Exhaust pipe analysis

[dvogtvpe]

2 strokes are very sensitive to any exhaust change. putting a longer head pipe before the expansion chamber adds torque , shorten it and it gets more peaky. even silencers make a huge change in a 2 stroke, anyway enough about 2 strokes.
I believe that while the pipe becomes smaller the velocity of the gas's leaving are increased and during the cam overlap it helps empty and fill the cylinder better , fill the cylinder better and increase the running compression , higher compression = higher torque. most people are familiar with compression during cranking. there's no way of actually knowing running compression unless you can monitor the air going into and out of the motor.
We all think of filling the cylinder is done on the down stroke of the piston. but during that short time of valve overlap the exhaust is actually helping suck air/fuel mix in.
I just thought I'd throw this out there as an example so people can better understand how an exhaust can help or hurt you and on a little single or twin it can make decent increase's
heat wrapping the pipe or heat coating also increase's velocity of the exhaust.
mickb72; the 3 stage pipe you have should probably stay in the shed it could have a cool factor hanging on the wall.

just so you know that there were also fuel mixture and timing changes tried while messing with the pipes. some pipe changes required fattening up the fuel mix and some required leaning it out. in the end I was able to get around 28 degrees of ign timing on premium unleaded pump gas with the head with a plug over the exhaust valve .
It'll be headed for the track to see how it stacks up, customer is happy with the gains considering its twice the hp his old worn out 12 had. I used a stock head gasket but shaved the head .030 and decked the block .010. there's still enough room above the valves to go to a thin copper head gasket around .030, I should have put some playdough on top of the valves with no gasket to see how much room was really there. could maybe get away with no gasket like our pro/super motors


There is not a right tube diameter or length. It all depends on the application

[J-Mech]

I know that was for mostly explanation.....
So, why did the little bit of back pressure bump the torque?

I think it may have had a venturi effect. Small area of low pressure caused the exhaust gas in the moving through the stack to draw harder on the higher pressure on the engine side of the lollipop. When the upward motion on the piston stopped, it "drew" or "sucked" a little harder as the high velocity air was still moving. Causing a little better sweep of the cylinder air. May have even caused it to move fast enough to overfill the cylinder before the down-stroke. So it was starting off with an overcharge. I wonder what the pressure difference was before and after the lollipop, and closer to the top of the stack.

How fast is the valve cutoff on the exhaust cam lobe? Pretty quick I'm guessing. I wonder what it would have done to it changing the carb spacing along with the other stuff.

[Sam Mac]

If you want to see some works of art when it comes to exhaust systems check some of these out.

https://www.google.com/search?q=f1+e...w=1280&bih=878

[dvogtvpe]

J-Mech; I suspect the pipe diameter was incorrect for the motor. on our pro/super 50.5 motors we run 1 5/8 or 1 3/4 depending on a few things. I used 1 3/8 on this one, a .010 over -16 at 4000 rpm . I'd bet if the pipe size were 1 1/4 or 1 1/16 it would do nothing , probably hurt it. that's why it was so sensitive to the length when I cut it off .
I think (and I'm no rocket scientist) and your somewhat correct . a good or proper sized exhaust the hot exhaust gas's start to speed up as they try to expand and leave the pipe . torque is created by filling the cylinder better. Make sense?

Sam Mac; they are a thing of beauty , I've been studying those pipes for a couple months now looking for a specific design that was used at the cylinder head that I'd like to copy to one of our open rpm motors.

[dvogtvpe]

If you're going to get into performance work you need a degree wheel and a dial indicator , photo of the lollipop in the pipe.
I've had problems holding point adjustment at times on these motors. and if you adjust the points enough the screw slot wears out. I like to put a hex head screw in the points . easier to tighten and the slot don't wear out.
those are my tiny tidbits of worthless information for now.