How did Harley overcome the old style ugly cross-over pipe on the newer bikes? Yes I've got that ugly crossover exhaust equalizer pipe on the stock exhaust and am told by others and forum members that it's necessary to equalize exhaust pressure... OK I agree with that basically because I feel that without equal back pressure on each exhaust that one cyl may be overworked or underworked... I am assuming..

So the question is... I don't see how Harley's newer bikes have overcome.....or changed the look of the equalizer pipe... Is it no longer necessary?

 

The crossover pipe on newer models is installed down lower, between/behind the mufflers.

 

The 'crossover pipe' is still there, but they did a better job of hiding it on the 2004+ models.

It is now kind of hidden behind the pipes/frame, but in the below picture you can probably see where it mounts to one pipe and runs back to the other pipe.

 

Thanks for the views! I never noticed that before while taking a quick look at the newer bikes.. Now I guess I need to try and find some pipes (newer style with cross-over" that will fit the old 99..

My stock pipes are hanging on the wall. Was thinking about re-installing them and chucking the drags.. since the drags are loud but not a deep sounding like I desire. Also the drags my be hurting performance.

 

Exhaust scavenging is the reason for the crossover.

It's easier to do a google search for "Exhaust scavenging" than it is for me to type the reason...

okay a summary

Exhaust gas leaving the cylinderforms a pressure wave & travels down the pipe. The wave reverses direction when it hit a junction or the end of the pipe in the case of drag pipes. If the reversed wave hits the exhaust valve as it is opening againit will push itself past the valve and blow the incoming air/fuel charge out the cylinder, big problem. Easy fix is to set the first junction a particular distance from the valve ensuring that for the rpm range of the engine the returning exhaust wave will turn again against a closed valve and help suck out the next exhaust charge, thereby sucking in the air/fule mixture. The sucking is an ideal which is difficlut to get. It is easy to setup the crossover a distance that will keep the exhaust charge from blowing out the air/fuel. As mentioned, many custom exhausts and drag pipes don't take pipe length into account and can really mess things up. Thunder headder & Supertrap have equal length pipes to the first junction (a curve will slow down air compared to a straight pipe) & they both did their homework in the muffler design to keep down unwanted reverberations (echos).

 

WTF? I have never heard that theory, and I will search it later, but It seems common sense the crossover is simply to provide a sufficient chamber to allow an even backpressure, and I am wondering if torque cones dont work just as well. Anyway, I think dynos show that the good aftermarket pipes without crossovers, and sufficient backpressure devices (baffles or cones) do increase performance. I have never heard that a straight drag pipe without backpressure design ever improved any performance except at rather high RPMs. I will google as suggested.

 

Well, if you have a 'tuned exhaust' the cross over pipe isn't necessary, and maybe the following will explain that in a little more detail.

After a cylinder fires and the piston cycles “up” - exhausts gas vents through the exhaust valves, along with the remainder of the “explosion”, exiting the cylinder into the exhaust system.

This explosion creates 2 pulse waves… a Sonic Pulse - and a Thermal Pulse.

When the faster Sonic Pulse leaves the exhaust it has created a vacuum in the pipe behind it, which air pressure will enter the pipe to fill the vacuum. This air will collide with the Thermal Pulse and cause it to reverse. The timing of this event causes exhausted gases to be returned to the cylinder while the exhaust valve is still open therein not allowing all the spent gas to escape and also polluting the environment for your next stroke cycle with fresh air/fuel mix.

This “Event” is call Reversion andcan degrade performance.

The cylinders fire opposite of each other – or nearly so on a V-Twin engine (such as a Harley), so when one side is firing, the other side is idle. Having the cross over pipe allows the pulses to dissipate back and forth between the tubes instead of - or before exiting the exhaust and therein greatly diminishing the effects of reversion which improves performance.

You do see a lot of bikes with duals and no cross over pipe.

These are usually "Tuned Pipes", meaning their length and size are tuned to work with the S/T Pulse in such a way that reversion can not return to the engine before the exhaust cycle has ended.

 

Just to add...

The crossover pipe works like a H-pipe or X-pipe on a performance car exhaust. Even on older cars with "true duals" I always installed H-pipes and the gain in low end torque was noticable. Exhaust scavenging is the answer.

 

So, how does the theory of back pressure as an important or needed characteristic of an exaust system fit into the anti reversion equation (if at all)?

 

I think it has to do with unburned fuel and air escaping the cyl too fast as with drag pipes or baffleless exhaust.