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.

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.

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!:-


*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.

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..



c

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?

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

[timwilky]

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

Originally Posted by timwilky

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

What and what? (flowmaster? CFD skills?)

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...

[timwilky]

Originally Posted by ThEGr33k

What and what? (flowmaster? CFD skills?)

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

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

Originally Posted by Berlin

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...

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...



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.