Michael_Rudy

Well i was thinking of making a set of expantion chamber type headers but though that would involve to much science to it. So i thought to myself.. hummm.. michael.. why dont i have megaphone headers. So my though was to make or by myself 2 mega phones starting at like 2 or 2 1/2' and have them getting slowly get larger to like 4 inch at the collector. I looked at the SDJ designed headers and it looks like theres slightly do that. Has anyone ever built a head like this?

fastrotaries

Like a two stroke exhaust?

trooper55

thats for two stroke motors, im not sure if a rotory motor would work the same if you did that

Cheesy

A two stroke type chamber isnt too hard to figure out there are lots of books on it, a tapered header would also help with the increasing boundary layer thickness which reduces the effective diameter of the pipe, ie the increase in area from the taper counteracts the decrease in area from the boundary layer.

Michael_Rudy

no well what i wanna do is basically have 2 megaphones coming straight from the exhaust port into like a 4 inch collector, so instead of having your header the megaphone it is all in 1

Doc Holiday

iTrader: (2)

You know what I'd like to know. Everyone says that the primaries on a rotary should be really long (collect at the back of the car). This goes against everything I've ever learned with piston engines. Why do the rotaries defy the laws that other internal combustion engines abide by? Why would I stick 100 inch primaries on my bridgeport when I want to make the engine more efficient a 9000rpm? Heres a kicker. Why is it when I find pictures of Scheepers or Padilla doing wheelies at NHRA events, their collectors are at the front of the door on the RX7's? This suggests maybe 24 inch primaries for their high RPM engines, not 100 inch primaries. Someone want to tell me what thats all about?

rotaryengineering

I don't know alot about headers but, I took my best shot at making something I thought would work. I used 2 X 2 1/2" OD tubes mandrel bent exactly like a Racing Beat header and then used some custom flanges to join the header to a Flow Master Y. The inside of the header is 2 3/8" from the block to the Flow Master Y. I spoke to Stan from SDJ and he preferred the step approach from smaller to larger but, for Drag Racing I don't think it really matters at 9,000 or 10,000 RPM's.

We'll see I plan on going to the local track in a couple of weeks. I'll post my results when I get them.

Thanks
Albert

Doc Holiday

iTrader: (2)

I have some math formulas that use exhaust port opening time (in degrees) to determine length. Then, after the length of the pipe is determined, I have another calculation that determines the pipe sizing according to the length and the displacement of the engine, rpm, blah blah blah. According to said math formulas, a 12a engine that wants the header tuning to have its biggest gain at 9000 rpm (high rpm drag racing) would require roughly 24 ingh length tubes with 1 7/8 ID pipe. A 48 inch primary put the tuning influence lower, somewhere around 5800 rpm.
This math was derived from formulas used on otto cycle piston engines, but I find it hard to believe that rotaries respond much different from this. I guess my disbelief is because I cant find any information proving otherwise. I'd really like to find it, so if someone has technical data to disprove this math, I want to see it. Follwing the belief of "super long headers for rotaries" seems common place on internet boards.

Mahjik

iTrader: (1)

You might take a look at the design this shop is using:

http://www.kcraceware.com/

Michael_Rudy

i can't remember exactly what it said in the racing beat catalog thing, but it was some where around 18 inches for a short primary header

Mahjik

iTrader: (1)

The header that KCRaceware created showed an increase of 10hp over Racing Beat's full race header.

jgrewe

iTrader: (1)

I've been looking for info on this same subject a lot lately. One thing I remember from a previous racing life was a short header that some friends made for a GT3 car. If I remember correctly the primary was like 11" long and it dumped into a 3 or 4" exhaust pipe that was really long(it made a couple U bends under the car). They had done all kinds of math, sacrificed a chicken to some 'god' and came up with three choices. 11", 22" or 120". They built the short one and with the engine porting they had done this car would pull guys with 5X's the money in their engines. It was the only thing the car did well for a while so it wasn't better corner exit speeds.

Rexrecycle, if you're out there refresh my memory.
(thats who's car it was)

I've remembered stuff wrong before guys so feel free to punch holes in any of this. Just throwing stuff out there, it might open someone's mind to other options that people regularly don't think of.

IanS

iTrader: (4)

My exhaust sytem is the RB Road/Race setup only it collects right above the rear axle (custom made piece) and seems to work really well. If I wasnt so broke I would have already done some dyno testing to see how it looks. This is on a mild street port (the company who ported it used RB templates) 12A with stock nikki carb.

riceburner1r2001

iTrader: (4)

how much is that kcraceware header going for.

Doc Holiday

iTrader: (2)

I made this short style header myself to see if it would do anything. My previous header used 48 inch primaries, this one has 20 inch primaries. To be honest, I'm not sure it really changed anything on the motor, but it runs better on the nitrous. I have a 12a bridge port with a Holley.

Kenku

Well, just as a two cents kinda thing, we have one header that collects about 25" away from the block, and Downing's book says they were collecting about 11" away from the block on the Kudzus, but that partly because there was no room. The RB short primary header is something silly like 35", but hey, we had it sitting around so on my street car it went.

So, basicially, I think you're probably right. The idea with the long primary setups has always been to tune to a different harmonic than the short primary setups. Damned if I know why.

Doc Holiday

iTrader: (2)

Unfortunately, when I made this new header, I changed more than just the primary length. My old header was made from a Racing Beat kit. Racing Beat uses 1 7/8th inner diameter primaries. It was 48 inches long, and I shifted the car at 9000 to 9500 on my bridgeport 12a. Now, I made this new header with 2 1/4 inch outside diameter thin wall tubing. Its near 2.1 inch inside diameter (ballpark, sorry for not being exact). I made it shorter and bigger to help it breathe in the upper rpms while on the nitrous. The big tubes might have hurt the power on the motor (though I cant really notice a change) but they really helped when I use the nitrous. The motor used to feel labored when loaded in higher gears with the small header. Not now with this big one.
Thin wall tubing REALLY adds to under hood heat though. Holy cow what a difference. I wrapped the header tubes to calm the heat back down. It would get so hot under the hood, than when I would shut the car off, you could hear the fuel boiling in the float bowls of the Holley.

Kyrasis6

People and their advertising centered thinking processes. What you need and what to design is completly dependent on the application. THE LAWS OF PHYSICS DO NOT CHANGE!!!

Do you want high end hp, or low end torque, will the engine only be running between 7,000 and 9,000 rpm or will it be running from 1,000 up to 7,000, there is rarely ever a right or wrong or a single answer to anything dealing with a COMBUSTION engine.

As far as I understand it the Otto cycle is basically the 4 cycle process and therefore many Otto cycle concepts apply to rotaries. Both piston and rotary engines have an intake, compression, power, and exhaust stroke.

One thing I am not sure about is if rotary engines have scavenging? I'm pretty sure they don't but someone correct me if I'm wrong. In a piston engine or any engine where scavenging exists there will be additional concepts to consider, hence tri-Y header designs and so forth. Scavenging is also the reason why few ppl run zoomies on piston engines.

Anyways there are 2 basic concepts for any gas traveling through a pipe, this goes for intake, exhaust, rotary, 2 stroke, diesel, whatever. Smaller pipes are better for low end power, larger for higher end power. This is because larger pipes require more gas to be traveling through them in order to maintain a good pressure pulse in order to evacuate the exhaust completly.

Similar concept to longer and shorter pipes, longer pipes provide a longer time for the pulse to travel down and back up to the pipe, so long pipes are good for low rpms, short pipes are good for high rpms.

Now if you have a street engine with some radical port work as I understand it on rotaries bridge and periphial porting (sp?) will dramatically raise the powerband into very high rpm ranges. You may want an header design with long primaries to make it more drivable at lower rpms, but if the car is strictly for drag racing you want the engine to start and stay at the optimum RPM for hp and therefore you arn't going to worry about being able to pull away at an intersection so you go with very short header designs. What you use depends on what you want and if you understand the concepts they stay constant although dyno tuning is always going to allow you to tweak the system and find flaws in the system. With piston engines because of port shape sometimes the engine will never make good power at a certain rpm, or a certain type of header will work better than others.

As for megaphone headers and the like. Do a lot of research before you expect to get the results you want. One of my instructors at school did a lot of development for Formula One and NASCAR one of the things he said was critical was the angles of the megaphone and that you actually want a small difflection angle at the end in order to balance the harmonics. He said he started his research by reading a lot of old (lWWII period) motorcycle books and he went from there.

lol, sorry if that sounded aggressive, it wasn't aimed at anyone, just stressing some points.

Doc could you send me some of those formulas? I may already have them but if not they would be nice to have.

Thanks,
K6

yelwoci

K6
I think you've hit this spot on.
All the Exhaust calculation mathematics that I've seen are based on piston engine work.
There are 2 parameters for racing and one more for road:
1. Gas Flow
2. Reverse Pulse Scavenging
3. Noise (both environmental and quality (eg MGB, Alfa V6)

Trouble with rotary is that I don't think it works with 2, at least not like a hemi/penta head
I think it wants no pulse, just 'constant' mass flow away from the port.

One reason why a drag racer would use short pipes, is cos that is what everyone else with a V8 does.
On my racer I want the NOISE and heat as far away as possible!!!

Mind You the 787 Le mans car has very short headers, but thats the nature and restrictions of a rear engined car.

Yelwoci

Kyrasis6

Regardless of what it wants the system will follow the laws of physics. In order to maximize evacuation of the exhaust gases the pressure pulse returning from the exhaust system much reach the port just before it closes.

If the exhaust system is designed wrong it is possable that the opposite happens and just before the port closes exhaust is pushed back into the port.

crispeed

There's no set length for a given combination. From my experience header length goes with rpm useage which is mostly determined by port design. I've also found this to be the same for intake length. Peak power is always made with shorter headers. Wide powerbands are made with longer designs. You have to decide what works best for you. With that said on our all motor PP 13b the header is around 12" with a 2.5" id collector. Peak power is at 10K rpm and stays flat to 10.8k rpm at which point even above 11K rpm the power only falls off by 20hp. Peak torque is also concertrated in the rpm band. Torque is pretty flat from 9k rpm to 10k rpm.

rarson

This is purely speculation, but I would think exhaust runner tuning would be very similar to intake runner tuning. That is, maybe focus should be place on volume instead of length?

Everyone knows that when you go too large on the exhaust with a naturally-aspirated engine, you can hit a point where you make less power. But if you took the same pipe and made it half as long, then I think you'd see a power gain (with a raised powerband, perhaps). I've seen many cars make the most power with no headers. Thinking of it this way, without a header your pipe is basically "infinite-diameter." But the length is also "infinitely short" so to speak. When using large pipe, the decrease in velocity causes the exhaust to slow down until it can't get out of the pipe. So cut the pipe shorter before it stops, and then the rest of the exhaust has plenty of room.

What I'm getting at is, if you're going to do like me and pipe the exhaust straight off the engine and out the front fender, you're probably better off going with a reasonably large pipe, though I personally think that a port-sized pipe, tapered out to a larger size, would be most efficient. Also, aerodynamically, there should be a bit of a low-pressure area near the rear of the fender, so I believe that angling the pipe a bit rearward and near the door would use this effect to draw exhaust gas out. The effect would be even better with widebody fenders.

mistert

i like where you're going with your header, but the problems i see are the very short runners will give up alot of power in the lower end. maybe with a monster ported motor where high rpm power is your only goal, but even with that in mind theres alot of room for improvement. i like your very gradual bends but you should try using a different collector

1revvin7 has been working on producing one of his own design. i've bought one of his turbo manifolds and a stud kit from him, and have seen the header in person. it's probably 13lb's or so lighter than the racing beat header i had, due to the thin wall thickness. i think it should hold up just fine because of the quality of the welds and the coatings (not shown in the pictures he posted up yet), inside and out. also the collector design looks first rate


https://www.rx7club.com/2nd-generation-specific-1986-1992-17/just-finished-custom-header-460413/

rexrecycle

We decided shorter was better. Why ? It took less tubing and was easier. (always my first parameters) We used 11 in x 2.5 in primaries that collected into a three in collector. we then used 10 feet of exhaust tubing snaked under the area where th passenger seat was . This was a full tube frame car and there was lots of room in that area. I also built my own muffler that had low back pressure but was still fairly quiet on the track(but not in the car). Iwas looking for that header the other day but have no clue what I did with it. If I find it I will post a pic.

Doc Holiday

iTrader: (2)

I read the thread in the link about the custom header. Looks pretty nice, I wish I had that kind of room, I have to put tight bends on mine to clear the steering arm in my RX3.

I'll tell you all what, if there is anyone on here that Im gonna copy its gonna be Crispeed and his 10 sec all motor car. He is obviously getting results, and there is no arguing with that.