Anyone know anything about exhaust design?
 

Ive done some searching on google and have found very little about the subject... well on V-twins that arnt bloody harley's. :rolleyes:

I have a bit of a hole in my power at 5K rpm (see dyno's below) and the cause is the exhaust system, or some part of it... I know this because other bikes that have after market systems dont have the same issue. :rolleyes:

So ive looked at it and I think the problem with stock system is the collector.

The problem im talking about is where the two pipes come together for 5-6". The law's of physics say that when a pressure wave hits lower pressure it slows down. This causes some of the wave to be reflected back up the system and towards the exhaust valves.

If this reflected wave was to hit the exhaust head at the right time to go back into the piston and then out of the intake valves on the overlap it can slow down the intake of clean air/fuel enough to lose power.

Right, does that all make sense? Soooo Ive looked at the Ducati setups and they go 2-1-2 and the single pipe part is only very slightly bigger than the header pipes (the part that fits to the exhaust port), no doubt to try not to cause the reflected wave.

Soooo, what I think I need to do is get the connection part of my stock collector (the 5-6" section where the pipes connect) cut it out and put a pipe in thats slightly larger than the header pipe. Keep the 2-1-2 style system I have but hopefully will lose the back pressure problem the stock system has. What do you think??? Will that work???

Last observation is that this maybe there by design, something to do with noise law's perhaps... its right at the right RPM to fit that.

The Collector in question, the top one, bottom one is RSV system, they have the problem, but worse, I think that is becuase the collector is way too large, even more so than my falco one!:-
http://www.apriliaforum.com/forums/a...4&d=1242724951

*EDIT* If you have a look at the design of the VTR akrapovic system you can see what im talking about, the pipes stay the same diameter.

http://www.akrapovic.com/typo3conf/e...6f24d1a997.jpg

Re: Anyone know anything about exhaust design??
 

Ooh! I love a bit of fluid flow dynamics and wave reverberation.

First think Nick, if you change one single thing in the system it changes all other parts. So if you reduce or increase the diameter of the connector or make it longer or shorter you will move the flat spot around. The flat spot will be somewhere in the rev range and the only way to reduce it so it's negligible is to use the Exup type system that Yamaha came up with. Various other manufactuers now use a variation on the theme to "tune" the rebound of the wave so it arrives at different times and so reduces the wave arriving back at the valve causing positive pressure at the wrong time.

The reason that its in the range it is is that the emmissions and noise are measured at about 4K revs and the power above 6K revs needs to be flat spot free so its a convenient place to put is just at the spot where emissions are measured. The positive pressure helps to get it through the test.

You can also fiddle with the inlet trumpet lengths ala Yamaha and several other manufacturers to reduce the wave rebound on the inlet side.

Now add in the diameter of the pipe versus length of the pipe. As each pulse that wasn't rebounded exits it creates a negative wave that scavenges the engine. The smaller the diameter of the pipe the faster it travels. the wider the pipe the slower it travels. A reduction in diameter will increase the speed of the gasses and an increase in diameter will slow gasses. Curves and welds will cause turbulence and reduce speed, increasing back pressure. Whist you're never going to get laminar flow you don't want too much turbulence.

Now, add in the number of cylindres and firing order. Pipes are linked so that the pulse going down one creates negative pressure and scavenging in the pipe it's linked to. This is why Twins have two into one into two pipes and not just one on each side.

So, What do you want the exhaust to do? Do you want top end power? Go long (have you seem how long and thin GP bike exhausts are?) Want more torque? make more restrictions.

Phew!

Then you have the old classics that ran megapphones. Not becasue they sound glorious, but becasue they help scavenge the engine. And taking it to extremes, you've got the expansion chamber to speed up the gasses even more for superior scavenging. This is why two strokes have such a narrow power band. The expansion chamber only works at higher revs as this is where it is tuned to scavenge most.

But expansion chambers can also be used on four strokes to great effect too. I've seem more than one of the top bikes with what looks like an expansion chamber in the exhausts in GP.

Here's a picture of a 650 single 4 stroke using expansion chambers on the japanese Supermono circuit..

http://www.corporateflyfishing.com/images/07exhaust.jpg

c

Re: Anyone know anything about exhaust design??
 

The thing is it can be dialed out, it has been with after market exhausts. I dont think it really needs anything as complicated as an Exup or expantion chamber.

What got me looking at modding the standard system is what people are doing with the Stock RSV system, that is here! It seems to work, everyone said they felt a change and there is one Dyno run which shows it does on last page.

Anyways, I think its basically the expansion type thing as the pipes come together that is causing the 5K problem. Think its worth getting a flea bay system and playing with it?

One other thing. What if you ran each exhaust independant, what effect would it have?

Re: Anyone know anything about exhaust design?
 

Now I see. OK, it looks like the link pipe has been added to scavenge one pipe with the exhaust pulse of the other, higher up the header.

Now this may have worked but you'd need to know the ID (internal diameter) and the length of the bridge before adding it yourself so you get the same result. a longer or shorter or thicker or thinner link would have a different effect. You also have to have the link to the other head facing back down the pipe so you get the required negative pressure.

Or you could lash something up and hope it works - Like Moto Guzzi do :)

Nick, I assumed that there was a link between the two pipes where the run parallel to each other. Is this the case? If not, any link between them would be better than no link between them.


C

Re: Anyone know anything about exhaust design?
 

Just happen to have flowmaster installed on one of my dev systems, If only my CFD skills were up to driving it.

Re: Anyone know anything about exhaust design?
 

What and what? (flowmaster? CFD skills?)

Re: Anyone know anything about exhaust design?
 

And there's more :)

Had a play with a few drawings on a fag packet and the simplest way of linking them (if they aren't linked) would be to cut out the centre part at the top and bottom of the parallel tubes and weld a flat plate across the top and bottom of them. Like this...

http://www.corporateflyfishing.com/i...priliapipe.bmp

Re: Anyone know anything about exhaust design?
 

flowmaster -> follow this link, I know I have the power generation functionality, but I need to learn what else, might be automotive as well. If not I would need to borrow licences.

BTW, it is a legal install.

CFD = Computational fluid dynamics as in get the design modelled.

Flowmaster say their exhaust tools would

Validate system pressures, flow rates and temperatures to ensure flow stability during any operating conditions
Optimisation of Positive Crankcase Ventilation Valve
Meet packaging, performance, cost and acoustic targets
Validate and optimise designs for the latest emission reducing powertrain designs

Re: Anyone know anything about exhaust design?
 

Nice piece of kit Tim! Do you have to know the variables for the system (temperature, initial flow rate. etc)? Must these be measured first with a prototype then remodelled with the software or are the automatically generated?


C

Re: Anyone know anything about exhaust design?
 

http://www.jetscreenshot.com/demo/20090405-g81-44kb.jpg

Im not sure how good this set-up is... What would be the effect of having a 2-2 vs a 2-1-2 or my current setup which isnt truly either... Close to a 2:1:2 I guess.

I know that some of the after market systems have a similar "merger" section but they look more like your bottom pic... I cant seem to find the damn pics after ages of looking Grrrr!!! They basically merge similar to mine but the two pipes merge further so it looke like one pipe similar to your bottom pic.



There is also the X crossover section Tigcraft use on their 60mm system...

http://www.tigcraftusa.com/user/04Dual1.gif

Which would work best im not sure... but the Tigcraft system seems to be well loved! :)



Oh and this is a mod the RSV people do to lose the 5K dip. Seems to work. Damn it. lol Could get an RSV system and do the same but my Blue flames cost me a small fortune so id be unhappy to not use them. :hackedoff:

Re: Anyone know anything about exhaust design?
 

Sorry, don't know, I would have thought base operating parameters would be reasonably well established. I have a meeting with our head of computational engineering who is a CFD god next week, (that's why I have his software on my dev systems) to talk through some operational issues. I will ask him for a few driving lessons

Last time I did any gas flow was back in the 1970s way before CFD was more than a pipe dream as the then mainframes were solidly booked for weekend/holiday FI runs

I would however be a little concerned as to how the gas flows flows can be efficiently optimised for scavenging/performance and fuel consumption etc. but the thing of most interest is how the hell do I get it to sound swwwweet.

Re: Anyone know anything about exhaust design?
 

accordingly, Aprillia spent more time on the sound of the new V4 exhaust than the performance side of things. 150 exhausts just to get the right note.

C

Re: Anyone know anything about exhaust design?
 

They are Italian...


After reading this ive decided that im going to try something similar to the RSV crew and put a jointer underneath from the bend on the back cylinder to just next to the header joint. This might work to slow down/redirect/ make weaker the pressure wave that apparently causes the lul at about 5K rpm.

Wish me luck... Though I think its going to be next month before I can get me a 2nd hand exhaust collector to play with :(

Re: Anyone know anything about exhaust design?
 

If it's a development exhaust, why not add butterfly valves into the joiners so you can play with the amount of opening or close them off completely so the differences show up. When you know whats best, weld the butterflies in place (open closed, semi closed).

C

Re: Anyone know anything about exhaust design?
 

Interesting idea... Im not out to lose top end though ha ha. :(

I think ill try the little pipe first. Ill keep you posted :)

Cheers for the help! :cool:

Re: Anyone know anything about exhaust design?
 

So it's been a while since this was updated... was any progress ever made?

Has anyone ever been able to lay hands on an M4 system to see what size pipe they use (that seems to be the most popular full system since it gives a solid 7hp gain across the board, when the bike is tuned).

If I can get the piping dimensions, I could probably fabricate one up myself using parts from summit racing (I've built several car exhausts that have turned out pretty nice). I'd just rather reverse engineer their system rather than do the trial and error method to get it tuned in.

I was thinking if I used the factory header pipes, and then upsize from a 1.5" factory pipe to a 1.75" midpipe, into a 2 into 1 collector that was 1.75" to 2"... throw a 1.75" crossover pipe about 6-8" up from the collector, and then flow into a 2"in, 2"out, 12" glass pack mounted about 18" back from the collector... that would give me along enough pipe to keep the bottom end, while the pipe diameter would increase top end and the crossover pipe would keep the "dip" from occurring in the mid-rpm's and widen the "power-hump" a little bit.

I am using "best-guess" engineering methods, rather than an a CFD and acoustic simulation software package. These engines aren't exactly high-tech, so I don't think it'll matter that much if it's not "perfectly tuned".

Honestly, the only reason I'm bothering to try this out is because the exhaust on this bike is wrecked beyond repair and I'm too cheap to buy an M4 or Yosh system for $750+shipping.

Anyway... if anyone has measurements on that piping, please, let me know.

Cheers.

Re: Anyone know anything about exhaust design?
 

I've got an M4 system.

BTW I don't think leo has the crossover and that doesn't suffer from the dip in the middle

Re: Anyone know anything about exhaust design?
 

Would you be willing to take some measurements for the D-I-Y types? If I can get some good measurements to fabricate from (I just need the piping diameters in the various sections), I plan on doing a write up in the DIY section including photos, tech-drawings, and a parts list (probably going to source most of it from summit racing and jegs).

Cheers

Re: Anyone know anything about exhaust design?
 

Yeah no bother, might even be tempted to make one or two myself if people are interested. What do you need just the ODs?

Re: Anyone know anything about exhaust design?
 

The Leo Vince doesnt have the balance pipe...but with one fitted to it the increase in torque is about 2bhp..

Found the M4 works best all round..the worst being the Micron and Hindle - bore size too narrow. Have just got one from the States and took off the Hindle that was fitted...plus 3bhp mid range and 1bhp top end..the lad said the actual on track difference was very noticable how the bike pulled.

Re: Anyone know anything about exhaust design?
 

The OD's would be great. If you want to take a little extra time and figure out the lengths of each section of pipe that would be even better... but I won't whine if all I get is OD dimensions.

I plan on leaving the first 4-8" of pipe in the factory sizing, so I don't' have to try and manufacture a flange to fit the exhaust port. I might change that later, but for now it seems the best bet. I believe the factory pipe is 1.5" OD. I'll step that up to 1.75" all the back to the collector... where it go 1.75"(x2) into 2"(x1)... the pipes will be relatively equal-length based on head-loss (I'm going to do a hand calculation for the flow restrictions in each pipe) and then install a cross-over pipe about 6-8" upstream of the collector joint. I'm going to do some additional research on cross-over piping... that's the bit that runs between the two header pipes right before the collector.. which tends to broaden the torque band a bit in the mid-range by allowing the two flows to balance each others pressures ahead of the collector.

If you care to, take a measurement on how far before the collector the M4 system installs that bit and I'll see if I can figure out why they put it there versus where the stock system install it (there is one on most of the 2003+ year models).

Thanks again.
Johnny

Re: Anyone know anything about exhaust design?