its a tuners trick...
one, remove exhaust, completely, so it doesnt affect any exhaust flow characteristics.
two, make a really long manifold.
three, start trimming bits off manifold until you feel it pulling at a desirable RPM. this entails a LOT of test rides.
four, once the intake length suits the intakes port times, you can design a tuned exhaust to suit that rpm.
the two effects together go hand in hand and boost performance.
it has nothing to do with letting fuel in faster or not, its all to do with pressure waves that reflect up and down the manifold due to the closing and opening of the piston port. get the length to suit the timing and the RPM. and then the fuel gets in faster regardless of the length. as its already flowing towards the intake port as the port opens. for a certain intake duration, theres a definite length that works best, at a specific rpm (or two, due to harmonics)
its like tuning a trumpet in a way...
i suppose i have to link it again...
http://www.amrca.com/tech/tuners.pdf
Intake-tract “tuning” will be vitally important no matter what kind of time-area is
provided at the port window, and it is all too easy to get the pulsations out of phase with
the piston by altering the intake timing. All alterations in intake timing should be
followed with a careful check to determine if matching alteration of the intake tract
length is not also required. Although this kind of work should be validated by actually
running the engine with a stub exhaust attached, as outlined elsewhere in this book, a
preliminary check may be run mathematically, using the formula for finding the resonant
frequency of the necked flask formed by the crankcase and intake tract provided in the
chapter on crankcase pumping.
and somewhere else...
And that effect will be unmistakable, for the engine will pull very strongly when
it comes “on the pipe”. You will also find that intake pipe length can move the stub
exhaust-equipped engine's power peak over a very wide speed range. My friends at
McCulloch, who acquainted me with the stub-exhaust testing technique, tell of tests they
have performed with an engine having an intake period of 120-degrees, and with pipes
ranging from 5- to 9 ½-inches in length they were able to move the power peak anywhere
from 3000 rpm to 8000 rpm. The 9 ½-inch length gave substantially the same power at
3000 rpm and 4000 rpm; at 5000 rpm, an 8 ½-inch was best; at 6000 rpm and 7000 rpm
the same power could be had with either a 9 ½- or 6 ½-inch length; and the same was true
at 8000 rpm, but with lengths of 5 ½and 8-inches. From these tests, it was determined
that the best compromise length for the entire range 3000-8000 rpm was 8-inches; a 5 ½-
inch length was slightly better for the range 6000-8000 rpm. It should be understood that
these lengths only apply to this specific example, and would vary with any changes in
intake timing, crankcase volume and intake tract diameter.
read that last bit twice!
the actual length will vary depending on engine, capacity, and any modifications made, etc etc etc... so you have to do your own experiments...sorry
