Vic, I decided to dig out my copy of "The Sports Car". I had forgotten what
a delight it is to read. I decided to copy an introductory paragraph about
supercharging because I think everyone will enjoy even if they do not give a
hoot about the subject.
"It is odd that supercharging is so unpopular with car manufacturers and
public alike. The stresses in crankshaft, connecting rods and almost all
the major moving parts are much higher when high compression ratios are used
to augment power than when the equivelent power is obtained by moderate
pressure supercharging. Unfortunately in the past, the designer of the
competition car, the sports/racing car, was not always satisfied with
moderate pressure supercharging. It is so easy to be greedy when
supercharging and the history of the supercharged car is a sadistic story of
of mangled components. As a means of discovering the weakest links in an
engine, suercharging has no equal, but this is no condemnation if the tuner
uses his brains and not his emotions in choosing the maximum permissible
presures." Is that not a delight to read? I love it!
Mr. Campbell goes on to state increased compression increases engine power
and efficiency, the down side is rapid increase in peak cylinder pressure.
A 6:1 ratio gives peak pressure of about 500 lb/sq. in., 10:1 yields
pressures of about 1000 lb.sq. in. The gain in torque is about 20%, gain in
power is about 35%. Increase in stress on rotating parts is about 100%. A 4
lb./sq. in. boost will produce about the same increase in torque and power,
but stresses will increase only about 30%. He pressure definitions are
somewhat vague, but appears to consider anything under 5 psi as low boost,
and moderate boost seems to go up to about 15 psi.
As to the qustion 3 main forged crank vs. 5 main cast, I would suppose it is
largely a wash, a close call favoring the 5 main. It looks as though the
change was made in interest of reducing costs, otherwise we would be talking
about fitting a 5 main forged crank in every high performance engine being
assembled. The 3 main crank is not a wet noodle!
Happy 'Beaming
Bill
----- Original Message -----
From: "Victor Hughes" <v.hughes@student.canberra.edu.au>
To: "Barbara Blue" <the_blues@worldnet.att.net>
Cc: <alpines@autox.team.net>
Sent: Thursday, March 08, 2001 11:59 PM
Subject: Re: More Power from the 1725
>
>
> Barbara Blue wrote:
>
> > By the by, I have seen a couple of references to the effect that some
> > engines are not up to supercharging or turbocharging. Colin Campbell in
his
> > "Sports Car, etc." claims that supercharging puts less strain on an
engine
> > than high compression, high revs and so on.
> >
>
> Forced induction whether by mechanical supercharger or exhaust driven
> turbocharger does give a motor higher compression because it forces
fuel/air
> in. The more fuel/air, the bigger the bang at ignition, the more power is
> produced but also the more strain on the bottom end. With 4 Cyl motors,
those
> with 5 mains are (other things being similar) generally better able to
cope with
> this than those with 3 mains.
>
> However a supercharged engine probably won't have to rev as hard as a
motor
> modified by 'conventional' means to produce the same sort of power, so
that
> would be less strain. Also, it may be more tractable (compared to say a
> multiple carb motor with a lumpy cam) and so can be driven at lower revs
when
> you're not after all that power.
>
> Nevertheless, if I were fitting Forced Induction I'd want the best bottom
end
> possible.
>
> Vic
|