Steve,
Forget pressure ratio (or boost). What the engine needs for
more power is an increase in density ratio which is a
function of both pressure and temperature.
For the sake of argument, let's assume that "doubling" the
displacement of an engine is achieved by being able to pump
twice the air mass through the engine at a given RPM. That
theoretically would allow burning of twice the fuel and
potentially developing twice the horsepower at that RPM.
The formula for Density Ratio (D.R.) is
P2/P1
-----
T2/T1
where:
P2 = Compressor Outlet Abs Press.
P1 = Compressor Inlet Abs. Press.
T2 = Compressor Outlet Abs. Temp.
T1 = Compressor Inlet Abs Temp.
Now, ideal (adiabatic) outlet temperature (T2i) can be
determined for compressing dry air by:
T2i/T1 = (P2/P1)^.283
To get actual T2, you must take into account compressor
efficiency:
T2i-T1
T2 = T1 + ------
Nc
Using:
T1 = 70 degF (or 530 degR absolute)
P1 = 14.7 psia (sea-level atmosphere)
Nc = 65% (typ. for centrifugal compressors)
you can now solve all this mess for the Boost (psig)
required to double the density of air going into the engine.
Guess what - it comes out to 32 psig!
Obviously, changing compressor efficiency, intercooling,
radiant heat losses in the compressor and intake tract,
conductive heat gain from exhaust turbine and fuel
atomization will all work to change that number, but those
are what I'd lump into "thermal losses" for this estimate.
Hope this helps. Looks like Doug's original post was closer
to right after all, altho' apparently he didn't know it at
the time!
Jim
66MK1
------------------------------
Date: Wed, 13 Dec 2000 15:43:35 -0800
From: Steve Laifman <Laifman@flash.net>
Subject: Re: inquiry 121200b
Doug Mallory wrote:
> Hello Craig,
>
> Tuesday, December 12, 2000, 8:12:37 PM, you wrote:
>
> CW> PS: Vortec superchargers are C.A.R.B. legal, bolt on
horsepower. They act just
> CW> like you have doubled the displacement.
>
> Not quite... It would take 30lbs boost to double
displacement.
> My "STOCK" Merkur runs 15.
>
> --
> Best regards,
> Doug
mailto:RDMALLORY@earthling.net
Doug,
It would only require 14.7 psi(g) boost to double horsepower
(as that is 14.7 psig
(GAUGE) over the existing 14.7 psia) giving you the
necessary 29.4 PSIA (absolute)
you refer to. Of course this is race car tuning, nothing
streetable. Neglecting
thermal losses, of course.
Steve
- --
Steve Laifman < Find out what is most >
B9472289 < important in your life >
< and don't let it get away!>
http://www.TigersUnited.com/gallery/SteveLaifman.asp
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