Orma & netters
At one time when I was drag racing in street and stock classes, I was
studying about the feasibility of turbo or supercharging, especially when a
buddy had a '56 Chevy truck with a B&M street blower or supercharger, pushing
about 16# of boost according to the panel gauge. He always had a cutting
out/popping back problem due to leaning out during actual acceleration. He had
the hardest time understanding the dynamics of how actual acceleration involved
more factors than sitting static during a burnout. In the course of studying
to understand his situation I came across a test performed by B&M where they
used several different cams in the same engine and performed dyno tests to see
which worked best. They found that the most dramatic differences in
performance increase were made when a cam altered for the artificial
aspiration, supercharging, was used. The typical long duration, high lift cams
used for most normally aspirated hotrods actually performed at or even below
stock cams. They reasoned that the large overlap of the valves caused too much
of the air/fuel mix to be pushed out of the exhaust valve and not retained in
the cylinder. The cam was designed to help overcome the lack of efficiency of
the engine in order to increase its "breathing". With the supercharger this
was not necessary, since the blower was providing over 100% of the CFMs capable
from the engine to start with. Simply stated the cam needed to help the blower
provide more power, by delaying the closing so that more air could be packed
in, instead of trying to assist it in moving more air from outside. They found
the best cams were cut with this in mind and were quite different from the
traditional stock or performance cams. Another interesting fact they
discovered was that even though the boost numbers were high with the stock and
performance cams, all the pressure was remaining in the intake, not getting
into the cylinder. When they changed the cam to one designed more for the
blower, the boost number went down, but torque and horsepower went up.
Literally more air was being packed into the cylinder, not just into the intake.
Also something to consider when selecting your turbo: choose one where the
CFMs are higher, not necessarily the pressure capability is high. Especially
since you are wanting to add performance over the entire range and extract more
horsepower from the engine, you are best to have a turbo that can move alot of
air, not one that just makes pressure. Racers have found that high boost
numbers are not the whole story. Lowering the boost pressure, but increasing
the volume of air delivered to the intake manifold actually puts more air into
the cylinder without elevating the intake air temperature as much due to the
lower amount of "squeezing". This reduces the possibility of detonation. Orma
you may also want to install a blow off valve protection on the intake manifold
to prevent over pressurizing, and also consider some form of spark retard at
lower RPMs, while under boost, the same as the 2300 motor in the Mustang has.
The engine can't tolerate these pressures for long periods in the lower RPMs
and will lead to piston, rod or crankshaft failure eventually.
Colin & Bev Rainey
KR2(td) N96TA
Sanford, FL
[email protected]
or [email protected]
http://kr-builder.org/Colin/index.html
