1badFB

iTrader: (1)

I have read that the rotary motor can benefit from the scavenging effect. Basically this requires the individual exhaust pulses to remain seperated for as long as possible before the exhaust is combined and enters the muffler. Racing Beat headers claim that the scavenging effect is good for 25-30 hp. Is the racing beat setup optimally configured, or would a custom dual exhaust where each exhaust port has its own tailpipe and muffler be more effective. I am just thinking about my options when it comes to the exhaust system and i want to get the most out of it. I had the idea of using a racing beat header, but instead of letting the 2 exhaust pipes from the header join, i would just run 2 single pipes back to 2 seperate mufflers. Would there be any advantage to this or should I just run the regular Racing Beat setup? I know that if I wanted to use a presilencer, or 2 that this may not be the best setup, but im not tooo worried about having them if this setup would have any advantage over the Racing Beat setup. Any input is appreciated.

85rotarypower

If you go custom, you will NEED to know what you are doing. I've heard bad things about uncollected exhaust on rotaries. Its actually counter productive. With an uncollected exhaust, there is no scavaging. Go with the racing beat road race system. I believe its called that. Its a dual exhaust all the way up to the muffler where it collects. It offers the biggest performance gain of any system I have looked at. I don't personally have the system or even any racing beat parts at all, but I have heard nothing but good things about racing beat systems.

rotarygod

iTrader: (1)

A true dual uncollected exhaust system works very well on stock to mildly ported rotaries. My personal favorite system is the long primary (collects at the back near the muffler). If porting gets large this is the best way to go. Too many 2nd gen guys think huge ports and dual exhausts work great together. Um, no. Show me a bridge or peripheral port engine running a dual exhaust and I'll show you a car down on power. If your engine is stock, almost any system will benefit you. Just get that damn cast iron box off of the side of the engine! The exhaust is probably the best way to get lots of power relatively easily.

LongDuck

The idea behind exhaust gas scavenging has more to do with exhaust gases that are passed into the intake charge to help with atomization and pressurization of the pre-compression air/fuel mixture - in the classical sense.

I think what you're referring to here is 'scavenging' of the exhaust gas pulses that are emitted by the two ports, exactly anti-phase from each other. In a dual exhaust setup, you have ZERO scavenging effect since the pipes never join. In a scavenged system, like the Racing Beat headers for 12a and 13b, the joint of the 2 head pipes is such that the tube lengths are equal between either port - this allows for each exhaust pulse to 'stack' one behind the other, and as the air velocity slows as it moves down the pipe, this helps to reduce constriction at the junction of the 2 pipes. Keep in mind that you're reducing constriction without reducing muffling effect and still keeping noise at a minimum.

As each port is opened, the high velocity, quickly expanding gases exit the port and are routed into the headpipe. When the gas volume stabilizes and cools, there is no pulse coming from the alternate exhaust port. The next exhaust pulse comes from the other rotor, and you have a slight pressure drop that is quickly filled by the rapidly expanding gases from the 2nd rotor. This continues throughout the powerband, since you're isolating the gas pressure spikes as much as possible. Some exhaust system theories suggest that this timing of exhaust gas pulses helps to 'extract' the gas from the opposing exhaust port, but this is unlikely, since the exhaust gas is under it's own 'power' due to burning and expansion. More than likely, it's just being vented into a slightly reduced pressure environment at the exact moment that the exhaust port opens, helping to reduce constriction.

Does it work? Definitely. It is the single biggest performance improvement that you can make to your car, and should be the first change that you put together, even before intake, which is the 2nd thing that you should do. HTH,

karism

Speaking of scavenging..

Have any of you ever though about building an expansion chamber exhaust?
I think it will help alott,the only problem would be to fit it underneath the car.
Anyone agree?

IanS

iTrader: (4)

What is expansion chamber exhaust? Got any good links to explain?

Feds

A racer friend of mine claims 5 foot primaries is the way to go. I believe him as his stock port/stock FI 13b is putting out around 180 hp.

karism

Ians:
Expansion chamber exhaust,are used on 2 stroke engines(In dirt bikes,jet ski`s etc)
It is used to help the engine ,it ads a big amount of scavenging,seeing that it pulls the exhaust gas out.On an Rotary with alott of overlap,it will allso help pull in the airfuel mixture into the combustion chamber.

Read more on it here :http://auto.howstuffworks.com/question636.htm
Hope this helps you to understand it more?

Karis

IanS

iTrader: (4)

yes I understand it more now. That would be a very interesting idea to make it work. From the link provided, it showed a pic of one for a dirtbike. Looks pretty big. I am thinking it would be REAL BIG under your car. Of course, if the shape didnt change the outcome, then couldnt you go with an oval shape instead of round? That would at least help with the clearance issues. My ideas

Ian

Jeff20B

iTrader: (13)

A rotary is not a 2 stroke piston engine, so I doubt it would help us as much as is helps dirt bike engines. Remember that our exhaust pulses are strong enough to drive large turbos.

Here is a question for your exhaust experts how about scavenging on a 20B? While we're at it, how about for a single rotor engine? Heh.

REVHED

I think that also has a lot do with the fact that a rotary flows as much as air as a piston engine twice it's size.

REVHED

Are you on crack? Exhaust gas dillution of the intake charge is BAD. It reduces volumetric efficiency by taking up space that could otherwise be filled with fresh mixture as well as making the mixture harder to ignite.

No, inertial scavenging still exists in an uncollected exhaust. Towards the end of the exhaust cycle gas flow through the exhaust port slows down but the high velocity gasses present in the exhaust system help pull the remaining gas out through the port. That is scavenging in a nutshell.

I don't know if you're trying to describe pressure wave tuning but that made absolutely no sense whatsoever. Pressure wave tuning is based on the positive and negative pressure waves that travel back and forth between the exhaust port and collector and which help propel exhaust gasses through the exhaust. Changing the length of the primaries tunes the rpm at which it has the most effect.

LongDuck

REVHED - I am not on crack. Thanks for the critical analysis.

karism

Mmm,i see your point Jeff.
It isn`t a 2 stroke engine,but there ia quite a few things on both engines that are similar.

Allthough the intake is not drawn through the crancase,i still think the exhaust will help to some extent to get more air in.Besides,both of them has no valves,and that could indeed be very similar with exhaust tuning.

I dont really think that pulse tuning has such a great effect,as it would on a boinger.However,the is no real facts about any sort of tuning of exhaust lengths.

Have any of you ever seen a paper on tuning the exhaust,and not just the way that everyone*thinks* it should be?Well, i havent.

I think there is no gain possible with putting a 3 in 1 exhaust on a 20B.It would be a waste.Same with a single Rotor engine,you can get an estimated uncollected lenght,but it wont be as good as a collected one would be.

The whole expansion chamber exhaust is just and idea,i think it might work.Just have to try it someday.

"Exhausts are like woman,you will never know enough of them"