MRP exhaust

[perrym]

I was checking the mods that Scootdawg did on his scooter. Getting food for thought, so when I do get rid of my trusty 49CC 2 stroke moped to move up to a a 150CC 4 stoke scooter I will be able to squeeze out a little extra umph out of it.
I noticed that the MRP exhaust is a 2 piece. Now we old 2 strokers love to play with them cause they can be fine tuned by shortening them or lengthening them to increase top speed or low end. Now remeber all of this is based on 2 stroke experience, but I believe you can get similar benefits on a 4 stroker. It is a lot of trial and error, but even a 1/4" can make a difference.
Right now, I think Scootdawg has it at the shortest point. It would be a cool experiment to so what affect it would have to lengthen it a 1/4" at a time. Especially since he has all the toys to measure the increase or decrease in acceleration and top speed.

[Admin]

Perry, I wish I know more about exhaust. I've got a name of a guy who has a pretty sophisticated shop where he works on racing cycles such as Ducati. I'm not sure that there is much of a way to play with the length as it's a tapered fit and it needs to be fairly tight. From what little I do know, two strokes are much more reactive to changes than four strokes ;D

Perry, you know that Allen reads this board sometimes. Dawgs, be prepared, there could be a war of the strokes.

[perrym]

Lee, can't you loosen the 2 clamps on the muffler and pull it out a little without moving the header pipe? No need to weld anything. On the 2 piece Bi-Turbo exhausts for old school mopeds all we needed to do was just move the muffler part either rearward or forward to get the exact top speed/acceleration trade off. Now remeber, it was only a small difference, usually only a couple of MPH. Shouldn't have too much of an exhaust leak. Even 4 strokers are receptive to changes in the exhaust system, but not as much as a 2 stroker.
Can't wait to hear what Allen thinks of my little experiment.

[Admin]

Yeah, it can be done. It's going to have to wait for a while. I have an rx150i I need to finish a final review on. A Tank 250 Touring in the driveway awaiting assembly, a Veypor Motorcycle computer to install and a Kymco Xciting 250 on the way soon to deal with. Dawg is drowning in his own self-created scooter sea.

[lewtwo]

I think the idea behind a two stroke "expansion chamber" exhaust is to help evacuate the cylinder. With a two stroke there is not much time between one exhaust puff and the next. The general idea is to have the gases moving fast enough so that that they leave a reduced presure area behind them for the next exhaust puff. If they slow down too soon then the presure equalizes. If they go too fast then they get sucked back. Get the harmonics right and it works great.

Four stroke engines have a relatively long time between real exhaust strokes. Getting the gas harmonics to work can require a lot longer pipe. Example: You may have seen some "paper clip" designs for four stroke scooters. There is a place in England that build exhausts for four stroke Piagio engines that puts a kind of spiral loop between the engine and the mufler. Also four strokes have a real exhaust stroke so it is efective to just make sure the exhaust is not restrictive.

Spiral exhaust cgi.ebay.com/PIAGGIO-VESPA-GT-125-210-cc-SPORTS-EXHAUST-PM-TUNING_W0QQitemZ8033915671

Question: So why not just drop the mufler and run a short exhaust pipe open to the air. ?

Answer: If the pipe is too short then when you shut the engine down it will draw cold air into the exhaust and warp the valve stems. Yes I know that they do that in dragsters. Ever see those balls in the ends of the exhaust pipes ? Looks ain't the only reason they are there.

Other than that straight pipes are noisy and scooters are not supposed to be offensive.

(this post has been modified because I can not type worth a $#!~).

[ooopseyesharted]

Perry,
i was wondering what kind of 2 stroke scooter you have?

[lewtwo]

I was wandering around lost on the net and ran across this page about increasing performance on the old Honda Elite 125/150 line of scooters. About half way down the page is a piece on the paper clip exhaust design.

www3.telus.net/dougsimpson/Elite_4.html

It also refers to a page that has a piece of software to calculate the optimun length/size for one's exhaust system. So I downloaded the software and plugged in the numbers for a 150cc at 7500 RPM. 20 Inches at 1 inch diameter, 4 inch cone, 20 inches at 1.4 inches diameter before you get to the atmosphere or mufler (and thus the paper clip design).



So you tinkers out there (Lee, Stan, Limeracer, Perry ... among others) have any thoughts ??

*

[90GTVert]

Yeah, I've seen this article before. Because there are good performing systems readily available, I didn't feel the need to buy 1.25" dies to make a system. No doubt that with some patience, some ss tubing and the correct dies, you could build a good system. Stan

[lewtwo]

If the numbers from this program are correct (44 inches) then all the performance exhaust systems seem a bit short . . . but it is supposed to be a very complex problem.

I have been looking for more information on subject (interesting stuff). Three things that I have found:

1) Two things at work: Soundwaves and gas movement.

2) Two Strokes: The timeing can be very critical (we all knew that) because it is used to push fuel/air charge back into the combustion chamber. I thought all they were trying to do was clean out the exhaust gases. OK ... I stand corrected.

3) The intake side of Four stroke engines can be tuned if you have a carb on each cylinder (da). This helps you get more fuel/air charge into the chamber at higher RPMS.


I ordered a book on the subject: "SCIENTIFIC DESIGN OF EXHAUST & INTAKE SYSTEMS"

Online References:
www.burnsstainless.com/TechArticles/Theory/theory.html
www.challengers101.com/IntakeTuning.html

[kevperro]

lewtwo: I used to have a copy of that book. I was looking for it last week and couldn't find it. It is a great resource and you are right.... there are multiple factors at work. The book will help you glimpse some of the complexity, give you some mathmatical models to work with and leave you at the same place you started.

You really need an engine on the dyno and a lot of time to tinker to build the optimum set-up. Even then you are making engineering choices about what RPMs you want to optimize for, what head is optimal (valve size, lift & timing) and tuning the intake is much of the same work. Overall you have to have a good R&D budget to come up with something really good and then it is applicable to only one engine set-up. ;-) I've often thought a computer controlled variable length/resonance chamber on the exhaust or intake would be the ultimate set-up but I'm sure it is cost prohibitive, especially on the exhaust side where you are working with high temps and expanding/contracting materials.

[lewtwo]

kevperro said:

lewtwo: . . .
You really need an engine on the dyno and a lot of time to tinker to build the optimum set-up.

Yep, that and an unlimited amount of time and money would be a big help . . . course then I would probably have other interests.

I have decided to turn the longbo into a project bike (the elite 125 was too much of challenge to find parts for). I am kind of curious if the paper clip thingy would work. I have located sources for the madrel bends. I can not tig worth a $!#t but am a fair hand gas brazing. I want to read up a bit before I decide what to do to it.

I had not thought of computer controled chamber but something like a trombone slide had crossed my mind: "Now everyone in the key of B flat".

(Yes I know I am sick).