A Back-Pressure Discussion

Weldangrind · 03-15-2010, 04:12 PM

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?

03-15-2010, 04:12 PM	#1
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	A Back-Pressure Discussion 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? __________________ Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer
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