You might check out the following site;

http://www.uucmotorwerks.com/html_pr...torquemyth.htm

After reading that information you might see why my WideGlide (with it's'stepped tunedpipes', no internal baffles, and with the air/fuel mixture optimized) produces a noticeable increase in power throughout the operating range.

 

Ok, so I read the thread, what is the conclusion, that a smalll amount of even shock absorbing back pressure is the ticket? I don't think anybody is saying no pressure (drag pipes) are optimum, correct?

 

If you want power at a particular rpm, say 6500, and only at that rpm, you can set the length of your straight drag pipes to give you the ideal pressure waves for that rpm. At other rpm the exhaust will work against your performance, but if your goal is to get down the track the fastest and you are geared to run at 6500 then drag pipesare your ticket.

 

Let me preface this by saying that all of my building/tuning in the past was based on cars, specifically Ford V-8's, but the general principal caries over to any internal combustion engine.

It's all about pulse timing which creates "scavenging". The exhaust reverses when it hits a restriction (turns, baffles, etc.) and heads back towards the motor. It then reverses back towards the exhaust exit when it hits another restriction. It you can plan the changes in direction correctly the first exhaust pulse can travel ever so slightly ahead of the second pulse and help "pull" it along. Kinda like 2 cars drafting on a race track. Because the exhaust pulses are continous as long as the engine is running, the 1st pulls the 2nd, the 2nd pulls the 3rd, the 3rd, pulls the 4th, etc.

Back in the old days, exhaust was just a pipe that carried burnt gases back behind the driver. With racing technology getting better and better somoeone figured out the concept of scavenging and started developing tuned exhaust systems. One of the common features of most (not all) tuned systems is a crossover of some sort. The idea here is that if you can create scavenging in one exhaust pipe, then by connecting the two pipes, the pulses from both exhaust banks can scavenge off each other as well, therby increasing the effect and benefits.

Even with a crossover, there is tuning that can be done. Example: On some of the Mustangs that I built / helped build I would add a H-pipe, some would get a X-pipe. Why? The "H" was tuned to help with lower RPM torque and power while the "X" worked better for higher RPM power. On a street only car, I'll always add a "H" and on a street/strip car I'd add a "X".

Totally open "drag pipes" work well for wide open high RPM use, but there are no significant restrictions, therfore the exhaust pulse never reverses it's direction significantly. It just heads south at full speed. Because of this, the low to mid RPM ranges suffer, but wide open throttle (WOT) actually benefits.

Even the length of the open drag pipe has an effect on top end power as well. There is an old drag racer's trick for tuning the length of an open pipe using just a crayon... but I'm getting way too far into car tech, not bike tech.

 

By the way, as just a little trivial knowledge...

When fuel injection first became popular on cars, the throttle body looked like a stripped down carb with a couple of injectors bolted in it. Now a days if you open the hood on any modern FI car there is a plenum that branches into a bunch of runners that are significantly longer than the runners on a carb intake. Wonder why? Because of the same principle of scavenging, except on the intake side. Longer runners help pull the air into the heads faster at low/mid RPMs. You'll notice that when manf's started going to the long runner fuel injection all of those "Tuned Port EFI" badges started popping up. Same deal applies.

Long runner intake = street / low to mid RPM performance
Short runner intake = strip / high RPM performance

 

You needs some back preassure.

Worked with a friend that dyno's bikes. He has shown a lot of people over the years that run straight open drags. If they have alittle back pressure( Holds a 2X4 over about a 1/4 of the tail end of the drags. That they bike actiually gains horsepower. The board is kinda like the baffles in the newer style performance pipes and mufflers. Same as open drags using torgue cones.



Like years ago you hear some of us old timers talk about how we run a bolt from the back side of our open drags to create that back pressure. Also helped when the man slid a stick up the pipe to check and see if they could write you a ticket.

Just some thoughts and school experience.

 

(raises hand) Teacher, may I be excused to go to the bathroom? Hot dog!!! School is in session today Jack!

 

I have a 94 sporty, and i I just replaced the headers with some cheap drag pipes that matched the bends of the stock header pipes, cut them down and put my cycle shack mufflers on. Without the crossover pipe, I had no noticable loss in lower end torque which is where the crossover is suppose to have the most noticable effect, and felt the overall throttle was more responsive without the crossover. Anyhow if you wanna try my route, it's cheap enough, i got my dragpipes from Jireh cycles for $30.

 

There are 2 ways to deal with the cross over. The crossover does help the bottom end on stock and stage 1 bikes.
The problem with the cross over isn't that it's there but that Harley started buying the head pipes from the cheapest bider in the late 90's and instead of getting a nice welded in cross over the new supplier just drilled a hole in the side of the pipe and shoved the crossover it the exhaust track by about a 1/2" which totally screws up the flow of the exhaust system.
You can cure this problem 2 differant ways, first way is if you want to retain the the cross over which is a good idea if you have a stock or stage 1 bike (if you remove the crossover on a stock or stage 1bike you will fell the differance on the bottom end) On my wifes bike I used a tubing cutter and removed the top of the head pipe and then ran a hole saw down the inside of the pipe and removed the portion of thecrossover pipe that was protruding into the exhaust track and then had the local exhaust shop weld the pipe back together.
On mybike I cut the crossover off as short as I could get it to the head pipeand thenbuilt a wood plug I could pound into the crossover an thenIused a hole saw to cut therest of the crossover out of the head pipe and then I used another hole sawone sizebiggerand cut a plug out of another piece of pipe and then had my local muffler shop weld the plug over the hole.
This has worked very well on both my wifes bike which is a stage 1 bike and my bike which makes 85 HP at the back tire.

 

There should be no more confusion after mrb's posts.

A benefit to amotorcycle is there is no hood restricting the design of the intake!! Stupid plenum causes erratic airflow, no taking advantage of the mass or energyof air.