i have a 12a 85 nothing done to it besides 151k and a k&n..it needs pipeing done and i wanted to go header to strait pipe to 2 bomb mufflers...is this alright or do i have to pay 700 bucks for a stock unit?

Stepdaddy

 

Haha, you could probably do it, but DAMN would it be loud...Ask me how I know

~T.J.

 

some backpressure=better scavenging of exhaust gasses.

If you can afford the RB system, buy it.

 

i cant thats why imgoin loud and cheep i guess

 

Cheap mufflers won't last with rotary exhaust heat. And it will likely be louder than you think - we're talking VERY VERY loud lol, not just Honda fart can loud.

My advice is to wait until you can afford the RB system, because it's worth every penny and lasts forever.

 

A suggestion would be to get a header (RB $170), flange it to a Magnaflow Resonator/Muffler as a presilencer($60) and flange or weld that to the stock connecting pipe onto some 2" piping then a Magnaflow muffler (up to $100) and enjoy the music Magnaflow has stainless steel packing for longevity against the rotary breath. This will cost signifantly less than a full RB setup.
You may also not get a header but ditch the stock catalysts.. those are the real power chokers, get the RB cat replacement pipe or have one made.

 

The need for "backpressure" is a myth and should not be confused with pressure wave tuning.

For a given tuned pipe length (pressure wave tuning) and diameter (to maintain good exhaust gas velocity) the system will always work best with the least restriction as possible. This is especially evident with ported motors.

 

Read and learn

http://personal.riverusers.com/~yawpower/

 

I'm not going to get into the backpressure/no packpressure debate again as it's been extensively covered in other threads. Sufficith to say I'm in the camp of mechanical engineers who believe exhaust tuning INCLUDING backpressure enhances engine performance, especially as the engines were engineered to be connected to an exhaust system and backpressure was taken into account.

 

The rotary on the other hand was designed to be used without an exhaust (way to much overlap for its own good), and will in all cases produce more horsepower with as little backpressure as possible...

Honestly, doesn't it make sense that the largest pressure drop from the combustion chamber to the exhaust will produce the fastest exhaust escape?!?Pressure tuning is just taking account the sound properties to further increase the pressure differental during the closing of the port, lowering reversion. Backpressure is bad any way you look at it, IMO.

 

Then I suppose you can explain in scientific terms exactly why backpressure is benificial to an engine?

 

Was that directed at me? I've read the stuff on that site and nowhere does it talk about backpressure. Pressure wave tuning yes but not backpressure.

 

You're splitting hairs.
There is no actual "backpressure", but the shape and locations of restriction in an exhaust system do provide varying pressure wave characteristics. Look at an "expansion chamber" exhaust as used on 2-stroke motorcycles for example. The effect of controlling the expansion and subsequent compression of the exhaust flow actually causes the exhaust to be "sucked out" (scavenged) from the exhaust port at the proper TIME.
This is at the heart of exhaust system tuning. -WG

 

Someone with a dyno, put their car on it with a stock exhaust system, then with the RB system, then take the exhaust mainfold off... Be sure the exhaust doesn't set the front tire on fire... And you'll defenately need new idler arm bushings after that.

 

Backpressure exists because of viscosity. Although exhaust is a gas it can also be viewed as a fluid. It exhibits a certain amount of cohesion, and adhesion. Adhesion and cohesion are the causes backpressure. The exhaust “sticks” to the walls of the pipe, near the walls of the pipe the gas moves slower. Cohesion then steps in… the gas that close to the gas near the walls “sticks” to that gas and so forth. Thus, larger pipes are slowed less in the center than small pipes. The force that tends to slow the gases down builds up over distance…. And viola BACKPRESSURE!!!

The intricate balance (and possibly where Mantis’ belief that backpressure may actually be good) comes when compromising between velocity and backpressure. A 2” pipe as X lbs/(ft*in^2) and on a particular engine has V, m/s velocity down the pipe. Now increase that same pipe to 3” and the back pressure may drop 10–15%, but the area of the pipe is now 2.25 times as large so the Velocity adjusted to the lesser amount of backpressure could be half as fast. This could be as evil as backpressure (if not more so, depending on the velocity V).

You generally want to stay away from about 2-4 tenths the speed of sound on the upper end, but fast enough for the decrease in backpressure to have effect.

 

im not disagreeing... but why was it designed to be used w/o an exhaust?

--eric

 

It wasn't. People tend to think of automotive systems as modular ("dis guy makes de engine, dis guy makes de chassis, dis guy puts 'em togedders...").

In reality the engine is designed for it's application. TO say rotaries were designed to run with no exhaust is like saying piston engines were designed to be vertical. In reality each engine is tweaked for it's application. Subaru runs piston engines in Boxer configuration (horizontally opposed), jeep uses inline 6 vertical engines, many imports use slant-fours (diagonally oriented, transverse mounted engines) and rotaries in our ReXes were designed for 70's tech exhaust systems - then emissions regulations changed and most manufacturers were forced to choke their engines with primitive emmissions systems (remember the gutless Camaros of the early 80's? HUGE displacement for a measly 150-170 HP) We free up power designed into the engine by putting on tuned exhaust systems like Racing Beat's.

The only engines I can think of off the top of my head that were designed specifically for no muffler are small aircraft engines, where the added weight isn't worth the lower noise level and so the engine is tuned to run without mufflers.

Anyone who doesn't think exhaust tuning matters should ask themselves why headers are made with equal-length runners instead of just enough pipe to get from A to B. The answer is exhaust tuning matters.

You want SOME backpressure but not too much (and the hasty bolted-on emmissions systems of the early 80's were far too much, but manufacturers had no choice), and in equal amounts per combustion chamber so each has equal scavenging/evacuation characteristics.

 

dp

 

You still haven't answered why some backpressure is beneficial.

BTW, to think the Racing Beat system is the be all and end all is naive. The RB muffler is great for the street because it's quiet and obviously works well but it's not the best in terms of outright horsepower. There's quite a bit of power to be had by replacing it with a basic straight through muffler. A lot louder but you'll have more power because there's less restriction and less backpressure!

In all honesty, this is very simple stuff that has been proven years ago. The less restrictive you make an exhaust system for a given and pre-determined pipe length and diameter the more power it will make. Ask anyone who has experience in racing or with 'large' ported rotaries. It becomes a balancing act between making power and keeping the system under the legal noise level.

 

im no expert.. but ill add my 2 cents into this....

basically, the idea of backpressure is to balance the intake pressure with the exhaust pressure. you want an even flow or pressure throughout the whole engine. so if you have bigger ventures and ported intake ports etc. which would increase the intake flow and decrease pressure, with a stock exhaust port and cat(s) and muffler... then the exhaust is basically the choking point, and vice versa. so its different for every car, depending on your mods. the best thing to do is to get some dyno time at a local rotary shop. my buddy with a miata tested his car on the dyno and the results were that the car made more hp with a muffler than w/o.

and to fatboy7... the rotary was definately not designed for no backpressure... any car designed for street use had to take into consideration the cats and muffler.

--eric

 

Ok, I was taking it a bit too far... but the overlap on rotaries in general is huge for a "street" car, which works best with as little backpressure as possible, and proper exhaust tuning. Backpressure with overlap creates reversion. Exhaust gas in the intake is never a good thing. Because of this, if you properly tuned the lengths of two pipes dumped out behind the front wheels, it would outperform its counterpart - dumped to the rear of the car. Not by much... cause pipe doesn't produce nearly as much as do mufflers and such, but the gain would be measureable. Thats what I meant by no exhaust... not truly NO exhaust, because without any pipe, you can't pulse tune.

 

REVHED:

I said it quite a few times in this thread.

SCAVENGING.

 

The SE 13B has nearly 60* of overlap, whereas most modern street cars have closer to 20*. 60* is much closer to "race" standards than "street"... Thats one reason why emissions can be such a pain, and why our engines produce much more power with a free-flowing exhaust.

The Renesis has NO overlap, (but larger ports aka more "lift" in piston terms), so it will probably be more tolerant to backpressure, and fewer gains to be had through exhaust work.
(now combine their ports with our overlap, and you'd have one hell of a race engine!!)

 

I'm going to try this from a different angle, so stay with me.
Let's start from the intake side. When a rotor finishes its intake cycle and closes off the intake port, all of that incoming air must come to a abrupt stop. This causes a pressure wave that travels back up the intake runner. In an SE, the runner length and dynamic effect chamber are designed to deliver this postive pressure wave to the other rotor just before it completes its intake cycle. So you have the monentum and velocity of the air flow with the added "bump" from the pressure wave to FORCE extra air INTO the port just before it closes. This is pressure wave tuning. Everybody with me? Question one: Would you want something to restrict this process? If you say yes, please explain.

Now let's go to the exhaust side. We want to use the same idea to SUCK the exhaust OUT. When the exhaust opens we are going to use the velocity and momentum of the gas, along with a pressure wave to suck out as much gas as possible. This is where the header length
comes in creating a negative pressure wave. Question two: Would you want anything to restrict this process? I say no. Backpressure would simply try to force the exhuast gases the wrong way, back into the combustion chamber.

So why do say 2" or 2.5" pipes work better than 4" pipes? (for the street) Is it because the smaller pipes have more backpressure? No, it's because if you get the pipes to big you lose the velocity and momentum of the gas and the scavenging effect is less.

 

Thank you for articulating my point on scavenging