General exhaust tuning guidelines/tips

[Fidelity101]

ou want to get use of scavenging in your exhaust system, primarly 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 effeciency and power so you basicaly 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.

[PercentSevenC]

I think it would be good to link to the research Racing Beat has done on the subject:
http://www.racingbeat.com/Tech/Tech.Exhaust.2.htm

I'm not sure about the collected diameter you posted, though. That would indicate a collected diameter of 2.75" for a typical 2" primary setup, which is big. 2.5" OD is the largest I would go for a street-driven two-rotor, preferably 2.25" or even 2" depending on your porting and so forth.

[Fidelity101]

Originally Posted by PercentSevenC

I think it would be good to link to the research Racing Beat has done on the subject:
http://www.racingbeat.com/Tech/Tech.Exhaust.2.htm


I'm not sure about the collected diameter you posted, though. That would indicate a collected diameter of 2.75" for a typical 2" primary setup, which is big. 2.5" OD is the largest I would go for a street-driven two-rotor, preferably 2.25" or even 2" depending on your porting and so forth.

Awesome link!

the particular collector above was suggested for another board member on his application. so depending on your power goals it will be different.

Just because you have a turbo doesn't mean you can just put any exhaust on, it does make it easier for exhaust tuning but when your not in boost your exhaust velocity is low but then again if your not in boost your probably not driving the car very hard at all.

I will post up a chart later that depicts what diameter you should use for your HP goals.

[proz07]

i remember looking into this a littlebit but what about say a turbo setup can you use this there too?

z

[drewski86]

With a turbo you really just want to get the exhaust out as efficiently as possible. You don't want to go any smaller than the turbo exit diameter. Pre-turbo most use 2" OD pipes and keep runners equal length. The thing with turbos are you can always raise boost to overcome poor exhaust.

It is also optimal to have the header run out a few inches from the motor before the first bends and to keep the bends smooth. There is a lot of theory that floats around about exhaust design. Some speak highly of expansion chambers while others say it won't work since it is a 2-stroke design. Some say run the primarys back to the rear diff. (>90") before merging for a road race setup, while all of the ITS (stockport) and EP (streetport) cars I've seen run ~24" primarys.

[Ender]

Originally Posted by drewski86

The thing with turbos are you can always raise boost to overcome poor exhaust.

You can try. When I had a bad exhaust set up on my TII I could crank it up to 12psi, but it would fall to 6psi within 2 seconds of reaching peak boost, so the extra boost was essentially rendered useless just because of restrictive exhaust.

[drewski86]

Originally Posted by Ender

You can try. When I had a bad exhaust set up on my TII I could crank it up to 12psi, but it would fall to 6psi within 2 seconds of reaching peak boost, so the extra boost was essentially rendered useless just because of restrictive exhaust.

What do you mean by "bad exhaust"? I would say you had other issues than just bad exhaust.

[Ender]

Originally Posted by drewski86

What do you mean by "bad exhaust"? I would say you had other issues than just bad exhaust.

It was restrictive. As soon as I replaced the muffler that was in the middle of the system with a piece of flex pipe it boosted fine.

[drewski86]

Originally Posted by Ender

It was restrictive. As soon as I replaced the muffler that was in the middle of the system with a piece of flex pipe it boosted fine.

I guess everything has limits.