Im no exhaust Guru.. in fact im new to the bike scene all together... but my boss.. drives a Harley Road King... and he told me it was all about back pressure.... and different size pipes with different baffles create different back pressure..
He took like 8 different shower drain plugs that had different hole patterns on the end and had them welded around the inside edge of the exhaust and made a bracket to try out different ones to give different back pressure.. and he found ones that worked great for his bike.. and he had it checked out by a Harley certified Specialist.. and the guy said that the bike had a lot more hp than stock just from that and a few other little tweaks he had done...
That is becuase, frankly, harley guys are not the brightest gearheads in the automotive world. Ever had one try to say they can take your crotch rocket in the 1/8th or from a dig because they have more torque?! I have :Laughing rolling:.
No offense but thats one of the dumbest things I have ever heard, and sounds like something a 16 year old would do to their neon.
I can absolutely promise with 100% certainty that his little mod is costing him power. Restriction in the exhaust is bad for low end hp, top end hp, low end tq, top end tq, EVERYTHING. It is a restriction, and the engine has to overcome it!!!! The ideal exhaust system for a bike would be something like the pinebox or motowerkz pipe with the cat deleted and a proper dyno tune. Noise(and costs) makes that not an option for some people though.
What a combustion engine needs is
scavenging, not "backpressure" - and scavenging is due to the shape, length, and diameter of the header primaries - not some random crap blocking your exhaust flow. Exhaust gas does not come out in a stream like a hair drier, it is a series of rapid back to back pulses, and each pulse literally sucks the next pulse of air out from the engine. Scavenging will increase the force of this "in-between each pulse vaccum", literally sucking the exhaust out of the engine and helping it make more power.
Anything after the collector, where the headers scavenging is not tuned to take affect, is a restriction and will therefore cost hp. That includes the cat, baffles, valves, any rough pipe transitions, and especially shower drain covers(wtf?!?).
Backpressure is one the the most widespread engine myths, and I have seen so many misconceptions over the years that it is not even funny. Sure, you get some backpressure from a properly tuned header system, but that is not the goal - just a side affect of the exhaust design needed to create scavenging.
A lot of the misconception comes from the fact that sometimes an ECU cannot compensate the A/F ratio after eliminating a lot of backpressure, so the engine ends up running lean or overcompensating and running rich. That is why you get the people who run open headers without a tune on their bike/camaro/civic complaining that it runs like garbage now...and automatically assume it is because of not enough "backpressure". No, it is becuase your injector pulse width and timing have not been tuned to take advantage of the free flowing exhaust!!!
If backpressure was a good thing, dragsters would run exhaust valves and 1" moped pipes instead of massive open headers. Do a search for exhaust cutout dyno gains if you do not believe me - companies like QTP make electric Y-shaped pieces for exhaust tubing that opens a cutout on one side, essentially turning a tame street car exhaust into open headers. Back to back dyno gains,
with a proper performance tune, show increases in both horsepower and torque with an essentially "0" backpressure exhaust system. Popular belief would lead you to think that hp will increase but tq will plummit, but that is just not true. That is why almost every performance exhaust part for any combustion engine from snow mobiles to diesel trucks always advertise with claims like "low backpressure!", "free flowing!", and "no restriction!".
Sorry for that rant, but If I can prevent just one person from cramming crap up their ninjas pipe to create "backpressure" for more torque, this long rant would be well worth it!!!
