I discovered an interesting fact today.. I had a complete new straight through exhaust put on today. My muffler I bought for the back has a removable silencer, and after I put it in to test the volume levels, I noticed my car was quite a bit faster! It accelerated better than with the silencer out. What gives?

 

You sure that isn't your butt dyno giving you a false reading?

Either that or you have a very crappy presilencer that's really backing things up

 

No engine likes backpressure.

Rotaries especially are sensitive to backpressure. They hate it with a passion.

I think your butt dyno is misleading... loud cars tend to feel slower because you have all that noise. Quiet cars seem faster because the engine doesn't "seem" to be working as hard for a given acceleration.

That and the carburetor tuning is pretty sensitive to exhaust backpressure with these engines (see above - rotaries *really hate* backpressure) so your carb might work not as well with the silencer out. You could rejet the carb, but then it would run worse with the silencer in.

 

with a less restrictive exhaust the motor will run leaner. so you need to richen the fuel jets to compensate, and that will make more power.

 

I dunno.....

I'm kinda thinking the rotary runs better with a little backpressure myself. My engine is pretty solid, but with hardly anything for an exhaust. Stock manifold into THIN AIR.

I was working on another local guy's 85 GS, with what I believe was a stock exhaust, and I noticed more pull at high RPMs.

I've heard it tossed back and forth that backpressure is good......and bad. I say it's good.

Hell, even when my cats were clogged all to hell, I felt more acceleration.

 

We've had this debate before. I maintain that proper 'backpressure', or power-pulse tuning, scavenges the exhaust out of the engine better by creating a vacuum at the end of the exhaust cycle - which is why every header worth it's price has equal length runners - and also on the same principle as the dynamic effect chamber Mazda engineered for the 13B intake to create a pulse that sort of pushes air into the engine giving a slight amount of precompression.

 

Do not confuse backpressure with velocity.

An open exhaust manifold with nothing behind it can have *more* backpressure than having a pipe on it.

 

Can you give us a breif explanation on that peejay. Ive definitly heard it before and have read about it in exhaust tuning articles. Something about pressure traveling back and forth like sound waves?

 

Yerp.

 

That's exactly the deal with this "debate"...People confuse "pressure wave tuning" with "back pressure". -Two totally different things.

 

Or they think well, 3" is bigger than 2.5", therefore the 2.5" has more backpressure? And for some reason the engine makes less power at lower RPM with the huge pipes so therefore engines like backpressure at low RPM right?

All the while neglecting to figure that exhaust gases have mass and velocity and thus they have inertia, and when the pipe is too small yes there is back pressure, but when the pipe is "just right" there is minimal back pressure, and going bigger than that causes the exhaust speed to slow down, therefore there is less velocity and engine power drops, because velocity is important.

See what happens when assumptions are made based on not seeing the whole pictute?

 

Exactly. If I could remeber where that writeup was that I read about this subject, I'd post the link. I'll see if I can find it.

 

That's it right there. It's all about exhaust scavenging and velocity.

 

Pressure wave tuning: varying the legth of the primaries so that the negative pressure wave arrives back at the exhaust port at a certain rpm. Nothing to do with back pressure! A rotary will never make more power by making the ehaust more restrictive.

 

agreed, but zero restriction (i.e. no header or manifold) doesn't allow pressure tuning at all, therefore you lose the scavenging effects.

 

If anyone still has questions, read up.

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

 

You're right. I guess what I'm trying to say is, for a given and pre-determined pipe length and diameter (to allow for pressure wave tuning and good exhaust gas velocity) the less restrictive your exhaust is the better. Adding a muffler, cat or other restriction (ie, increasing the backpressure) will never give you more power.

 

It can't physically be all or nothing. Any pipe will put some restriction on - even if it's rediculously miniscule. I think PWT is best obtained with a pipe that is as small as possible without impeding exhaust flow.

BTW< I've been wondering how the RB header/double presilencer/RB muffler set-up fare in all this. Is it restrictive? (...much?)

 

I read in some performance handbook that there was an optimal exhaust diameter but that the length had no forseeable cutoff for making power. I believe that book was; How To Mod Your Mazda RX-7. They said that, if I'm not mistaken, a 2.5-2.75 diameter exhuast is ideal but that if possible to run it longer than nessesary because the output just keeps climbing.

I will have to go and re-read it again and post a second time here...

 

But the tuned pipes guiding the exhaust enable scavenging, unlike no pipe - and it's not neccessarily true that any pipe will cause some restriction...

think of the volume of exhaust. It comes out of the engine in pulses. A tuned pipe system will guide it and use the pressure waves to create one contiguous flow of burnt gasses from exhaust port to tailpipe. A pipe too small will cause too much backpressure, and instead of one smooth flow you have pressure trying to push some of the exhaust back towards the engine. A pipe too large,or runners improperly tuned, however, will keep the exhaust fragmented into individual pulses instead of one smooth flow.

A loose analogy: Soldiers marching in step efficiently moves a steady flow of men because their footfalls and arm swings never interfere with one another, but fill in voids left by the men around them. A large group not in step cannot move smoothly in a tight formation without bumping forearms or occasionally stepping on each other's heels.

 

A proper collected exhaust with the proper header will help pull air out of the engine. Each rotors exhaust velocity puts a small vacuum on the other tube. Now you have LESS pressure at the exhaust port than using no pipe at all. It wasn't a flow problem. Too large of a pipe will reduce this effect. As a general rule if you can't decide what pipe size to use, use the smaller one. A 2 1/2" collector is fine for street use. 3" works good on high rpm highly ported engines but makes poor low end power. You need low end in some form for street use. A true dual exhaust will not scavenge at all. It tunes by organ pipe resonance. Scavenging is when one pipe helps pull air out of another pipe. Obviously when they don't connect they don't do this. With the duals your rpm tuning will be determined by the length of pipe. Even though a 2" true dual system basically has the total area of a single 3" pipe, it will have much more backpressure than the 3" due to lack of scavenging. The 2 1/2" collected will match the flow and scavenge. Bigger isn't always better. The stock carb was tuned around the stock intake and exhaust. Any time you make a change, the carb may need to be retuned. It doesn't mean that the less restrictive setup isn't the better one when tuned.

 

lol, this thread is Deja Vu all over again.

Look at a Supertrapp silencer. The number of discs you use allows one to "tune" the system.

 

A Supertrapp is basically a glasspack with baffles on the end... the "tuning" is done by adding backpressure. They really suck. You might as well jam steel wool in your tailpipe and hole it in place with a couple nails.

 

I don't like supertrapps either. If the end is completely left off they flow decently well but at that point they aren't really mufflers anymore.

 

A Supertrapp is good if you can do without the disks and end-piece. Otherwise, it's only good as an apex seal catcher.