SureShot

FWIW - I drove my NA to the muffler shop with no collector & just a temporary heat shield.
Torque was way off & high end revs were way off.

mazdaspeed7

You do realize that at 3000 rpm, there are 50 ignition events and exhaust pulses EACH second. That would mean that there is only 1/100 of a second between a pulse from the front rotor and the rear rotor. And that time only gets shorter as the rpm's rise.

But thats an eternity for physics. Think of an F1 engine. 19000 rpm, and they still have intakes and exhausts tuned to take advantage of the pulses. at 19000 rpm, there is only 3/1000 of a second difference between the pulses of opposite cylinders.

mazdaspeed7

Ill clarify a little on my above post. The time period between exhaust pulses is so small, we cant see it with our eyes, but its still an eternity as far as physics goes. Intake and exhaust tuning relies on this, as well as the whole field of acoustics, and part of fluid dynamics.

Id love for someone to present an intelligent argument against collected exhausts. Come on Icemark, show me what you know. Im waiting.

Howi

iTrader: (2)

ok... first of all, i don't really know why some of you are arguing over the different types of pipe designs. you should all know that each system can be engineered to be fitting to your specific car, whether it's a 7 or not. i find the extremely generalized statements that some of you make kindda silly. you just can't simply sit on your *** in front of a computer telling someone that you've never met over the internet that a particular exhaust system is the best for their car, which you have not driven or conducted any tests on. it's not that simple.

now, with that in mind, i'd like to address some of the benefits/problems that are GENERALLY associated w/ some of these exhaust systems (i'm only concerned w/ performance here, not sound).

NA:
join pipes can work really well on an NA. the difficult part is - how? the basic idea behind a join-pipe system is that when 2 individual pipes come down, each carrying their own wxhaust mass and velocity, ideally speaking, they can be joined into a single pipe w/out suffering too much from turbulent and vena contracta. if this is done intelligently, the end result is a single pipe that is able to carry nearly all of the exhaust gas to the atmosphere (let's assume it can carry all of the exhaust gas as before the joint).
now, since Q=VA (Q=flow rate, V=velocity, A=cross sectional area), A would be smaller after the joining of the pipes. since Q has to stay constant (or nearly constant), V would have to increase to balance the equation. it is this increase in velocity that helps the performance of an engine. as we all know, velocity in exhaust gas can help suck exhaust gas out of an engine.
now i'm sure we can all see how complicated it can get to design an join-pipe system well. some of the influencing factors include: the diameter of the 2 downpipes, diameter of the single pipe, the design of the joint, the quality and shape of the bends, the material of the pipes, and the craftsmanship of the welds.
not to mention that it is extremely difficult to fabricate a near ideal join system for your car, as soon as you make a modification to your car (from as small as changing an air filter or as big as a new turbo), you'd throw the balance off and the exhaust becomes, again, less ideal.

turbo:
the turbo only allows 1 exhaust pipe to come down from it (obviously), so let it flow out smoothly as 1. the most influential factor in a single system is the diameter. and no, bigger is not always better.
why do some larger diameter pipes create the boost creep problem? becuase they are oversized. a single system w/ an oversized diameter will not provide suffient exhaust velocity to help suck out the excessive boost. at the same time, a pipe too small will restrict flow. again, you gotta find the balance for YOUR car, and only your car, not everybody else's. ideally, the best diameter is the one that lets your engine vent all of the exhaust mass (m), while keeping the diameter at minimum (while letting nearly all of the exhaust out) to create maximum velocity (V). this will give the most amount of momentum (momentum=mV) in your exhaust to provide the suction needed.


finally, whether you have a single or join, NA or turbo, i personally do not see any benefits by splitting it again (dual muffler) from a performance standpoint. maybe someone intelligent can design a system that will somehow make an engine perform better. i don't know. i'm not the fuild mechanics god so i will not deny the possibility. but untill it has been proven, don't do it if you wanna go fast.
conclusion, do less arguing and more testing.


howi

mazdaspeed7

You completely skipped the exact point Im making. Collected exhaust make more power because the positive pressure wave from one rotor returns the other rotor as a negative pressure wave, and it arrives back at the chamber, resulting in a low pressure area inside the chamber, and the exhaust port closes, trapping that low pressure(in some cases, negative pressure, or vacuum) inside the chamber, which draws in more intake charge because there is a larger pressure differential. This is what happens at the tuned rpm of a collected exhaust. The farther away from that rpm you get, the less effect you get.

There is a very fine line between a good exhaust, and a perfect exhaust. But even still, you cant expect that to be perfect. Flow differences between single and dual muffler is entirely dependent on the mufflers used, and the qhality of the y pipe. And dual mufflers makes the car that much quieter, which many people go for. If I didnt care about noise, and was just goign for power, I would dump the exhaust right after the collector, using a megaphone to improve the scavenging effect even more.

Keep on topic here. Your whole post was beside the point. We're not discussing how good an exhaust needs to be to make power, thats common sense. A wel designed and made exhaust will make more power than a comparable one of lower quailty work. Second, we're not here to argue about why one muffler is better than two.

Howi

iTrader: (2)

excuse me mazdaspeed7.... did you say stay on topic?
i couldn't find anything regarding exhaust waves or exhaust pulses in the original thread, could you?
i did not mention your name in my reply at all, i was not targeting you. i didn't "skip" any point you made because i was not even making any points regarding what you said.

i typed out some "common sense" to explain to rxspeed87 my opinion about his idea about X-pipes, and that 1 muffler is better than 2 therefore X-pipes is a bad idea.

i hope my reply answered your question rxspeed87

howi

NZConvertible

That's a really long limb you're out on there. In fact I can hear it cracking...

Sorry to go off-topic again, but... I don't think you were around when Soul Assassin was. Over 500rwhp, 10sec 1/4, dual rear mufflers. Fast enough for ya?

Howi

iTrader: (2)

really??!! did he really have a single and then split into 2 at the end? odd..... i wonder how that works....

howi

Howi

iTrader: (2)

i don't really get what you're trying to say here mazdaspeed7. correct me if i'm wrong, but are you saying that when +ve pressure comes down a pipe from a rotorhousing, and then into the joined single, it would create a -ve pressure in the other pipe and help suck out exhaust for the other rotorhousing? please clarify.

howi

Icemark

Okay, you have two different directions going in this thread. I keep telling myself that I refuse to get suckered back into something that I have covered countless times, but Mazdaspeed7 you are forgetting several important points and your childish goading is going to get the thread closed. I am sorry again you didn't bother to read past posts.

#1 The output timing of the rotors is almost identical (were you got 180 degrees off I am not quite sure- perhaps you were referring to rotor position rather than ignition event??) at any rate it is close enough that in the reactive chamber (exhaust manifold) that the exhaust exits react as if they are at the same time. Why??? How do the do that???...

Because of
#2, the amount of unburned, still burning, and completely burned mixture continues to expand and ignite in a closed cell (such as the reactive chamber). As a hint this is why the tuned length expanding exhaust pipe diameter works well on a rotary when the system is a single.

So, you have an exhaust timing event close enough to be considered on top of each other and continuing through each other when the two are combined.

That is why the FE has such a big HP increase. Because the explosive exhaust events are kept separate, rather than forced together as in the previous generations of rotary engine. In conjunction with the separate events, the exhaust gas is re-routed back in from each separate rotor (again lowering combustion temps as rotaries need to keep from burning themselves up) rather than forcing back in under the same open window (or more accurately exhaust ports).

Lets go a step further and ask anyone using a true dual, if the flames coming out on lift throttle are at the same time or alternating (as in a V type engine).

But the premise of this thread was how to make a piston engines exhaust design work on a rotary. How to time the exhaust pulses to further help the scavenging effect. Well if the exhaust event continues in the header or reactive chamber, and how can we time the pulses velocity to stack up on each other? We can't. If one rotor is running slightly hotter than another the continuing exhaust event will cool enough at a different positional length than the number 2 rotor, throwing off any length tuning that we could do. Our best hope would be to join the two only after the event has finished its primary burn (which is what the RB collected header does), and go from there with the minor loss. Yes you could immediately go from that collection into a y-pipe, (technically making the famed x pipe) but the collection already has robbed the HP.

So, now this thread has been covered to death, and can be ended.

Icemark

He is using the concept/analogy from piston engines were the exhaust pulses can be tuned to stack up on each another to force out the exhaust.

The idea is that there is a void between exhaust events. If you can time the next cylinders exhaust event to fill that void, the exhaust gases should escape faster than if they all dumped into the exhaust at the same time.

Of course that doesn't quite work as well for a rotary where the burn events continue into the exhaust events.

Icemark

Mazdaspeed7 wanted this added (although it should be under the general rotary tech support and not 2nd gen section):

Icemark

You know what fine, I'll reopen this thread.

Valves have nothing to do with the operation (hench valveless 2 stroke motors don't have the same comparible exhaust that a rotary does either), Of course backfiring too, maybe that is why exhaust temps reach 1800F... oh wait... they don't reach even half that with a piston motor... even with Japanese inline 6 running rich.

However I have answered all I will in this thread. If you want to prove that a rotary engines exhaust works the same as a piston feel free. It really should be done in general rotary tech instead of the 2nd gen section however.

I'll leave this thread to some other mod who can see it going no where and covering no new ground to again lock it.