> One method I have read about, but not tried, for simple supercharging is
> very interesting. It is also simple to do. Basically find an air pump
> from a large american V8 and use it to supercharge a small 4-cyl via a
> preassurized airbox. It appears that this makes about 3-4# of boost,
> and since it is supercharged it will make that at much lower rpm. I'd
> send info but I don't remember where I read it.
>
> --
> James Nazarian Jr
That's a bloody stupid thing to do. An airpump is much like a centrifugal
supercharger, it has an impellor and is driven by the motor via a belt. Now
the main thing with centrifugal superchargers and turbochargers is that the
tip velocity of the impellor needs to reach speeds near the speed of sound
for them to be efficient (or rather the efficiency increases the closer they
near that figure). This means that the centrifugal supercharger and
turbocharger's impellor tip velocity routinely reaches 120,000RPM+. Will
that airpump be capable of that?
Okay, so the airpump won't do 120,000RPM.... but it probably runs on a 1:1
ratio with the motor, or a little higher. You're pumping losses will be so
great that little noticeable increase in power will be achieved, if any.
The power created by that airpump in order to create boost will be negated
by the process of it creating boost!
Now why do centrifugal superchargers and turbochargers run such high speeds?
Simple, because the output pressure increases as the square of the rate of
rotation. The ideal limit to the rate of rotation, as stated above, is the
speed of sound because the turbulence generated at this speed is so great
that efficiency drastically declines. This can be helped with the addition
of an outlet diffuser, but again... the speed of sound is pretty fast, and
there is rarely a need for the tip velocity to exceed it. Back to the first
sentence of this paragraph, centrifugal superchargers and turbochargers need
to spun very fast in order to create large amounts of boost and be
efficient.
Impellor design plays a BIG role in the efficiency of a centrifugal
supercharger and turbocharger. Typically today all centrifugal
superchargers and turbochargers have a curved tip which seems to be the most
efficient and also producing the least air turbulence at the outlet of the
unit. I seriously doubt the Detroit auto makers put much R&D into the
impellor tip design of their air pumps.
And now to chastise James a bit for proposing such a preposterous idea!
James doesn't your signature read "A complex system that does not work is
invariably found to have evolved from a simpler system that worked just
fine." Well, then why use a air pump to do the job of a centrifugal
supercharger or turbocharger!
;-)
Now that I have hopefully dissuaded anyone from putting an air pump on their
MGB to create boost, let's discuss the various forms of forced induction and
their benefits and downfalls on the typically long stroke, low redline
British motors. In this overview, I'll refer to overall efficiencies
because well if you want to know every aspect of forced induction and the
differences between each route you are best advised to go purchase some
books and not base your entire knowledge on my email.
I strongly believe that centrifugal superchargers and turbochargers should
not be adapted to British motor cars, highly balanced and uprated racing
motors being the exception. Typical redlines for our cars fall below
6000RPM, meaning much of the benefit of a centrifugal supercharger or
turbocharger is never released because of the short time in which you
actually have usable power from them. Obviously this was already explained,
and it is obvious that if we can increase the rate at which the engine can
run the more we can get out of these methods of forced induction. You will
release little, if any, increase in performance below 3500RPM in either
case.... and then it is only a matter of about 2000RPM before your redline.
A roots supercharger or a lysholm twin screw (as Dave Munroe has on his MGB,
from Hans Pederson) provides a much more usable option, and don't need to be
spun nearly as fast as the impellor driven compressors... Power comes on
shortly after 1500RPM all the way through the redline... although beginning
to taper off around 4500RPM (more so in the case of the roots).
A turbo must be properly sized to your motor, too tiny of a turbo and you'll
just wear it out and create problems with your exhaust. Too large a turbo
and you may never fully realise it's potential. Centrifugal superchargers
are slightly more forgiving in this aspect as they're engine driven.
Blah Blah Blah, enough of this. Just don't use an airpump as a centrifugal
supercharger. Don't do drugs either.
Kai
1974 Triumph TR6 -- Supercharged using an Eaton M62 blower, now being fuel
injected with a Saab turbo throttle body increased in diameter by two
millimeters and a GM throttle plate installed, Electromotive programmable
ECU, and six injectors machined into the TR6 intake manifold.
http://www.pil.net/~felix/supercharger/
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