Hello,

If you were able to build an exhaust sytem out of stainless for a 13b 6port unmodified for carbureted induction, how would you do it????

Let's pretend that the exhaust could be perfectly straight and the goal is to have a wide range of power from bottom to top (RPM that is).

For the current time no restrictions are neccessary such as pre-silencers, cat's or even mufflers.

Would you have long primaries?

What diameter pipe would you use?

What length main would be best?

Sleeves in or out?

Individual pipe's or into a "Y"?


My project consist of a 13b in 64'VW as my name states. I have access to 2.5 inch stainless steel. I intend to build:

Long primaries 4' +/- that are 2.5 inch in diameter right off the flange.

Into a nicely made "Y" pipe that is also into 2.5 inch.

I intend to make the "main out" as long as possible maybe 10' +/-

I will have segmented radius bends (which I know are not as good as smooth ones, but my buget is in the negatives, but I can always replace them later.) I will keep these to a minimum.


So the real question is who out there really knows how to utilize the benifits of back pressure, sound pressure, free flowing transitions and proper diameters, to extract the most power (throughout the entire power band). The good thing is that I can keep it real simple for now, until the reality of the sound level out weighs the benefits of the system...Thanks for all your help and I'll keep you all posted with my out come soon...

 

If you have made any up grades to your stock set up (I.E. Racing beat headers or any other improvements) tell me what you did and how it improved your system.

The goal is to see what response simple changes make to our engine.

This engine can't be any easier, just two huge ports. There is a lot of energy being thrown from out of the exhaust, that's why turbos work so well on these engines.
but How do we tune the exhaust to meet our demands for an N/A system. Later I'll be asking this question for a TURBO system.

I want people to be able to look at this THREAD and get some info on what to do with their exhaust to get the most from their engine. I hear so many different answers like:

Use 2" primaries 2.5" is to big, no sleeves causes excess heat, short primaries are better than long BLAH BALH ect...

I want facts and real evidence of what works. In the mean time I am very close to finishing my straight as possible exhaust to see how it works...

 

basically you're right it is simple in concept, there are a few things in construction you need to look out for though.

this is IMO, i don't claim to be a genius, although i have played a pilot before.

sleeves. they should stay in, they have some anti reversion properties that should be kept. also they are already too big so making that area bigger would be bad.

header material. thickwall mild steel, like RB uses seems to be the best on the street. the thickwall tubing keeps the noise in. for racing use you should seriously look at stainless.

diameter. mazda uses 43mm ID pipe on its racing engines, 1.7" although the "sleeve" on those is also 43mm, so its a better match. if you look at a stock exhaust manifold the sleeve is smaller than the opening, more anti reversion. the RB header is smaller, mostly due to the funky bend it makes.

length. this is the easy one! traditionally there is the long system, which offers a broad power curve, or the short which is peakier. lately people have has success with a mid length (50" ish) system

collection: basically the diameter after the collector depends on the hp level. a sub 200hp car seems to be totally fine @2.5", its easier to muffle too, which is nice.

the muffler. these do not need to be straight thru, but they must be able to flow enough to support the HP, without causing a restriction which can be quite a lot of air!

and then the biggest problem of course, besides the time this takes, is just fitting it under the car!

 

exhaust tuning is a very delicate process, in bends and scavenging effects of the exhaust which will in turn give better bottom, mid range and best top end response.

ideal for rotary applications you can look towards 2 stroke expansion pipes to see how the scanvenging effect compares to power ranges. generally for better bottom end "grunt" you had a larger chamber with more bends, for top end pipes you had a more straight pipe with a smaller expansion chamber.

but no one that i know of has experimented with expansion pipes and rotary engines, why that is i do not know, considering people have built 4 rotor engines from bare chunks of metal. expansion pipes in theory should work for naturally aspirated rotaries but for turbocharged rotaries the piping should be the same diameter all the way throughout.

you could consider the precat and main cat as expansion pipes but most of the time "performace" exhausts omit those areas while the stock cats were restrictive and likely didn't offer much to the table.

2 stroke piston engines also suffered from lack of bottom end and these expansion pipes are how they help to overcome that lack. they also use reeds which act as one way valve to end the resonance in the intake manifold which pushes air in the reverse direction after the piston is moving back down on it's path, while not as drastic in rotaries, it is still an issue as to why they are so sluggish on the bottom end and less than stellar for fuel consumption.

hell, i'd like to see someone gut out a large main cat with 2.5" in/out and place it at the end of the header to see if it does in fact offer anything as far as gains. i'm also curious if mazda did attempt at seeing if reed valves offered any benefit.

 

A valved 2-stroke is a very different animal than a rotary. When the piston uncovers the exhaust port the gases begin to exit trough the open port as well as a powerful sound wave. As the piston continues to move toward BDC it opens up a set of transfer ports that allow the air/fuel mixture that has been compressed in the crank case by the descending piston to enter the chamber. At this point there is a path trough the carb and past the reed or disc valve under the piston into the crank case trough the transfer ports into the cylinder and out the exhaust (rotaries cant do this). The scavenging effect of the pipe will actually pull a little but of unburned air/fuel into the exhaust pipe. As the sound wave bounces off the diversion cone and travels back toward the port it pushes the unburned air/fuel mixture back into the cylinder . Because the reed valve will not let the charge travel back out trough the carb the pressure builds inside the cylinder and crankcase giving you a much more dense charge. A expansion chamber on a rotary wouldn't have the same effects that it does on a 2 stroke.

 

i'm not one for ruling something out just because it doesn't add up in "theory". yes they work very differently but there is a bit of similarity in their operation as well, mainly in the free flowing air that enters and exits the engine at will.

 

people post about them here and there, there was one guy who claimed big gains with an expansion chamber, but no pics or any info other than his claims.

ISC also sold an "expansion" chamber, it was a 4" dynomax bullet, but it doesn't do anything, jacks car has one, could you tell?

the concept is probably ok, but results are few

 

I wouldn't make it out of stainless. Stainless is a PITA to work with and it changes size a lot more than mild steel.

 

Paul Yaw did. The info is probably gone from his site, now. IIRC he did it because he made special "port plate" style tongues for a header that reduced/eliminated the rapid area increase in the exhaust port, and a side effect of this was that the exhaust got tremendously louder, so he studied expansion chambers and did some figuring and made one. Again, it was more for quieting the system down than a power gain, but anything you can do to quiet the system down without a muffler should result in more power since you can get away with a less restrictive exhaust system.

Not quite. Unvalved two strokes effectively have 100% overlap, and the timed backpressure you can get from a properly designed exhaust system will act as a kind of pressure-wave exhaust valve. Rotaries don't have a problem with the compression stroke blowing fresh air charge out the exhaust port, because rotaries aren't 2-strokes. No matter how far you port your exhaust ports up, you're not going to get them into the compression stroke

 

Alright guys,

This is my home made exhaust!!! I busted my A$$ making this (had nothing but a hack saw, vise, and a tig welder).

I know it's not perfect in any way, it still doesn't fit perfectly under my bug but I'll get there. This is just for my own experiance and fun.

I believe that it will work well and even sound cool (nice low tune).

Later I will make one with 1.75" or ones that will match up with the sleeves better and then into a 2.5" main. I think this will give me better back pressure utilization and quiet down the noise...

 

With no presilencers in the primary pipes, it's gonna be loud.

 

but it'll last more than 6 miles!

 

does that go forward? if so this is actually good, put a U bend on that and bring it back, it seems like longer exhaust is better.... mazdatrix's EP cars have the mufflers sideways, and so the exhaust ends up being something like 2 feet longer than the car, and not only is it quiet, but they gained power

http://www.mazdatrix.com/pictures/fa.../ep120502f.jpg

 

Yes,

I did intend on making it as long as possible just to see what the sound and power levels would be like. Right now I'm running my 6-port stock 13b carbed with a motorcraft 2100 (1.21 for a 351 windsor) and since it is not turbo (yet) and I can actually get away with a pretty long set up, I'm just going with the simplest straight system first. Once I get an idea of how it performs, I'll then chop it up to try and fit it up and over the trans (my bug has quite a bit of room for a good custom exhaust) and route it to the back of the car.

The goal is to get a good over all response from my exhaust and to eventually make it nice and quiet. I have so many plans for this project... but my budget is making it move slowly, and my progress depends on how well I can fabricate everything during my lunch hours at work...

 

i've been talking about redoing my exhaust for at least 6 months, maybe more like 9?

i think longer should be quieter, but yeah find out, i don't know anyone who has done a 40 foot exhaust!

 

Cool,

I don't know about 40' but a realistic 25' feet is what I do propose. I can't say I've seen anything that used an exhaust that was that long, but then how practical is something like that on a front mount engine. I wonder how that heat will travel through the stainless piping, and I'm also concerned about backfiring or fuel getting in the piping before ignition and then "BOOM" it lights...

One thing I do know is that resonant frequecies get lower as piping gets longer regardless of diameter (to an extent) and with the exception of temperature, with 25' of pipe I should have good back pressure throught the full band. I wonder if I'll have increased throttle response??? But I shall keep this thread up to date and I will continue this project in hopes of making a difference in the Rotary and VW scene...

 

Generally a non-issue. Don't worry about it.

The only good backpressure is no backpressure. Ideally, you'll have a vacuum in the pipe during the final degrees of the exhaust cycle.

 

Alright,

So I just placed 7'10" primaries (yes you read that right) on my 13b. I fired it up with just the primaries going strait out to nothing but the open air and If I could just describe the sound as "HOLY MOTHER !@#%$" it had the rumble of a drag racer. I hit the gas and noticed the acceleration (RPM) was much faster than with short or no exhaust. I wish I had the means to graph the power curve, because I know that this setup had to have increased the power.

More on this later and when I have a chance to complete my system, I'm currently at the hospital with my newborn baby girl.

My question to anyone looking through this thread is...

Has anyone ever done a system (exhaust) like what I'm doing:
LONG 7 foot +++ primaries, to try to extract all wavelength backwaves and got it on dyno record??? If you've seen anyone with something cose to this please post what you have here.

I really would like to have hard evidence of of systems that works. If any of this draws some curiosity, look at "rotarygod" and his thread on (Here's how to figure out intake runner length), it backs up alot of what I theoriezed would be the optimum length to extract more power over the entire band....

 

A typical "long primary" system is 90-110" long primaries. They seem to go in and out of fashion.