Hi all,
The shape of the exhaust ports on the TII engines is oval (about 2.25" wide, and 1.75" high). I'm trying to make a custom header for my application, and wanted to buy one of the pre-made flanges. I went to MazdaTrix web site, but their flange opennings look circular. I called them up, and the guy said (and i quote): "what you're worried about, you don't need to worry about..."
It seems to me if the port hole and flange hole don't match up, you'd have a sharp edge in the exhaust flow.

What am I missing?

 

The ports are round where they meet the manifold. Are you thinking of the port shape inside the housings?

 

The RB flanges and the factory gaskets are a LOT larger than the factory port exit.

This is good. You want a mismatch like that (small going into large) to reduce reversion.

 

ok. i just went out there and looked...
Blue TII, you're right that the engine side of the port is round. I had made my measurements on the manifold.
I guess PeeJay's answer explains the mismatch.

I just always thought that you wouldn't want to have a sharp change (even an increase) in the path of the exhaust (or intake), b/c of excessive turbulance.

Either way, your answers solve my problem with the MazdaTrix flange.

Thanks

 

Uh, bullshit - it also causes turbulence which impedes smooth gas flow.


-Ted

 

Uh,I though manifold "step" is a matter of tuning choice. Most agree for full out top end flow a matchport is best. Alot of people do use step to fight reversion caused by pressure waves bouncing back and forcing exhaust back at engine (like opposite of tuned intake systems using pressure waves to supercharge).

 

You guys need to take some fluid dynamics classes, or go learn exhaust system tuning from some experts in the field.&nbsp "Reversion" really only poses a "problem" when you have a "Y"-split in the exhaust system; reversion is the term that defines gas flowing the opposite was of normal exhaust gas flow out of the engine.&nbsp I'd like to see this possible with a "single pipe" system!&nbsp Smooth transitions are the RULE in terms of building exhaust systems.&nbsp I know there's a lot of funky exhaust "systems" that tout minimizing this reversion effect, but I don't see Formula 1 cars messing with this crap.


-Ted

 

Yes, race exhaust/intake is tuned for a small operating range. Everybody I have heard recommending step was doing so for street aplications. Manufacturers spend alot of money tuning intakes and exhausts to utylize the pressure waves or decrease their effect.
Reversion waves are a pressure wave exerted in the opposite direction of flow-whether or not they force flow backwards, they still affect flow. Right?

 

ok, my 2 cents' worth (maybe 1.5 since i'm new to rotaries)
Clearly, doing a step increase in flow cross section limits reverse flow more than forward flow. So, it's at best a compromise. This was just strange to me b/c with 4 stroke engines you take care of reversion flow control with valve timing. So i had never seen it done like that.

I think the concensous here is that doing a smooth transition increases max flow, but also increases reverse flow.

I wonder if this setup is common among 2 stroke motors as well, since they don't have a valvetrain either.

-Mazda

 

I'd to hear the explanation on how the exhaust gases reverse direction, because the exhaust "cycle" is suddenly stopped?&nbsp This is exactly what happens when each rotor face "dumps" the exhaust gases out the exhaust port...


-Ted

 

OK Ted, heres how a reversion wave is set-up in an exhaust system. The apex seal moves accross the exhaust port and the rotor forces the exhaust out w/ a great force and very abbrupt opening (compared to piston engines valve opening). This strong pulse combined w/ the extended exhaust stroke duration is what allows rotories to spool up such large turbos, right?
Now, this exhaust pulse carries a sonic shockwave/pressure wave (no duh, ever heard a rotory running w/ no exhaust?) that travels at the speed of sound w/ the exhaust. The exhaust is most dense at the port/runner walls (basic flow phsyics) so when that shockwave hits its first restriction (sharp manifold bend, collector full of slower moving gas or in this case a buch of slow moving gas stacked up in front of the turbine wheel) and bounces back it will affect the dense gasses at the wall the most. If there is a step present it can break up the shockwave and relieve the affects of the reversion wave.
Why do header manufacturers use steps in the runners before the collector? Because, before the rpms the collector is efficiently scavenging (using these same pulses) the steps will reduce the negative affects of pressure waves. Torque step- helps power below the point of collector scavenging rpm. Why would manufacturers waste thousands on design/materials and production of this if it was irrelevant?
Hell, this is the same basic principle that 2 strokes use to make a tuned pipe scavenge exhaust. Look at that pipe- little, big, little. Why don't they keep a nice consistant diameter for maximum fluid flow? Because thats ignoring ALL the other pysics of exhaust systems.
Now, having said all that; I'm port matching all my stuff too. I don't care about the low rpms, I want max flow. Just trying to let you know why some tuners mandate "step" for the street. Ian

 

Excellent post

 

Nice argument, but I find quote a few misconceptions...


I'd like to hear argument and calculations that it does travel at the speed of sound.&nbsp The 700mph+ Mach I wall is something of a barrier which you don't want to break; if the exhaust is indeed breaking Mach I, won't we all hear a "sonic boom" somewhere?


This I find hard to believe.&nbsp Fluid dynamics and boundary layer theory claims just the opposite - "fluid" (i.e. gas) velocity vectors are highest in the center, away from the walls, which implies that fluid/gas concentrations are in the center.&nbsp "Density" is a misnomer, as it takes a Cray computer running fluid dynamics simulations to find out where the true highest density is - I can safely say that highest "flow" is easily in the center, away from the walls.&nbsp Thus the rate change &#916 is highest in the center.


Turbine wheels are not part of the argument.&nbsp This is fine and dandy except you're totally ignoring the scavenging effect from the other port.&nbsp Due to the high velocity exiting from the other port, it pulls the exhaust gases from the current port away from the combustion chamber.&nbsp What you're arguing is correct, but you're describing a static system which isolates the current port (and exhaust gas) from everything else - the other port has a LOT to do with the dynamics of the "whole system".


Sure, it'll break down the coherent exhaust pulse, but you're still ignoring the scavenging effect from the other port.


I've never seen a top-tier motorsport exhaust "header" that used a "step" in their exhaust system.&nbsp Care to drop some pics on your proof?&nbsp Consumer grade exhaust systems are not valid proof of your argument - it's just about as believable as Borla's intercooled exhaust tips BS.


That's right - what you're describing are "expansion pipes" - I got no beef with those, but a "step" is no-where near a design as a typical expansion pipe.&nbsp Adhere to the golden rule of 6:1 on the expansion pipe (see Corky Bell's Maximum Boost), and you have a smooth transition that breaks up the coherent exhaust pulse.&nbsp Expansion pipes are places at exhaust nodes that would accelerate the exhaust pulses due to it's expanding nature - these are tuned for specific RPM's, as exhaust nodes constantly change versus RPM's.&nbsp If you put an expansion pipe where a known exhaust node is, you get greater velocities of the exhaust gases exiting the engine, and therefore greater scavenging effect from other rotors/cylinders.&nbsp This is no where near what a "step" does.


You seem to have some idea of what all the fancy-schmancy exhaust designs can do, but you still need to brush up on their intricacies[sp?].



-Ted

 

I like apples

 

na strawberrys

 

? How can the sonic component of the exhaust pulse help but move at the speed of sound through the exhaust? What am I missing here? No, I don't know the mph speed of sound is through X degree exhaust at X pressure, but I assure you its still moving at "the speed of sound"- by definition. No I've never heard of a sonic pressure wave creating a "sonic boom" either- how would sound travel faster than itself?

You say fastest moving gasses would be at the center of the runner. How then can you refute the cooler slower moving gasses at the walls would be more dense?

Yes, in a spooled turbo running in its efficiency range there is an strong scavenging affect from the other exhaust pulse and the turbine wheel rotating w/ inertia-I am talking about when a turbo is being accelerated and is providing lots of backpressure- ie stacked up exhaust at the turbine wheel. This part of why huge exhausts work great on turbos, right? Lots of pressure differential between sides of the turbo to accelerate the turbine quickly up to its efficient operating speed.

Ignoring the scavenging affect? So scavenging happens at all rpm ranges no matter what the runner to collector length is or what the manifold backpressure is?

No, a top tier (race application) header would NOT use a step as they will tune it to provide maximum power at the (narrow) operating rpm and a step does cause a bit of turbulence and therefore a bit of power loss at maximum flow. As I said, everyone I have heard mandating step does so for STREET applications- your usual consumer application. Thus consumer grade.

OK, where did I say the step HAD to be 90 deg to the runner. Look at the step Mazda designed into the stock turbo 13B exhaust port. ~45 deg vertical step from less than 1" high port to 2" outlet. Expansion-yes, but why didn't they choose a smooth transition as you say is best? You address the first part of a tuned pipes funtion on a two stroke- expanding exhaust by increasing the volume of the system to create a low pressure area at the port. If this was all they wanted they would utylize a megaphone pipe like a 4 stroke. But the tuned pipe is reduced at the other end to reflect the sonic pressure wave back at the exhaust port to keep the incoming air/fuel charge in the cylinder. Ever wonder why turned pipes have a smooth expanding front section and a more abbrupt conical section reducing the diameter? Ah! It reflects the pressure wave, but in this case back at the engine- as it is facing the opposite way of a "step". Sounds like the same pysics principal to me...
Yes, I do know just a little about the pysics of exhaust systems; but it seems enough to explain the principle of step and why it is used. I am alway open to learning more. So school me if you can.
If you think the sonic energy shock wave of a rotary periphreal exhaust port opening is not enough to affect exhaust flow at all, think of the level of noise of the engine w/ just a turbo and that of the same engine w/ nothing after the exhaust port. Where did all that sound energy go? Now think of the level of noise of a two stroke w/ a turned pipe (and no muffler at the end) and the noise level w/ nothing after the port. Pretty damn close to as loud, but that little sonic energy that has been reflected back at the port has served a vital and sought after affect on the intake charge.

-edit- wow, its late. I don't know what the hell a "turned pipe" is either...tuned pipe.

 

I'd like to see proof that this is the case; I am not about to take your word that exhaust gases travel at the speed of sound - can you provide proof of such claims?&nbsp Any object (yes, gas can be construed as being an "object") hitting Mach I will cause a sonic boom - see below.


An exhaust pulse is <> a "sonic pressure wave".&nbsp There is a reason why engineers make sure piston speed does not break Mach I in a piston engine; breaking Mach I in a piston engine causes all kinds of havoc due to the sonic boom created in the combustion chamber.


Sure, colder on the outside; hotter in the center.&nbsp Sure, slower moving gas on the outside; faster moving gas in the center - boundary layer principle.&nbsp Does this mean it's the most dense at the outside?&nbsp No, it doesn't.&nbsp Let's look at the ideal gas law - PV=nRT.&nbsp You're still ignoring P (pressure) and R (gas constant) - in this case the word "constant" is a misnomer, since you're talking, literally, and about a combusting exhaust gas that changes it's characteristics while it's burning.&nbsp To make an assumption that lower temps + slower velocites = more dense is an ignorant assumption.&nbsp Like I said, it'll take a super computer to computate the gas densities due to all of the variables, and I doubt you nor I could do this with a pencil, paper, and an HP-48S.


Who the hell put the turbo into the whole argument?&nbsp I thought we were stricting talking about the port step, independent of what was downstream?&nbsp I always assumed it was an NA application, or at least elimination of any turbo system that would introduce any change of variables (i.e. back pressure) to the port "system" itself.&nbsp If the turbo were introduced, and you're going to argue increased back pressure, you're still only increasing P, pressure.&nbsp Velocities are still going to stabilize to some x velocity, which makes the whole system still pretty stable.&nbsp Now you're going at argue changing conditions where boost is being built, and velocites and back pressure are going to change?


Sure, to some degree.&nbsp Scavanging effect might be optimize to a certain ("narrow") RPM band, but it's still there.&nbsp With the rotary engine, with it's 180&#176 phase between front and rear rotors, you're still going to have some kinda scavanging, unless you're going to throw a variable length exhaust system into the whole mess?


Narrow RPM operation is a bit overemphasized here.&nbsp I'd bet these race teams would also like to have a broad power band if they could attain it.&nbsp Drag racers that run 2- or 3-speed automatics need to run a broad of a power band as possible due to not being able to shift into the "sweet spot" of the RPM range - these guys all run smooth headers with smooth transitions.


Cause Mazda isn't looking for "best power" - the whole concept of being able to design a mass produced automobile is about compromises.&nbsp NA's have the port insert that kills sound.&nbsp There is also a port air system that is built into that exhaust port sleeve that's for emissions.&nbsp Care you explain how that adds into your whole argument?&nbsp I'm not a Mazda engineer that designed that whole exhaust port configuration, so I can't refute or support what you claim or cannot claim.&nbsp I do know that Mazda went into a LOT of trouble on that exhaust port sleeve system to reduce noise (on NA's) and for the emissions system (port air).&nbsp If you don't believe me, there is an SAE paper on exactly this subject you can read.

Bottom line, if I had to design an exhaust port, it would be smooth for best power, period.&nbsp I don't have to worry about noise or emissions, which Mazda did have to worry about.&nbsp This is all I can tell you at this point.


It seems you know your 2-stroke exhaust theory; you have a leg up on me in this area.&nbsp As a save grace to my ***, we're still talking about the 13B port step and not some 2-stroke piston engine.


Like I said before, only the Mazda engineers who designed the whole system can tell you what they were thinking and can back up their R&D with data.&nbsp To assume it's some kinda performance design is a bit presumptuous?


Like I said before, 2-stroke engine theory doesn't really apply in this case.&nbsp All the SAE papers I've seen about rotary exhaust tuning really do not address the exhaust pulse and "reflected" energy for performance gains.&nbsp Mazda did know about the port inserts decreasing noise.&nbsp Mazda did know that higher exhaust pulse energy being very conductive to turbocharging.&nbsp If what you're describing is true, why don't we all run 2-stroke type exhaust systems for better performance?




-Ted