| Author | Message | ||
     Rick_A |
Exhaust sound waves are used to aid in efficiency as well Blake. It is a functional part of assuring a better fill. It only becomes detrimental if the pulse is timed incorrectly. Generally, in a motor that has the exhaust tuned for high rpm...the pulse comes in too late(?) and allows some of the intake charge out the exhaust at lower rpm. If it is tuned for low rpm it happens the other way around...the wave causes some of the exhaust gasses to re-enter the cylinder. That's how exhaust tuning comes into play...and why it's generally always a compromise. Using reversion waves to an advantage you get more pressure (fill) in the cylinder from both sides...more than would otherwise be possible. That's basically gives you an idea on how your jetting is affected with different exhaust systems. | ||
| Rick_A |
I know exhaust scavenging helps...I'm not debating that...but exhaust pressure waves are used to help fill as well. | ||
| Hans |
Ha, found it: Neglecting the initial higher currents....the Typical starter current draw will range between 140-180 amperes (written in the the 2000M2 manual). It means that the starter motor is 12 x 140 till 180 is roughly 2000 Watt. It makes clear that a full 18 amp/h battery is completely empty after less than six minutes continuous cranking because the capacity is even not 18 amp/h at very high currents and a battery, slightly older than factory fresh, does not reach that full potential anymore. Hans | ||
| Aaron |
"Generally, in a motor that has the exhaust tuned for high rpm...the pulse comes in too late(?) and allows some of the intake charge out the exhaust at lower rpm" I've been deciphering some of the formulas for exhaust length (well, the ones that work), and they're all designed to put the negative pressure wave generated by the outlet at the chamber during overlap, not out in the middle of the exhaust stroke. Which kinda makes sense to me, I mean, if you put it in the middle of the exhaust stroke, the pressure would be transistioning back up as the intake valve opened. You want low pressure while the intake has the opportunity to flow. You're right, though, you potentially end up with some fuel in the exhaust system. But in a performance application, so what. Something you see on a dyno sheet of a poorly diffused exhaust is that a few thousand rpm below where the exhaust is working, as evidenced by the torque peak, you don't get fuel in the exhaust. Instead you get the opposite, exhaust shoving out the carb, as evidenced by the stand-off fog at the carb opening, the hole in the torque curve, and the rich indication on the a/f meter. There's a positive wave right behind that negative wave. The better designs are considering the waves generated by the steps and putting those at the chamber at advantageous times, too, either to stack up on the outlet's wave or to broaden it for a wider powerband. It's something that seems to be overlooked in many of the formulas floating around out there btw. "Using reversion waves to an advantage you get more pressure (fill) in the cylinder from both sides...more than would otherwise be possible." Okay, you lost me ... how could reversion in the exhaust help fill? | ||
| Blake |
Rick, "Exhaust sound waves are used to aid in efficiency as well Blake." Really? That is interesting. What frequency are the sound waves that are beneficial for enhancing engine performance? | ||
| Blake |
Hans, Thanks for the correction. Dang! Over 2 HP just to crank the engine! | ||
| Rick_A |
It can be used in the respect that an intake charge drawn out the exhaust can be forced back into the cylinder before the exhaust valve closes during overlap. That would aid in fill and efficiency, would it not? The frequencies that are beneficial??? Got me. | ||
| Aaron |
I ain't buying it. If it shoved back in right then, before the exhaust valve closed, it'd push an equivalent amount of fuel charge right back out the intake and you'd get no net gain (remember the intake valve is open and getting more open all the time right then). Plus you'd get a stand-off and associated richness. I don't see any stand-off or rich spot when I'm at the torque peak. | ||
| Blake |
Rick, So you are saying that the exhaust is first drawing out fresh charge by scavenging, then before the exhaust valve closes it stops, reverses and shoves it back in? You might want to calculate the amount of time the exhaust and intake valves are open concurrently. Then calculate how far the intake charge could flow out past the exhaust valve, come to a stop, then reverse and flow back past the exhaust valve into the cylinder without exceeding Mach 1.0 passing by the nearly closed exhaust valve. | ||
| Joey |
I just had a thought! What if we calculate the ideal exhaust length for RPM, and fabricate some sort of a sliding exhaust extension? You could tune it for power or efficiency, just like you would the carburetor! Any thoughts? | ||
| Aaron |
Oh, it could be done, but the complexity of being able to change the length on the fly, cabling/servo motors/whatever, would be a nightmare. | ||
| Sarodude |
A certain class of kart racer uses a contraption called a 'Slippy Pipe'. If you're familiar with 2 stroke expansion chambers, there's the divergent cone (closer to the engine / exhaust port) and the convergent cone. Well, in a slippy pipe, the convergent cone is made to slide back and forth with some sort of pull cable / spring setup routed to a cable usually mounted to the steering wheel. The intent is to change the timing of the backstuffing (reversion) pulse to better suit low RPM running on these karts with TALL gearing and no gearbox. I'm talking about slippy pipes on the carb page of a 4 stroke twin oriented website. Uh, BUELL! Is this on topic now? -Saro | ||
| Glitch |
Hi all, first time poster, long time lurker... On the view of having a telescoping exhaust pipe, this has been done on some high performance bikes, by having a water jet spray in the tail pipe, cooling the gases and "fooling" the bike into "thinking" it has a longer exhaust. Pretty cool huh? | ||
| Carlk |
Isnt this what Yamaha's exup valve does essentially? | ||
| Rick_A |
The waves that cause reversion and are traveling the speed of sound...it is created by sound, after all. ...and that's exactly what the EXUP valve does...it times the reversion waves to be beneficial at all RPM and broadening the power band, despite having big cam overlap. | ||
| Aaron |
Are you sure it doesn't time the negative waves? This ain't a 2 stroke, where the exhaust is part of the valvetrain. I think, too, you're confusing the speed of the waves with the speed of the exhaust, two very different things. | ||
| Hans |
It is difficult for me to imagine at what place exactly the advantage of the resonance takes place. The waves travel with sound speed but they will have trouble to work upstream as the exhaust gas is supposed to stream out with even higher speed out of the exhaust port. That causes the big bang. Could it be that the profits come from the shock waves from a former exhaust pulse? Or looses the exhaust gas speed at the end of the exhaust expulsion stroke letting the "sucking" wave in, just in time to suck the last gas out of the cylinder when both valves are open ? Hans | ||
| Aaron |
Exhaust velocity is generally designed for around 300fps. Pressure waves in an exhaust system average around 1700fps. | ||
| Blake |
Rick, "The waves that cause reversion and are traveling the speed of sound...it is created by sound, after all." Are you asserting that an explosion is caused by sound? Funny, I thought it was the other way around.  The exhaust flowing forcefully out of your tailpipe does not derive its energy from sound. Hans, I don't know for sure, but I imagine that if a shock did exist in the exhaust system it would manifest itself at the exhaust valve/valve seat. I'm not sure a shock exists or not. Interesting point. | ||
| Rick_A |
The exhaust flowing forcefully out of your tailpipe does not derive its energy from sound. Of course not, but I know for a fact that negative waves are derived from sound. From animations I've seen the individual pulses actually reverberate up and down the exhaust until they lose their energy. I thought reversion waves and negative waves were referring to the same thing. What's the difference? Thinking on it, there is probably little practical application except for factory motors...and Yamaha has put more research into it's application in 4-strokes than anybody else. It makes sense in a factory street motor where there isn't the luxury of jetting rich to compensate... Maybe this needs to be moved to the KV Exhaust section? | ||
| Blake |
"I know for a fact that negative waves are derived from sound." Really? How do you know that "for a fact"? Good suggestion. I'll move it to the exhaust section. | ||
| Aaron |
Well, maybe this is about semantics, but to me, reversion means a positive pressure wave in the exhaust system arrived at the chamber during overlap and caused the intake charge to reverse itself and go back out So a "reversion" is the undesirable result of poorly timed/diffused exhaust wave travel. What I try to do instead is to get a negative pressure wave to arrive at the chamber during overlap. That yanks on the intake charge and gets it moving before the piston even starts down, greatly helping the cylinder fill. Get a nice bump in the torque curve. Time it for a high rpm and presto, lots of horsepower. The problem is, to make that negative wave as strong as possible, you basically have to make it narrower. Then you end up with a terrible positive pressure wave (causing a reversion) timed to arrive during overlap a couple thousand rpm below where you've got your negative wave targeted. Causes a major hole in the powerband and a lot of blubbering because the intake charge gets really rich when you send it through the carb 3 times (in, out, back in again). Damn thing will spray gas out the carb opening from the reversion, too. Dig up a copy of FMJ's race bike's dyno sheet to see a dramatic example of this, I believe we put it on the Dyno results page somewhere. We made that pipe purely for top end power. Zipper's makes a nifty little device for the carb called a "Thunderjet" that addresses the mid-range richness caused by these kind of exhaust systems and keeps it from blubbering, although it can't overcome the poor cylinder fill. FMJ didn't have one on his bike though. I can put a diffusing device on to broaden the negative wave, but that costs amplitude and thus peak hp. The best diffusing devices btw are the ones that don't generate any back pressure. Anyhoo, I've never heard anyone use the term "reversion" to describe negative pressure waves arriving at the chamber. What's reversing? | ||
| Rick_A |
Cool...I see what you mean. Blake, it's just a well known fact. It's created when the exhaust valve opens after combustion. There's two basic factors considered in exhaust tuning and design...whether 4 or 2 stroke...the gas speed and negative pressure wave (created by sound). The pressure wave travels up and down the pipe...and thus can affect the cylinder as a positive or negative pressure. I think a single pipe is the only one that generally can't create a positive wave (with the excepion of two stroke expansion chambers)....'cause as I understand it, two or more waves converging and reflecting their energy are what creates it. I thought that if the exhaust was tuned for peak power that the negative wave would create a hole in the power range due to leaning it out by sucking out the charge instead of the positive wave pushing it back in. I guess it depends on the pipe, though, right? It seems like drag pipes tend to do that. I guess that might explain why opening my SuperTrapp up gave me a punch in the upper midrange and 'caused a very rich condition below that. I like having a tuneable exhaust system...for the future when I may actually fully be able to use it to my advantage  For now it's just trail and error  | ||
| Rick_A |
Anyone experiment with different octane fuels? I've heard that mixing low octane with high octane nets the best properties of both. I don't know whether that's myth or not, but I experimented by mixing 1/3 87 with 2/3 93. I noticed a bit more low end snap. Oddly enough...it seemed to run noticeably cooler. I don't know how that'd be possible. Next I'm going to fill it with 91 and see how that works. My usual fuel of choice is 93...but a lot of sources suggest that the lowest octane fuel your engine can run knock free on, is what should be used. I know for a fact that 89 and lower does no good, after...in a pinch...a buddy gave me a tank of 87 once telling me it was 91. I understand that octane level simply represents resistance to burn (anti knock)...but can using a higher than necessary octane level be detrimental in any way (besides price)? | ||
| Pammy |
Is this on topic? Air moves from one point to another by virtue of pressure differences. The,change induced, pressure difference (suction) is normally associated with the piston traveling down the bore of the intake stroke. The better a head flows, the less suction it takes to try to fill the cylinder.The piston traveling down the bore, exerts a suction, at the most, of 1 psi on the intake port.On a winston cup motor, the exhaust will exert a suction of 6 to 7 psi on the cylinder.This happens during the overlap period. Much of this suction is applied via the open intake valve to the intake port. The exhaust system can draw on the intake port as much as 500% harder than the piston going down the bore.Under these circumstances,the exhaust is the primary element of induction,not the piston traveling down the bore. The pressure pulses moving thru the exhaust pipe have the property of inertia,which is the tendency of moving objects to resist any change in their motion. The high pressure blasts of gas that leave the exhaust port tend to keep moving through the primary tube,and the intertia of these gases,if sufficiently strong, will draw additional air/fuel mixture through the open intake and exhaust valves during overlap. When the exhaust valve opens, escaping high pressure gasses burst into the exhaust pipe and a pressure pulse is created.This pulse,moving at the speed of sound,quickly reaches the end of the pipe,where a reflected wave lower than atmospheric pressure is created. This reverse wave travels back up the exhaust pipe toward the exhaust valve,also at the speed of sound(which varies with temperature)but generally considered to be about 1200 to 1300 ft/sec. For a particular pipe length and engine speed, the low pressure pulse can be (fine tuned) so that it reaches the port during valve overlap. This reflected wave flow,in turn, initiates air-fuel flow into the cylinder through the open intake valve before the piston begins the intake stroke. Adjusting the length of the primary tube to optimize reverse wave scavenging is called Resonant Tuning. If the primary pipe is relatively short, the resonant effect occurs in the upper rpm range. If the primary pipe is relatively long, the resonant effect occurs in the lower RPM ranges. And so it shall be... | ||
| Buelliedan |
Rick, I believe your have your facts backwards. Sound does not create pressure waves. Pressure waves create sound. If a tree fell in the woods and there was no-one there to hear it would it make a sound?? Answer, no. It requires an eardrum vibrating to create sound. In a exhaust the pressure waves are what make sound not the other way around. | ||
| Hootowl |
Dan. Good man. Now, what is the sound of one hand clapping? | ||
| Buelliedan |
Jeff, I believe that sound would be "whoosh"  | ||
| Cjmblast |
Pammy, I don't know you, but you are my hero !! Awesome explanation ! CJM  | ||
| Reepicheep |
Great description Pammy, thanks! |
