not just another tuned runner question... question on theory

[sam85iroc]

Hi all,
I'm new, so nice to meet you! (I hope) and sorry in advance for using the English units of measure

So here it is- we are all familiar with the eq. 84000/rpm=length, but I wanted something more accurate so I did alittle brainstorming and please let me know if I'm off my rocker here...

now, the theory is the valve closing causes a shockwave that reflects up and down the runner at the speed of sound and ideally the valve is just opening when the wave arrives.

speed of sound at 130*F = around 1200fps
crankshaft rotation per full cycle= 720 degrees
example cam intake duration: 250 degrees
example desired torque rpm: 3,333 (for easy calculations)

the amount of time the wave has to travel is the time between the valve's closing in one cycle and it's opening in the next cycle, or "total degrees minus duration" --> 720-250=470 for example

and at 3,333 rpm each degree takes 0.00005sec
thus it takes 0.0235sec to rotate the crank 470*
and at 1200fps, sound can travel 28.2ft in 0.0235sec
so the first harmonic wave would be 14.1ft (wave travels up then back down covering 14.1 * 2 = 28.2

working thus, i crunched the numbers to get the following equation:

x=24a(50/b)
where:
x= ideal runner length
a= 720-cam intake duration
b= target rpm

All good so far??? pracically like the 84k/rpm=lnth but with cam variable..

question 1: ^^^assumes we want the wave to arrive at valve opening.. do we want it there right when the valve opens or do we want the wave to arrive close to the end of valve duration to push in that last bit or at max lift for the highest gain???

question 2: what duration numbers should be used?? typical listed duration value of @0.050in seems quite "open" still.. my gut tells me to use duration @0.006in, at which point the valve is much more "closed"

any input is appreciated. and sorry again for the inches, feet, and F*

 

[Lou Scannon]

Well, you do have to take into consideration the number of cylinders sharing a common plenum.
Also keep in mind, tuned port length is a double edged sword. It helps in a narrow tuned rpm band (can have higher harmonics also, I suppose), everywhere else it hurts.

 

[GregLocock]

You could probably answer question 1 by studying the torque curve of an engine with known geometry. This will also give you some idea of the damping of the resonance.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[sam85iroc]

GregLocock, thanks, that's a great idea!
Hemi, I totally forgot about number of cylinders.. back to the drawing board..

 

[BrianPetersen]

I believe the effect that you are talking about - the reflection of the wave while the intake valve is *closed* into the next cycle of the engine - is secondary in comparison to how the runner reacts while the valve is open and drawing in air, especially when you consider the interactions with other cylinders.

What you want is for the exhaust system to generate a negative pulse that travels through the cylinder to the intake runner during the first bit of intake valve opening, then that travels out the runner to the plenum and gets reflected back to the cylinder as a positive wave which in turn increases the velocity in the runner and gets reflected back to the plenum *again* as a negative wave, and that gets reflected back to the cylinder *again* during the time between BDC and intake valve closing.

The last bit of intake runner before the intake valve has to have a high velocity to increase the "ramming" effect, and this is not quite so inclined to produce peaks and valleys as the resonance effects are.