Does back pressure play any part in how a rotary runs?(speaking about the exhaust). Or is the whole concept about back pressure not applicable?

 

Not applicable. Backpressure is the devil. The only time it is ever used for a good purpose is on the GSL-SE to open the secondaries (5th and 6th port actuator sleeves). The later 6 port engines used the air pump.

The less backpressure, the better.

 

So by putting on an exhaust system with no real back pressure in it, is a good thing then, a little loud, but good?

 

Put down the crack pipe. You need proper exhaust tuning to scavenge the exhaust pulses out.

 

Read the articles on the exhaust cycle.

http://www.yawpower.com/techindx.html

Conway

 

He didn't ask about the technical stuff, so I didn't mention it. lol crackpipe.

Backpressure is still the devil. Ask peejay, Manntis Sterling, even Banana boy (Carl), they'll say uh something to that effect.

 

Backpressure= bad
Velocity= good
you don't want one, you need the other.
If you have to ask, just go buy an RB setup, you'll save money in the LONG run, not to mention ear bleeds from noise of homemade exhausts.......

 

That is true but it has absolutely nothing to do with backpressure. You want no backpressure at all. It is still possible to scavenge without it. In fact the best scavenging occurs when you have little to no backpressure. Scavenging is the ability of a collected exhaust system to suck out gasses from the other pipes using their momentum. It is also not possible to have a scavenging exhaust if they do not collect.

 

Well either way, good or bad, I now have an exhaust system which is basically a straight pipe, with a muffler that doesn't exactly muffle....... Looks good, sounds... not bad, more power?? Not too sure, doubt it...

 

To get all the gains out of a very free-flowing exhaust system you usually need to adjust the carb tuning as well.

 

What about 2 strokes? or are you speaking strickly rotary here?

 

And by collecting do you mean backpressure? If there's 0 backpressure, how can they collect? I don't understand how scavenging could exist if there is no backpressure.

 

Backpressure is a result of resistance to flow. You can never completly get rid of it. The smaller the pipe the more of it you get. Backpressure is bad anyway you cut the cake.

Scavenging is the result of pressure waves. Like sound, they travel through the exahaust regardless of how much air pressure (back pressure) there is in the pipe. Inertia resulting from high velocities are the source of pressure waves. So velocity playes a major part. Velocity increases with decreasing pipe size.

There is a optimum pipe size where the velocity is still high, and the backpressure low where the most power is made in the powerband. Increase the size further, and low-rpm power will suffer because velocity drops without any real benifits up top because backpressure decreases ever so little, decrease the size and you may gain a tiny bit of low-end due to increased velocity, but you'll start sacrificing the top end because backpressure will start to bite you.

It is possible to have scavenging without collecting. Anytime the size of the pipe changes size pressure waves are going to be both reflected back and trasmitted. When the exaust exits the tail pipe, its as if the pipe suddenly becomes infinately large, so a negative pressure wave will travel back up the pipe, and can be used to scavenge.

Scavenging with a header uses the same principle of relected waves. The pressure wave travels down the pipe to the collector, which appears as sudden increase in pipe size, sending a negative pressure wave back. Headers are more effective, because if the collectors are in the right location (at at or near the right RPM), another negative wave is simultaneously trasmitted by the other port.This wave combines with the other wave, and together they are stronger than the wave in the uncollected exhaust.

 

So, scanvenging occurs because when the pressure wave leaves the exhaust tract, it creates a vacuum condition thereby pulling the gasses out more efficiently from the housing.
So, 0 backpressure IS desired, afterall, then.

 

but, to add to what I said just now, if the exhaust tract is too long, and the 2nd pulse comes before the 1st one reaches atmosphere, (which would be the case with any streetable exhaust) doesn't this create backpressure? I suppose this is where the collection point at the headers resolve this, as you said above. Does this mean that a straight dual exhaust is not as good as one that collects at some point? RBs street port exhaust setup is an example of one that doesn't collect. At least, I don't think it does. Yet, it's touted as being better...
I'm still confused!

 

I think that not collecting is the best way to go, although I'm not 100% on that. It just seems that if you can get as little backpressure as possible, you dont really have the need for scavenging.
****, on a side note, never drink twice what it takes to get you drunk.

 

NO YOU CAN NOT SCAVENGE WITHOUT COLLECTING THE PIPES! You can tune it though. It is called organ pipe resonance tuning. This is not backpressure. That is merely tuning the powerband based on pipe length and diameter. The acoustic pressure waves create high and low pressure zones and depending on when these arrive back at the exhaust port determines if they will help or hurt at that rpm. That is not scavenging. There are 2 types of pressure waves in an exhaust and only 1 of them contributes to scavenging in the technical sense of the word. Many people get this mixed up or confused.

Scavenging is when the exhaust gas inertia not sound pressure waves from one pipe helps to pull out air from another pipe. They obviously have to collect to do this. Depending on how well the system is designed, at certain spots in the powerband the other rotor may have such an effect that it helps pull air out from the opposite side. How is there still any back pressure at this spot? There isn't but only for a split second. There is no way in the world that this can happen on a true dual exhaust.

Get reading about how this works. There are many good internet articles about backpressure and several good books also. I have alot of them.

 

You know there is no actual backPRESSURE.
There is RESTRICTION, and it's effect on exhaust pulse velocity and duration, but no real backpressure save for the 14.7 psi of the atmosphere.

 

ok rocket scientists so, what size pipe, at what lengths, collecting to what size pipe works the best for a large bridge-ported 12a motor?

 

Back pressure plays a big part. The more back pressure, the worse the idle and the less power you make.

That's all. No engine "likes" back pressure.

My old streetport would brap brap brap like a bridgey through the restrictive (yet very quiet) ex-Lincoln muffler. Uncorked the header (three wingnuts and it was open exhaust, gotta love it) and it'd idle smooth as glass. Ran lots leaner too, especially on the top end.

 

Porkchop Sandwiches!@ http://media.ebaumsworld.com/gijoe-porksand.avi

 

Holy cow, look whos back! Yes guys, think of a collapsed cat, MAJOR "backpressure". However, like anything else internal or external on a car, controlling the airflow is the key to sucess IMHO. In this case, as was stated before, VELOCITY is what we really want. "Backpressure" is the result of a restriction, this also lowers the velocity of the slug. Proper tube sizing, proper collectors and pipe lengths, minimized bends all play a part on the exh velocity and scavenging capabilities. Too huge a pipe dia? low pressure or low "backpressure" for sure, but you also lose a ton of velocity cause the expansion into the larger pipe= lower velocity. It's not a simple science as there are many factors that play into a proper exhaust, just like any other airflow control device. Looks are decieving and what looks right aint necessarily so when it comes to things like this.

 

Thanks GavinJuice!

Now i know!

 

but becareful as if you run an open exhaust it will be much hotter at the exit and if you have stock steel apex seals you can fry the tips and then no power at all. have had personal experience with it and ti was costly.

 

Going to an open exhaust will make your engine run leaner which requires you to rejet your carb. Letting it run too lean can cause excessive wear of the apex seals like you describe.