RX-7Impreza

Ok here is my idea,

i know that on a dual exhaust piston engine the x-pipes work to create a sort of vaccum which draws the exaust out of the chamber. this is dont by creating resonance in the pipes (when the burst of gas from one bank is crossing the X or H the other bank should simultaneously be evacuating) this "resonance" is found by moving the pipes towards and away from the engine until you get that sweet spot.

so if a rotary engine has true dual exhaust (headers) will this work the same? I figure since there are no valves in a rotary it would even work better by "sucking" ALL of the exhaust out of the chamber.

has anyone tried this? does anyone know if it will work?

Justin

mazdaspeed7

It works, but I dont know exactly how much. I know that rotaries will always make the most peak power with true duel. It also makes true dual a lot quieter. The noise reduction os from the exhaust pulses cancelling each other out somewhat.

The intake pulses are much stronger in rotaries than pistons, so intake *can* make a huge difference in power. Im not talking about just a filter, though. Changes in the manifold, such as ported manifold, etc. Runner length is extremely important in rotaries.

The exhaust probably is affected the same way, but there has never been enough different designs on rotaries to really know what works best. Most Rx7's have single exhaust, or single exhaust with the y-pipe and 2 mufflers. Very few people have true dual, and I only know of one person with an h-pipe on a true dual. I think his name is BlackRX7 on the forum.

C. Ludwig

iTrader: (27)

Every engine's exhaust (piston or rotary) will create pulses as it's expelled. This means that the exhaust doesn't neccesarily flow continuously but in a "pop-pop" sort of fashion. You can feel it and hear it. These pulses are obviously correlated to RPM. The higher the RPM the shorter the frequency. The way collectors work is to use the pulses of the cylinders or chambers to offset each other and "pull" more air out than any one pipe could flow on it own. The challenge is in where exactly to place the collector. The location is critical because it will take so long for an exhaust pulse to travel a given distance. Supposing we have a true dual header on a rotary, there will be one spot along the path of those two pipes where we could place a collector and use it to increase exhaust efficiency for any given RPM. In general the higher the RPM, the shorter the primarys (dual section before collector). After the collector splitting the exhaust again is really not neccesary. Look at Trans-Am cars. They run a single outlet. Fortunately for us the header manufacturers take much of the guess work out of the equation for us and place the collector where they think it will most benefit the powerband the engine will be run at. However, if you purchase a 13B street header from RB and intend to run it on a 10k RPM P-Port motor it will not produce as much power as a header designed for the higher rev range. The same is true of the after-market, or factory, H and X pipes offered for V motors. Each is designed with the idea that the collector needs to be in a certain spot to be a true benefit.

With that said, using a header properly designed for your application is about as big an improvement as you can make. There would be no real benefit to splitting the pipes and then merging them again. If you'd like to design your own RB gives some vague measurments in their catalog.

I'd challenge MAZDASPEED's claim that true dual always makes more power. If that were true we'd see more of them at the racetracks. Speedsource builds arguably the fastest IT cars in the country and they use a collected exhaust.

Chris

Felix Wankel

I've read that true duals collected near the rear axle make the most power. It seemed to work well on my first gen.

SpeedRacer

In general, the closer the exhaust collects to the engine the better the low-end power, compromising high-end. Thats why most people don't have true dual exhausts, because you WILL lose low-end, but it will produce MAX power at high-rpms compared to a collected exhaust.

mazdaspeed7

For one, racing bodies have rules, one of which is often noise regulations. True duel exhaust is MUCH louder than a single exhaust. On a single exhaust, the pulses interfere with each other enough to greatly reduce the noise. Theres another reason too, single exhaust is lighter. Half the pipe, half the mufflers, etc. Rotaries bave sound level requirements as it is, usually much more stringent than piston counterparts, which means bigger or more mufflers, and also more weight.

Blue Goose

You want the X/H pipe where the exhaust has the most backpressure. Need an easy way to find out? Let Uncle Jake help! Go get some black automotive lacquer based spray paint from autozone or wherever (NOT high temp). Jack up the car and put it on jackstands. Spray the exhaust for 18-24 inches, starting about 6" behind the header. Drop the car, let paint dry, then run it for about 30-45 minutes or until hot. Jack the car up and look to see where the paint is glowing the brightest. That is where you want your crosspipe. BTW: Put some newspaper over the exhaust before you start painting to control the overspray, and don't worry about the paint; as long as you didn't buy the high-temp stuff, it should crack and fall off within 1-2 weeks...

Bambam7

I have to put in my 2 cents here.....Sorry Mazdaspeed7- We are on the same track here with our cars, but I have to disagree with you.
If you have equal length headers, and a properly tuned pipe diameter/length (to which the detail I have NO clue).
The alternating exhaust pulses, once combined (I don't know where th optoimal locatoin is) create a complimentary scavenging effect, where the velocity of one pulse travelling down the pipe pulls the nest pulse down with it (the nest pulse being from the alternate rotor).
This will create a definate peak in engine performance though, just like matching the alternation in the reversion pulses in the intake manifold (you've felt the sudden kick at 5600 rpms right?), location of the collector is crucial, as is the length of pipes before, and after it.. The peaky nature may not be good for racing...... maybe the optimal pipe geometry doesn't fit under the car.... Who knows, I just know the principal is undeniable.

mazdaspeed7

Well, I am learning how to weld, so I wont have to pay someone to do it for me. I WILL try a crossover pipe once I learn how to weld, just to see how it is.

Bambam7

One thing I am not sure of is whether a collected pipe (2 into 1) is better, or an H pipe is better (2 equilized with one cross pipe)
So far, to me, H pipe configuration is harder to calulate....I may figure it out eventually, but for now, my Biology reading is calling.....

rxspeed87

some numbers from a boinger when they put a x-pipe in the system
doesn't seem to make it peak out but instead take the existing powerband and make it broader and higher.

I know it is a piston motor though still a lot of the same theory I would htink works


I have all the stats on the motor if you want them
but just to let you know it is a 355 running with 1 5/8 header into dual 3" pipes
hey if you want what the same setup did with a h-pipe instead I can put those on here also if you guys want


rpms 1 5/8 w/x-pipe
3000 384.7 404.7
3100 380.4 398.7
3200 377.0 398.2
3300 373.4 401.0
3400 370.9 404.9
3500 370.0 406.9
3600 372.8 409.3
3700 381.9 412.1
3800 392.3 415.0
3900 397.9 415.6
4000 403.9 416.7
4100 410.8 418.8
4200 415.9 421.4
4300 419.5 422.2
4400 419.9 421.2
4500 418.7 418.7
4600 413.9 415.2
4700 410.3 412.9
4800 406.2 411.1
4900 401.7 407.4
5000 385.7 401.1
5100 389.5 392.5
5200 384.6 385.3
5300 380.4 380.5
5400 376.4 377.0
5500 372.4 374.3
5600 370.3 371.0
5700 366.0 368.6
5800 359.6 364.9
5900 350.1 357.2
6000 339.9 345.6


ok so I have far too much spare time on my hands big deal

tesla042

erhm...isn't it, the further away the exhaust collects, the better the low-end? since the pulses are further apart, the pulses travel further down the line...

-Tesla

RX-7Impreza

ok i figured this out when i was trying to go to sleep last night

the problem with this convergence of exhaust is not the frequency increasing because with the frequency increase the speed of the pulse increases so they would never interfere. the problem lies in the duration of the exhaust pulse. at low rpm the pulses need to be a certain distance from the engine to avoid each other.

picture this. imagine the exhaust pulses as 6 inch blocks with 6 iinch spaces in between. when tuned correctly they will avoid each other completely. at a certain rpm. but as the rpms get higher. the pulse is shorter because the engine is "throwing" it out of the exhaust for a shorter period of time. so now you have 4 in ch blocks with 4 inch spaces. with the calibration of before there are more pulses in the pipes at the same time so the point at which they cross will no longer allow them avoid each other.

this may make no sense to you but it does to me

the reason it increases the power band on a v8 is becaust they have a relatively short on in the first place (5k or so) so the interference is minimal throughout the powerband. with us we are running through 7k. the interference is also more substantial because our pulses are very strong. so the dilemna is "top end or low???"

i couldnt go to sleep until i at least thought of one insane idea so here goes. the difference in the harmonic balance at high rpm and low rpm is only a matter of inches in distance from the block, so to get this to work at high and low you would need to have a progresssive system to where the exhaust has to go farther to the X at low and shorter at high. my idea involves flaps that cause the air to go in a slight bend directly before the X and this goes away as rpms get higher. to open the flap you could use the exhaust pressure itself to push it back

yeah i know this is a crazy idea but i wouldnt have thought of it if i didnt think that there is a lot of power to be tapped with this. and mazdaspeed is right that you will get the most overall power with true duals. the convergence will give a higher peak but it will suffer in other areas


my .02

Justin

RX-7Impreza

bump

C. Ludwig

iTrader: (27)

Instead of constructing any kind of variable length exhaust system you can use water injection to make the exhaust think the tuned length has changed. Water injection has been used for many years in 2-stroke exhausts. By building a system orientated toward top-end performance you can use water injection to restore some of the lost bottom end. For strong top end the tuned portion of the exhaust will be located more closely to the engine (sorry SPEEDRACER, you're wrong). This is conterproductive to low-end performance because the pulse frequency is longer for low-RPM exhaust pulses. The tuned portion of the system needs to be further away from the engine for strong low-rpm performance. With water injection we can spray the exhaust stream and cool it, thereby slowing the speed at which the gasses travel. This makes the gasses see the tuned portion of the exhaust as being farther away than they really are. Once a certain crossover RPM is reached the injection system is shut down. The gasses heat up and travel at a faster rate maintaining proper timing with the tuned section of the exhaust. I've used many systems like this on two-stroke race motors utilizing nitrous solenoids, Mikuni carb jets and commercially available RPM controllers.

MAZDASPEED I rethought your contention on the true dual system particularly the argument that a roadracer might not use a true dual system because of it's weight. In the case of an ITS car here are some numbers to consider. Minimum weight for RX7 and driver in ITS is 2650lbs. Assuming the car is making 200 crank hp we arrive at a power-to-weight ratio of 13.25lbs/hp. Now if a true dual system adds only 3hp we can add 39.75lbs to the car and maintain the same power-to-weight ratio. I'd certainly doubt that an extra 6ft of pipe and one race/bullet muffler weighs 40lbs. Not to mention the fact that the Speedsource cars are built underweight and carry ballast to make weight which can be dumped to negate any small weight gain which might be incured. When I see two time national champions using true dual systems I'll think about it.

Chris

Roy James

damn, you guys just messed my junk all up, i had it all nice and planed to run straights back from true duel...... hehe

Bambam7

Hold on dude, gotta light it.... Ok, toke... toke... whheeww.... Now let's think of something...... Pipes made of bimetallic slinkies that elongate from the heat during high RPM's, making the convergense point further down.... sweet, at lower RPM's they cool a bit, and shrink back to optimize low end!!
Dude, wanna eat the roach?

Rotortuner

Sorry, But no. The farther the collector is back the more high end pwr there is, thats all there is to it. If you don't understand it, it is discussed on P. 22-23 of the "how to modify your rx-7" By jim downing. He says that they did not have a SIGNIFIACANT difference. but he quoted Jim Mederer from racing beat as saying, "for street ported engines longer primary tube leangths are better". I also can personaly vouch for the fast that farther means higher pwr.

CJG

C. Ludwig

iTrader: (27)

If you'd map an exhaust pulse it'd look mostly like a sin wave. There are actually multiple spots along the pipe where collectors can be added to some degree of comparable success. Racing Beat says the same thing in their catalog. You can use either the long or short primary system. The measurements they give for a short system indicates a simple header as we know it. The long primary systems end up looking like what most of the roadracers are using with the collector near the rear axle. It's quite possible that the long primary system makes a bit more power somehow. Perhaps is due to the increased volume of the longer system? No idea really. That's why Downing and places like Racing Beat get the big bucks! The fact is however, the higher the RPM the shorter the frequency of the pulses. If we're making incremental adjustments (not moving the collector six feet) to fine tune the system they'll be made shorter for higher RPM power. Again the pulse is like a sin wave. Move it too close and you're on the other side of the curve and you've missed the sweet spot.

Chris

RX-7Impreza

cludwig you beat me to it. but i must say again that it is the pulse duration not the frequency of the pulses that causes interference. (just for clarification)

Justin

RX-7Impreza

yeah, anyways....... the idea i had was an actual possibility. it could work, but it is a matter of $ vs GAINS and Weight vs Gains and also Time vs gains. it is possible if an easier way could be thought of. (like the water injection) but thank you for your insight bambam

Justin

mazdaspeed7

This is kinda off topic, but since we're talking about getting every litle bit of hp out of the exhaust... Anybody here familiar with 2 stroke engines, especially dirt bikes? They use an expansion chamber to create a vacuum in the chamber, that sucks in the fresh air/fuel for the next power stroke. Could something like that be adapted to a rotary? That would improve VE a good bit if it was tuned right. The exhaust would close with a vacuum in the chamber. That vacuum would draw in more air than normally possible through the intake, increasing VE, and HP.

RX-7Impreza

i know what you are talking about. (the huge bubbles in the exhaust of race bikes) but i am not familiar with how it works exactly

Justin

C. Ludwig

iTrader: (27)

The expansion chamber in the two-stroke exhaust is designed to create a revision in the wave. A two-stroke's exhaust port remains open for part of the compression stroke. The revision is used to force the air/fuel mixture back into the combustion chamber until the piston can close the exhaust port. The idea is to shape the chamber to make the waves bounce back and forth at a given frequency, flowing away when the piston is moving down to create a vacuum and flowing back in for compression. A great deal of a 2-stroke's peak power and power spread is dictated by the pipe. Again a pipe can only be tuned for a given RPM band, just like X/H pipes, header collectors, etc. As described earlier, water injection can provide a much broader powerband than just the pipe could. Water injection was pioneered on 2-stroke GP bikes a long time ago. When I was racing jetskis we used water injection ALOT. My last boat had 4 different points along the pipe where water was injected and 1 computer and 2 manual controllers controlling the whole deal.

What would that do on a rotary? Good question. I couln't answer for sure but I would guess not a whole lot. The port does not remain open like it does on a 2-stroke so the revision and the restriction it creates would do more harm than good. I guess.

IMPREZZA, as RPM rises frequency does too. Think about it. RPM is a measurement of frequency.

Chris...that's me in the pic

pp13bnos

iTrader: (1)

I was under the impression that true duel is the best for peak hp. I'll ask Rob what he thinks about a H or X pipe tomorow. CJ