23Racer

These were used back in the late 90's in 4 stroke headers built to fit into tight spaces to compensate for unbalanced or wrong size primary pipes. With all the focus on header and pipe length tuning and possible mis lengths, would these be a possible crutch to assist in tuning NA rotaries? I have them on a set of headers for my other car and thought, hmmmm.......

Comments?

Eric

peejay

Stepped headers is nothing new in the piston world, seems to go in and out of fashion.

23Racer

Peejay, they are not just stepped headers, they have a built in anti reversion cone which substantially reduces the back pressure pulse being seen by the engine. My other set for the Cougar are stepped with the pipes growing in size the further away from the port outlet, but these do that, plus have the cone.

I really need to ponder this.

Eric

RotaryEvolution

iTrader: (30)

sounds like a gimmick but try it and see.

anewconvert

iTrader: (1)

It's not a gimmick. The idea is that when the pulse reaches the collector it slows down. The gas behind this is still moving at the accelerated velocity and starts to pile up behind the collector. In effect this causes a reverse pulse back up the primary to the port. At certain rpms this reverse pulse can get back to the port at the same time as the exhaust opening event, limiting exhaust flow. Essentially the opposite of scavenging.

The cone gives the gas any area to expand into and limits the velocity of the reverse pulse at the port. Instead of the all the energy from the pulse trying to push its way into the port some of the energy is routed around the cone and dead ends into header wall. Seeing how sensitive NA rotaries are to exhaust scavenging I am actually kind of surprised this topic doesn't come up more often.

peejay

Yeah, it looks like the tailpipe tip anti-drone devices I make (cost: $2.99 at AutoZone) except the goal there is the opposite.

GSLSE-YA

This is interesting, though it is better to time the reversion so that there is no reversion at the desired rpm range. But as you mention, it's hard to figure out the length of primaries and or spend the time fine tuning the exhaust to figure this out. Although the exhaust port is always open because the apex seals are only 2-3mm and the exhaust port is much larger. So any reversion would be bad on a rotary with peripheral exhaust port.

I Think the reversion has an effect, but probably not nearly as much as having a crapy collector angle. Using the low pressure from one primary to scavenge the exhaust from the other primary exhaust port probably has a greater effect. The scavenge effect has the same problem though, how long should the primary be for the rpm range desired.

It would be best to have the exhaust from one primary passing into the collector (in doing so, also passes the other primary pipe) while the other exhaust port is towards the middle to end of its exhaust cycle (in order to help scavenge). Why at the middle to end of the exhaust cycle you ask? For ported rotaries we have intake/exhaust overlap correct? If we have scavenge effect at the end of the exhaust cycle, wouldn't it make sense that we will also scavenge some intake simultaneously (scavenge is just another term for vacuum)? This might help draw or vacuum in some air/fuel at the very beginning of the intake stroke. After all, the intake port has started opening before the exhaust port is fully closed.

But if the scavenge effect happens at the end of one exhaust cycle, it also means that it is also scavenging at the beginning of the next exhaust phase. As I mentioned earlier, we all know the apex seal is only 2-3mm thick, so during the end of one exhaust phase there is simultaneously the beginning of the next exhaust phase.

Does any of this make sense to anyone? I'm getting dizzy just explaining it.

23Racer

I completely understand what you are talking about. What I also think is that, just like a 2 stroke, rotaries are very sensative to exhaust tuning and you need to take the whole system into consideration when you test for differences.

I look at the generally recognized best practices approach systems with the larger radius bends out of the keg, 1 7/8" or 2" primaries down quickly to an extraction (expansion) zone or chamber with an H pipe before the chambers, then back to a single 2" or 2 1/2" single pipe out the back. These types of exhaust always have a lot of energy when you stand behind them and feel the pulses coming out of the back. They are generally, loud, raspy and harsh sounding, which again reflects lots of energy transfer out the tailpipe. My system is a Racing Beat Header, extended back through 2" pipes all the way to the rear subframe, then collected through a Burns style venturi collector into a single 3" pipe. Then through a pair of mandrel bent 3" 90 degree bends then through a race 3"ID Borla then out the back.

With this system, the exhaust pulses are softer, the engine note has always been quieter and sort of like the sound is on Dolby with the peaks and valleys removed. Almost like a turbo car. So what I am wondering is if the 3" pipe setup is too much even with the substantially reduced back pressure. Do I loose too much exhaust energy by expanding it to 3" from 2" and leaving it at that all the way out? Is this reducing the scavenging effect of the exhaust pulses in the merge collector? That is why I am looking at Megaphones or anti reversion cones or what, to get the energy back. My old 2 stroke racing karts and motocross bikes had a certain ratio of primary pipe to expansion chamber to tailpipe ID's for them to work correctly. Also, when I go back and read the Jim Downing, Dave Emanual book, the exhaust diameters are a lot smaller than mine.

Ideas, comments would be appreciated.

Eric

23Racer

Further information to help muddify the discussion. This information comes from a World-Class Pro-Stock bike header producer and really stops to make you think;

if you reduce tube diameter you need to step it at least every 8-10 inches or so and it can be shorter if you work with final choke diameters. The megaphone and reverse cone help to keep power going past power peak for what would normally be a too small header. Stepping quicker helps everywhere. Last point I would make and I have made it on several other posts, just because the factory/engine designers designed a exh port that is say 3.2" long to the header interface does not mean that that is the best or proper place to have the exhaust experience its first "sudden" step. The PS bikes I have worked on (Suzuki's and Kawasaki), I have to do a lot of work to do what I would call properly exit the head. Most stuff I have seen has a very large pipe coming off a very small port. A serious loss of exhaust energy given up early in the event. IMO.


And then he added this;

It helps to not think in terms of "port" verses "tubing". The tube becomes the port. You do not introduce tight radius for the same reason you do not do it in the ports. The only way to overcome the loss, especially in the transition area and the first 6-8" of the header, is by using larger tubing diameters. And the problem keeps going down the toilet. "Well, if we made a bigger header, it would make a little more power",(because we overcome the flow loss up front). But the larger tube now needs an even tighter radius to clear the obstruction... And you are now so far away from what the engine wants for a header, it really does not matter what you do from there, the engine won't respond to changes.

The smallest diameter tube allows for the largest radius. It can even be smaller in area than the port, especially if it allows a larger radius. (the application can trump this though)

The first 6-8" of the header is just as important as the first 3 or 4" in the cylinder head. The same rules and thinking that a head porter uses apply to the header. Most headers I see break all the rules.

The last thought is this, the reason the first 10" of the exhaust track is so important as opposed to just the port in the casting? The gas particles, the mass...only gets that far before the ex valve closes behind it, and it no longer is connected to its pressure differential, if you will. At that point in the exhaust track, everything changes. But anything you do to the "particulate-mass" flow in the port while the valve is open that violates flow, will cost you power and you can not get it back. You want to get the "mass-flow" as far away from the ex valve and the cylinder as you can. Larger "anything" in this area does not usually help.

I do understand that many applications force the header to break all these rules and larger is the only answer if you need maximum power.

Quote:
In my world, reversion is the biggest problem. I define reversion as pressure waves from the other cylinders in the system going the wrong way at the wrong time. They hurt a lot. Venturi collectors, 421 tube layouts, and controlling sudden area changes have all helped to control them. It is no longer beneficial to try and stop them in the primary tubes, and your observation is the reason, I think. A "reversion dam" also hinders a negative wave also, and that is not good!
I currently use a large step, ie-3 tube sizes, in some pro-stock headers I build, but it is not at the head surface, I move it out about 6 inches. They can work for some tuners but I see them as a way to assist their need for a rather large header.
ProStock,and subsequent benefactors,has been under a relatively steep learning curve over the last 4 years or so. Since Anderson tapped into to Cup technology, advances in ring seal, valve train/Cam etc, take any focus away from headers. The need is for a header big enough to not get in the way of the other advancements. This is good! Eventually they will be back into the headers searching for any more power thay may have left behind in the rush!

Quote:
I think almost all my efforts over the last ten years with PS type engines is to increase the velocity and for a good % of my customers it has been a distinct advantage at times when others are simply making everything bigger. An exhaust system that is only helping with pressure wave tuning is identified not by a lack of horsepower but rather by a rather sudden drop in power after the peak. A exhaust system that also incorporates or tries to add velocity at least in the first third of the system is identified by at least the same power level but it has a much broader curve and carries much farther after the peak.
I have found over the years, and this is what I started this thread with, is that I can only reduce the area of a port I am given to build a header for by about 10-12% before the power falls off. So my conclusion is that maybe the ports themselves are too large and why can't we try smaller ports and go from there! It has been a frustrating journey


Stops to make you think.

Eric

GSLSE-YA

I agree with this. although a fairly big difference is that the exhaust valve closes the port in a piston exhaust, where as a rotaries is constantly open. There is never a time when exhaust is not flowing out of the exhaust port.

My current header only uses 1.75" primaries, then steps up to 1 7/8" at about the 8" mark. Then collects initially into 2.5" and tapers to 3". I personally think 2" initial primaries are on the big side. It's like you said in your post, it is better to have slightly smaller initial primary but have a larger radius turn.

Let us know what your results are, good or bad. I gained almost 20whp when I switched from a RB road race header and garbage collector to a much better designed header and collector.

Cheers!

j9fd3s

iTrader: (3)

+1 to this! if you get a set of calipers out, and start measuring the port and header its really obvious with an RB header, this junction isn't good.

IMO, the FC sleeve exit is too big (bigger than the MFR engines!), and the RB header is a good ID (Mazda uses 43mm ID for the MFR engines, RB is 43.9mm), but they start in the center of the bend, etc.

so that is pretty simple, and getting a primary length that the engine is happy with is pretty simple too (although i seem to be too lazy to do it). cut and try...

aside from that, since we aren't sure what theory even applies to the rotary, its hard to say what works and what doesn't. for instance i'm not sure velocity matters much after the collector...

Rub20B

Making a long story short, it seems that the stock TII and FD ech sleeve inserts are too much of an aburpt expansion, so the question is, what expension ratio works best for a properly built 13b PP engine and what initial ID pipe should be used directly after the rotor housing flange surfica

j9fd3s

iTrader: (3)

the attached is what the MFR port supposedly looks like. notice the exit matches the header pipe @43mm....

i'm not sure its exactly to scale as the MFR port is bigger than the FD/FC. the 500-3700mm refers to header lengths tested, although they don't supply enough info to really choose one