N/A intake idea, OK or hokey beyond belief?

[Wayland]

Thanks for weighing in, Andrew.  I think that the runner length/airbox on the n/a vanagon was selected for convenience of installation, not performance.  I think that a shorter intake runner length resonates at a higher rpm, not a lower one.

AFAIK, the Vanagon intake manifold box is the same as the A2 NA cars, but the cover is different.

[Vanagoner]

Sorry, need help with "A2", what is that in the U.S. market?
[ and I just noticed you have a Kubvan! what fun.  I found one here but haven't tried to buy it yet]

[Vanagoner]

Ok, apparently the long runner idea is nothing new.  Here is an historic 30" setup-

and the explanation that goes with it-
http://www.chrysler300club.com/uniq/allaboutrams/ramtheory.htm
.
and a good overview of "acoustic supercharging" (with equations)-
http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=466

[Wayland]

Sorry, need help with "A2", what is that in the U.S. market?
[ and I just noticed you have a Kubvan! what fun.  I found one here but haven't tried to buy it yet]

A2=MkII, 85-92

[Rabbit on Roids]

most super long runner intakes are meant for SUPER HIGH rpm use. think of a tunnel ram on a gasser? does it make any torque or power down low? dont think so...

[Vanagoner]

The engine pictured above is tuned for 2800 rpm.  It is in this car-

which is not the paragon of high rpm engines.

[Rabbit on Roids]

still tho, that intake pulls harder at 6000 then any manifold with shorter runners. generally long runner intakes are meant for high rpm use. and just cause that car is tuned to 2800, that manifold still has way more top end than the non-cross-ram intakes. im not trying to disprove you, im just saying that they are generally for super high rpm use where no torque is needed.

[Vanagoner]

I don't have all the answers either, still trying to figure this out. I would welcome information that supports the long runner= high rpm thing, but I think both of us can be right on this one.  Just as 16 1/2" came up on some formulas as a good equivalent to 33", so too 3000 rpm target is also a 6000 rpm target.  Harmonic waveforms occur most strongly in octaves, so we can both be right. 
At this point I'm just trying to understand the benefit of using a long runner vs. a short one.

[8v-of-fury]

Fab up both and try them out See first hand what it does

[truckoSaurus08]

You should make a variable length intake like these.
http://www.autozine.org/technical_school/engine/tech_engine_2.htm

[Rabbit on Roids]

kinda hard to make a variable intake when we only have one intake port per cylinder. those ones were so good because each cylinder had 2 ports.

[Vanagoner]

T-saurus, that is cool, never knew about that (I'm stuck in the '80s).  Looks like variable intake is for n/a engines what vnt is for turbos.
I figured out why the long tubes (from wiki)-
 "When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased."
So choosing a long tube length over a short tube length with the same rpm target yields more force.
8v, right you are.  as soon as cold lets up (have no inside workspace) I'm going to play with a fixed long tube prototype for grins, to see if it makes any difference.  Maybe think about variable intakes or learn to play trombone later...   
Thanks guys, that VIM stuff is mind opening-  looks not so hokey after all.

[Rabbit on Roids]

but one important thing, the fuel and air are not even mixed until its time to fire. so long runners are not going to help mix the fuel/air charge in this case. they may make it a little easier to breathe for the engine tho.

[VWSmokr]

most super long runner intakes are meant for SUPER HIGH rpm use. think of a tunnel ram on a gasser? does it make any torque or power down low? dont think so...

Have to argue that point, at least as to the causes of crap low rpm performance. Look at the runner diameters sizes on the so-called "tunnel ram" manifolds; they're huge! The low rpm air velocity through those manifolds is terrible (carbs or f.i.), but they don't run RV cams with them either. Race stuff.

My German Ford 2.9L gasser has short/straight runners & smoothly hits its factory redline @ 5800rpm, sounding like it's ready for much higher revs. The same basic 2.9L in Ford Ranger, Bronco II,et. al. had intake runners at least twice as long, a bit more torque from 800-2200 rpm, and sounded like it was all out of useful power and revs at 4200 rpm... typical small truck motor, before electronically-controlled valves, dual intake tracts, etc.

The same runner length differences were evident in the EFI Mustang 302 V8s vs. the EFI 302 V8s in the trucks.

Superchargers and turbos take away part of the low rpm advantage of the longer intake runners vs. shorties, but only when they start to cook up some pressure.

In the 1980s Oldsmobile did some experimental multiple-runner-lengths manifolding for their V6 version of their horrible 350-inch V8 diesel, in hopes of broadening the rpm range over which the V6 produced usable torque. It worked quite well, but GM did not figure the extra expense was worth it, and they made more profits selling buyers up to the POS V8 diesel engine. (Oddly enough, the cast-iron head version of the Olds V6 diesel wasn't generally too bad, just never developed & offered to the public in a version with power to match its weight and displacement, only 85hp from 4.3L!)


J.R.
SoCal

[Vanagoner]

From Audi of America's website- 
Variable Intake Manifold
"Depending on engine speed, intake manifolds of different length are advantageous. These requirements can be met by installing a variable intake manifold: at low engine speeds a longer pipe increases torque and improves responsiveness. At higher speeds the shorter intake manifold is effective for high performance. Variability is the basis for an optimum torque curve over the entire engine-speed range, as well as for good acceleration and elasticity."

I agree Kevin that a long, small diameter pipe doesn't seem like the thing to use because it would be like breathing thru a straw.  But apparently if you get it right you turn the intake into a horn, focusing and delivering pressure waves to the engine at the right times, and flow is increased. It is about the energy of sound.  A final illustration of length as a function of pitch (rpm) comes from the physics of music-
"The pitch produced by a pipe is a function of its length, the wavelength of the sound produced by an open pipe being approximately twice its length. A pipe half the length of another will sound one octave higher. If the longest pipe, C, is 8 feet (2.4 m) in length, the pipe one octave higher will be 4ft long, and two octaves above (middle C) will be 2ft long. "