A Back-Pressure Discussion

[Weldangrind]

My son and I are building an old XR200R for him, and we're currently designing the exhaust system. Of course the original muffler and spark arrestor are rotten, so we've managed to create a new system by using the stock head pipe, the clamp portion of the muffler, some Gio exhaust tubing (thanks TurboT ) and a Gio 125 Pit Bike muffler.

TurboT had a 125 Pit Bike muffler on his 200 Beast quad, and it forced the rivets out and blew apart due to excessive back pressure, IMHO. I arrived at this conclusion by measuring the diameter of the holes in the restrictor plates, calculating the area of the 15 holes, and comparing that figure with the area of the head pipe.

I'll spare you the math and say that the head pipe area is 0.74 square inches, and the total area of the restrictor plate holes is 0.29 square inches. For clarity, there are two of these plates in the muffler, one at the inlet and one at the exit.

I began to wonder what would be a good amount of back-pressure, since I want the bike to start and run well. I'm already aware that I can tune the carb to compensate for additional airflow and avoid a lean mixture, but I'm curious about the magic number. The math tells me to drill each hole out to 0.25" for zero back-pressure, but that will likely obliterate the restrictor plates.

I don't want to remove the plates completely, because I don't want a straight pipe. Our riding area has a 96db noise limit. The muffler is a decent design, in that it has a beefy aluminum cap at each end, and aluminum tube for the outer structure and decent perforated steel tube for the core. The perforated tube is wrapped in fiberglass.

After some searching, I found this explanation: http://www.uucmotorwerks.com/html_pr...torquemyth.htm

What do you think we should do?

[waynev]

Not sure if the "monster" muffler is too big ( physically ) for your bike but it has been working great on my 200cc beast.

[TurboT]

Mate I'll give you a fine good guy rate on the exhaust tubing. (Free for being a good guy)

I didn't much like the article. I thought it was wordy but didn't really go anywhere, other than throw out a bunch of scientific gibberish.

I know we've had this discussion a bit, but the more I think about it, the more I think you're better to go with limited backpressure and more flow. I use my YZ as an example. The only thing I can see in my exhaust is a screen (spark arrestor) that would be blocking any flow. I haven't been able to see any restrictor plates, and the head pipe is a lot beefier as well out of the exhaust port. I'm aware this motor is a bit more advanced, being 5 valve allowing more fuel and air in at once, but with that said, I think better breathing should be your goal.

I'm going to open up a theory discussion with the king on this, to see his thoughts. Will update when I've had that chance.

-TT

[suprf1y]

No amount of 'backpressure' is either required, or desired. The article is a simplified version of the truth, but the important thing here is choosing the correct diameter, and length of exhaust tubing. A muffler is required only to quieten the exhaust, and is a necessary evil. It's all about exhaust gas velocity.

So since your tubing diameter and length have already been chosen, run as free flowing a muffler as you possibly can (stand), and hope for the best.

[Weldangrind]

Originally Posted by TurboT

I'm going to open up a theory discussion with the king on this, to see his thoughts. Will update when I've had that chance.

I recommend that you capitalize King. Please give him my regards also.

[TurboT]

Originally Posted by Weldangrind

I recommend that you capitalize King. Please give him my regards also.

Well, after some prodding on the King(he doesn't like to talk theory much anymore, after 40 years in the shop discussing performance with every backyard expert out there he's tired of it) he gave me these minor tidbits.

"The ideal exhaust for a motorcycle would be straight out with spark suppression to protect the forests.. "

"motorcycles benifit from tuned exhaust and no back pressure just do not burn down the forrest."

He seems more concerned about the forrest than I'd given him credit for. This is obviously one man's opinion, which we all know can vary on different topics...but I'd tend to trust him on this one, as I've spent many moons polishing exhaust ports to remove resistance on air flow, and looking at my YZ it is virtually like this, as mentioned. I don't struggle to start it so I think you're okay with very little restriction in your new exhaust.

[katoranger]

I have noticed that your exhuast can greatly affect where you usually torque is.

My escort wagon with the thrush turbo muffler (Redneck fart can) was almost useless at low revs. When I got it replaced with stock parts the torque was down low where I could use it.

So what I am saying is the backpressure may have robbed some horsepower, but I gained big in driveability. Depends on how you want to use the bike. If mostly high rpm than little restriction is probably just fine.

Not and expect, just my notes. I have seen similar results on other vehicles.

[suprf1y]

It has nothing to do with 'backpressure'. Restricting the flow with a muffler does not work on anything.
It's all about exhaust gas velocity.

This is obviously one man's opinion

No. It's fact.
It's not only motorcycles.
Everything benefits from a tuned exhaust.

[TurboT]

Originally Posted by suprf1y

It has nothing to do with 'backpressure'. Restricting the flow with a muffler does not work on anything.
It's all about exhaust gas velocity.

No. It's fact.
It's not only motorcycles.
Everything benefits from a tuned exhaust.

Well, if you want to get really picky, 2-strokes need some backpressure. That doesn't mean it's not 'tuned' though. On a 4-stroke, I'm inclinded to say as little as backpressure possible that keeps the noise level down.

D, have you considered the Monster muffler on the XR200? The can is longer and wider, and would allow more room for sound deadening inside? Won't look as cool as the pit bike pipe though.

[Weldangrind]

I agree with waynev and TurboT that a Monster muffler would be a good choice. That said, I have a Pit Bike muffler (and a spare), so I’ll incrementally drill out the holes until I reach the outer limit and give it a try. If unsuccessful, I’ll buy a Monster muffler.

I agree with the King that ideally we would have no back pressure, a tuned exhaust and a spark arrestor, which is why I’m not punching out the plates entirely. BTW TurboT, we should do the math on the Monster pipe.

I agree with superf1y that there is no benefit to back pressure in a four stroke. My son was on a jr. dragster team, and I noticed that every four stroke exhaust pipe was a straight pipe that stepped up in diameter a couple of times. IMHO, this is to prevent the restrictive effects of a single diameter pipe and maintain velocity. I know from some plumbing experience that there is a point where straight pipe will equal the restriction of an elbow.

I agree with TurboT that a two-stroke benefits from an expansion chamber to boost horsepower in a specific rpm range. I rode a two-stroke that did not have an expansion chamber, and it lacked the usual power band.

Katoranger, IMHO your Escort wagon could have maintained the usual torque and had an increase in power if the induction had been optimized. Simply swapping to a high flow muffler increases airflow without proportionally increasing fuel to match. From what I’ve read, the stoichiometric14.7:1 air / fuel ratio is only ideal for economy. Maximum power is usually achieved in the 12:1 to 13:1 range. Like you, I’m also not an expert. :wink:

Thanks for an interesting discussion guys!

[katoranger]

Originally Posted by suprf1y

It has nothing to do with 'backpressure'. Restricting the flow with a muffler does not work on anything.
It's all about exhaust gas velocity.

No. It's fact.
It's not only motorcycles.
Everything benefits from a tuned exhaust.

That is probably correct. The "turbo muffler" flowed alot more than the stock one. Everything else was in stock form, just rusted out.

Allen

[katoranger]

Good little bit on expansion chambers too, but if you go down to the bottom it has "tuned pipe".

I just learned something here. On my car the actual length of pipe was not changed, but the effective lenght was. The larger diameter muffler acted like a shorter pipe.

http://en.wikipedia.org/wiki/Tuned_pipe

Again. Back to the how do you want to use the bike. Where to want power. For me having power at lower rpms in a street car is good. On a trail bike I would like low end power too. If racing then maybe top end is better.

[Reveeen]

Another consideration is heat retention. Sometimes, when an exhaust valve cools too fast, it can warp. This has more to do with system length than flow.

[2LZ]

A little back pressure is quite useful to help your power curve.

Example: I took the stock-style (stage 1) exhaust off my '74 Ironhead chopper and mounted some sweet straight pipes. Sounded super nasty but all my mid-range instantly vanished. Put in some restrictor cones in the ends of the pipes and it all came back.

Concerning 'how much' is enough, I don't know. I once saw at a run, a guy had adjustable washers he could turn like a carb butterfly with a hole in the middle toward the ends of his pipes. He said he custom tuned the back pressure for his cam, piston and tuning setup and tighten them at a certain angle that he liked the best. Looked pretty neat.

[katoranger]

Like the supertrap exhausts. They are tuneable with those discs.

Allen