Tunable intake manifold question

[Mazda Ebrahimi]

The chamber volumes before the throttle DO make a difference.  Most 
modern production intake systems are based on a three degrees of freedom 
model consisting of (in order as air enters the engine): the snorkel, 
air chamber, zip tube, plenum, runner, and cylinder.  The stuff above 
the throttle is mostly effective at lower RPMs (let's hope they are not 
restrictive at higher speeds!).  You can treat every volume and its 
following tube length (as in the plenum and the intake runner, or the 
air chamber and the zip tube) as a spring mass system, where the mass is 
the mass of in the plenum plus the runner, and spring rate is:
	K = nPA^2/V  where n is the polytropic constant, A is runner 
cross sectional area, and V is the volume.  The resonance frequency of 
such a spring mass system is:
	
	f = c*sqr(A/LV)/(2*PI) where c is the acoustic velocity.

to tune the system, you want the frequency of your pulses (derived from 
engine speed) to match this number at your target speed.


Having said all that, most of the equations are from some hand notes I 
took while I attended a class on cam design.  Since I don't design 
intake systems for a living, I don't recommend you actually believe what 
you just read without further investigation.  I'm sure there are plenty 
of people here with more experience that can shed some more light on the 
subject.  Enough with the fine print!!

Best Regards, Mazda



Mark Eidson wrote:
> 
> I believe tuning only works on the runners between the plenum and the valve.  me
> 
> At 03:38 PM 2/5/97 -0500, you wrote:
> >I have examined some of the plans for the variable length/variable volume
> >intake systems used by Honda (in the Legend, Vigor and 1st & 3rd gen.
> >Integra). All of their variable intake systems adjust the length or width
> >of the runners between the intake manifold and the ports. I was wondering
> >if it would be worthwhile to design a variable length add on (for a car
> >without any variable stuff) to the intake system which would fit between
> >the air intake scoop and the TB. Adding a variable intake before the TB
> >appeals to me because I only have access to an electronics lab (and some
> >hand tools).
> >
> >
> >Thanks  in advance.
> >
> >
> >Wen