Sent: Saturday, December 20, 2003 9:06
PM
Subject: [FlyRotary] Dumb intake
question
Greetings,
OK, I have my dunce cap
on, since I ignored the previous intake lectures.
Say you have TB that
has two butterflies, and each butterfly feeds one rotor (primary and
secondary). There is no connection between the
two rotors. Is the length of the intake runner important for
full throttle operations?
I assume it is, and
recall that racing manifolds can have a short, almost non-existent
runner. I believe Ed used one of these originally, and got
less than exceptional performance. If I recall correctly, it was decided
that it would have been fine at racing RPM, but not 5500 or so, where he
wanted to run.
Assuming it's important,
is there a rule of thumb for the length of the runner for a particular rpm
range? I'm wondering if I can keep my TB in the (turbo free) left cheek,
and get enough runner length to optimize operation at
about 7500 rpm. Probably a silly idea, but my only other option is
still trying to sneak it under the cowl to the right
cheek.
Thanks,
Rusty (I can already hear
Ed screaming <g>)
Sigh! If you would
only come to class, Rusty {:>)
Here is what it appears
like. Since you are using an Open TB with two throats and no connection
between the two rotor, the answer is still YES. For good
performance you do need to pay attention to your manifold length. In
fact, at higher RPMs the appropriate length is even more
crucial.
But, since they are not
interconnected, can not use the DIE equation. However, Here is what the
calculations come out to using just a pipe from intake port out to your
throttle body. I assume from your comment that you don't have a lot of
length to deal with. I further assumed you do want to get some help from
Intake "tuning".
The only way I could see
to keep your runner length down and still get some benefit from pulse tunning
was to calculate an intake length based on a pulse being generated by the
closing of an intake port (the "A" pulse if you had paid attention in class
{:>)) and arriving at the next opening of that same port. That is not
the best possible arrival time, but longer timing - say to the closing of the
port to overcome reversion makes the length infeasible.
So based on that - it
looks like your length from port to butterfly inlet of TB ranges from 13.1"
for 6500 RPM to 11.5 RPM for 7500 RPM. This provides time for the pulse
to be generated by the closing of the port and to arrive after that port opens
again after bouncing back from the throttle body once. So from that
Length you would need to subtract your block to port distance of 2.5" and the
length of your throttle body.
Ordinarily, I would say
that the prop load (with a 2.17:1 PSRU would likely keep you from reaching the
rpm range where this tuning would become effective. But, perhaps with a
2.85 ratio, its within reach.
Hope this
helps
Ed
Anderson