|
It all depends, Tom. I started out with short
runners and a TWM twin throat Webber with injectors (50mm each throat) as that
was recommended by the racing guys. Turns out (at least in my case) it was
the wrong combination for an aircraft engine turning in the 5000-6000 rpm
range. Changed to 21 inch runners (copied Tracy's Dynamic Manifold - with
which he won his first Sun & Fun 100 race). My static rpm increased
from 5000 to 5600 rpm, rate of climb increased 300 fpm and top speed increased
from 185 mph TAS to 194 MPH TAS.
The inertia of the air/fuel mixture is an important
factor in stuffing the combustion chamber. A inertia of a fast moving
airstream will continue to fill the chamber even after the rotor/piston starts
to come up from BDC. Smaller Dia and longer tubes generally favor
the lower RPM (5000-6000). Now if your engine is going to turn 7000+ rpm
then I would agree larger dia shorter tubes are called for. But with the
original 2.17 reduction drive 7000 rpm was a dream - now with the 2.85 its
within reach. I have hit 6800 rpm straight and level with the 21 inch
tubes. So I may revisit the first intake I took off and have sitting on my
bench. Even the Mazda LesMan race engine had approx 12-17" of tube length
(they had a variable length intake). Bill Eslick tried very short runners
(maybe even shorter than Atkins) and found performance disappointing, he
lengthened his intake runners and gained considerably power.
As with most things, a balancing of characteristics
is called for. Too small a dia tube and you are restricting flow - too
large and you drop airflow velocity. How important it is depends a great
deal on what your operating regime is. Big and short for high rpm, smaller
and longer for lower rpm.
One thing I have found is that it does not matter
if you have an intake capable of producing 250HP at 9000 rpm if your prop load
keeps your engine from getting above 6000 rpm, that intake not only does not do
you any good, but is probably hurting power production at your 6000 rpm
range. NOW if we had constant speed props it would be a bit different
story. You could hold the pitch fairly flat reducing prop load on the
engine and wind it up into the higher rpm ranges (producing increased power)
before loading the prop with more pitch. Or have a gear shifting gear box
{:>).
Ed A
----- Original Message -----
Sent: Friday, March 18, 2005 8:27
PM
Subject: [FlyRotary] Re: Intake
Ideas....
Hi Dale,
20 inch runners are contrary to the experience of Tracy and
Paul. Both found that going from long to short runners made a noteworthy
improvement in performance. The pics I've seen of the
Atkins intake, like Paul's, has fairly short runners. I know
sucking on short straws is easier than sucking on long straws, sucking without
a straw is even easier. Other than allowing a
single throttle-body to feed multiple ports and being an approach to address
possible airflow confusion, I'm questioning the necessity of
runners. I see I'm not the first to raise the
question and maybe the only one who doesn't know the
answer. I wanna know if its just a matter of unquestioned
precedence but not really necessary. It's something
else I'd like to try.
Tom
Dale Rogers <dale.r@cox.net> wrote:
Tom,
It
would _run_ ... but you would be giving away an
important benefit
derived from having 20 or so inches
of individual runners: maximum fill
of the combustion
chamber at 6000 RPM.
I'm building an NA, so *I*
can't afford to give that
away on my engine.
Dale
R.
> ----- Original Message ----- From: Tom
>
>
> Imagine taking their plenum, cutting off the runners, and then
bolting this plenum onto the side of your motor over the intake ports
without using runners. Where the ported ends of the runners appear on the
inside of the plenum you have ported entries into your intake ports. Oh, and
then use an Ellison as a throttle-body. Is this wrong?
>
>> Homepage: http://www.flyrotary.com/
>>
Archive: http://lancaironline.net/lists/flyrotary/List.html
Do you Yahoo!?
Yahoo! Small Business - Try
our new resources site!
|