Intake runner length and pulse wave tuning

[Thom]

Hello,

As this subject has apparently not been much covered yet on here, I was wondering what the knowledgeable PH collective would have to say about the theory quickly depicted on here.

Without being an expert I'm surprised that runner diameter & possible taper may have little to no impact?
Since the speed of sound varies with intake temp, with what intake temp do most OEM tune their intakes to?

I'm not sure but it may seem logical that the higher the pulse wave mode tuned to (the 6th in the page linked above), the higher the loss of "load", no?

Any input greatly appreciated.

Edited by Thom on Tuesday 12th March 15:43

[jimbob82]

forgive me but what do you mean when you say "load" ?

taper and diameter have little effect IIRC. length is the "key" variable in this case. if you have a pipe 10 feet dia or a pipe 1 inch dia the speed of the wave will be the same, the wave is "bigger" because of the increased diameter but the speed and distance it travels is the same.

the difference in the speed of sound isn't great enough at normal temperatures to concern manufacturers, I wouldn't have though it was feasible or cost effective to tune them for different continents/temperatures, I believe they just tune to a standard 25dC s.of.s or something along those lines...

[Thom]

The intensity of the (sine) pulse wave isn't related to its length, but if longer runners are usually said to improve low/mid range torque compared with shorter runners, then there must be a loss of "load" (or ram effect) even if the positive pulse wave reaches the intake valve exactly at the moment it opens?

What I suspect but am totally unsure of is that peak torque may diminish the higher the resonant mode corresponding to peak ram effect.

Take a 2V engine running a given camshaft.
Let's say that with a long runner intake manifold it delivers peak torque at 4k rpm for the 3rd positive pressure wave, and that with the short runner intake manifold it delivers peak torque at 5k rpm for the 4th positive pressure wave. Are both peak torque values going to be necessarily "about" the same?

[jimbob82]

Thom said:

...if longer runners are usually said to improve low/mid range torque compared with shorter runners, then there must be a loss of "load" (or ram effect) even if the positive pulse wave reaches the intake valve exactly at the moment it opens?

there are more qualified people here than me to answer your questions, but, I think your confusing inertia charging with runner velocity...though you may not be but that's how it reads to me

As I understand it, a longer narrow runner increases low/mid range torque whether it's tuned or not, this is because the velocity is increased as the valve opens and the piston moves down the bore. tuning the length increases the effect. The old 1725cc Rootes engine as tuned by Holbay had large valves and seats with a belled out throat area but a restrictive port and runner to achieve this increase in velocity, AFAIK they weren't tuned, just restrictive.

Thom said:

Take a 2V engine running a given camshaft.
Let's say that with a long runner intake manifold it delivers peak torque at 4k rpm for the 3rd positive pressure wave, and that with the short runner intake manifold it delivers peak torque at 5k rpm for the 4th positive pressure wave. Are both peak torque values going to be necessarily "about" the same?

I would say that, with the shorter runner @ 5krpm the runner would be wider/fatter to compensate for any loss of load that may occur, though I think the "loss" would me minimal anyway depending on the type of manifold/plenum used.

I think the 5krpm tuned setup would produce maybe a little more torque, albeit not much. But if my thinking is correct, the increase in piston speed would increase the pull and as such increase the velocity.

Someone please correct me if I'm wrong...

[Thom]

Sorry for the poor wording - I actually make the difference between air flow and pulse waves.

jimbob82 said:

As I understand it, a longer narrow runner increases low/mid range torque whether it's tuned or not

This sounds like a bit of a contradiction to me, at least from what the Maths are telling me, which is that "longer" runners will feature in say a [2000,5000] rpm range more points of harmonic resonance than "shorter" runners for the same rpm range, while "shorter" runners will feature more harmonic points in a higher rpm range, say [4000,7000]. All other parameters being equal, of course.

If we assume that there is no loss of "load" (or whatever it actually is) between say the 4th time the pulse wave reaches the open intake valve on the long runner set up, and the 15th time the pulse wave reaches the open intake valve on the short runner set up, then why don't both set up make the same torque at the same RPM, if we assume runners are not tapered?

Edited by Thom on Thursday 14th March 14:17