Exhaust pipe diameter?

[dracothered]

Ok, the pipe coming off of my 2 stroke is 1" diameter so the piece I welded to the stub I had is 1" diameter. Now what I am wondering is how much will it hurt the HP output if I reduce the exhaust pipe to 3/4" diameter back to a muffler or at least something to provide the needed back pressure that a 2 stroker wants to see to run right?

My limited knowledge of a muffler is the holes in the pipe must equal in total the diameter of the exhaust pipe. These holes cause the noise wave to be broken up and also provide some back pressure. I know a straight through muffler has less back pressure than one that has a chamber that the exhaust goes through. What is the best way to get the wanted power with a more throaty sound to it?

 

[GearNut]

Keep your 1" head pipe. Attach it to a big fatty expansion chamber like the one in this thread:http://motorbicycling.com/showthread.php?t=43969

The small diameter pipe coming out of the tail end of the expansion chamber is called a stinger. It is small for a reason, you want that. The stinger only needs to be about 2" long.
Have the stinger exit into a larger diameter pipe say about 1 3/4" OD. and as long as you can stand it to be. The long 1 3/4" pipe will not affect the performance gain of the expansion chamber but it will greatly deepen the exhaust note to a very throaty sound.

 

[dracothered]

Keep your 1" head pipe. Attach it to a big fatty expansion chamber like the one in this thread:http://motorbicycling.com/showthread.php?t=43969

The small diameter pipe coming out of the tail end of the expansion chamber is called a stinger. It is small for a reason, you want that. The stinger only needs to be about 2" long.
Have the stinger exit into a larger diameter pipe say about 1 3/4" OD. and as long as you can stand it to be. The long 1 3/4" pipe will not affect the performance gain of the expansion chamber but it will greatly deepen the exhaust note to a very throaty sound.

That design of an expansion chamber is just not going to fit the look I am going for (To modern looking).

 

[dracothered]

What is the down side of just going straight pipe with some back pressure introduced into the mix?

 

[rustycase]

imo, 'just going with a straight pipe' IS creating back pressure in the exhaust system.

...it is the expanding cone which draws the gasses FROM the cylinder, and the converging cone which sends the pressure wave back into the cylinder, all dependent upon particular angles, and length of the belly of the chamber and the exhaust tip.

If you want to avoid the tuned pipe LOOK, perhaps you should look at the form of a factory stock Motobecane 'cashew' exhaust pipe... from what I have read over at mabecanemobylette website, if they are opened up just a little they approach the power level of some tuned pipes.
..worth a try???

 

[maniac57]

imo, 'just going with a straight pipe' IS creating back pressure in the exhaust system.

...it is the expanding cone which draws the gasses FROM the cylinder, and the converging cone which sends the pressure wave back into the cylinder, all dependent upon particular angles, and length of the belly of the chamber and the exhaust tip.

If you want to avoid the tuned pipe LOOK, perhaps you should look at the form of a factory stock Motobecane 'cashew' exhaust pipe... from what I have read over at mabecanemobylette website, if they are opened up just a little they approach the power level of some tuned pipes.
..worth a try???

The tuning effect these pipe give is NOT an issue on a four-stroke engine. Four-strokes do not need or benefit from the cone shape. Simply build it to look the way you like and don't over muffle the engine. Your holes vs. dia. formula should work very well. Bigger chambers seem to give a deeper sound. If you are welding, experiment with making a Flowmaster type muffler using shapes to direct exhaust flow between baffles in the can. I've had great luck using shaped baffles in homemade mufflers.

 

[dracothered]

The tuning effect these pipe give is NOT an issue on a four-stroke engine. Four-strokes do not need or benefit from the cone shape. Simply build it to look the way you like and don't over muffle the engine. Your holes vs. dia. formula should work very well. Bigger chambers seem to give a deeper sound. If you are welding, experiment with making a Flowmaster type muffler using shapes to direct exhaust flow between baffles in the can. I've had great luck using shaped baffles in homemade mufflers.

Ok, but I have a 2 stroker....

 

[rustycase]

LOL
Did I botch the whole thing?
I thought we were talkin abt a 2 smoke!
Sry if I goofed...again!
rc


always get a 2nd opinion on my ranting and raving, especially while I'm waving my arms around in the air!

 

[dracothered]

LOL
Did I botch the whole thing?
I thought we were talkin abt a 2 smoke!
Sry if I goofed...again!
rc


always get a 2nd opinion on my ranting and raving, especially while I'm waving my arms around in the air!

We are talking about a 2 stroke here.

 

[rustycase]

Well, ok, then!

IMO, you must decide where you wish the powerband to be in your rpm operating range. This can be modified considerably with the configuration of the Xchamber/exhaust system.

If you want max hp, it's gonna have to be at top rpm, which will basically leave you with a machine that is un-useable for just about any other purpose.

If you hope for the best torque at lower rpm, these little 2 smokes seem to be doing well with approximately a 20 inch header pipe that is about 15mm in diameter... I think it is... someone will come along with best measurements..
I wouldn't feel bad running a header at 15, 17, or 19mm... somewhere in that range. best results will be if the header expands at a 2% rate before the cone, but that's a difficult task to fabricate.

If this might be your intent, the header could drop down to beneath the motor, and you could have an expansion cone down there, where it is not so visible.. which I believe you said was part of your overall goal.

Perhaps you could try a somewhat flattened cone, and body for an Xpipe?

I have NOT seen where they were effective, yet if squashed a tad, might be a little more attractive????

Best example of a proper Xchamber is the japanese dirtbikes.
They are exotic, high performance machines! ...and it seems they really try to avoid flattening their Xchambers, keeping them round as possible.

A short Xchamber is very pipey, all at the top end, the longer ones will bring the torque at a lower, more useable rpm.

Have a look at some of the tuned pipes for mopeds...
mopedarmy has great pics.
I have never tried one of those center tapped stinger outlets, but it looks interesting, especially to shorten up a system.

The moped guys also do fairly well with a muffler corked onto their stingers if it's not too restrictive.

Good luck!
rc