| Author | Message | ||
     Koz5150 |
As I was sitting here at work at 1:20 am on a Saturday night I was curious as to what I need back presure on my exhaust for? Now please read this whole post before answering. I know that stock bikes will loose power from running an open exhaust and I have been told that you will also burn up your valve seats if you do this. But what about people who are bolting on an exhaust like the Force? Isn't that basicly a straight pipe even with the "street baffle". I am just curious as to what would happen if I bolted a straight pipe on the end of my exhaust for my internally stock 2002 M2. Thanks for your answers in advance. | ||
| Wyckedflesh |
The free-flowing exhaust causes too much scavanging out of the cylinder making it lean out the mix. Or atleast that was how it was explained to me. So even running a straight pipe or the Force pipe without rejetting to richen the mix, it still leans it out. | ||
| Koz5150 |
So if I just run a richer mix I won't hurt the bike? | ||
| Wyckedflesh |
Yup...  But you will need to play with it a bit to see how much you need to richen it up... | ||
| Koz5150 |
I will be taking my bike to a few HD open houses and just love having a Buell that sounds more Harley then the average Harley... edited by koz5150 on April 25, 2004 | ||
| Unibear12r |
Even if you get the mix right you'll still loose power over at least part of your rpm range. Most likely a gutless midrange in exchange for more top end. Someone else would have to explain exactly why tho... | ||
| Outrider |
Koz...Are you thinking of giving Dyna a blast of his own medicine with the loud pipes?  | ||
| Dyna |
LOL..wont even come close. BTW, it doesnt take much to create a little backpressure. You dont need much at all. Something as simple as a bolt run up thru the pipe near the end is often times enough to do it. | ||
| Outrider |
Dyna...should have clarified that it was your Dyna, not your Buell, with the ear-splitters. Koz...Just do your mods and then take it for a dyno tune at All Pro. They will jet it for best performance and proper air/fuel ratio. Plus you will have the print out to prove it. Truthfully, having read your profile, I don't recommend doing anything to your bike. Smooth, mellow and fast beats obnoxious anyday of the week! edited by Outrider on April 25, 2004 | ||
| Aesquire |
It's not so much the back pressure, (although less is better, as a rule ) it's the length of the tubes, and the musical note that results. (resonant frequency) There are back pressure pulses that reflect back up the pipes, and negative pressure pulses. The trick is to have the negative pulse arrive at the combustion chamber with the right timing to help suck out the burnt gases & encourage the incoming fuel/air charge to rush into the cylinder, without sucking too much unburnt gas out the exhaust. ( or cramming the intake charge back through the carb ) Cam timing & port design are key factors too. That's why drag pipes are not good. ( except for the exact rpm that matches their length/cam/port combo ) Dyna's right, any discontinuity in the system can create a "harmonic" or pulse trigger. Usually where the pipes hit the muffler, or where they end in the muffler, as on the XB12 stock muffler. That's why home built pipe setups rarely, if ever, perform as well as one built with a dyno to give feedback as to what the pipe is doing to the combustion process. (also math & voodoo ) Oh, yeah, less back pressure may be louder too. | ||
| Charlieboy6649 |
OK guys, now I have a few questions. My wife's bike is a stock 9R with Jardine pipe. Every now and then I get a sputter out of the engine, a hiccup, and a back fire. It comes literally up through the intake! I know cuz I feel the hot air come out of the airbox. My concerns... My truck was doing this until I had the carb tuned and the backfire burned up the air filter. I had to replace it. I know that could be happening here. Also, do you think opening up the intake would help this??? Snorkel removal and race filter? Please help. | ||
| 800mxz |
Charlie, need to richen up the fuel curve I think if you go to a race filter, it will be even worse. | ||
| Buckinfubba |
charlie...get the race ecm and filter together if you don't already have them. If you do already have them you may have an intake leak which is causing a lean backfire..... or possibly the TPS is misadjusted | ||
| Charlieboy6649 |
Thanks guys. I kinda figured... I'll be making the wife save her pennies for that, I have my own bike now! Thanks again. | ||
| Blake |
I lost all semblance of adequate back-pressure once. The results were not pretty. Damn double bean and cheese burrito. | ||
| Charlieboy6649 |
EEEW! | ||
| Fullpower |
Koz. the concept of " backpressure" is misunderstood some. the exhaust from a piston engine is not a steady stream or flow, but rather a series of pulses. one of the functions of a well designed exhaust sytem is to break up, or smooth out the pulses, to reduce the tendency for the pulses to "reflect" and travel back up the pipe toward the exhaust port. with a straight pipe, the exhaust pulse will "see" the end of the pipe as an impedance mismatch, and part of each exhaust pulse will reflect back up the pipe, and at a particular engine speed will actually arrive at the open exhaust valve, and reduce the engines scavenging. this engine speed depends on cam timing, pipe length and diameter, and probably several other factors. this is the flat spot or torque dip found with drag pipes. having a stepped header, or a large volume exhaust can are 2 ways to reduce this effect. neither method increases the average pressure in the exhaust system. less exhaust pressure is better. |
