Vance writes: (Indented paragraphs)
Dick:
Thinking about the
seemingly low HP gains, I would think that for 6 lbs of boost the final
HP would be ((6lbs + 14.7lbs)/14.7lbs) * 105 HP = 148 HP
Vance---I don't know if normal atmospheric pressure was figured into the
Moss calculation of their final boost pressure, but I will assume it was
(gauge pressure). If they sized the pulley for this six pounds of boost,
I'll take their word for it.
That comes out to be 43
additional HP on our motors. This formula assumes that the compression
is adiabatic, which it is not.
Vance---Factoring in adiabatic efficiencies for boost pressures is
beyond the scope of this forum. Those interested in the formula and
charts for this sort of thing can find great stuff in books, as written
by Todd Curless (The Theory of Turbocharging) and by Hugh MacInnes
writings in TURBOCHARGERS, which has chapters comparing various SC
compressor designs as well as turbos.
We know heating up the charge air by compressing it will lose some
density. When writing that one can expect a "10 hp gain per pound of
boost" I didn't make it up. This is a direct quote from Kastner. His
kits were set up for a "safe" boost of six pounds, and he engine dyno'ed
this setup at 165 hp at the flywheel, (where he claims to have taken all
of his measurements).
Allowing for some heating of the incoming air/fuel
charge, 39 HP seems very reasonable indeed. In fact, I am surprised that
only 4 HP is lost between the parasitic drag and the inevitable heating
of the incoming air. There must be some additional HP gained somewhere,
perhaps from the use of the Holley carburetor, otherwise that would be
the most efficient supercharger I have ever heard of.
The HP gain is not a
fixed multiple of the boost pressure, it is a proportionate gain on the
original power of the motor. The rule of thumb you cite must (?) be
derived from a recognized standard mule engine or some such, and could
not be applied to any other motor without additional correction. At
least, that is what it seems like to me (which is always questionable).
You are right about that. A 25 horsepower motor could not expect to see
35 hp if it been given one lb. of boost. The 10 hp gain per pound was
based on Kastner's findings on an otherwise stock 2.5 liter TR motor,
using a compressor A/R ratio which did not excessively heat up the air.
References to Kastner's work can be found in some pages of MacInnes
book. Interestingly, Kas used the Weber side draft carb when boosting
the TR7, but chose the 2 inch SU for the TR6. (Better mileage)
As was previously noted, that amount of HP (39HP) can be
gained from less expensive, simpler modifications. On the other hand, a
simple pulley swap on the supercharged motor and suddenly you are
running 180 HP with no loss of drivability. Try doing that with a
naturally aspirated TR6 engine.
You're preaching to the choir, here! Whether changing pulley sizes, or
adjusting the wastegate spring pressure, it's all about how much power
do you want, and are you ready for it?!
Dick Taylor
....Typically, one can expect about 10 additional hp per lb. of boost,
up to where air density is compromised at higher boost pressures. (At
least this is true with turbocharging.)
<snip>
Dick
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