Japan8

port timing is in a post I made on here quite awhile ago.... here you go... https://www.rx8club.com/showpost.php...3&postcount=17 Merry Christmas... I didn't tell you to search.

That thread in general was good... read if you have some time... https://www.rx8club.com/series-i-tech-garage-22/turbo-kits-aux-ports-4372/page2/

rotarygod

Since the Renesis rotors each have 2 exhaust ports, 1 shared and 1 not, it is the center one that is already throwing the chamber out of balance. The key to retuning it's length differently is to bring balance back to the system. There is more to tuning than just length. You need to consider how much is going through that pipe and what effective resonance it will have as a result. My idea does not necessarily make the center port twice the length although it could be up to that. The center runner, even siamesed, is still smaller in area than the outers. Diameter of the pipe also plays an important role in tuning in relation to length and what flows through it at what speed. As to what I mean by what flows through it, I am really referring to the effects of having air in the pipe twice as often which would result in a higher velocity of gasses through the center runner even though the total flow volume may be lower. Yes this is possible. To get an idea of how this effects tuning, you need to know someone that plays the flute. Blow through it gently making one note. Now only change how hard you blow through it making it much harder. You will get a much higher note and nothing else changed. You don't necessarily blow twice as much air but you did increase velocity through the flute to change the resonance point. For this simple reason alone you need to retuen the center runner to bring it back all into harmony. I know how it all works, but putting it into words isn't as easy.

Exhaust tuning is absolutely not a waste of time unless you don't care about getting the most out of the engine. Then it would be a waste of time. The Renesis exhaust ports open and close more gradually than a peripheral exhaust port which more closely resembles the characteristics of a piston engnies valves. Valves do not just pop open. The ramp speed of the camshaft profile affects this and it is relatively mild compared to a peripheral rotary port.

DARKMAZ8

Wouldn't unequal length headers sound awful?

r0tor

iTrader: (1)

but once the valve opens, you have the exhaust gases being pushed through a port of fixed sized by a piston of fixed area that starts from zero velocity, reaches a velocity peak half way, and then slows back to zero velocity. The result is a rather well defined pulse of exhaust gas - which produces a well defined frequency (and natural frequency) and a good shot a getting a good clean resonance at 1x, 2x, 3x, ect times the natural frequency.

on the rotary, you have the rotor sweeping by the port - meaning you have gases going through a continually varying port area from continually varying rotor area for the amount of time the port in open. Off the top of my head it looks like instead of getting a good clean pulse, you get a distorted and noisy looking pulse - which IMO limits the potential for resonance.


I'd sooner spend time on a making a header with equal effective length runners (effective length meaning equal resistance, meaning you have to take into consideration pipe diameter, bends, amount of flow, and physical length)... but hey, this is just my little theory

globi

Actually there are more crucial differences:

- A rotary engine and a 4 stroke piston engine both do have a solid rotor or piston expelling exhaust exhaust gases. A 2 cycle engine does not.

- Not only does the 2 cycle engine have more overlap - the exhaust port actually closes AFTER the intake port closes. This is why the expansion chamber design is so crucial. http://www.liebold.com/rumipages/esp...twostroke.html (There's no use for an expansion chamber in a 4 cycle engine - be it rotary or not.)

- 2 cycle gasoline engines have lower compression ratios, do have a high volumetric efficiency and have therefore a significantly higher initial exhaust pressure.

rotarygod

The problem is that exhaust ports/valves don't work like that except on a peripheral port. Even then velocity is not constant and is not at it's peak when the valve/port is open at it's peak. It is possible to have higher velocity at port open and closing but less than this at full open. It is all dependent on design. For the most part, the strongest pulse is when the port/valve first opens and it generally decreases from there. On a peripheral exhaust port this is very sudden and the pulse is strong as the port reaches it's full open timing very quickly. On a piston engine or side exhaust port, this is a much more gradual opening as it happens over a larger degree of timing. This makes the inital pressure pulse weaker than what a peripheral port would be but by no means insignificant. It it weren't of any significance, headers wouldn't matter on any engine.

rotarygod

Almost every car from the factory with the exception of a few have unequal length exhaust runners.

TeamRX8

iTrader: (25)

IMO it won't work the way you seem to think it will, but that's easier to say than prove. WRT the Renesis engine, the exhaust energy of each rotor has two paths to follow. If both paths are not equal in all respects the energy will bias itself toward the path of least resistance until balance is achieved. You're attempting to take a single port per chamber theory and apply it to a dual port per chamber situation. What you propose will exacerbate the imbalance, not reduce it.

The exhaust ports of a dual port chamber are cojoined at the hip. You can't treat each port as a separate chamber entity. It's not going to happen that way.

globi

'not sure whether this has been discussed - but the fact that there's no overlap appears to be a good explanation why headers don't seem to lead to significant gains on the Renesis.

The major reason why header increase power on a piston engine is because it improves scavenging. Scavenging increases the amount of burnable gas in the combustion chamber, which generates more power.
It doesn't surprise me that the 6 can get 20HP with a header and the RX-8 doesn't. http://www.cp-e.com/imgs/maz6s_hdr_hp.jpg

If I was to the design a header for the Renesis I'd mainly try to make it as light and simple as possible.

yiksing

So does less power necessary to expel exhaust gas mean better engine response? would the rotor be easier to spin faster? These all all very new to me!

globi

Well you slightly increase power. (Which translates to faster engine response).

This is how these 4 cycles work:

1. - power to suck the A/F mixture
2. - power to do the compression
3. + power from burning the A/F mixture
4. - power to expel the exhaust gases

Major power increases are accomplished by increasing number 3 which means getting more combustionable gas in your engine. Obviously you can increase power by reducing 1 and 4, but they are already neclectable.
Here's a picture of the 4 cycles: http://www.grc.nasa.gov/WWW/K-12/airplane/ottoa.html (compression and power stroke is what is relevant: See 2, 3, 4, 5 in that picture).

Back to the header design: I guess my question would be, why not simply have 2 main pipes leading to the engine. Why not get rid of the 3rd pipe and lead the exhaust gases from each side of the siamese port back to one of the main pipes immediately after the engine (after all they come from the same exhaust chamber anyway).
In other words: 4 very short pipes leading into 2 mainpipes and then into one. If nothing else it would at least be lighter than the 3 pipe design. Besides it would be easier to model - just need to calculate the length of the 2 mainpipes.

The 3 pipe header is essentially the same as if you would add 3 exhaust pipes on each cylinder bank of a STi.

Richard Paul

iTrader: (5)

I don't want to get into this since I have no rotary experiance. I'll leave it to the experts and watch. I have some math that will show that the reduced pressure will help the fill. Just replce the piston events with the proper rotor cycles.

I also don't know if the way I installed the shortcut here will work. If not go to page 5 of the Axial flow thread and get it here.






https://www.rx8club.com/attachment.p...chmentid=19461

MadDog

I started down the path of fabricating my own header about a year ago. I can't lay my hands on my original calcs right now since I'm not at home, but needless to say, I decided that it wasn't going to be practical to design for scavenging. I even found some real test data where we had measured the speed of sound in hot exhaust gases! As I recall, the length of pipe needed to tune for sacavenging was going to be greater than 10ft because the exhaust pulses come relatively slowly - basically one exhaust pulse from each rotor per eShaft revolution. You'd have to design the header so that the exhaust pusles were tuned to a SUBharmonic of the header. I remember thinking that piston engines had the advantage here because they have more exhaust pulses per second than rotories do.

Of course, I could be full of crap. What the hell do I know?

globi

If inlet port and exhaust port are open at the same time, reduced pressure in the exhaust can pull fresh gases into the combustion chamber.
In the case of the Renesis inlet port and exhaust port are apparently never open at the same time. (Which means no matter how much you pull on the exhaust side, it won't help pulling fresh gases into the intake).

Nevertheless reduced pressure in the exhaust will reduce the residual gas (from the last combustion) in the intake and therefore will help the fill. So a good header design will increase power even if inlet and exhaust port are never open at the same time. However it won't help the fill to the extent it could if there was overlap - where an appropriate header design can pull all the residual gas out of the combustion chamber.

jeffrx8gt

hey richie you are a genious, im not mechanically able but sort of understand and appreciate your info. question for you tho does it help just replacing a k&n airfilter without doing anything with my exhaust

tuj

Since there is no overlap, the only thing to do is flow improvement, by designing the headers to avoid interference during positive pressure. Its possible to tune for improvements at one particular rpm and see significant gains there, but usually no where else. Many do not realize that the pressure in the headers actually goes negative for part of the timing. Yes, the flow in the exhaust is actually going backwards sometimes. By tuning the header lengths to work together, you can get the out-going pressure pulses to line up and increase the pressure, or you can use interference to cancel out some of the negative pressure.

I'm not sure there is much to be had in headers here. The scavenging effect is the most crucial in terms of adding power, and it doesn't come into play here. This is a complex subject, because the pressure in the header varies by 3 variables:

-location of the pressure measurement in the header
-time during the combustion cycle
-rpm the engine is operating at

jeffrx8gt

hey japan 8 where did you get yours m symbol its cool
much appreciated

Richard Paul

iTrader: (5)

No

Richard Paul

iTrader: (5)

There are two waves, pressure wave and partical wave. The partical wave does not reverse. The pressure wave will reverse when it hits any sort of change. Not just the end of pipe. If there is a sharp increase in pipe dia it will act like it met the end.

rotarygod

I know what idea you have!

IZoomZoomI

we should set up a rx8 header fund for charles hehe

globi

If you'd increased the size of the intake and port areas such that you'd end up with overlap, a header would be more effective and especially top-end horsepower could be increased.
Unfortunately fuel efficiency and idle stability would suffer and emissions would go down the toilet.

(Note that overlap is not engine-type specific. Either piston or rotary engines can be designed with or without overlap. Also, the Renesis is possibly the only rotary engine without overlap).

tuj

Of note, every modern piston engine has overlap. Overlap doesn't necessarily ruin emissions or fuel efficiency. In the early days of engine development, it was thought that overlap would fail spectacularly.

Beodude123

I was wondering if you could get the tuning right on the pipe length / diameter if you could actually create a slight vaccuum inside the chamber after the rotor closes up the exhaust ports. While it probably wouldn't be as efficient as actual overlap, if there was a partial vaccuum inside the combustion chamber when the intake ports were uncovered, you would get more air into the engine....

I doubt this would even work, since somebody else said that proper tuning length is 10+ft...

globi

True. The Renesis is possibly the only engine in a passenger car without overlap.
Overlap doesn't necessarily ruin emissions or fuel efficiency and it might actually improve fuel efficiency at higher rpms. However, I believe it's fair to say that if engines wouldn't operate above 2500 rpm, they probably wouldn't have overlap. Overlap is mainly a compromise in order to gain horsepower at higher rpms.
Unless fuel is injected after the exhaust valve or port is closed, a small amount of the air/fuel mixture can always leave through the exhaust port (rotary or piston).

In the case of the Renesis Mazda obviously wanted that the unburnt air fuel mixture would get 'recycled' in order to improve fuel efficiency and emissions. If there was overlap the fresh gases could push the unburnt exhaust gases out of the combustion chamber. The piston engine doesn't have a moving combustion chamber and therefore doesn't deal with the same extent of unburnt fuel. I used to have an RX-3 and it measured about 3000ppm of unburnt fuel at idle, a piston engine from the same time measured about 300ppm. Actually the fact that the RX-8 still does have an air pump indicates that even the Renesis still deals with that issue to some extent despite side exhaust ports and zero overlap.