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Re: [T3] too much mind time (runner lengths)

Hey Toby-

> To further complicate things, isn't runner length only good for a certain
> rpm range?

Exactly!  It's a phenomenon just like resonance, which depends on a certain
frequency: in our case, engine speed...

> I believe the longer the runner, the more of a boost in low-end
> torque will be given to the engine (within limits, of course) whereas a
> short runner will benefit higher rpm's.

Yes and no.  With everything else equal, that is exactly correct.  However,
you can set up the runners to hit a given RPM band using the 2nd or 3rd
reflection of the wave, not the 1st (kinda like an overtone series in
music...).  So, you can shorten or lengthen the runners and end up with the
same band in the end.

? Plenum size I'm not sure about but
> a large one is nice for that 'reserve' of air when the throttle gets
> :)

There are certain "rough" ranges that are good.  Too big isn't good either.
I forget the numbers offhand, but from memory I *think* it goes something
like this: in general, it is about 100% of applicable cylinder volume for a
2-cyl plenum, 80% for a 3-cyl plenum, and 60% for a 4-cyl plenum.  You
should increase or decrease it slightly depending on your goal RPM band,
too.  Unlike runner length, I don't know of pretty, precise formulae to
determine plenum volume.  I know the rules-of-thumb are something like this,
but I'm not positive of the numbers themselves... I'd have to check...

> I've only read a little about this phenomena and I really need to read
> as I'd like to build my own self-tuning system like some modern cars
> have...soon as I get my other 'wanna-doos' done ;)

Self-tuning?  You mean variable runner length?  Those are trick...

Take care,

>     Toby Erkson
>     air_cooled_nut@pobox.com
> >-----Original Message-----
> >...Whatever the case, it really doesn't matter.  For a low-performance
> >that is restricted in other ways, 1/8" or even perhaps 1" difference is
> >big deal.  You're better off changing the exhaust and (somehow) enlarging
> >the throttle body before tuning the runners just right.
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