Calculated Accurate Exhaust Size
 

I am fully skilled with normal piston engines and how to tune them to perfection, but have not got a clue about rotary engines.

What I am looking for is a formula to calculate values on exhaust system sizes for wankel engines. Piston engines has formulas to calculate a near perfect size for the engine and how it is supposed to be runned.

This is not a easy question to answer I know. But I'm a perfectionist so a guess isn't good enough for me.
The rotary engine is bound to have a diffrent characteristic then a otto piston one.

As much info as you can give me on exhaust tuning for rotary engines is good, give me rumors and such so I can maybe use that data to draw a conclussion.

Try this site

www.ausrotary.com

I've found much better information for N/A applications in their technical section than here. Just search for exhaust. There are two threads I can remember specifically you'd like. One for the ideal exhaust on a bridge port and another for a street port. Ironically they both need the same thing.

TII :: 3.0 - 3.5
N/A :: 2.5 - 3.0

These are the norms of what people put on their cars. If you're making something really specific or looking for large horsepower (basically anything seriously deviating from stock) then I'm useless.

Most people run one of these 2 setups. Rotaries are seriously restricted by exhaust flow. The Racing Beat full exhaust system has been dyno proven to add ~50hp over stock just bolting it on.

The exhaust pulses are different for a rotary but that should affect only the tuned length of exhaust pipes if you want wave reinforcement.

The diameter of the pipe is chosen to get a specific gas velocity. That is based on volume of exhaust gas which is based on HP.

Your pipe diameter should be directly related to your HP regardless of engine type.

ed

Order a Racing Beat catalog. It has a good general guide for exhaust diameter and length sizing, as well as other helpful rotary engine technical tips.
http://www.racingbeat.com/FRcatorder.htm

Yes, but people normally put too big of an exhaust on their car. Best performance is usually with 2.0" OD on an NA and 2.5 - 3.0" OD on a TII.

the only formula that would work anywhere near perfect would be to measure the volume of the entire exhaust system and know at what rate each component flows (muffler/cat/silencer/etc). i don't think i have ever seen any figures on complete exhaust system volumes.


if you mean port size or exhaust pipe size, i wouldn't bother plugging it into an eqausion because there is no "perfect" method of measuring exhaust flow unless you use equipment to measure the flow itself.

The basic equation that edmcguirk refered to is:

Velocity in Feet/Second = ((CFM Flow Rate / 60) / (Internal Cross-sectional Area in Square Inches / 144))

For those who are not so great with math:
Internal Cross-sectional Area = (Pi * Radius^2)
Radius = 1/2 Inside Diameter of the Tube
Inside Diameter of the Tube = (Outside Diameter of the Tube - (Tube Wall Thickness * 2))

Yes, you need to perform this equation for each of the different cross-sectional areas in the exhaust system if you use pipes of different inside diameter. Keep in mind that this is only a basic equation, and there are many variables, but this is about as close as you are going to get without an engineering degree or a very expensive computer program. The engine type doesn't make any difference, as this is a simple gas equation, although you could argue that the higher exhaust temperature of the rotary engine does affect the internal drag some because the Mach number changes with changes in temperature.

Rules of thumb for non-engineers:

CFM = Flywheel Horsepower * 1.5

Max recommended velocity for exhaust = 250 fps

Max recommended velocity for intake = 440 fps

* Evil Aviator is tired and is not responsible for errors in any of the above equations, so take everything written with a grain of salt, just like anything else you read on the internet. ;)

well with a piston engine the variables are not nearly as diverse hence i said what i said but with turbos and the exhaust chamber you have too many variables that will throw off air flow, not to mention the flow through the turbo. in a piston non force fed induction setup you have the exhaust ports and a tube, that's it, so there is very few variables and the calculation may be a bit closer to the actual flow rate.