OK Chris, here it is. Keep in mind it was
geared for the boys with SHOs. BTW, guess
I didn't get the brains of the family...
Well, it's more clear now that gains can occur with a cross-over & valving.
How much? I don't know but there will be a gain.
To find out the gains, though, one of you will need to be the guinea pig.
Do this at your own risk, obviously.
To get right to the point, if you guys are interested in this, the next
time you get on a dyno someone try two runs. One with the crossover as
is and one with it plugged. Theoretically you can either plug the
crossover in the middle (tough to do) or cap the end of each plenum. Do
so with car, however, as duct tape won't cut it. What you WILL see is
that low-end torque will INCREASE, as will high-end power also INCREASE
with a drop in mid-range torque. I don't know the amounts or at what
RPM this will peak at but this will occur it's simple physics. Finding
how much of a benefit, well, that is up to one of you.
Rather than drive the car with the crossover always plugged in order to
boost low-end torque, the opportunity exists to have a valve in the
crossover to open and close at selected RPM. The proper mode of
operation is always a) valve closed at low rpm b) valve open at
mid-range rpm c) valve closed at high-range rpm. There is the
potential that you might find (I'd rather doubt it) that the SHO would
faster with this plugged all the time. Or possibly if you drive your
SHO on the track and spend all your time in the 4000-7300 rpm range,
maybe this modification is great for you.
Without overselling this too much, the bottomline is that someone has
got to try this. Please do so safely. Those plenums will suck anything
flexible right into your engine so a little bit of thought is necessary.
How does this work?
Briefly:
1) What happens when the intake manifold plenums are separated from one
another (split-plenums so-to-speak) the intake manifold generates TWO
tuning peaks. The tuning peaks occur based on a natural resonance that
occurs between the intake manifold runners and feeder pipe. Imagine the
air in the intake pipes and in 'feeder pipe' (that between the plenum
and the throttle body) as 'masses'. Imagine the air in the plenum as a
big spring. At some point the masses can begin vibrating both in-phase
(same direction meaning moving toward the port at the same time) or
out-of-phase (opposite directions). Resonance is helpful as it provides
pressurized filling of the cylinder just before the valve closes. In
general the tuning peaks associated with a split-plenum design are lower
in magnitude than that from a single plenum but it may allow improved
area under the torque curve (I'm guessing).
2) What happens when the intake manifold plenums are all connected.
There is no longer two tuning peaks (yes, we get two tuning peaks but
that is due to long & short runners). Instead just one tuning peak
occurs. This single tuning peak occurs with greater magnitude than the
split-plenum design.
So what else can you do about this.
If it is really found that this type of operation is either valuable
(with plenums continually closed) or lends itself to getting the best of
both worlds (opening and closing a valve in the cross-over), there are
possible changes that can be made to TUNE the manifold to more optimal
rpm values.
How???
Well, as I said before the masses in the 'feeder pipe' and runners are
the key items. Changing the manifold plenum size by 50% (hard to do)
makes only about a 200 rpm shift in the torque peak. Changing runner
lengths is hard to do. Changing the 'feeder pipe' length by adding a 50
mm spacer between the throttle body and the manifold will also shift the
low rpm torque peaks by 200 rpm. That's pretty easy to do even for the
mechanically challenged like me.
In any case, my disclaimer. Do this at your OWN risk and please use
your head about this. I claim no absolute improvements.
There is the potential here, primarily from using a valve in the
crossover which would open around 3000 and close around 5000 rpm to
boost your low-end and high-end. With the valve operating properly,
there would be no difference in the mid-range, as this is how Yamaha
tuned our manifold.
This is a sound practice and is on a manifold that I've just designed.
The benefits are not great but they are measurable for the performance
minded.
Thanks to John Weidenbenner for rekindling this topic and researching
it. Possibly there is nothing more than a few ft-lbs of torque and 5 hp
on the high-end to be gained. But that is all my LPM & 80 mm got me and
they were nearly $450 with the cone filter. I can imagine that one of
you can put together a $300 kit that does this with relative ease.
