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From: phil@osiris.UUCP (Philip Kos)
Newsgroups: net.auto
Subject: Re: Superchargers
Message-ID: <257@osiris.UUCP>
Date: Wed, 24-Apr-85 19:32:05 EST
Article-I.D.: osiris.257
Posted: Wed Apr 24 19:32:05 1985
Date-Received: Sat, 27-Apr-85 00:24:32 EST
References: <337@ttidcb.UUCP>
Organization: Johns Hopkins Hospital
Lines: 59

It looks like efforts are being made to dispel some of the mistaken
notions about the operation of {turbo,super}chargers, but there are
still a few floating around so I thought I'd put in my 50 cents
worth (inflation, y'know).

A *charger will work on either side of the carburetor.  The pressure
differential which causes fuel to flow throught the carburetor jets
is created somewhat artificially in blow-through installations by
using the increased air pressure on the other side of the fuel, i.e.
venting the carb fuel bowl to the *charger output rather than to the
ambient atmospheric pressure.

The decision to use a blow-through or a draw-through setup is based
on whether the *charger can stand backfiring through it.  If it's a
blow-through design, it doesn't usually have to; if it's a draw-
through, it does.

The blow-through design also tends to be a little cheaper, because
the plumbing between the *charger and the carb doesn't have to be
fuel-resistant, while it does in the draw-through design.

Other design factors which are affected by the choice:

      + Blow-through designs allow the carb to be much closer to the
	intake ports, which decreases the possibility of fuel preci-
	pitating out of the mixture.

      + Draw-through designs feed the mixture through the compression
	process, which helps some in atomizing the fuel (as above,
	less precipitation).

      + Intercooled blow-through designs (assuming the intercooler is
	between the *charger and the carb) present cooler air to the
	carb, which may or may not help atomization (can anybody give
	me help here?  I'm just stating this as a difference, because
	I don't know whether it would be an advantage or a disadvantage).

      + Intercooled draw-through designs may worsen the precipitation
	problem - again, I'd like some more input.

There are lots of little details that determine the difference in per-
formance between the two kinds of *chargers.  The main functional dif-
ference is that superchargers are *always* on, while turbos have to be
"wound up" before they start boosting.  The two most common solutions
to turbo lag are (a) decreasing the mass of the turbine, and (b) pro-
viding a "closed loop" to keep the turbine spinning quickly, and bypas-
sing the turbine at small throttle openings.  Many V-engine turbo
applications (particularly in racing engines) use option a with two
small turbos, which is easily done because the exhaust ports are located
on opposite sides of the block.

Anyway.  This has gotten pretty long-winded (long-fingered?) so I'll
leave well enough alone.  I'd appreciate hearing from anyone who could
give more information on the practical differences between blow-through
and draw-through designs.

				Phil Kos
				The Johns Hopkins Hospital
				...!umcp-cs!aplvax!osiris!phil