rotarygod

iTrader: (1)

He was also referring to dual 3" pipes on a rotary.


Since the exhaust port area isn't even equal to a 2" pipe, not even when ported, I see no reason to go larger than that. Dual 2.25" is still too large. Don't try to second guess airflow. I haven't seen anyone do it yet. Dual 2" still flows more than the engine can and it will keep exhaust velocity higher. You'd probably make more average power this way than you would with a larger setup. In fact it's almost all but guaranteed that you would.

fcdrifter13

Ok cool, I am like new to the 7 systems. On my 460BB ford bigger was always better, and my fc seemed to like the big pipes. I will take ya'lls advice because of my lack of experince in this field.

Alex Saunders

fcdrifter13

FB II do you have pics of your setup. I looked on your cardomain thing and dint see any. I may be blind though.

alex Saunders

smnc

While I agree that 3" is too big for a rotary, exhaust scavenging isn't an issue in this case as we're talking true dual exhaust...

I like my dual crossover design...

I get scavenging, but I still get a duel exhaust...

fcdrifter13

You mean you are running like a h-pipe or x pipe. That would be kinda wierd I think, although I have an x-pipe from burnsstainless on my truck and it seemed to help over just strait flowmasters.

smnc

It's actually a little more refined. Basically, I'm going to run a dual header (not sure what kind), back to dual high flow cats (http://www.supremeperformance.com/Di...ab.aspx?tid=10), then possibly though dual RB pre-silencers, then into a collector (right before the axel). The it'll split back into dual pipes, and end in a pair of SuperTrapp mufflers(http://www.supertrapp.com/product_se...temID=449-2218)

This (maybe)


To two of these:


To these:


Two a collector like this:


Split back to a pair of These:

fcdrifter13

hmm that sounds like a sound idea. Are you running back into one pipe for clearence issues or for bette flow characteistics

Alex Saunders

smnc

The collector/split section is for exhasut scavenging purposes...

Crappy drawing:



Another dual exhaust thread:
https://www.rx7club.com/showthread.p...t=dual+exhaust

REVHED

Scavenging is still an issue even in a dual system. When exhaust port flow slows down towards the end of the cycle the inertia of the existing gases in the pipe from the previous cycles help pull the remaining gases out.

fcdrifter13

Hmm that looks like a really good idea. Although you should have went with hot pink pipes instead of blue. Thanks for the link BTW. I will use it well.

rotarygod

iTrader: (1)

You can't scavenge if the pipes don't collect. Yes there is inertia in a pipe from a true dual but there is more backpressure which puts the brakes on this inertia faster. This is due to the pulses slowing down and backing up on each other the farther they get downstream. These are pulses that contain deadspots that do nothing. Collecting the pipes gives you the low pressure of the opposite pipe to keep the air moving in the other one instead of a dead zone. That's what scavenging is. A true dual tunes by organ pipe resonance principle and not scavenging. A collected system does both. The collector location and style affect if it is useful or not.

I personally wouldn't use an H pipe or a crossover style X pipe on a rotary. The H pipe will do nothing for scavenging and the crossover X pipe isn't the best either. I'd at least collect it and run it for a foot or so and then split it again. Collect it before the rear axle and then split it again after the axle. This would work fine. Just give the gasses some more time to work on each other. On a V8 that uses H pipes and X pipes, you are trying to place these crossovers in a spot of high pressure. This will force some gasses over to the other side. This is due to the uneven firing order in each header on a V8. A rotary is perfectly spaced and doesn't suffer this fate. A crossover works differently on a rotary. The gasses won't mix as well and won't really scavenge.

I am personally not a fan of Supertrapps but that's just my opinion. There are many that have used them though. I feel the only way to really use them is with the end completely open. In this fashion though they don't really muffle well. I'd rather use a bullet style muffler than these. They are cheaper too. At least the rear mufflers are easy to change if you don't like them so try whatever you like.

smnc

Yeah I was gonna say what REVHED described sounds like inertia tuning, not scavenging...


That's almost EXACTLY what I had planned.
The SDJ style collector seems to work well, so that's what I had planned...

Yeah, mixed opions, but I like the idea of them, so, I'll give 'em a shot...

Besides, by running the hi-flow cat AND a RB presilencer, I should be able to get away with having them pretty open and still keep things fairly quiet...
Plus having duals will help as well...

REVHED

No, I was talking about inertial scavenging. Pressure wave tuning is a different matter entirely.

fcdrifter13

Ahh time to end the cunfusion. Link to the good stuff about header design. It explains alot of the ideas here. www.burnsstainless.com is were the article came from.


Before we delve into the dark art of exhaust theory, let's take a quick journey through the exhaust system from the perspective of the exhaust gases.

As the piston approaches top dead center, the spark plug fires igniting a fireball just as the piston rocks over into the power stroke. The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke. The gas pressure is still high (70 to 90 p.s.i.) causing a rapid escape of the gases (blowdown). A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (and is dependent on gas temperature). Expanding exhaust gases rush into the port and down the primary header pipe. At the end of the pipe, the gases and waves converge at the collector. In the collector, the gases expand quickly as the waves propagate into all of the available orifices including the other primary tubes. The gases and some of the wave energy flow into the collector outlet and out the tail pipe.

Based on the above visualization, two basic phenomenon are at work in the exhaust system: gas particle movement and pressure wave activity. The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases. The pressure waves, on the other hand, base their speed on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. At all times, the speed of the wave action is much greater than the speed of the gas particles. Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together, as in a collector for example, the waves travel into all of the available pipes - backwards as well as forwards. Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. Typically this phenomenon is controlled by the length of the primary header pipe. Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done.

Gas speed is a double edged sword as well, too much gas speed indicates that that the system may be too restrictive hurting top end power, while too little gas speed tends to make the power curve excessively 'peaky' hurting low end torque. Larger diameter tubes allow the gases to expand; this cools the gases, slowing down both the gases and the waves.

Exhaust system design is a balancing act between all of these complex events and their timing. Even with the best compromise of exhaust pipe diameter and length, the collector outlet sizing can make or break the best design. The bottom line on any exhaust system design is to create the best, most useful power curve. All theory aside, the final judgement is how the engine likes the exhaust tuning on the dyno and on the track.

Various exhaust designs have evolved over the years from theory, but the majority are still being built from 'cut & try' experimenting. Only lately have computer programs like X-design or high end engine simulation programs been able to help in this process. Practical tools like adjustable length primary pipes and our B-TEC and DynoSYS adjustable collectors allow quicker design changes on the dyno or in the car. When considering a header design, the following points need to be considered:

1) Header primary pipe diameter (also whether constant size or stepped pipes).
2) Primary pipe overall length.
3) Collector package including the number of pipes per collector and the outlet sizing.
4) Megaphone/tailpipe package.
There are many ideas about header pipe sizing. Usually the primary pipe sizing is related to exhaust valve and port size. Header pipe length is dependent on wave tuning (or lack of it). Typically, longer pipes tune for lower r.p.m. power and the shorter pipes favor high r.p.m. power. The collector package is dependent on the number of cylinders, the engine configuration (V-8, inline 6, etc.), firing order and the basic design objectives (interference or independence). The collector outlet size is determined by primary pipe size and exhaust cam timing.

For more detail on the specifics of header theory read ‘The Scientific Design of Exhaust and Intake Systems' by Phillip H. Smith’. For those that prefer quicker results, Burns Stainless makes designing a racing exhaust header easy. Our revolutionary X-design parametric exhaust modeling program provides you with the perfect starting point for any header project. Just fill out the Race Engine Specification Form, send it to us, and we will do the rest.

After a proper header design is constructed, the fine tuning can be done on the dyno with adjustable pipe sections (typically in 2" increments) and our innovative B-TEC and DynoSYS adjustable collector systems.

I hope thos clears up alot of things it did for me kind of.

Alex Saunders

Lt. Dan

The question you need to ask is whether ANY of that crap applies to a rotary engine....

Dan

Lt. Dan

I ran one.... lasted a couple weeks before a backfire broke a weld in the muffler... Then it rattled like hell......

Dan