As a general rule when building a hot rod exhaust system you should always put a cross-over pipe to tie both banks of the "V" engine. This balances out the exhaust flow improving the power of the engine. The cross-over pipe is a standard practice on a engine dyno as well.

That being said can someone please explain why this principle is not the same with the Harley V-twin engine? If changing from the factory cross-over exhaust to a true dual on my Road King while using the same slip on mufflers a increase in performance? Or is it the package deal of a air cleaner upgrade, emissions download, and exhaust change the gives the performance gains advertised?

 

You're going to get a lot of varying opinions on this but to me what's important is where the performance gains are in the rpm range.

 

It is the same on a HD V-twin. But like what Neckball said, where do you want the performance. Do you want 90 lbtq at 3000 or 4000? And to make matters worse, there are a lot of HD owners looking for things other than performance. Sound, style, less heat, etc... Factory head pipes with good slip-ons or a good 2into1 typically produce more torque than duels up to 4000+ and duels typically produce more HP than 2into1s over that. Now the real money is on a professionally TUNED 2into1 exhaust!! JMO

 

highman, I would really like to hear the answer on your first question... and why are we so quick to eliminate the crossover?

 

The principle is the same on an HD v-twin. Eliminate the crossover, all other things remaining the same and lose some torque. Rinehart true duals are a perfect example, plus add in the fact that their head pipes are larger. A well tuned two into one system will outperform true duals in low end torque. The HD stock headpipes with slipons will in most cases outperform most true duals in torque. Now, if you are looking for horsepower especially in the upper rpms, true duals can be the ticket.

 

The "crossover" on the pre-'09 touring bikes isn't really a crossover in the strictest sense. It's more like a hybrid 2-into-1-with-extra-bleed-pipe-on-rear-cylinder than anything. I like the idea of the Bub 7 dual with crossover, but have yet to see one in person. I don't know how well-executed the design is. Does the rear O2 sensor get just rear cylinder gases or does it get washed on a regular basis with stuff from the crossover and/or fresh air from the tailpipe? Anybody know that answer?

 

O.K. so I have learned that the factory crossover is beneficial despite aftermarket sales ads leading the general consumer to believe it is a power sucking OEM screw-up. That is no surprise here and thanks to all posters for the clarification.

In summary here is was I have learned. Please be free to correct me if any of the following is miss-stated.

-A well tuned factory header and good slip-ons will give you better usable torque (2500-3500rpm range) than a true dual system.
-A well tuned 2 into 1 system provides better usable torque than the factory headers. I may be stretching a bit but the factory setup somewhat simulates a 2 into 1 system and gives the nostalgic dual system touring look. ???
-The performance gain of a well tuned true dual system is typically in high-end horsepower. Which is not a benefit to the everyday rider because there will be some loss in usable torque in the touring RPM range. The benefits are more in the enhancement of the distinctive Harley sound, a reduction in heat to the rider, and providing the preferred look many consumers are after.


I hope I'm not to far off base with my summary but feel free to correct if so.

 

http://www.fuelmotousa.com/


these guys are sponsers here and they offer modified slip on mufflers specifically designed to be used with stock head pipes . i have been very happy with mine , i have a k and n filter , modified rush slipons from fuelmoto and a powercommander . my bike had dyno readings of 67 hp and 76 tq with a screaming eagle air filter and harley performance download ,it now has 76 hp and just shy of 91 tq. give jamie a call .

 

I would characterize your summary as quite fair. About the "loss of low-end torque" with the dual pipes, it's not necessarily a net loss from what you had, but it will be a relative loss compared to what you (should have) gained elsewhere. The shape of the torque curve will be less flat at first, starting from lower rpm.

 

I think you hit the nail on the head!