fcboy89

i was wondering, altho the 7 is an old car, why dont they get rid of the y-pipe and make it more like say...an h-pipe? or is it solely based on the fact of the path of the tranny? ive always wondered that. ive seen single exit exhaust, but i was wondering it any companies made two pipes, with two exits, sorta like the 350Z.

HAILERS

RacingBeat. See their site.

turbo80cid

I think an H-pipe only helps engines with 2 banks of cylinders.
v6, v8.....etc

slpin

iTrader: (9)

H pipe?

do you know anything?

turbo80cid

if you have an NA I think Racing Beat make a true "dual" exhaust. One pipe per rotor.
or maybe that was Mazdatrix?

HAILERS

http://www.racingbeat.com/FRmazda2.htm

http://www.mazdatrix.com/r-ex86nt.htm

Sideways7

iTrader: (7)

He's talking about a true dual that has an h-pipe to equalize pressure in the event of a problem. My old 5.0 had a setup like that.
Anyway, the only true dual system for our cars that I know if is the RB system listed above, no idea if it has the h-pipe or not, though.

ColdSeven

yeah this the only way it works...go single

GTU_FAN

read a thread called weight reduction in this section there's a bunch of talk about exhaust design in there...

Icemark

a cross over pipe as used in some V6 and V8 applications would actually be no better than stock. On a Rotary, for maximum power you would want seperate pipes as far as possible.

It is not like some lame piston motor that a crossover pipe (H pipe) would help due to the minor exhaust pulses.

GTU_FAN

Hey Icemark care to give your reasons for above statement in the weight_reduction thread? I put a lot of time into that one about this lol

slpin

iTrader: (9)

hey mark, watch it!! my 496 stroker will make more power than 100% of the rotarys in this board NA. and more power than 99% of the forced 13Bs while I am still NA. :P

My5ABaby

iTrader: (7)

It's still lame.

Motion in the ocean baby... motion in the ocean...

Icemark

Link???

I am sorry, although I have a personal disgust for large OHV motors, I merely meant, that the advantage of a using a crossover pipe in piston motors, is that the exhaust pulses are so low, that the crossover pipe helps move the exhaust down on V and flat type engines.

You don't have that issue with a rotary as the exhaust pressure pulses are stacked up on each other. In fact if you could keep the pulses apart until exit from the exhaust pipe it would net more HP, as then you don't the exhaust pulses stacking up at the same time.

See the timing of an exhaust on a piston motor using different cyls (usually fired sequentially on opposite banks) benefits from the exhaust pulses stacking up (6,5,4,3,2,1 for example) which helps push the exhaust out. If the cyls were without a H pipe, you would get 6,4,2 and on the opposite bank, 5,3,1... too much time between each event to get velocity.

Not so with the rotary. There the events all stack up in order on each rotor, with the timing so close that it can rip apart cheaper manifolds and headers. So if you collect them together with a crossover pipe, the events are actually magnified forcing some of the exhaust back into the combustion cycle lowering HP (sort of a reverse DEI set up).

So with a V6 or V8 or flat 6 motors, a crossover pipe is good. For a Rotary it is bad bad bad... as you don't want the exhaust being forced back in.

GTU_FAN

[QUOTE=Icemark]Link???
[QUOTE]

Here ya go... https://www.rx7club.com/2nd-generation-specific-1986-1992-17/weight-reduction-552078/

Icemark

What do you want me to say, Nihilanthic doesn't understand the difference between a rotary engine and a piston engine when it comes to exhaust. His replies in that thread are strickly based on Piston engines (IMO probably V8's mostly, by his comments) and the poor exhaust volocity of them.

Whearas my comments are based on building countless exhausts for the FC.

For example I know without any doubts, that the most HP on a non turbo will be made with a 2.25 inch exhaust. That even moving up to a 2.5 inch on a non turbo and you start loosing power. The only safe place to start increasing exhaust pipe size is about 8 inches in back of the rear converter mounting, and even then you must be carefull through the bends not go wide.

Groshuk

Hey SLPIN I would hope your lump of crap 496 makes more power since it is 6 times larger than a rotary, if you really want to brag though you better be making over 1500hp with no power adders on pump gas. You can get conservitively 250hp out of a NA 13B and your engine is the equivalent of just over 6 13Bs so I'll let you off with 1500 although it should be closer to 1800. I think you're on the wrong board if you want to bash rotaries.

ericgrau

iTrader: (1)

I'd have to see some visual diagrams regarding all these claims about single exhausts, dual exhausts and H exhauts.

How much would I pay a mechanic to get a 496 installed (motor, tranny, parts, labor, everything)? Or give me the cost of all parts and the number of hours of labor involved. What is its horsepower? What is its weight? Where is its CG located (or tell me half its length, if that's easier)? What is its city/highway mpg in an RX-7, while getting above horsepower? Does it pass smog?

Personally I'll go with a locomotive steam engine over a V8 or a rotary any day. With one of those babies I'd get way more torque than either .
http://cprr.org/CPRR_Discussion_Grou...m-engines.html

Nihilanthic

Wow, we go from roll bar drama to outright, OBJECTIVE ignorance on your part.

True duals do not give scavenging, and add weight because you have two times more piping you need (past the point at which you'd collect for scavenging anyway) plus a second cat and muffler, etc. The only way dual pipes would scavenge is if you found a resonant frequency (or a multiple of it) of the exhaust gas pulses at the RPM range you want to.

Exhaust pulses are exhaust pulses are exhaust pulses, rotaries aren't magical, and not that different, just misunderstood.

Im sure you understand how 4-2-1 exhaust works on 4cyls or a bank of a v8 (thats a single plane crank...), right? Each cyl fires (individually) every two revlutions of the crank shaft, but as far as the entire engine goes, one of them will fire every half revolution of the crank shaft. The way 4-2-1 headers are paired, each pipe in the "2" part of it (between the first pairing and the second where the main exhaust pipe goes towards the cat, or directly into the cat in some cases) sees a pulse go through every 180*. What they try to do is have it set up so that the vacuum after the one pulse leaves (and has the pipes nice and hot) is present when the pulse from another cylinder is coming down the pipe and help 'scavenge' and pull it out with resonant pulse tuning.

Guess what? With a 13B, those two exhaust ports fire (well, one of them does) every 180*, alternating, meaning you only need a 2-1 header to collect and scavenge, and becuase its for a higher rpm range and generally a peaky powerband N/A youd want it fairly close to the engine so you can use a single exhaust from that all the way to the muffler.

Now, unless you have a scientific, factual, objective reason why Rotary engines dont benefit from scavenging, please dont call me stupid, ok?

ericgrau

iTrader: (1)

I've always leaned towards a single exhaust, but I'd still like to see some kind of diagram from either side of the argument. Or a more clear explanation with less use of the words like "it" and a more specific explanation of what happens to each and every thing involved (parts, exhaust gas, etc.).

Here's what I've always assumed: exhaust pulses aren't simultaneous so they can share one path almost as easily as they could use 2 different paths. Plus you can always make a single exhaust wider to match the increased space a dual exhaust gives. Only reason I see for a dual exhaust is to increase flow without the risk of muffler(s) hitting the pavement (as an upsized muffler would). The stock Y-pipe accomplishes this already.

Nihilanthic

The Y-pipe is actually a bit of a restrictive because splitting airflow is extremely difficult to do without imparting turbulence. Its much easier to collect.... and the thing is I really dont see why youd WANT two pipes, because if the exhaust gas cools down it slows down too. You want to hold the heat IN, and having twice the surface area for it to cool with isnt gonna help... and the weight is a matter to.

Also, speed of sound is a given if you know the temperature of the gas in question... you have to do some fairly simple algebra (for a high school graduate anyway) to find out the length you want if you put into the equation the rpm you want to tune for the peak of the curve and if you know the temp of the exhaust when the engine is at running temperature.

As far as how it actually works is basically you want to have it so that at the ideal rpm range you're tuning for, one pulse from one rotor (or a cylinder on a piston enigne) goes through its pipe and into the collector right after another pulse is there, so the 'vacuum' behind it helps to suck it out.

Two-stroke piston engines work more like how intake pulse tuning works, which is reflected pulses from a closing valve to impart more force and shove more air into an open valve, but in reverse: most two strokes dont have a real exhaust valve and theyre trying to use controlled reversion to prevent over-scavenging - they dont want to blow the charge out of the cyl and waste fuel and lower cyl pressure.

Also, rotaries are not two strokes... so that kind of tuning doesnt really work.

EDIT: https://www.rx7club.com/showthread.p...ning+resonance <- this has already been covered in '03 by one of the more knowledgeable people here

ericgrau

iTrader: (1)

1. A Y pipe isn't that restrictive. Even if it is, a bigger Y pipe will solve any problems.

2. You don't want to hold heat in. Cooling exhaust contracts it. It slows down b/c there is less exhaust (and therefore less restriction). However, even a dual exhaust won't cool the exhaust much anyway.

3. I'm afraid the tuning is still confusing. You seem to be missing some explanation around the "vacuum" and other parts.

As an engineer who works with fluids, I can say the simplest way to get power is to make your pipes, cat and mufflers bigger... up to a point. After that you don't get much additional gains. No other method can do any better, though it may save weight, money, etc.

Nihilanthic

The restriction comes from TURBULENCE. Getting the proper shape to split a flow of air without imparting turbulence is very hard. Have you looked at air horn designs for ITBs or porting designs? It has to be smooth, NOT AN EDGE, and contoured. Most Welded Splitters are not very effective at that, youd have to cast a coupler to use or use a CNC machine or a talented person with a griding tool and a lot of time. Once you have laminar air flow, the best thing to do is leave it the hell alone! The way to make air flow is to give it a place to flow where it wants, not try to make it go where you want it to.

Yes, you do want to hold heat in. You heard of header wrap? Even if it contracts, it still slows down, meaning the exhaust 'behind' it is still going to run into it and find it to be a restriction. And yes, twice the metal with twice the surface area would give it more of an opportunity to cool down, having TWO pipes to radiate heat and TWO heatsinks to conduct heat away.... and twice the surface area to create friction. Yes, air can find friction when flowing against something, even though its very insignifigant.

Its an inertial thing - if gas is flowing into a tube through an oriface that is suddenly closed, it wil continue to go, and there will be a low pressure area behind the compression wave of hot fast moving exhaust gas because the oriface is closed and the only gas thats filling that area is from the pulse itself being pushed back by the pressure within the pulse itself. Proper pulse-tuning fills that with more high pressure hot air so its a laminar flow and just gets the hell out of the pipe, and helps to suck more pulses out along with it into the vacuum too.

Also, as an engineer, tell me why twice the weight for a theoretical decline in flow is a good idea? You'd have to go way out of your way to make a splitter that flows well and doesnt impart any turbulence, and theres no reason to not just use a single, tapering pipe!

Icemark

No, quite the opposite.

Something that you are missing that the exhaust volcity slows in a bigger pipe, and something that it appears both you and Nihilanthic are missing is that on a NON turbo, the exhaust is still burning and expanding as it goes down the exhaust.

So since fluids don't expand, fluid dynamics are limited to base exhaust engineering on.

Icemark

Ah, yeahm objctive ignorance??? who seems to have the ignorance here???

Tell you what, my research is matched exactly by racing beat, and several racing teams.. and what did we all come up with independently of each other???

True dual duals flow better and produce more HP on a non turbo rotary engine.

So until you have spent countless hours dealing with mutiple designs and figure out what really works best, well, you can feel free to try and think that you understand the exhaust flow on a non turbo.

Come out here, put up the money for more dyno time, and I'll show you in person what produces more power.