5 exciting new HORNEY pics to whack with.

[jdbubb@ix.netcom.com (Dan Bubb)]

Pulse tuning refers to the pressure (or vacuum) pulses generated in the
inlet and exhaust tracts as the valves open.
In the case of your ITB's as the intake valve opens a vacuum pulse is
generated in the inlet port and travels to the end of the horn where it
is reflected back down the inlet tract as a pressure pulse. (this is
just like what happens in organ pipes) If the pulse is timed correctly
it will arrive at the inlet valve just as it's closing causing a light
supercharging effect. I've heard on the order of 10% if the system is
tuned correctly.
All engines generate the pulses, but the intake manifold or ITB's need
two things to take advantage of the pulses.
1. the end of the inlet tract needs to shaped so the pulse has a strong
reflection. Your ITB's have the abrubt cutoff required. An A1 8V
manifold, as an example, does not since the runners sort of slide into
the side of the plenum.
2. the inlet tract needs to be the correct length so the pulse arrives
at the intake valve at the correct time. There are two formulas I'm
aware of to calculate length. One is the formula developed by Chrysler
in the mid-60's. The second I picked up in Racecar Engineering and is
commonly used in racing today. With your tract length of 12.75" the
Chrysler formula indicates you will get the max torque increase at 6500
RPM. The Racecar Engineering formula indicates 8200 RPM!! You can expect
that you will see a boost over a range of ~1000 RPM centered on the
calculated RPM. 
I built an intake manifold with 18" runners for a Fiat 124 engine I
converted from carb to CIS injection. The boost in power from 5000 to
6000 RPM was very noticeable.
There is no doubt that your ITB's give the absolute minimum intake
restriction (very good for power!), but I'm thinking that unless you
intend to shift at 7500 to 8000 your horns may be a little short. At
some time in the future you might try something like a 4" longer runner
just to check the effect. The downside is that if you're running an
aftermarket ECU (and you would just about have to be) then to maximize
the benefit of changing horn length you would have to remap your ECU.
Understand, I'm not critisizing your setup at all. I'm just saying that
changing horn length gives you the ability to shape your torque curve to
some extent and you may find a more driveable power curve with longer
horns.
In my experience changing intake runner length (on a setup that really
uses pulse tuning) will have a much more pronounced effect on the shape
of the torque curve than changing cam timing.
Dan

Mike Blain wrote:
> 
> Explain "pulse tuning" please.
> MB
> 
> Dan Bubb wrote:
> 
> > That is very nice! I especially like the little vacuum plenum with the
> > plastic tubing. Very neat!
> > So, when is one of you guys with ITB's going to stick a ruler down one
> > of your throats and measure the distance from the valve head to the end
> > of the stack???
> > Just curious cause I have a formula that will tell at what RPM you will
> > get a torque boost from pulse tuning.
> > Dan
> >
> > Mike Blain wrote:
> >
> >>LOL,
> >>
> >>you'll see.
> >>
> >>MB
> >>http://www.mk1scirocco.com/mikepics.htm
> >>
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> >