Inlet tract length

[ace124]

On my 1756cc with standard cams im running 45DCOE's 36mm chokes on a 130TC inlet manifold suitable modified to accept 45's. I run the longer weber push in ram tubes and a meshed airbox on backing plate filter system, like ITG type.
Ive been playing with an inlet tract length formula: Optimal inlet length (In) = (90 x 1100)/ RPM ( for max torque).
Stock 131 2L cams in the 1800 seem to peak at around 4000RPM so that would give me an optimal inlet tract length of: 24.5".
This is way longer than what i have, no doubt due to the short 130TC manifold.

Would adding length to the ram pipes compensate for my short inlet tract length?
Does this formula calculate from the edge of the ram pipe to the inlet valve?
(In which case i can increase the ram pipe length)
My torque curve is pretty flat, but given i use more low / mid range torque, rather than top end, im a little concerned im giving away much torque in the required area by running the short manifold.

thoughts please.

[Guy Croft]

Sorry for the slow reply!

This is way longer than what i have, no doubt due to the short 130TC manifold. Yes, those type of calculations can lead to rather 'bizarre' predictions for rampipe length. They may be right - who knows? Only dyno test will show.

Would adding length to the ram pipes compensate for my short inlet tract length? Yes, the effective inlet (tract) length is manifold and port (to valve head) + carb (or throttle body) + spacers + rampipe and for a given torque curve you can run short manifold and long rampipe or vice-versa and it won't alter the torque characteristic. The reason we must consider the total length is that the tract filling is a function (in part) of the pressure difference between the valve throat and atmospheric at the rampipe end. This is true whether the rampipe is inside an airbox or expsoed direct to atmosphere. Naturally the airbox - if fitted must be at atmospheric - which it won't be if the feed pipe/duct to it is too small. It is important to note - in addition - that the length of the ex primary pipes has an equal bearing on this.


Does this formula calculate from the edge of the ram pipe to the inlet valve? I don't know but it should..

My torque curve is pretty flat, but given I use more low - mid range torque, rather than top-end, I'm a little concerned im giving away much torque in the required area by running the short manifold. On a given engine (ie: the same overall setup) a longer inlet tract will give stronger mid range torque and chop off peak power and a short tract will do the exact opposite. I did extensive dyno test on this when refining the GC NHRA 2 liter 8v in the 90s.

GC

[ace124]

Thanks Guy,

I'll be making some variable length ram pipes, and booking some dyno time at a friends workshop to see if there is much in it.
thank you