Inlet manifold length

[090]

Can someone please give me a laymen's version of inlet manifold length formula's . The twin pipe manifold is quite long and I feel it would be better to shorten it up , but how far? I would prefer a mid range bike to a top end (power) bike.
Cheers.

[paul]

long +plenty low down
short +mid to high 
i was told

[Lozza]

there is no laymen's version

[Bamford#69]

Hi,
Iv'e got a short one, and I know your one is longer,(standard) dont shorten yours until you borrow my one , try it  for yourself,

[090]

Thanks guys. I will keep it close to standard then.

[norm17]

whilst I agree with Mr. Lozza that there is no real "simple" formula that is effective it is suggested that 90/rpm(in 1000s) can be used for a four stroke inlet manifold length (result in inches). Likewise other sources state 84000/rpm for the same calculation.
This "length" is from the intake valve head thru to the end of the carb.
This then needs to be balanced against port/tract diameter, divergent angle of the tract, which pulse you are using etc. which all leads back to Lozza's statement that there is no easy formula. Suck it and see appears to be the most effective method.

[Lozza]

nobody ever calls me 'mister'   You could try making a 'trombone' section manifold and test would be my advice   

[pancho]

on a 4  stroke for maximum negative pressure at the inlet valve opening time the entire inlet tract length from valve seat to end of bellmouth is 14 inches for 4000 rpm. if you want to work it out for different rpm, the pressure wave is considered to travell at approx. 1100 feet per second depending on temperature. This of course does not mean maximum torque will be achieved at that rpm because of lots of other factors. Exhaust pipe length for 4000 rpm is at or near 48 inches [regardless of whether a megaphone is fitted.] Please excuse my non metric brain, as i tend to make weired conversions on occasions, cheers wally.        If you're working on one of those strange 2-cycle machines, i'd be mystified!

[Lozza]

Why wouldn't positive pressure in the inlet as the valve opened be far more desirable Resonance in the tract will fluctuate between +ve and -ve

[pancho]

yeah your right lozza must be time i put my old brain to bed cheers wally.

[lukeb1961]

nobody ever calls me 'mister'   You could try making a 'trombone' section manifold and test would be my advice   

Emeritus Professor Sir Lawrence of Newcastle? 

[Lozza]

Wally ,that sounds like  a formula from Phil Irving Tuning for Speed?

I got a long way to go before I get the Doctorate Luke, unless I download one off the net

[TT5 Matt]

make serveral manifolds of various lenths and see what suits your riding style

[sudman]

I have done a lot of real world testing on a dyno jet dyno with single 4 strokes & the intake length is very much over all including the air cleaner boot, so all we tune now is between the carb & the air cleaner with the same effect as changing the manifold length.

[Tim754]

And toss all those above if you supercharge your machine