Inlet tract length

What is the standard inlet tract length on a Norton 650cc, (that is the length from the valve seat to the needle valve)...?
Has anybody experimented so establish what is the optimum length?

Possum said:

to the needle valve)...?

Click to expand...

:?:

The length that matters would be from the valve seat to the wide end of the bellmouth.

Not sure what you mean by standard. May be different for models with monobloc or concentric, single or dual carbs.
My '64, with 376 monoblocs, has following:
* Valve seat to head flange = 70mm (as best I can measure it with valve installed)
* Spacer = 25mm
* Carb = 75mm
* Bellmouth = 18mm although due to the way they screw onto the carb they only increase the tract length by 10mm (I have 50mm "effective length" bell mouths on mine)
So... total on standard '64 SS is 180mm.
The path to optimising with depend on a number of factors. What rpm to you want maximum torque? My understanding is longer tract lengths lower the rpm at which maximum torque is produced.
Will you have the steel inserts in the head which reduce the inlet tract diameter?
What cam will you have installed. Valve opening and closing times as well as lift will have a significant influence.
Phil Irving's book "Tuning for Speed" (ISBN 0 908031 29 7) covers this subject in great detail.
Cheers
Rob

Phil Irving states that the overall inlet tract should be 90" divided by the RPM. But I can find no info on where in this length is the optimum position for the carny. Hence my reference to needle jet.

"Phil Irving states that the overall inlet tract should be 90" divided by the RPM".

Can't say I've heard that one before, or seen it in print.
Where did ye find ?
And how would that formula work, 90" divided by say 8,000 or 10,000 rpm gives a rather small number !
And if it was that simple (ignoring the 3 zeros) inlet tracts would all be MUCH longer... ??

Over the decades, carby placement has varied all over the place, seemingly with very little difference.
Veteran machines sometimes had the carby at arms length on a v-e-r-y long manifold,
whereas some more recent machines had the carby right on the inlet valves doorstep.
Heat insulation is one reason for a bit of distance...
Concentric and monobloc Atlas's had different lengths of spacer, purely due to these 2 carbs being different lengths themselves, and very little overall spare room to fit them into.

Fast race bikes almost always had more length to the inlet tract, if they could manage it.
Dunno that it makes much difference for road bike use pottering down to the cafe though.
hth

Irving said usually 10-13" on racers, in Tuning for Speed.

Irving, P. (1996). Tuning for Speed, Turton & Armstrong, UK. [bottom of p.142 to top of p.143]
Which states '...90" divided by the RPM in thousands...'

So if 5500 RPM is taken as the average onset of vibration, then the inlet tract needs to be 16-3/8" [90" divided by 5.5)
Or, if max. revs of 6800 is used for the calculation, then the inlet tract needs to be 13-1/4" [90" divided by 6.8], which is close to the figure quoted by Triton Thrasher.

But, that doesn't indicate where in that inlet tract length is the optimum location for the centre of the carby...
Or, the optimum length for street use...

Possum said:

Irving, P. (1996). Tuning for Speed, Turton & Armstrong, UK. [bottom of p.142 to top of p.143]
Which states '...90" divided by the RPM in thousands...'

So if 5500 RPM is taken as the average onset of vibration, then the inlet tract needs to be 16-3/8" [90" divided by 5.5)
Or, if max. revs of 6800 is used for the calculation, then the inlet tract needs to be 13-1/4" [90" divided by 6.8], which is close to the figure quoted by Triton Thrasher.

But, that doesn't indicate where in that inlet tract length is the optimum location for the centre of the carby...
Or, the optimum length for street use...

Click to expand...

As far as I understand, it is as TT already stated, the measurement is taken from tip of bellmouth to intake valve head. Location for centre of the carb is not relevant. That's why it's common to make adjustments using shorter and longer bellmouths.

Rohan
I've attached scans from "the book" - hope it helps




Cheers
Rob

Al already quoted the page number, but thanks.
I'm still pondering if my old early sidevalve slogging along at 2000 rpms needs a 45" manifold !!

Rohan said:

Al already quoted the page number, but thanks.
I'm still pondering if my old early sidevalve slogging along at 2000 rpms needs a 45" manifold !!

Click to expand...

Only if you see some benefit from a resonance effect increasing cylinder filling.

If your 2,000 rpm is not at full throttle it won't work very well anyway.

I think 2000 rpms is actually somewhat beyond full throttle,
these early old bangers were very tall geared,
the engine pully drives the rear wheel direct.
And it took a l-o-n-g time before they figured out how to make an engine rev.

P.S. Where would you put a 45" manifold !
And where would that place the carb ??

But we diverge from 650SS's and Atlas's.

Rohan said:

P.S. Where would you put a 45" manifold !
.

Click to expand...

I'd put it on your bike, rather than mine.

The theoretical optimum length of the inlet tract and exhaust pipe depends on the standing wave relationship - wavelength equals speed of sound divided by the frequency. It is probably only an optimum for a very narrow RPM range. In my own case I know what makes a two into one exhaust work best. The inlet tract is 'suck it and see'. If power delivery is strong and smooth right through the usable rev range - that is good.