[alfa] Plenums, runners and power

[webb.p@xxxxxxxxxxx]

With a few recent posts about plenums and runners WRT Spica, carbs and EFI, I stole this text from another list I belong to (without the authors permission).   Some interesting insight.   I've exchanged some interesting emails and ideas with a few listers on this subject.  I'm almost ready to prototype the Milano Verde 3.0 plenum/runner change.  I'll post pictures when I'm done.  Meanwhile, here's some (albeit opinionated) jargon from someone experienced in (professional) engine building.  Please no flames about American vs Japanese vs Italian, or NASCAR vs. F1, oval vs road.  It's all good so long as it's fast!!

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It is a well known phenomenon (at least in the pro race and OEM circles
I travel thru in North America) that a large plenum and short individual
runners will help an engine achieve maximum PEAK hp (which is all that
Mr. Shurvinton cites -  a position I have not nor can challenge).

HOWEVER, what does the entire torque curve look like 'tween the plenum
versus IR setup he mentions???  A review of the entire curve will be
interesting.

Plenums are generally sized/used to RESHAPE the torque curve - seldom

if ever will they boost it.  The guys I work with use them to fill in
the hole that can occur if/when an IR manifold "sees" a plenum (be it
an air box, a finite plenum or the "infinite plenum of the atmosphere").

The hole that's filled is the one that develops between the N1 (torque
peak) and the N2 HP peak and the "fill" comes from shaving a bit off the
N1 and N2 values.  Basically, there is only so much area under the curve
you can work with and playing with 20-30 year old manifold designs will
only yield the same torque curve shapes they created 20-30 years ago.
This is why the 4 pots supplied today (Honda, Nissan, Toyota, et al)
have plenum equipped manifolds, may of which are active (variable plenum
volume and/or runner length).

If you are going to run an engine over a SMALL power range, by all
means, tune for the peak and hope to god you can keep the engine in the
power band.  However, if the car only has 4 or 5 speeds and it gets

short shifted or parked in corner by a back marker, a peaky engine can
fall on it's face and have a hard time recovering - this same situation
relates to sport driving on the street.

Since the IR setup doesn't communicate, you may see a better idle with
carbs - not something I'd challenge.  However, since the M/S board is
dealing with EFI and EFI cleans up idle quality, why not take advantage
of what's been learned with runner AND plenum tuning over the past 20
years?

Case in point:  Take a Sentra SER and run it around town behind a Focus
SVT.  The SER will run up the ass of the SVT up until 90-100mph and is a
pleasure to drive with its wide torque band and good power curve.

The SVT will outrun it above 90mph but where can you run an SVT where
it has the legs to shine???  Once you get to drive a car with low end
torque - be it a V-8, a 6 or even a 4 (witness the SER), any driver
(male or female) will probably develop a liking for it (sort of like
crack cocaine for motorheads).


The F1 and motorcyle folks take full advantage of the HUGE plenum/air
box with  short runner manifold systems and they have to run at
12-18,000 rpm.  A "4 pot" will also likely respond favorably to a
large/no plenum with short individual runner manifold system at WOT and
high RPM.

The Celica GT with the Yamaha tuned 2ZZ 1.8L I-4 is a perfect example
of this - it don't do much below 6000 but from 6500 to ~7000, hang on.
However, even Honda has used VVT and an active manifold to tune their
plenum equipped 2 liter 200hp motor to have a relatively flat torque
curve and a wide (4200+rpm) 90% torque band width.
However, a plenum will enable the engine tuner to shape the torque
curve more favorably - especially in the mid range.  Whereas it can be
shown that an IR system will out peak power a plenum equipped system, it
could be shown that a plenum equipped 305" small block with a "tunnel
ram" fed by 2x4bbls will make more peak power (but less low end torque)

than the same engine with a Hilborn injected 305" small  block having
the typical longish but big diameter "trumpets".

If memory serves correct, Honda even uses a plenum on their 240hp 2
liter S2000 engine (1.96hp/cid and emissions legal).  I wonder why???

The road racers clearly preferred the Hilborn system (wide RPM band
required with excursions into being "parked in the corner") while the
drag racers and their ability propensity to keep the engine at 8500 plus
clearly preferring the Holley/tunnel ram setup.  The Honda piece
addresses the 4 pot scenario.

The point is this:  Engine tuning is not merely a thing of who makes
the most power.  It comes down to who makes the most USEABLE power over
the broadest operating range for the intended operating environment.
For every 1.6L 4-pot that can be cited that simply MUST run an IR setup,
instances can be cited where a plenum setup will be a better compromise
for that particular situation.


The perceptions of reality that Mr. S and I have are and will clearly
differ, perhaps irreconcileably.  Yet, this much is certain.  Bill
SPENDS money to practice his hobby and the industry values his patronage
and support.  On the other hand, I am fortunate enough to MAKE money
with via my participation in the industry via the engineering services &
products I provide to pro race teams.

Different perspectives and different cash flow directions - simple as
that.

In closing, we're finally gettin' the drivers to realize that a high &
wide 90% torque band width will make the car go faster than a huge
torque and hp peak for oval track, road race, marine and street use -
even the drag racers are finding that a wider torque band helps the car
recover and not fall flat.

And just for a closing reference, the 1.6 hp/cid that is referenced in
the note that prompted this reply has a way to go in comparison to the
2.11hp/cid we make in the 355" plenum equipped NASCAR motors and the

2.6hp/cid plenum equipped 500" Pro Stock motors and the 1.96hp/cid 4 pot
engines I deal with.

As far as a 1.6L engine mentioned in #30523 goes, the power listed is
pretty good for a street piece but we have crate motors you can buy with
a credit card that make more than that.  As far as a 1.6L engine goes in
North America, if we wanted a starter motor that strong, we'd call
Denso, Delphi or Visteon.
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