a few people have hinted at the reasons why a single plenum works better, a few have got some misconceptions.
first and foremost, throttle butterflies are a restriction.
not a massive restriction, but a restriction all the same.
if youre chasing the ultimate power, you dont want ANYTHING hindering you.
that 3% could cost you
£5000 to get back lol.
not really relevant on road engines, but its good practice to make as few compromises as possible.
id rather not settle for 199bhp when i can get 200bhp with a little effort and no extra cost
secondly, lots of the TOP cars in motorsport are running ITB's.
i cant argue that theyre not, i can argue the reasoning though.
first off, the majority of racing series' are running restrictor cones.
if youre restricted to a certain amount of airflow anyway, then youre not quite as fussed about a lesser restriction upstream.
ITB's are easier to just bolt on as you can tune lengths more easily.
they are also available straight off the shelf.
#
they run ITB's
(of sorts) in Formula 1, but they are using rather a more complex setup than your simple butterfly, sliders and all sorts
(not got a clue what they are running these days, but theyve run sliders and barrels in the past) which means there is zero restriction at full throttle, and race engines spend most of their life at full throttle or zero throttle, there isnt a great deal needed in between
now for the specific reasons why a plenum is better.
.
.
.
air enters through the throttle body
(a single large one) and fills into the plenum.
the plenum then feeds the 4 individual runners which head to the ports.
valve opens, large vacuum appears, air flows down the runners.
if youre designing an inlet manifold for a specific application then youve got your tuned lengths, youve got a trumpetted runner opening inside the plenum too to smooth the flow.
the difference is that youve got a plenum full of slow moving air.
.
.
there is no such thing as ram effect on road cars as most people think of it
(fast moving car scooping in air at speed), it doesnt happen much below 100mph, and below 150mph its as good as pointless with regards scooping air into the airbox from outside to
"ram" it.
F1 cars get a ram effect, but they also run a complex airbox which converts their ITB's into what is essentially a plenum setup.
if you just feed your throttle body sufficient air for the engine to run, then it will enter the plenum at a low speed, and within the plenum you aim to get it even slower, and also to
"ram"
(using the velocity inside the inlet, not the speed of the car, dont confuse the 2) it in there as best you can to increase the pressure as much as possible.
then when the valve opens youve got a vacuum in the engine, and a positive pressure zone in the plenum, and youve effectively got forced induction
how else can you acheive over 100% volumetric efficiency on an engine? you simply cant recreate this on ITB's without adding in restrictions
(butterflies, non-flush trumpets, etc).
standard inlet manifolds have a vacuum in the plenum so any vacuum your engine creates is negated.
its
(one of) the reason(s) why engines only tend to be around 50-70% volumetrically efficient from the factory.
its not in the manufacturers interests to chase perfection in fluid flow when they can get the power back more cheaply elsewhere
there are holes to be picked in my argument, but there are just as many to be picked in using ITB's.
the fact of the matter is that for ULTIMATE performance, a plenum manifold is superior.
ITB's are just cheaper and easier to bolt on and run.
at the end of the day, how many people possess the skills and equipment to design and build an inlet manifold with complex fluid flow analysis to get optimum plenum pressures and runner profiles without spending an absolute fortune?
(for the record, i myself dont entirely, but ive got access to a lot more than your average person through work and contacts)
ITB's are easy and work well, plenum inlet manifold is just showing off, and is something of a challenge for me to play with, and ultimately going to make me more power.
i could just as easily hinder myself with it, but its something else to keep me busy on my never-ending MR2 project
P.S.
jrleech
- once ive gont mine built and working how i want it, i fully intend to produce them to order.
its going to cost me enough to do it probably, so if i can recover my costs by sharing my work il gladly do so
