Has anyone experimented with varying the diameter of the tailpipe section (including silencer) to dial in a 2-stroke pipe?

The books I've read seem to indicate a trade-off between enhanced pipe action and power output (reducing diameter) and motor overheating with a too-small pipe - has anyone found this to be a problem in real life?

My 175 is currently using a 1.125" I.D. tailpipe/silencer on a too-small (in my opinion) Jemco CR125 up pipe, and I'm going to try reducing it to 1" I.D. to see what happens (I've got to make this pipe work, it's the only game in town I know of). The bike started out low on power and I'm making progress, but I've got so much going on now with jetting/timing/compression/etc. that I'd like to prioritize things a little.

Ray

If you are reducing the stinger diameter, you are increasing back pressure of the entire system. This should (this is just me thinking out loud) force the charge back into the cylinder earlier and therefore giving you more bottom end.

This is just an opinion so I may be wrong and someone will correct me if I am.

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I think (and hope) you're right. :) I did a bit more reading on it today, and I found this on page 101 of Bell's 'Two-Stroke Performance Tuning':

"... the stinger is in reality a bleed pipe. It's function is to restrict gas flow out of the exhaust and create back pressure by slowly bleeding off the exhaust gas. This serves to assist the positive pulse wave in pushing any spilled fuel/air charge back into the motor."

It's almost like the backpressure gives the sonic pulses something to push against, or even some additional and/or earlier push as well. The text makes it seem as if a smaller stinger enhances performance over the entire tuned range of the pipe, but I always thought that it was mainly a low-end boost too.

I sure hope the 1" I.D. stinger tube helps. Right now the pipe seems to do very little below 6,000 or 7,000 rpm, and starts to kick in right as the exhaust port is signing off, it seems. I've heard it's a clone of the Mugen CR125M pipe.

Ray

I wonder how much top end you will lose due to the smaller stinger. This is definitely an interesting project you have :thumbsup:

check out slippy pipes for shifter karts youll find it interesting. but anyways reducing the diamiter will bring up peak hp and narrow your powerband because the stuffer wave will be much stronger and earlier.

http://www.iwt.com.au/MOTA.HTM check out this awsome program i have built 4 pipes using this software all of them worked great

Thanks for the slippy pipe tip - those things are cool! I thought briefly about something like that a long time ago, but I had no idea they were in wide use. It would be outstanding to use a 'temp slippy' to fine-tune a design before committing to a center section length - I bet most pipe builders already do something like this. If I have to lengthen my center section - as I'm almost certain I'll have to do at some point - I'll try it myself.

A smaller stinger making the pulse stronger is good news in my situation, but earlier is not so good. :)

I couldn't get the MOTA demo to download, but I'll try again later. I've been looking for a Windows pipe program on and off for a while now; thanks also for that info.

Ray

Probably a more effective approach to gain low-end and bring the pipe more in line with the low-rpm porting would be to shorten the HEADPIPE length. If you can cut say 10mm out of the straight-section from the headpipe you will be bringing the reflector cone closer to the exhaust port, reducing the time it takes to get there, and pushing the "tuning" of the pipe lower in the RPM range.

If you've got the cash, maybe contact Jesse Williams @ williamsmotowerks. He is a master pipe welder and has software based on the engine parameters that will make a pipe that will WORK.

Stinger diameter can be 55-70% of the head pipe diameter, 65% is a good number to shoot for but it's just one variable, Tighter will give more punch down low but can also drive the motor into detonation real fast. A rule o thumb question is how long of a wfo pull are you going to make? I've made stingers (or restrictors) for 660' racing that would burn a motor down in 1/4 mile.

It's a balancing game as highmarker said!!

On your 175 (what is it again?) I would bet a well designed pipe would have around a 22-23mm stinger ID....

Maybe even smaller depending on your power output.
However the entire pipe design changes to some degree with a stinger change.

Lots of great info, thanks to everyone!

My "CR175" headpipe is 39mm, and 65% of that is about 25.4mm, or 1". Right now I'm using a 32mm/1.125" I.D. pipe, and I've got a 1" I.D. tube on the way that I'm going to slide inside the existing one. I didn't know about the tailpipe being a specific percentage of the headpipe diameter - really appreciate that. I just lucked out on the 1" pipe being about right. :)

The Jemco MR175 pipe (very different from the Jemco CR125 pipe I'm referring to in this thread) uses a 23.5mm/.925" tailpipe/silencer I.D., and this pipe works quite well below @ 7,000 rpm. I'll also try some 22mm/.875" I.D. tubing, but as the up pipe installation did a number on the left-side engine cooling, I'll keep a careful eye on temps during testing.

In the woods, I'm WOT for maybe 30-40 seconds max at a time, and on the street I'm rarely WOT at all, trying to keep the noise down.

I'll definitely contact Jesse Williams - it seems like there's so few pipe builders, it's always great to find another one.

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Based on what I've read and the way the 6 pipes I've tried on the MR work, I'd have to disagree with this. At higher rpm things happen more quickly, and the 'rev' pipes I've seen - including this CR125 pipe - are all very short. At lower rpm, the exhaust port is open longer and the 'blow up' interval should be delayed (longer pipe) to prevent jamming exhaust gas back into the cylinder, which is what I think is happening now.

The pipes I have which work well at low rpm are all long, with longer cones and center sections - the ones with the most low end also have really long headpipes before the first cone, but these pipes have very little high end. My two best pipes, which work well everywhere, have either a tapered head pipe or an additional smaller divergent cone closer to the exhaust port, in addition to longer cones and center section. I've also got two pipes which are pretty much identical except for center section length, and it seems the shorter pipe loses low end without any real gain up top - but these are low-tech vintage pipes.

I guess 2-stroke exhaust system design is all about tradeoffs, and I think for woods riding it may be better to lose unburnt charge out the tailpipe at high rpm (longer pipe) then push burnt gases back into the motor at low rpm (shorter), but that's just my totally non-expert opinion.

Sorry for the long post.

Ray

Do you have engine specs? I can design one for you...just requires some info aka porting timing inlet system etc..

I still dont know what bike this is!?

Thanks very much for the offer of help! Sorry about keeping you guessing... it's a 1977 Honda MR175 motor in a 1974 Honda CR125M frame. Nothing is stock except the gearbox. rear brake, and kill switch. :)

Here's the porting I'm using now, along with time/area specs and comments (the rear transfers look better on paper than in the cylinder):



Here's porting that might work well with the pipe I'm using now - all I have is the diagram, sorry:



... and here's that pipe, on the bike:



I'd like to get another cylinder done with porting optimized for 9,500 rpm, modifying this pipe if needed. The 8,000 rpm peak of my current porting works great in the woods, using a modified OEM pipe - I have a 1975 Honda MR175 (complete bike) with this porting, which I'll keep on that bike - but I'd like to go for more top-end power on the '77 motor in the CR frame.

Ray

36MotoMarc said:

I have to agree with this. The problem you will run into if you start reducing stinger diameter is increasing piston crown temps. Go a little too restrictive and you'll end up melting the piston quickly.

Chokey said:

Ruducing header length always always always hurts low end on an already developed pipe for the vary reason described....it shortens the time it takes for intake and exhaust pulses take to get back. This HURTS low end and helps top end.

rayivers said:

I'll try to get some basic pipe designs out for you in a bit...it's piston port, right?

One of the porting arrangements is a single oval port the other a T port...your running the first one, right? No 5th intake port called boost port either?

harrperf said:

I'm sorry, I didn't explain myself well. My point was that it is safer to alter the headpipe length than to make the stinger too restrictive, which has the effect of increasing piston temps. I wasn't implying that the correct headpipe modification was either/or...

Chokey said:

Ah...

stinger dia mostly depends on output of the motor...I doubt this motor is really pumping out too much!

Maybe 20hp all tuned in?

OK, great - thanks again for your help.

It's piston port, no boost port, mostly rectanguler exhaust port w/slightly rounded edges and sides (the diagram shows it fairly well). If I did try the 'T' exhaust port, it would use two drilled auxiliary ports rather than trying to add a bridge.

The stock MR175 motor develops 14hp at the rear wheel. It seems that if the stock CR125M put out 20 hp at the crank, this motor should be good for maybe a bit more than that, no?

Ray

Hey folks!

I'm guessing when you folks are talking about the 'stinger', your talking about the length between the end of the pipe, and the beginning of the silencer... correct?

Assuming thats what your talking about, whats up with the shorty silencer... Its supposed to add low end torque? But that dosn't really make sense to me unless it has a smaller diameter 'stinger' and silencer tube itself... (if I'm referring to the right thing as the stinger)

Can anyone comment on that for me?

Thanks :ride:

rayivers said:

Well give me a bit to model it up...

I'll make a couple iterations and you can wade through what you like...