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MGA Performance- Part one

To: mgs@autox.team.net
Subject: MGA Performance- Part one
From: William Eastman <william.eastman@medtronic.com>
Date: Tue, 05 Nov 1996 08:11:58 -0600
I tried sending this post Monday morning but it was too big.  I have broken
it in half and will attempt again.

Thanks for the request for my opinion.  I am not that familiar with MGA
engine modification but I am fairly familiar with the B series motor having
spent a good deal of time inside my MGB earlier in my automotive life.

There are three main areas that effect engine output.  Volumetric
efficiency is the ability of the engine to bring in a fresh fuel air charge
and completely expel the exhaust gases.  Increased volumetric efficiency is
a good thing.  The next two topic are "bad" things in that they use power
but they are necessary evils.  Pumping losses describe the work done by the
engine to suck in and blow out the combustion charge.  When you get right
down to it, your engine is just one big air compressor.  Due to one of
those cruel quirks of nature, pumping losses tend to increase with
everything that increases power - i.e. higher compression, higher RPM.  The
third area is frictional losses.  This is the classic engineering balancing
act.  Your old, worn 1622 is pretty much already optimized for low
frictional losses.  Unfortunately, that optimization is at the cost of long
term reliability.


>From a theory standpoint, the most important variable for engine
performance is intake air.  There is actually a formula for calculating
power output from an engine.  It is about two blocks long with 300
variables but the most significant variable is intake air pressure.  This
appears as both a linear multiplier and as an exponential term.  Anything
you can do to increase the pressure of (or decrease the resistance to) the
intake will be a good thing.  It both increases volumetric efficiency and
reduces pumping losses.  Scavenging the exhaust also helps both sides of
the equation but, since the driving force for exhaust evacuation is
pressure from the cylinder which can be quite high, not atmospheric
pressure which is fairly limited at around 15 PSI.

Concerning camshaft selection.  For a street engine, I believe that you
should not look at maximizing peak horsepower.  Rather, you should attempt
to maximize the area under the torque curve.  This means a cam that is
fairly short duration to preserve low end torque with enough lift to get
air in and out quickly.  Due to the advances in metallurgy and, more
importantly, oil formulation in the last 35 years, cams can survive higher
surface pressures now than they could back then.  This allows you to run
stiffer valve springs and control higher valve velocities.  Higher valve
velocities can allow you to either rev the engine higher- not recommended
for a street engine- or open / close the valves faster and higher at the
same engine speed.  Remember, lift and duration are only two variables for
a camshaft.  Cam lobe center angles are very important for engine
performance.  If this is too narrow for a given profile, you will get a lot
of flow reversion at low RPM.  If it is too wide, you will effectively
loose more of your compression stroke because the intake valve will still
be open.

Now for a quick shot at traditional wisdom.  I have heard from just about
everywhere that the long stroke, small bore (called under square) design of
the MG engine limits horsepower.  If you look at the equation Torque = BMEP
X Piston Area X Stroke, you will notice that, for a given engine
displacement, the area to stoke ration can be anything and the power output
will be the same.  Theoretically, a larger bore / shorter stroke engine
will see less change in combustion chamber volume during the cycle and that
would allow slightly higher BMEP for a given peak pressure, but this
advantage is pretty small.  An MGB engine is around 80 mm bore by 85 mm
stroke.  My Acura Integra is 81 mm bore by 85 mm stroke.  The GSR Integra
is 81 X 83 and redlines at 8200 with 95 hp / liter net!  That is the second
highest specific output of any normally aspirated production engine behind
the Ferrari F355- not bad company.  The GSR engine would be interesting in
an MGA except I think the bastard runs backwards- just my luck!  Anyway, in
my opinion (and Acura's), an under square engine is not automatically a low
performance engine.

Best Regards
Bill Eastman




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