Intake manifold design question - Runner diameter and length

[ktm]

I have been doing quite a bit of reading over the past 2 weeks about intake manifold design. The heuristic regarding the design is that long, smaller diameter runners help low torque due to inertia (high velocity and large volume of air) while short larger diameter runners help high rpm torque. I am not concerned at this point about frequency tuning.

 

I read Justinolson's, Monzter's, Rontyler's, and Prox's threads on intake design as there are some GREAT discussions contained within. Ron's original intake looks like it flows quite well and provides nearly even flow distribution to the runners. He is using 1.5" ID runners that are 6 inches long (Ron, if you read this, is that 6 inches from the flange to the front side of the plenum or is that 6 inches TOTAL length to the backside of the plenum). BRAAP stated that the stock manifold runners average approximately 7.5 inches in length and they measure approximately 1.25 ID.

 

I have looked at some aftermarket RB25DET and 2JZ intakes and both of them utilize short, large diameter runners, especially the 2JZ intake.

 

My question is about a blend of the two: large diameter runnger (1.5 ID) coupled with a longer runner such as the stock length of 7.5 in. Do you get the best of both worlds? If not, what would be the downsides?

[RTz]

I am not concerned at this point about frequency tuning.

 

You are, you just don't know it yet The length of the runner is all about frequency tuning. The length determines the timing of the returning pulse. Longer puts the return at a lower RPM, at the expense of high RPM restriction (function of CFM though a given size).

 

(Ron, if you read this, is that 6 inches from the flange to the front side of the plenum or is that 6 inches TOTAL length to the backside of the plenum).

 

6" from face of head to inlet of runner (including the velocity stack).

 

Keep in mind, my prototype was built for a show car first and foremost. The difference in performance between a 6" runner and a 7" runner would be minor in this case, but the effect on appearance, with this particular manifold, would be pretty dramatic. A compromise was made at 6". Otherwise, I think a 7" runner would have been a better choice.

 

Also, in retrospect, I believe the 1.5" runner was a little oversize for the cam we settled on. A 1.4-ish runner would have bolstered the bottom-end with no loss on the top (and quite possibly would have shown a gain).

 

My question is about a blend of the two: large diameter runnger (1.5 ID) coupled with a longer runner such as the stock length of 7.5 in. Do you get the best of both worlds? If not, what would be the downsides?

 

Its commonly taught (and mirrors my experience) that runner diameter is the single largest influence on where the torque peek falls. Runner length, in normal scenario's, seems to have more impact on the area under the curve. In other words, how flat or peaky the curve is.

 

Keep in mind manifold design is both art and science, and a person could invest an entire career in manifold design. Its not always black & white... and the 'intuitive choice' doesn't always prove so.

[proxlamus©]

http://www.velocity-of-sound.com click on Intake Calculator and play around with different lengths and diameters.. it will change your peak RPM rather dramatically

 

=)