Intake Shape? Why is it not like a Exhaust header?

[JSM]

Why does a Intake manifold have equal length runs straight to a can verses flowing smooth like a header does into one or two exit pipes?

 

Wouldn't it make sense to have the same nice flow the header does?

[thehelix112]

Excellent question. For forced inducation applications I would hazard a guess it is to do with a few things.

 

1. Equal length runners. This is much more important for an intake manifold (IMHO) as unequal length intake runners can lead to differing ram effects at differing RPMs.

 

2. Packaging. Given the requirement 1, packaging an intake manifold with equal length runners that does not use a surge tank (and provide a simple, easy, place for the air flow to be controlled at) with a single T/B would be quite tricky.

 

3. Single T/B or Cost. It is much cheaper to put a single t/b on a car and call it a day than it is to provide individual t/b setup with equal length runners.

 

4. Throttle response. In a performance application, as you are talking, you want the t/b(s) to be as close to the head as possible for throttle response. The interference of the butterfly's (assuming you have them), reduces the ram effect.

 

In N/A applications you do have equal length runners in most individual t/b applications.

 

Dunno if that makes much sense, I'm tired.

 

Dave

[katman]

Uh, maybe because intake velocities are way lower than exhaust velocities? And if you look at a stock intake manifold versus a stock exhaust manifold there isn't toooo much difference.

[naviathan]

I see what you're you're saying. Although a single throttle body feeding equal length runners is a great thought, it's usually not feasable. If you center the throttle body then the two center runners end up curved to maintain length which can slow the air flow to those two cylinders, but at the same time a straight runner would cause the air flow to be faster to those two runners as they would be shorter. Now of course there is a happy medium in there somewhere, but from a mass production point of view it's much simpler to feed all 6 runners from a plenum with a single throttle body. A plenum setup also allows for easier setup of emissions controls, vacuum lines for accessories, PCV circulation, etc...etc... you get the idea. If you check out the ITB setups on here they are much more like what you're thinking. Usually double or tripple bodies feeding 3 or 2 cylinders per. This is more for a performance setup and requires a balance tube and seperate vacuum tank for feeding accessories, but very little allowable for emissions. I hope this helps.

[Tony D]

Have you seen some of the new tubular manifolds on Toyotas? Look like Aluminim Headers with very little plenum, and a throttle body hooked on a "collector" on one end.

 

But to the original question:

The difference between vacuum flow and pressure flow characteristics is a good reason.