tuj

Yes, as the frequency of the pulses increases, more air will end up stalling in the exhaust, which decreases the spacing of the rarefactions.

globi

This is a reasonable explanation but this also means that the whole thing is more complicated to model or in other words a relatively simple mathematical model to calculate what pipe would be optimal might not be as exact as you'd like it to be.

This is how you can look at it: Torque is pressure (brake effective mean pressure) times area times lever. On a given engine the area and lever is obviously constant. So the pressure is the only parameter that can affect torque. If torque is constant so is the pressure. The pressure (BMEP) is proportional to the amount of oxygen that gets burned and the amount of oxygen that gets burned is proportional to the amount of air entering the engine. So if the torque over an rpm range didn't change you should be able to expect that the amount of air that entered the engine per pulse didn't significantly change either.

TeamRX8

iTrader: (25)

theory is only that, if the magnitude of reality is insignificant then the fact that method A theorizes to be superior to method B is irrelevant

which makes arguing about things you have no practical experience with, pointless

\\Konig\\

iTrader: (4)

My eyes just fell right out of their sockets when I saw the length of that post.

GJ

globi

If that's the case then you indeed deal with a changing airflow per pulse but you also don't deal with a constant torque. Constant airflow per pulse can only be the case as long as the torque is constant. However one can say that the Renesis has a more or less flat torque curve.

What's also interesting is that the incoming air follows the piston or the rotor more or less.
On the exhaust side the air leaving the expansion chamber is initially way faster (supersonic speed) than the piston or rotor, which doesn't make it easier to model it (not that anyone claimed it's easy anyway).

rotarygod

Both of you actually have correct points. Airflow is completely dependent upon the efficiency of the engine at that particular rpm and load. The key to this though is based on efficiency at those spots. It's not always the same rpm or load for every engine so we can't say it is necessarily rpm based as different systems are designed for different rpm ranges. If you aren't at 100% efficiency, you aren't at 100% of the total combustion chamber volume. At least not on the fresh air side. This is why boosted cars need a larger exhaust. They have more air coming out due to the efficiency difference. Yes each pulse takes up a certain amount of space. How much space is determined by efficency. The pipe also puts considerable resistance on the flow of the gasses. The more pulses we try to add in the pipe per amount of time (as rpm's rise), the less room there is in the exhaust for each pulse as they get compressed together. Eventually we get to a point where theere is too much resistance and we start losing power. This is somewhere around 240-260 ft./second of average exhaust velocity. Aiflow is not constant as it is always pulsing. If we hit this speed but have not hit our target rpm, we need a larger pipe to flow through. This is merely dealing with the actual flow and not the acoustics aspect of it.

I do feel that the center exhaust pipe should be longer. Possibly twice as long as it has twice as many pulses in the same amount of time which is twice the frequency. Therefore we need to tune it to half of the others (not twice) and let harmonics do the rest.

tuj

I am baffled by this. If the center runner has the exhaust from both rotors in it, isn't this in some ways the equivalent of one rotor spinning twice as fast? If that was the case, conventional theory would say that high rpm operation = shorter headers. I know this isn't exactly what's happening, since the exhaust does not have twice the velocity, however wouldn't making the center pipe HALF its length be better?

That being said, doesn't it seem that by keeping the center flow seperate from the outer flows for longer makes contention worse? Without overlap, the tuning of scavenging is much more critical in terms of preventing reversion. Unless the phasing is exactly correct, the center flow scavenging effect will be competing with the outer runner for flow from the rotor.

rotarygod

From a flow standpoint, as long as all runners have the same total length to the back of the car (regardless of individual collector point) all pulses will eventually interact with each other from each rotor at the same spot. I don't just want the center runner longer, I also want to collect it twice as far back. This would mean that the distance from each port to the back collector (as there would need to be two) would be the same overall distance. The front and rear ports would collect at one distance and then the center port would collect twice as far downstream. This would make it exactly twice as long in total length and tuning. I don't want to make the center runner twice as long but then collect it at the same spot as the others although that would be an interesting experiment.

tuj

No, I understand you want to make a dual-Y header, I just don't get what you think it will achieve by merging the center pipe twice as far as the outer pipes. It sounds like a backwards tri-Y design. Normally, the thought is to keep out-of-phase pulses from interacting with each other until after the in-phase pulses have merged. Your design sounds like the typical design for a siamesed 4 cylinder engine. Unless you are going to use an extra capped-off runner to create an interference design? I've seen it on V-6 manifolds.

rotarygod

Actually you've pretty much got it figured out.

SureShot

In my observations so far, Mazda did a damn good job building a stock header, and the EPA did a good job of killing its potential.

Acoustic resonance tune is a big deal on both the intake & exhaust side of auto engines.
You see one of the best stock intakes in the 8.
Now we just have to engineer a good practical variable tune exhaust.
Anybody know of a quick acting valve that will stand up to hot/dirty/pulsing exhaust gasses?

rotarygod

The stock manifold actually isn't that nice. The problem is that the stock exhaust ports are far worse. As long as that's true, no header will do very much. You need to fix the worst areas first.

WantedTwo

iTrader: (4)

Ohhh my head. I need a nap. Great read though

TeamRX8

iTrader: (25)

based on some of the latest responses, some people either can't/won't read or they're brain dead ...

Wing5

^^^
http://www.youtube.com/watch?v=ZfmpZ3IIHOQ
Fast forward to 2:34. Nuff said... (What a great movie P.S: I have the same job as the flight deck crew )

Now, could you figure out how to put that on my 8, that'd be great, thank you.
Great read rotarygod ^.^ Makes me wanna go read some of my books on Bernoulli.

rotarygod

Your link isn't working. It shows up as javascript.

Wing5

works for me =/ http://www.youtube.com/watch?v=ZfmpZ3IIHOQ hopefully that works, even if its the same link

rotarygod

It must have just been the computer I was at earlier.

Wing5

So u were able to see it then? Whacha think? Similar to what you were discribing? Again thanks for taking the time to write all that up even if it was over a year ago ^.^

sosonic

Somehow I did not find this on my previous exhaust searches. This is great stuff from RG and a very good read.

paulmasoner

iTrader: (1)

RG, WOW.. that was a lengthy read but WELL worth the time involved. You have once again further developed my pre-existing knowledge, and enlightened me on whole new areas. One thing I'd like to tell you is; about the back pressure statements and your illustration of the straw(OMG LOL) and the compressor tank..... I have had the same argument with a co-worker of mine. Now i did not fully understand what i was saying as well as you do, but i KNEW that bigger is not always better in terms of efficient exhaust gas movement, to me it seemed like "dee-dee-dee" physics? Since I and my buddy are both firefighters, i explained it to him in a way he would understand. When we flow test hydrants with a pitot gauge, it tells us the pressure of water being discharged, knowing that and the diameter of the discharge we calculate actual volume(and of course we could then calculate mass if it was relevant). With this thinking, i was able to convice him that bigger is not always better, if the pressure doesnt change but the diameter does, the flow rate will change. And we all know that we want the exhaust out fast and efficient. The idea/fact of scavenging w/o port overlap had never crossed my mind as a possibility, but after reading, it too seems like "Dee-dee-dee" physics...

rotarygod

Strangely enough there are still people that call bs on that phenomenon yet it's already been proven to be true.

csl

Rotarygod, I need your advise but my english is not that good. To make it short, do you think renesis 6MT should remove cat to make the engine last longer (maybe can have less carbon stuck in the engine due to less back pressure)?

TeamRX8

iTrader: (25)

nmarz77

Not sure why anyone wouldn't believe it. It's been being used for years now on true dual exhaust systems using an h-pipe or x-pipe......noting mustangs in particular.