I'm building a custom exhaust for ducati 748 .i have my front cylinder taken care of ,but the rear needs to be much longer so that they are the same length .This is not a top shelf race bike ,but i would still like it to flow correctly within reason. my question is ,i have 11/16th or 40mm ID from the front cylinder it measures 45 inches before it connects to the muffler in the rear i can use 49mm ID ior 1 -7/8 f needed. the whole thing is under the bike and bodywork so you cant really see anything . how long does the 49mm pipe need to be in order to flow the same as 45 inches of 40mm. is the a calculation for this?
I'm no exhaust expert, but my no means a newb at this. From what i know & what makes sense,I don't think (someone correct me if i am wrong) you can substitute volume for length & that is why Ducati engineers go through the trouble of snaking the pipes down, around & back up for the rear cylinder. Having the same diameter pipe for each respective cylinder is paramount & you can cheat a bit with length. As you say, it is not a top shelf race bike & that being the case, first & foremost i would want the diameters to be the same & the lengths as close as they can be. I have had several different spaghetti style exhausts for Ducati's & they all had the same header diameter but the better ones had the same exact length. At the same time, the cheaper ones that had the same diameter just had a bit less rear header pipe. I'm just reasoning here, but you would think if the engineers could have made up for length with volume, they would have. As i said, i have seen spaghetti style exhausts from Sil Motor, Termignoni, Ducati Performance & Gio Ca Moto & they all head the same header diameters, but some went to great pains to get the length exact & some not.
thanks ,thats pretty much what i know so far ,and ducati had it right with the snaking of the pipes to make them even length .The situation is with the change of the stock swingarm to the s4r trellis style swingarm the area that the pipes would normally travel from the rear cylinder down between the foot peg and swingarm is no longer there.So the issue is to get the rear cylinder pipe to come out to follow the contour of the frame and still make up that 45 inches somehow. That is why I'm asking about flow and dia ,because if i can cheat the rear cylinder with a larger size to flow the same volume of air but in a shorter distance then it wont have as many bends as it would be the same Volume of air as the front cylinder. I could use the same diam tubing as the front cylinder but 45 inches would need to take more bends then i would like to have to make the distance . So that is why i m asking about flow ,I want the same velocity and volume in each pipe ,and if i can cheat with the rear cyl with a larger diam pipe then it wont have to take such a long bendy snake which is probably worse than getting the distance wrong . I would like to be as close as i can get ,or exact depending what i have to go through. here is some pics of what i got going on .and my dilemma .The front cylinder is now going through the swingarm (like a stock s4r) and directly up to the tail ,the issue is the rear cyl. here is the rear cyl and how i would like the pipe to start traveling ,i just dont want to have 7 hundred bends
Know anybody with a flow bench? I think you're thinking about it backwards - I think you would want to go with smaller diameter on the shorter pipe to get the same restriction (to get the same flow). I can't remember hearing anybody try what you're doing... Is there a tail on this bike?
Ahhh. I see. You hadn't really mentioned WHY this was neccesary & now i see. I don't envy you at all, but i know that there are guys that know this stuff well. Have you thought about calling that guy in N.H. who makes the custom Duc exhuasts? I don't know of your level of involvement/pride in this & who will actually be undertaking the work. What is normally suggested in the installation of an arm like you have? I know what you mean about the stock S4R pipes. I have one of those headers here, which is OK, but it's no spaghetti system, for sure. I try to use the blue tape for protecting. The standard masking tape glue is killer & the longer it is on & handled, the worse it can be. Not a news flash & not a major point, i know, but i cringe when i see standard masking tape on painted parts.
A larger pipe will have more volume capability but if you flow the same volume as the smaller sized pipe the larger pipes velocity will be lower. You also have to factor in curves in the pipe which changes the velocity. So what I am saying is volume and length isn't your only question but velocity is a big one.
i am having a pro welder do the work ,but he's not sure on the math for the flow as his exhaust are generally for show only there isnt anything regarding the install of this arm ,its a one off fabricated mounts etc. i know about the tape ,it was put on by the guy working on the exhaust the first time but couldnt get the job done ,i picked it up yesterday from him .Tape will be long gone after it takes me 4 hours to remove the tape residue.lol .....OH well at least he tried to protect it ,the paint wil be fine .
yea the velocity should matter more ,i just need to figure out the math .Does the velocity matter as far as Rpm ? lets say as a benchmark use 6 thousand RPM for the calculation of flow /diameter /length . There has to be formula for flow . I am trying to put the same amount of bends the stock pipes came with and no more .I'M actually using one of the major twisty bends for the rear cylinder (originally for rear cylinder anyway ) .So i can match the flow and length with the same dia pipe ,i just thought i could cheat the length with the larger size,,,now that velocity is mentioned i think ill scrap that idea and go with the same size pipe front and back
Which was sort of my point. Wether it be trying to get the flow, velocity, movement or other correct, i don't think it is possible to do by subbing volume for length or vice wersa. No, volume & length are not the only things to be concerned about, but they are the only tangible things he has to work with that result in a change. You can't adjust velocity. You can adjust the construction of the header to cause a change in velocity, but you can't adjust velocity. He can change the pipe volume & lenghth, but i don't think he will be able to get it "correct", however you want to quantify that, wether it be flow or velocity or other.
i am willing to sacrifice length for less bends ,i would assume it would be better to flow it as non restrictive as possible than worry about the specific length . Aren't the gasses on such a long distance pretty much spent and have given whatever backpressure is needed after the first foot or so? Like in a 4-1 ,as far as i understand by the time it gets to the collector the gasses have pretty much run thier course and you just need to get them out quickly and not restricted If i planned to dyno it ,or challenge bayliss to a race i would get an engineer to figure it out ,for this application I would just like them to flow and be close to even . I mean come on its a 748 ,it has the horsepower of 1 maimed horse pulling a trailer with an actual powerful bike on it ......zing.lol.
I googled it for you but I'm not ready for bed so I didn't read it. Here's some super nerd forum http://forum.onlineconversion.com/showthread.php?t=5632 More stuff; http://www.pipeflowcalculations.com/ http://www.eng-tips.com/viewthread.cfm?qid=147153&page=10 http://www.groupsrv.com/science/about31779.html
You have to think about the power pulses and scavanging. The pulses have to arrive at the collector at different times so they don't slow each other down (think merging onto the freeway). Also it is helpful if they come just at the right time so the pulse ahead of it can pull it down the pipe which helps with scavanging and keeping velocity up.
Yes, there is a marked correlation of exhaust velocity and rpm - gas velocity will increase proportionally with RPM. At 8000rpm, twice as much gas is being pushed through as at 4000rpm. The math and modeling on flow resistance can get extremely complicated. Look up articles on Skin Friction and then imagine the reverse - skin friction is generally about an object moving through a fluid, but you're looking for a fluid moving through an object, which isn't really any different. As you go wider with your tube, skin friction's effect will fall, however the overall gas velocity will be also fall because there is more volume for the same amount of gas to fill (this is why ricer coffee-can exhausts sound like sh!t). kneedragger49 has the wisest advice however - you'll probably be much better off going with the same diameter pipe, and then bending (or hell, looping) it to attain similar lengths. The biggest problem with using different-sized pipes is that the exhaust gas velocities will change differently depending on RPM.
WICKED THANKS DUDE, I may need a translator for it all ,but thanks. "the calculations of the coefficient of drag is subsequent to the algorithms of exhaust dispersement when correlated by degrees of volume " huh ? "you need bigger pipes to make bikie go fasty " ...ah now i get it ,,bitchin .
these are on a twin ,so each pipe is seperate from the head to the canister ,but good info to know on a 4cyl,,i have no interest in even thinking of tackling a 4-1 or 4 into anything other than a box that says Akropovic .lol
i completely agree ,the diff sized pipes idea is def off the list ,for the reasons already spelled out . thanks I have enough 49mm tubing to do the entire thing ,plus its bigger and lighter than the stock stuff(40mm).Only thing is it doesnt have any good bends and i really only need one good bend that comes from the head around the shock ,maybe in total 12 inches or so ,and since the pipe is aready 40mm from the head ,adding another 10 of the 40mm and then having it widen to the 49mm for the rest on will be fine . I think it will work out just fine . thanks for the info everybody ,ill post pics if anyone is interested
Unless you are running a two muffler design it matters and even then on a twin muffler design it matters as if the combustion chamber in one cylinder is getting cleared out better than the other, your fuel ratio is going to be different in each chamber and it'll be very difficult to tune.
its a two muffler design similar to stock but with diff cans.Each cyl has it's own seperate header/mid pipe and can by themselves.
