Most of you know the story of my exploding exhaust.

For those who don't, my cats were clogged and exploded one day on the highway, leaving me with only the stock header (collector) and whatever was left of the thermal reactor.

The car used to start, every single day, in this fashion:

Pull choke out, turn key, pump gas twice, instant start up, lower choke to 1500, and good to go.

Since the exhaust exploded, starting was a little more difficult, with a few stalls on the start up process.

Once I did the mechanical secondaries mod, starting the car became a real pain in the a$$.

I would have to leave the choke off, turn the key for like 15 seconds, turning the engine over. It would sputter to life, but then die out if you even TOUCHED the gas. It would die at least 5 times, before being able to run on its own, and letting me pull out the choke to keep it running.

The other day, we had a local Mazda Tech Session at a nearby dealership. I decided, since it was about 20 minutes away, maybe I should try to do something about my ridiculously loud exhaust, so I didnt have to deal with getting a ticket on the way there, or back.

I stopped at a local Pep Boys, and picked up the cheapest "Cherry Bomb Turbo Muffler" they had. ($17.99)

I didn't have the optional Rx-7 Welding Kit in my car, so I bought a spool of mechanics wire to rig it up.

Lucky for me, it was a very tight fit over the 3" of exhaust tubing that was left. I used the wire everyway possible to keep the muffler on there.

I didnt expect anything special at all with this muffler, just hoping it would be quieter than the open header sound I already had.

To my surprise, it sounded great! Like an entire new exhaust. Nothing like the cheap Pep Boys fart can exhaust I was expecting.

I drove back home, it all stayed together. About 4 hours later, I hopped in the car with my gf, who was already making comments about how junky my Rx-7 was, since it took so long to start. It fired right up.

Drove to the Tech Session, stayed there for 3 hours, and when I went to leave, with alot of enthusiasts around my car, it fired right up again, saving me embarassment. Alot of them said it sounds great.

The next day, over 12 hours later, I hop in the car, and it starts right up again.

Someone on the board said in another post, that ANY backpressure is bad for a rotary.

This proves otherwise.

 

Uh I think your wrong I know what your saying but, the production of horsepower is simply the faster you can get more air / fuel in and out of the motor more horsepower it produces.

 

hmmm, when i had just straight header it would crank right up no problem still. that happening to is weird. how quiet is that pep boys exhaust? i know anything lower than straight header noise is good, but i mean, can you talk in the car while driving? hahaha

 

Where is there a pep boys around here?

I'm guessing the weather had more of a effect on starting the car vs the back pressure.

 

My car started fine with just the 3 inch downpipe... Ive ran rotary open header before and never had them have any problems starting...

 

Nah. The weather has been pretty stable and warm these last few days. The exhaust sounds good. It's easy to talk while cruising, but don't even try at high rpms......

There is a Pep Boys right by Inver Grove Ford, (Robert Trail Road and 494E).

Basically, with the stock header being all that is left....I think combustion was quite odd in the engine. It really can't be much better than trying to run with an open block (or whatever this engine is called).

I notice a much smoother idle, and better response and performance. And, its none of that "I'd like to think there is performance, since I just spent money on the car" as there is with most tiny mods.

I dont feel like I have to worry about the car dying on me anymore....and there isnt as much hesitation in that flat spot of the mech/secs.......where my car would just about die before.

 

I've got headers, a presilencer, and straight pipe and have no problems starting up either. Of course, starting it is a completely different process than a car with all the smog crap. If its been started in the last 1.5 days, the choke is a "no-touch". If you touch the choke, it automatically floods the car. If it hasn't been started in 2 or more days, it needs the choke engaged until it fires up. If you keep the choke out, it dies and floods.

Unlike my streetcar, which will fire up with any amount of choke no matter what the circumstances.

 

Well.. my exhaust was straight through for a long time, and here's what I noticed. When I ran just headers it started hard, and ran rough, but as soon as I finished the straight-pipe to the back, everything cleared up. So maybe having no exhaust is part of the problem.

 

Ok, PLEASE, don't call it that, call it the stock 12a exhuast manifold from now on, its not a header by any means, its made out of nearly industructable nickel and doesn't create any length between the two exhuast ports before they collect.

DON'T take that personally, but yuck! Its not a header, and no one else would ever call it that on the furum I'm sure. And I don't think its a good idea to run it right out of that manifold under the car for a few reasons- insane loudness.

And even though its non-obstructed-in theory, its also really too short and will hurt flow. A header with no exhuast after it, will be be somewhat more rough but make more then a non-header'd car, and a header with tubing after that will help smooth it out exhuast pulses. The exhuast pulses are very pronounced in rotary engines and longer piping will smooth that out to some extent. I'm not talking about the advantages of long tube headers vs short tubes headers, I'm talking the overall length of the exhaust system will make a big difference when compared to right off the manifold to the rear of the car.

 

No thermal reactor on catalytic converter equipped cars.

But you are correct in assuming that some pressure wave tuning aids in scavenging.

 

ok, i'll address the back pressure thing. no car wants backpressure. it is a biproduct of exhaust tuning. in order to scavenge the chamber exhaust Velocity needs to be maintained. tubing size dictates this. at a lower rpm there is less exhaust so smaller tubing is needed to keep it from expanding and stalling flow as it exits. at higher rpms, there is more exhaust being released so tubing size needs to be larger to keep up velocity without creating unnecessary backpressure. now we all know that the exhaust tubing isn't going to change diameters for us to accomodate the best exhaust velocity. so tubing size must then be decided on to maximize a certain part of the powerband while hurting another. to lasrge and the motor falls on its face at low rpms due to the extra burnt mixture not being pulled out and continue into the next cycle equaling a less potent boom. but once the motor gets going it has room to breath and wails on to redline. too small and you have excellent throttle response due to the velocity scavenging out the exhaust and getting a clean chamber through the next cycle. once higher rpms are reached the tubing becomes restrictive to the volume the motor is releasing and it can't expell all the left over gases, and then takes up mixture space through the next cyle.

exhaust pulses are a little more difficult. from what i gather, you'd want them split evenly so the piping is seeing an even flow and keeping exhaust velocity consistent. since the rotors are doing this themselves by timing, giving each header piece an equal length will cause the exhaust pulses to even out and create the effect i just described.

oh yeah, cody don't expect that muffler to last to long. a rotaries hot exhaust tends to melt the fiberglass and blow it out the tailpipe. so don't be surprised if it becomes increasingly louder.

 

Proper exhaust pulse tuning that creates the right pressure wave to scavenge remaining exhaust requires backpressure, Jeremy that's what's used to maintain the pressure wave

 

from what i've read it causes backpressure. backpressure is not what scavenges. also back pressure changes according to rpm. if your saying that backpressure scavenges exhaust then tell me how, not that it does. i've never seen anyone explain that. what i see is velocity being kept up and thus pulling out or scavenging the chamber. backpressure is a biproduct of this but not what is causing the scavenging.

 

What causes the scavenging effect is the appearance of a slight suction at the exhaust port right before the port closes. This is a function of the pressure wave's propagation through the exhaust system, and involves the exhaust gas velocity, length and size of exhaust system, and a host of other factors. "Backpressure" may not be exactly the right word to describe it, but that is the term most often applied to what is essentially wave damping.

 

as exhaust exits the engine from the ports and into the header or manifold, it comes out in a stuccato of pulses (front rotor/rear rotor/front rotor/rear... you get the idea).

Without backpressure these pulses would just travel down whatever pipe, their sole velocity imparted by the motion of the rotor 'flinging' the exhaust out. Of course, some of the exhaust will be sitting in the indentation on the rotor face and wouldn't have time to flow down the face to the port and out before it shut. This leaves exhaust gasses taking up space when the rotor draws in air and fuel, limiting the amount of fresh charge that can be brought in by diluting it with combusted gas (read: no useable energy left in them that hydrocarbons).

However, with the right amount of backpressure you create a pressure wave. As each puff of exhaust comes out of the engine, it enters not dead space but the exhaust immediately in front of it (which has been held back slightly by backpressure). This turns the pulses into a smooth, steady flow of exhaust. Now, when the exhaust comes out of the port there is a pressure wave drawing out that annoying pocket of exhaust gas, in essence vacuuming the chamber clean of gasses before the port closes.

A very loose example, strictly for visualization purposes: think of racing cars on a track as the pulses of exhaust. They can all flow independandly in the same direction; however, when one is immediately behind the other it creates a kind of vacuum that helps pull the rear car along. Not identical to a pressure wave within a confined space, but it gives you the idea.

Now there is room for a full charge of fresh air and fuel to enter through the intake port, thus giving you better bang during combustion, doing more effective work, and giving you more power and better fuel economy.

 

Mantis- great theory, but explain how that applies to how well a header works? (a real header)

 

i was runnin open headers on the way to the muffler shop... started and ran fine... realy shity respons under 3500 4000rpm now i have straight pipe got my power back and now my car sounds like a sick chainsaw.
instead of an airplane... but its cool if you like airplanes...

 

Simple: A proper header tunes the pulses to get optimum pressure wave - that's why equal-length runners and the positioning of the collection point are so critical to a header's design.

 

Ok, if my car had cats......then, what is DIRECTLY after the stock collector/header/manifold/thing bolted to the engine that the exhaust goes out of first.

Looks like a small cat. What is it? Its still there.

 

It's a pre-cat, a sort of catalytic converter that assists the main cat.

 

That would be one of the pre-cats. IF you have a '84-85 12a model, it would normally have 2 pre-cats and one large main cat with a air pump line going to the main cat. If you just have the first pre-cat, it would be pretty loud and not good since that exhuast is not really treated so I'd be more worried about exhaust getting in the cabin formost, and the first pre-cat has the most potential to being clogged if the engine is running too rich, or driven too hard, or is too old and naturally plugs up. Which could explain why you have poor power to some extent.

Why don't you get your exhuast system fixed right the first time? Don't take this the wrong way, but as constructive direction, but how much or little money do you have to spend on this? Someone could hook you up with their old exhuast system for free probably in the twin cities or you could buy a new system or have a exhaust shop make you up something. I am a firm believer in not wasting time with half-assed exhaust, one time deal and is good for 3-5years probably with no service issues. To me its just too stupid to play with mechanics wire and pipe clamps and flex pipe and expect it to last longer then 1-3weeks on a average piston engine, let alone a rotary engine before you spend another $20 and again and again...

 

hence the more bullet proof systems, like oh let's just say Racing Beat full exhaust...

Incidentally, a lot of the weight of our exhaust systems is there because of the heat and after-combustion inherent with our port designs. The Renesis new side exhausts supposedly does away with this.

Going back to pressure wave, the same is true of intake which is why 13Bs have the dynamic effect chambers - a sort of header for the intake system.

The Renesis goes one stap further, having an additional valve that opens at about 7250 RPM that shortens the intake acoustics so the wave doesn't have to travel as far, thus keeping up with the higher-speed air demands of the engine

 

I think some of you have the concepts of backpressure and pressure wave tuning mixed up a little.

What you're describing there Manntis isn't actually pressure wave tuning but inertial scavenging. Towards the end of the cycle exhaust flow out of the port slows down but the existing high velocity gasses in the header help draw out the remaining gasses in the chamber.

Pressure wave tuning is a completely different thing. When the exhaust port opens the sudden change in pressure creates a pressure wave which travels down the exhaust pipe. This wave helps move the exhaust gasses in the same direction it's traveling.

Now when the pulse reaches an opening in the exhuast such as the collector it actually becomes negative and causes the wave to revert back down the pipe towards the exhaust port. This negative pressure wave also helps move the ehaust gasses away from the exhaust port. Ideally this negative wave will arrive back at the exhaust port right at the end of the cycle.

This is the good part, by changing the length of the headers you can time the pulse to arrive back at the port for a certain rpm thus optimising exhaust tuning for that rpm.

That's why I don't see how backpressure will help performance in any way. There will always be some inherent backpressure due to the exhaust pipes but any extra backpressure by the way of mufflers or cats etc will always hurt performance. It will only lead to more pumping losses for the engine as well as excessive exhaust gas dillution of the intake charge especially on ported engines.

 

Dragsters = Zoomies = Straight pipes coming out of the block and out the side of the car = no backpressure at all.... They don't have any problems...

An RX-7 Isn't a 2-stroke engine, so there's no reason to have backpressure like on a dirtbike with it's expansion chamber. So, technically, backpressure is robbing high end power on all Rotaries.... but you lose low end if you straight pipe it, which is why you put a full RB exhaust on it. They've figured out a good balance between resriction and free-flow for normal street and race applications...

 

In addition, I know this from fact and not from reading a book, the further you move the collector of the 2 exhaust ports to the rear of the car tghe farther you shift the power band upwards. Meaning further you collect the 2 pulses the higher the rpm's will be when the power band peaks, up until a point then its the principal of diminsihing returns