V-type engines with multiple plena

[Matt Cramer]

At 04:55 AM 9/28/2001 +0000, you wrote:

(snip)

>My second question has nothing to do with EFI, so sorry for asking it, but I 
>was wondering why in a V-shaped engine, many performance automobiles 
>(ferrari, lamborghini, etc.) used two plenums instead of one.  I understand 
>the notion that the larger volume needed when running only one plenum would 
>result in slower throttle response, but is that it?  Also, when running a 
>forced induction type setup, would a dual plenum design still be a worthwile 
>design?
>
>thanks in advance for any light you guys can shed on the subjects
>
>-santi u
>
	Here's my theory on why they don't use a single plenum on the extremely
high performance engines with multiple cylinder banks.  It has to do with
the intake runners; they are often set to be a certain length to use a ram
tuning effect, and the fewer bends they take, the lower the friction
losses.  You could increase the diameter of the runner to offset this, but
it would lower the velocity a bit.
	So, the best runner design would be nearly straight, and using something
like five centimeter runners isn't an option.  On a V engine, keeping long
runners close to straight would either call for runners that cross in the
center of the engine and then extend out over opposite cylinder heads, or
ones that gently curve straight up, resulting in a very tall manifold that
would give you hood clearance problems.
	So, if you want tuned runners and a single plenum, and don't want it to
stick up half a meter above the engine, you'd have to bend the runners into
something that'll compromise how well they flow.  Consider the 5.0 Mustang
manifold off Fox-chassis cars, for example - the runners line up in the
center of the manifold, take a very sharp bend upward, then kind of fold
back on themselves to put the plenum above the driver's side cylinder head.
 That's got to put something of a crimp in their flow.
	So, if you want the fewest compromises in runner design, it would make
sense to have the runners from one bank lead to one plenum, and the runners
from the other bank lead to their own plenum.  And this does seem to have
something to do with EFI, as the only long runner, dual plenum intake I can
think of that wasn't injected was the Chrysler cross-ram of the early 60's.
	By the way, I think the Viper V10 intake does this for a different reason:
It has a long runner and a short runner for each cylinder to give it ram
tuning at different RPM.  The long runner goes across the lifter valley,
while the short one makes a U-turn and heads to the cylinder bank under it.

Matt Cramer
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