From: (Norman H. Garrett III)
Date: Wed, 28 Aug 1996 19:48:27 -0500


The outlet temperature of a blower (turbo or supercharger) is only
critical in the way that it affects flame front propagation (i.e. does
it cause the air/fuel mixture to pre-ignite/detonate). When the piston
comes up and compresses the air/fuel mixture, by design, it gets close
to the self ignition point of the air/fuel mix when based on 87 octane
fuel in the stock motor configuration. Marginal gasoline or too
advanced timing will cause detonation in a stock motor because we
designed it to be at a tolerable threshold of detonation as a stock
engine - meaning the compression ratio was optimized for 85+/- octane.

When you force induce an engine you affect the combustion process by
raising the starting pressure of the compresssion process (the piston
begins compressing air/fuel mix at a slightly greater starting
pressure than with a "vacuum" engine filling scheme) and by raising
the temperature of the combustable mix.

The pressure difference, even if you get it up to the full boost level
of the blower doesn't affect detonation in a major way, but there is
some component. The heat does make a big difference.

The Miata engines (1.6 & 1.8) do not appreciably detonate at 6 psi of
boost if the temperature of the air in the intake manifold plenum does
not exceed 240F (source: Miata Club engine dyno testing). In the worst
case of a 95 degree 88% humidity level day with underhood temperatures
at 142F, after full heat soak and full boost loading for an extened
period of time the Sebring M45 blower put out a maximum temperature of
216F with no detonation. Ignition timing was at the stock Sebring spec
of 8 DBTDC, two degrees down from the stock Mazda settting. Severe
tests on an engine dyno (where everything can be monitored and pushed
to its limit) confirms these calibrations.

Since detonation was not an issue, the added cost and complexity of an
intercooler was not necessary thanks to the low heat output of the
Eaton blower. The temperature of the blower discharge has nothing to
do with engine life outside of detonation concerns. The coolant in the
cylinder head is hotter than 216F, the exhaust gasses are at 1450F, so
the metal in the motor doesn't care about 74 more degrees of inlet air

These tests were done on an Sebring 6 psi system three years ago. Now
Sebring has an "S" trim blower that has lower outlet temperatures.

Jackson Racing made up an intercooler for the Sebring/Eaton kit - it
was much smaller in volume than the Autorotor piece. It made more
power by dropping the inlet temps 30F or so. It was not as good a unit
as the Autorotor because this piece, like the old BBR unit, mounted
BEHIND the radiator and was washed with pre-heated air.  The whole
trick in heat transfer is temperature difference and BEGI did a good
job getting the intercooler in front of the radiator/condensor where
it sees ambient air. To get there, however, they had to use discharge
pipes so small in diameter that I'm not sure that they are just
gaining the heat back as 300+ cfm gets crammed through the smallish

Anyway, even though there was more power with the Jackson prototype
intercooler, the throttle response was aggravating compared to the
straight-shot Eaton M45. There is a term called "throttled volume" -
that is the amount of air between the intake valve and the throttle
plate. The Jackson intercooler, although very small and direct, caused
a slight dull spot on tip-in (cracking open the throttle) that was not
worth the added power. Making the throttled volume greater by using a
larger intercooler would have made more power but it wasn't worth it
in the eyes of those who drove the car before and after.

As a test, a smaller pulley was run on a stock Sebring kit. It
developed he same 8 psi as the Autorotor without the predicted high
outlet temperatures. Actually, he car had a J&S active knock sensor on
it and even during the 97F days this summer it never pulled more than
3 degrees of timing out from the stock Sebring setting of 8 DBTDC. The
proof is in the pudding in the lab - if the detonation isn't there,
then the heat must not be all that high. It was instrumented it one
day and saw nothing over 240F on the same 142F underhood sort of
day. Taking 3 degrees of timing compensated for the added 24 degrees
in outlet temp.

Look at it another way. If a kit has an intercooler, it should have
lower charge temps. If it still needs an MSD retarder as standard
equipment, then here is still some threat of detonation going on and
the temperatures must be high. So where is the benefit of the
intercooler? If it was really cooling things down, the engine would
not be near the threshold of detonation.  In some cases, a blower can
superheat the air to begin with nd a intercooler is needed to get
things near enough to 240F for an MSD to cover for the detonation that
couldn't be avoided. The net result is not that much more power and a
whole lot more complication and cost and reduced throttle response via
increased throttled volume. The MSD available on the open market is
much more aggressive than the special Sebring MSD unit, which means
more power is being given up on the timing issue.

The stock Sebring 6 psi kit does not require intercooling or ignition
retarders to function as a result of its low heat output.