It's not that the runners are more restrictive that makes them work better,
it's simply the
smaller size. Increased restriction actually isn't there...at the low rpms.
At those low
rpms the smaller runners move the gasses faster, both on the intake and the
exhaust. If
the runners were larger, the fuel mixture would seperate on the intake side,
and the
exhaust gasses would not exit or scavange. Rev the engine up beyond the tuned
range, and
then those small passages become restrictive.
You are absolutely right about many people slowing their cars down by putting
on too large
an exhaust. One can also of course put on one that is too small. The needs of
a 1200cc
engine running at 4,000 rpm are different then a 3000cc engine running at 8,000
rpm.
Remember too, the more precisely you tune the runners, the peakier the
performance. This
is why a well set up two stroke with tightly tuned expansion chambers has gobs
of power,
but in a very narrow range. Out of that range, and the engine can't peel the
skin off a
grape.
Longer runners also use accustical properties of lower rpms. The time between
pulses is
longer, so you need a longer runner length to allow the sound waves to travel
in and
achieve resonance.
Steven Fooshee wrote:
> Daniel1312@aol.com wrote:
> >
> > I have heard of some rubbish in my time but this is incredible - absolute
> > rubbish.
> >
> > In a message dated 15/11/00 04:20:16 GMT Standard Time, sfooshee@home.com
> > writes:
> >
> > << The generalization
> > is that you want a more restrictive exhaust and a smaller carb/intake for
> > higher
> > velocity at low engine speeds. >>
>
> Okay, so besides your subtle disagreement, what's the answer? I
>qualified it as
> a wide sweeping and incomplete generalization because I didn't feel like
>typing
> that much.
> In my experience, engines with smaller and/or longer intake tracts
>have made
> better low end torque versus high speed power. Mazda's Le Mans winning
>prototype
> used a variable intake (copied, I believe, by Ferrari's F1 program) in which
>the
> intake would shorten as engine rpm increased. This allowed the longer (and
>more
> restrictive) runners to increase the velocity of the intake charge at lower
> engine speeds while still allowing the short runners required for higher rpm
> power. Mazda has also used this with the '89-'91 normally aspirated car's
> variable dynamic intake effect system, and Porsche's variable intake is very
> similar. If the intake charge is stagnant (too big an intake for a given
>engine
> speed) then the fuel doesn't mix well and doesn't burn well.
> I don't pretend to understand all the physics of engine design, but
>my short
> experience has shown me many people make their cars slower by simply opening
>up
> the exhaust. I know that 2-stroke engines are desperately dependent on
> backpressure for torque, and rotaries are very similar to this. My old SAAB
>was
> in dire straits when the manifold flange rusted off, it wouldn't discuss power
> until 3500 rpm or so.
> Assuming that the questioner is limited to modifications below
>fabricating some
> means of variable cam timing, most any change which will increase low end
>torque
> will sacrifice top end power.
> Even if I'm wrong, why call it _incredible_ rubbish with no evidence
>or opinion
> of your own? If I'm wrong, correct me, but don't be an ass.
>
> --
> ~
> -Xavier Onasis.
> '87 RX-7 TII: Live fast.
> '90 GS-500E: Die young.
> MCMLXIX Sprite: And leave an exquisite corpse.
> If God dropped acid, would he see people?
>
> ~
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