Okay

 

what do you think/know

 

Whoever this "everyone says putting a slip on will hurt your bikes performance" is is a dope. Carbed bikes generally needed to have larger jets installed to accommodate the increased airflow through engine or you'd run lean and not develop the power you could. What I've read is that FI bikes are a bit more flexible, but would benefit from a good tune with a Power Commander.

Some backpressure helps with a 4-stroke engine to optimize the fuel charge in the cylinder, just not to the extent that a 2-stroke benefits from it.

 

Some backpressure helps with a 4-stroke engine to optimize the fuel charge in the cylinder, just not to the extent that a 2-stroke benefits from it.

Huh? Backpressure HELPS a 2-stroke? Then why can't they have a normal muffler?





The exhaust valves open at the end of the power stroke because doing so means that the gasses push themselves out. If the piston had to do it all then it would take energy and the engine would make less net power. Also, by the end of the power stroke the angle of the conrod with the crank is near 180*, and more pushing won't produce much rotational power. You gain more by reduced drag in the exhaust stroke than you lose by less power in the power stroke.

Exhausts and intakes have another effect: resonance and pressure waves. A piston engine doesn't produce a constant flow, it makes on-off-on-off flows. These make pressure waves that bounce around the system. The resonance varies with RPM, so at some RPMs a high pressure wave can hit the valves (great for intake, free boost) and at other times a low pressure wave hits (great for exhaust, free gas suction). Hence why nobody runs pod air filters on new bikes, they lose that resonance that the rest of the system was tuned for. Also why a 4-2-1 (two weaker waves) exhaust behaves differently than a 4-1 (one stronger one) exhaust.
My old Mazda MX-6 v6 took advantage of this and had two servos in the intake manifold that changed the shape of the chamber by opening or closing an additional tube at varying RPMs. It was called VRIS, IIRC.

 

i think you're gay.

That video is crazy.

 

wow he must have rubbed you the wrong way, and made such an impact on your life that you have to follow him to different sections to insult him.

 

Just the drastic reduction in weight can't hurt.

 

I've designed the exhausts for my University's Formula SAE car for the past two years and from what I've been able to find, backpressure is the biggest myth going in terms of exhaust systems.

 

^^ then how come different exhausts can produce more or less more than one another?
also, i know for a fact that the people running straight pipes on their Harleys lose power by doing so, i've seen dyno charts for them.

 

Indeed back pressure makes torque. Its useful for the street, and makes wheelies easier.

The thing is, 99% of this forum likes to think this is tracklife.com where back pressure doesn't mean much and they don't take other things into account. These guys like being so vague so you can come back to them looking for more answers. Silly squawk-boxes aren't they...just sit back and watch.

 

My scratching the surface understanding is that at lower exhaust flow rates inertial forces dominate. Therefore, at these lower flow rates, it is advantageous to have increased backpressure. This in part is what leads to greater torque numbers. However, at higher flow rates, the frictional forces dominate. At these higher flow rates, where friction is one of the main contributors; it’s advantageous to have a less constrictive exhaust. The result of less backpressure at higher flow rates is an increase in HP.

 

Unless you're on a race track, I bet the HP difference on most bikes is almost unnoticeable. Contrast this with the different sounds the exhausts make, and you can see why people put them on anyway.

That is if there is a loss in hp. :beach

My new bike has a 2 brothers exhaust, but no power commander. I am not feeling like I need more though. So if there is a loss(or a gain), the sound seems to be the more psychologically motivating factor for why people do it.

I've seen this question asked a lot too.(on numerous other forums) It would be interesting to have a definitive answer, but seems like for most folks it just won't matter anyway.

 

you would think headers would be the b***** relater to back pressure then mufflers.

 

Yes, and diameter per volume means more then that. In fact if you have ever REALLY built up a motor, you may have noticed that moving the collector 2" backward and forward on the primary can yield a 10hp difference on say a built chevy small block

Back pressure is a myth, velocity is not. That is why there are two parts to a header, the primary and collector. The collector is the point where the pressure waves collect and are multiplied. Now one of the earlier rules of dynamics is high pressure=low velocity and vice versa. if from the collector to the muffler you thin out the pipe you have created a high velocity low, pressure zone (ex-up, set, pair valve, ring a bell) this high velocity wave has the ability to to cause a vaccuum in the front of the collector and pull the exhuast down, increasing system velocity and reducing the parasitic losses of the valve train.

However, there is also a cut off point where you develop what is called a standing wave that basically bounces up and down the system....this wave completely nulls the advantages of a higher "back pressure" system and is the reason that you want a larger more straight exhaust for performance applications.

Fun fact, the more bends that you have in an exhaust the less horsepower the system will support. Just a note for people (like me) that have to have high-mount and/or underseat exhausts. Exhaust systems don't "make power" they simply support the motor, the motor makes the power.

 

google exhaust scavenging. this is the most important component of exhaust design and tuning. the pressure wave stuff is known as Hiemholz resonance theory.

 

Ok here is my take on the situation listed above..
I have a muzzy canister (clip-on). Just recently about a year ago just before the season ended in 2011, I had a PCIII and a custom tune w/dyno. After I had replaced 05' zx636 fairing and custom paint the catalitic converter where the clip-on links in warped my fairing.
Then I started feeling like (power/torq/hp) was at a loss. I had just replaced the plugs do I knew that was not the case. I then replaced the packing in wich there was some repair that needed attention. After all was said and done , the muzzy can ended up 1.5" shorter with new packing material. After my test ride it sure did feel like there was some torq to the bottom end.. Now exhaust flow was actually flowing better and not getting caught in the can where there was burned holes in the fiberglass material but it sure made a big difference.. Now was it back pressure or efficient exhaust flow at low rpms or high rpms.
Here was the old packing for my muzzy and packing was replaced with fmh packing from skyline Yamaha..

 

For once I'm glad a thread was dug up. There are some good explanations in here that I never would have come across otherwise.

 

First off, I’m a fabricator by trade and been doing it 17 years, built everything from small parts to large 100 ton bridge parts. Metal is my game.

Now to re-re the type of packing I used before I had to run off was "FMF" every one of those aftermarket exhaust claim to be the best and add horse power, in essence it does but you have to measure your gains and fine tune thus the pcIII and custom tune.

Everyone has heard of the jumper mod... the dyno theory and all the charts and graphs. Now I’m going to post and edit my last pic I uploaded to insure that w2hat it is we are talking about.

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Now back pressure I’m sure would bog down an engine at any rpms if there was enough in measureable amounts to work back the other way instead of releasing.

It’s like trying to ignite the burnt fuel the motor is trying to release. At any pressure if your valves were not let’s say working correctly and the stroke was taking it in instead of releasing then no... Back pressure does not make add or increase hp. releasing exhaust at an efficient rate would but you would need a custom tune to charge the fuel into igniting.

So repacking the muzzy did not add hp it just moved the exhaust in an efficient manner so as to compliment the tune...
Now I guess I should have done it before the custom tune but I am doing the jumper mod and back to the dyno soon as I can find one. I’m still not convinced as to shortening the CAN might have played a big part but all the packing material did was dampen the sound and the repair was added efficiency