Straight pipe / back pressure motorcycle?

Question

So it's been talked about all over the web. No back pressure = way less torque or no back pressure increases performance, or straight pipes burn out valves etc... I would prefer no finger waiving talking about how we shouldn't make a motorcycle loud, but rather an answer to if I put bends/curves, or wide pipe to small then back to large, will this create some back pressure? It seems like doing something like that could give you a bit of the pleasures of both worlds. Found nothing online for that yet just a bunch of people arguing over whether or not straight pipes are bad for the motor. Some swear it is some swear it aint. My personal experience has been more torque, quicker response, and just an over all tighter feeling engine when the mufflers are on. But on this bike the sound is remarkable and just want to find an alternative to get a little back pressure. Thanks for the help, and thanks for not lecturing about how bad loud motors are, and how bad straight pipes are...

Answer 1

less pressure is always better except in 2 stroke expansion chambers..

the back pressure people talk about is actually a sonic/acoustic reversion wave that resonates back & forth the full length of the exhaust system independent of the exhaust gas flow.

at certain rpm's it can act like a valve to keep the fresh charge from following the exhaust gas out. it can also push the burnt gas back into the combustion chamber to s***w up the incoming mixture.

torque cones or ports with flats around them like D ports can reflect this wave back on itself & keep it from entering the ports & combustion chamber.

it takes a lot of hours on a dyno & sliding pipes to get any individual exhaust system perfect for a narrow rpm range.

but in general open step pipes make the most power across a broad rpm range. for example  4 to 8 inches of 1 1/2 pipe into 13/4 into1 7/8 into 2 inch (1 7/8 id is hard to find) this is for engines under 100 c.i. for big engines use 1/4 inch bigger pipes all the way. same idea as the old megaphones from the 60s but the sharp edges on the steps seem to work better than the smooth megaphones.

another trick for straight pipes is to drill across the end & put a 1/4 bolt across the pipe end(called a splitter) this sounds like some kind of magic BS but it will give you more grunt off the line without hurting the top end power.

never heard a good explanation of why a splitter works or who first came up with it way back when. maybe it is magic, exhaust building is more witchcraft than science but a good system can put out 20% to30% more hp than a bad one with no other changes so its worth asking a lot of questions & doing some experimenting.

or just get the sound you like & then put in some torque cones to make it rideable like most of us do.

Answer 2

indian john hit the nail on the head here...

free flowing exhaust will make MORE power/torque as long as you tune it properly...

Answer 3

They sell torque cones that drop in the pipe right at the header.Some bikes respond well to just having very short baffles put in.Sometimes a 2" to 4" long baffle will solve a lot of problems.Still plenty loud too.

Answer 4

Pop off the mufflers and put on 21/2 inch dump pipes Yes that is bad for it and that is what 80% of driveway wrenches do

Now if you rejet for the pipes "Richen it up" then no problem the problem they have is they lean it out with the pipes it runs hot dont really get the fuel it needs to run on and bad sh-t starts to happen Just read your plugs till you get it right-The same if you put on a high flow air filter or run a V Stack you change the air flow you have to change the fuel volume

I run 2 inch wraped drag pipes on mine aint burnt nothing yet

Answer 5

Hi,

The length of Pipes is critical.

That's why some machines look like a  plumbers nightmare.

If these Pipes were straightened, they'd be twice as long as the bike!

Also, when fitting open Pipes, Carbs normally require re-jetting.

Other factors come into consideration as well, and as you say, diameter of Pipes is important also.

I owned an Aermacchi 250 Ala D'oro Racer once, and put it on the road.

First, I had to fit a longer Pipe, then change the extremely noisy End Can because I lost all power, and try several  types of Silencer before settling for a BSA Goldie pattern road model.

Even then, I had to drill holes in the Baffle one at a time until bike ran properly.

So, yes, varying the Pipe length and diameter has an extreme effect on back pressure, and so does the type of Silencer

I also had to fit a larger Carb, but that's another story!

Was it worth all the expense?

You bet!!!

Answer 6

Straight pipes are usually refered to as " drag pipes", whiich is about all their good for, wide open for a 1/4 mile, after that it's like Crash said, reversion etc.