across the pond) and about a 3 to 4 inch diameter pully with a
> tensioner pully mounted on a side bracket. It is aluminum, and the two
> "vanes" inside are of the rotating, constant mesh four lobe type. If you
> were to disassemble the thing and look at one of the two rotors from the end
> of the unit, it would look like a four leaf clover.
If I'm looking at the picture correctly, you've described a Roots-type blower
manufactured
by Wade, and this unit was originally for use on MG-T series engines. For
confirmation,
you should try to find the revised 5th edition of Philip H. Smith's _The Design
and Tuning
of Competition Engines_. It's also a very helpful resource in general. Lots
of theory and
practical information all rolled into one.
> 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
> much fun!!!!!
It will be great fun (until the engine destroys itself, which, regardless of
careful
assembly work, they often do.... <g>). I don't want to seem mean-spirited, but
there are
some things to consider when using a supercharger of this type (others in the
group may
have addressed these items, so please bear with me--I haven't gotten through
all the mail,
and most have to do with level of boost, and I have no idea what this unit will
produce):
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.
Cheers.
--
My other Triumph doesn't run, either....
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