Intake Design Exploration

[PercentSevenC]

Here are some more relevant posts by rotarygod:
http://www.rx7club.com/showthread.php?t=94362

Originally Posted by rotarygod

I have a custom built aluminum manifold on my 2nd gen. It has a 100 cu in plenum volume. The intake runners are slightly larger than the lowers but there is a taper down to their size. This is done to broaden the torque curve since the air accelerates as the volume decreases. I also have a 75mm Mustang throttlebody on it with my blowoff valve machined about a quarter of an inch in front of the throttle plate. This engine was designed for big power but still be street drivable. I'll have to post some pictures later this week. The mathmatical formulas are hard to apply to a rotary. Keep in mind that on a piston engine the air is stagnant longer in the intake runners than they are in a rotary. On our engines the intake ports are only closed for a very short amount of time. On a piston engine, unless it is a 2 cycle, the air in the ports is not moving until every other time the piston is at the top. Because of this the effect of plenum volume changes in relation to piston vs. rotary. A bigger plenum volume will raise the torque peak. Port runner length is also dependent on port runner size. The effects are due to a volume/velocity relationship. Theoretically peak power (n/a) is achieved at whatever rpm the engine is at when air velocity entering the engine is at .6 mach (60% the speed of sound). This is however offset by runner length as a runner of too much length can kill power. Its almost too complicated for me to get into without writing a huge book (already several out there on this subject) or giving a night school class. I'm afraid something I write will sound contradictory to something else. Lets just say my setup was trial and error and it works damn well! Oh btw a pp engines intake is always open. When the apex seal crosses the port it is briefly open to 2 chambers. Therefore it never closes.

Fred