Intake Design Exploration

[vex]

Originally Posted by RotaryProphet

Rotaries aren't that special; treat them as a two stroke two cylinder of stated volume (ie, 1.1 or 1.3 literss), or as a four stroke engine with twice the RPM. Port timing info is available, and with all of that info, you should be able to calculate runner size and length and plenum volume, as well as throttle body size for your given peak torque/horsepower point.

So Plenum is only concerned with volume and not geometry?

As an interesting note, by using exhaust and intake runners a couple of inches longer on one rotor than the other, each rotor will have a different peak power point about 500 or so rpm apart, which leads to a wide peak power. Helpful if you have a particularly peaky motor like a P-port or a big bridge.

But would that put a moment on the E-Shaft and cause additional wear or are the forces congruent with doing that negligible?

[RotaryProphet]

Originally Posted by vex

So Plenum is only concerned with volume and not geometry?

In as much as flow patterns are concerned, geometry is important, but as long as it's not shaped so as to -prevent- flow, you should be alright.

In an application where the throttle body is on the side of the plenum (facing forward in a rotary application), it's best to use a plenum four to six inches longer than the distance from the front of the first runner to the back of the last runner, and taper the plenum the entire way. This helps the air "slow down" earlier in the plenum instead of wanting to slam into the back wall, and helps the front runners breath. In a setup where this isn't done, the rear-most runner tends to ingest the most air, and the front-most one (from the throttle body's perspective) tends to ingest the least.

In a side-facing throttle body, the best solution would be a setup that tapered in both directions, with the widest portion in the middle, and some sort of diffuser to help the air with it's right-angle turn into the plenum, going either right or left. However, in a rotary specifically, since the middle ports ingest less air anyway, you can get very good results with a simple tube with a throttle body stuck on it, and let the "bad" shape direct the majority of the air into the big ports, where they're needed.

Also it's important to bell-mouth your runner entrance from the plenum, or better yet, use short velocity stacks actually sticking into the plenum.

Originally Posted by vex

But would that put a moment on the E-Shaft and cause additional wear or are the forces congruent with doing that negligible?

Considering that only one rotor is ever firing at a time anyway, you've always got more force (significantly) from one rotor than the other. The difference of maybe 10 horsepower at max between two rotor firings is negligible in comparison. No different than having a slightly low compression rotor and a good one.

It's really much more useful in V8 applications, where each pair of cylinders 360* off from each other is setup with peak power 500 RPM off from each other, creating, say, two cylinders making peak at 4500, two at 5000, two at 5500, and two at 6000, creating a very wide power curve. Obviously you're trading a reasonable amount of peak power (up to maybe 30hp) for this much wider band, but in situations where that's desirable (notably rally and drift racing, and some road racing), this is a good way to help. To really pull it off you need individual cylinder fuel and spark control, however.