BMEP and ft/min and tt clarification

[CM1022@aol.com]

Sorry if I confused some people with witing about BMEP as if it were the same
thing as ft/min.  To clarifiy, BMEP stands for break mean effective pressure
and is a general stat that engineers use to rate the efficieny of the engine.
It is the hypothetical average pressure in PSI that pushes down on top of the
piston.  It is a better aproximation of engine performance than simple
hp/liter ratios (which can be deceiving).  In reality, BMEP should be
calculated with a dyno and all kinds of other measurements, but for the sake
of time engineers use a simple equation:

HP (peak) x 13,000
rpm (at peak) x engine displacement in liters

This givens a good hypothetical stat.  For normally aspirated cars, a good
street engine runs 150-170psi.  A finely tuned racer with four or five valves
can get above 200psi easily.  But you would be supries, a Ferrai F-50 gets
175psi, if my info is correct.  Forced induction engines rate way higher.  The
tt is around 300, a very impressive stat. 

But high BMEP is not the only thing to look at.  I have found that
particularly high BMEP figuers are obtained through certain compromises in
other areas.  Another general engineering stat is used to rate internal stress
on engine parts.  Again, if you could work through lots of math and have some
very specific info, the result would be better than using this equation.  It
is a simple ft/min rating that measuers how fast the piston actually moves in
the engine.

Take max rpm x stroke (in inches) x .166

The number you get is ft/min traveled, and you can use max HP rpm or any other
rpm for that matter to see how much stress is on the engine at any given rpm.
Most engineers agree that with todays metalurgy, this chart applies:

ft/min
3000-3500   =   A good high performance street engine, with forged parts a
good
                       ide after 3300

3500-4000   =    Formula 1 cars, dedicated race cars, high stress engines
                        forged everything, even titianum to reduce valvetrain
weight
                        and spring pressure

4000 +        =    Some very stressed engine, like top fuel dragsters, made to
run
                        for short periods of time

This is a general idea, and the lower number is ALWAYS better.  Consider the
tt, which comes in around 3500.  It's not really pushing anything, but it is a
stressed
little engine.  Audi had to really reinforce the engine- forged everything.
The fact is that any engine would beneifit as far as durability/reliability if
the stat was lower.

As far as static compression ratio is concerned, it is really not a great way
to see what an engine's compression is like.  On the contrary, dynamic
compression is the only real way to rate compression.  But this involves vavle
timing info, and takes some more time to compute.  Remember, compression only
takes place when both valves are sealed.  I don't know anything about the tt's
valve timing stats.  The static compression is high for the engine, and
usually you can superficially go by that as a very general approximation
(turbo engines usually run less overlap).  The problem with high compression
is fuel:  you need the good stuff (leaded 100+) to start running higher
compression forced induction engines.  

The tt has a great engine I am sure, and I will learn about it more when I get
the info.  As for the 400hp claim as the max potential (ala MTM), I am sure
that is true also.  Again, the highest power engines in the world generally
are turbocharged inline fours of small displacement.  My Porsche 944 turbos
2.5 engine can produce upwards of 500 to 600 hp by the tuners.  A tuner named
David Chen is getting 450hp at the flywheel with a 3.0 liter 16 valve head
with a K-29, and at only .8 bar of boost!

Anyone have any acceleration or any other stats for the TT from any foreign
mags?  Love to see them.  The haldex diff to my knowledge is a control clutch
system, but there is so much conflicting info.  

Hear is real technology to put things into perspective:  The SwissAuto
experimental 250cc (that's .25 liter) turbo inline four puts out about 800PSI
BMEP, at has only 2650ft/min at a max of 16,000 rpm!!!!!!!!!  My my.  What is
the secret to those oh so low ft/min numbers? (hint: 1 inch stroke)

Cheerio Dave E.!  Glad to hear from you again.

Chris Morlock

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