I might be stupid for asking this and over thinking things alittle bit, but i gonna fabricate a header. I know that on pistion engines the length of the primaries and the diameter of the pipe have an effect one the power band and the amount of peak torque produced. i have been looking at off the shelf headers for my 87 GXL and all the brands seem alittle bit different in their approach, so i wanted to know are rotarys sensitive to the same variables or am i just putting to much thought into a simple concept?

 

what would make you think they are not effected by length and size?

 

the short answer is ... yes. output on a rotary is affected by all the variables you mentioned. you probably need to read up a little before you get to bending, cutting and welding. it'll be expensive experiementation regardless, but even moreso if you don't arm yourself with some background concepts and knowledge.

search "exhaust" using GtoRx7 and Rotarygod.

 

okay you want to get use of scavenging in your exhaust system, primarily the header by a low pressure area created in the exhaust system that can be used to "draw" the exhaust out the chamber. I know how to explain how reversion works on a piston engine but I'd have to look into it further for a rotary. BUT if you reduce reversion you increase volumetric efficiency and power so you basically build your header for RPM range.

now you want low power? then you need to utilize equal length tubing with a maximum variation of +/- .5" the bends (as long as they aren't crush bent or 90 degree bends) won't affect the flow, the length and tubing size will. By going from a 2.5" (outer diameter) primary pipe to a 2.0" (outer diameter) primary pipe you increase the exhaust velocity speed providing more low end power since you want the smaller tubing to maintain a high enough velocity when there is less exhaust volume at lower RPMS.

have you considered thermal coatings or wraps? With thermal coatings you can use those to retain heat in the exhaust system since as the gases cool they loose velocity, velocity increases scavenging at high speeds and reduces reversion (reverse flow at low RPM) and not to mention reduce underhood temp so you can get cooler air into your intake for MORE POWA! Thermal wraps do a better job but I wouldn't recommend them for a street vehicle since they tend to retain moisture and will rot your pipes alot faster.

smaller pipes better low-mid range power, so basically from going to a larger pipe to a smaller pipe you move your power curve more over to the left hand side of the graph (X axis is RPM, Y axis is exh velocity or power) steeper slope.

you generally want to keep around 240-260 feet per second when an engine is operating at peak torque.

to calculate the pipe size you need there is a formula

1. desired RPM at which max tq is to be produced
2. multiply CID of one cylinder (or rotor)
3. divide by 88200
the result is cross sectional area of the pipe required

4. now divide that by 3.1412
5. take the square root
6. multiply by 2
the result is the necessary inside pipe diameter.

by changing the pipe size you shift the torque curve around by going from a larger pipe to a smaller pipe your causing the engine to take away torque from one RPM and add it to another.

increase the length, adds tq below to engines tq peak, and vice versa.

the lower RPM range the header is required to work the longer the pipes need to be.

you can also use a collector to produce more tq below the peak but will decrease tq after the torque peak, typically you want your collector to be 3/4" bigger than your primary pipe diameter.

hope this helps!