Exhaust Back Pressure

[JayWizz]

Morning all

I have a Derbi Senda Cross City 125...

I've had to get a universal can, on account of not being able to afford the standard one, that's almost half the bikes value. I've bought one thats 45mm at one end and 38 at the other, which is the way round I intend to install it (in/out), for pressure purposes and I've taken the crap wadding out and crammed as much as humanly possible in there.

My question is, what if this isn't enough? Any ideas or tips to create some more pressure? If we could specify whether it's theoretical or from practical experience, would be useful. I have engineering and welding workshops on my doorstep so a bit of fabrication isn't out of the question. 

My initial thoughts are mostly along the lines of welding a big washer on the end of the outlet to reduce the flow, but it's not ideal really.

By the same token, if you think it'll be just fine, I wanna hear that too....truth be told I really do wanna hear that lol.

Cheers
Wizz

[Copycat73]

name the universal can .. web link.com
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[mentalboy]

I am under the impression that back pressure is not so much of an issue with four strokes, maybe some running difference if you'd changed the pipe but I very much doubt that you'd notice much difference with a 'universal' can.
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[Copycat73]

try tellin that to the exhaust valves as the lean mixture burns them out.
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[mentalboy]

[Copycat73]

[Suntan Sid]

This might help!

https://www.physicsforums.com/showthread.php?t=594484
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[Copycat73]

[_Iain_]

https://www.gomog.com/allmorgan/exhaustbackpressure.html

[JayWizz]

Wow great response, good to find an active forum.
Can is this one...
https://www.feked.com/universal-short-motorcycle-silencer-12-id-38-or-45mm.html

Was gonna go for the pretty bevel one but I need the extra OD for wadding.

I haven't upjetted as yet but I will keep an eye and do so if it turns out I have to. Might grab the senda 125R carb I've seen breaking and see if it fits, looks bigger.

Cheers for the links, I'll have a butchers

[Pol Anorl]

back pressure can affect a 4 stroke quite a bit, on an old bike i had i reduced the back pressure dramatically and it would barely hit 80 where as before it would do 110
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[_Iain_]

It's a cruiser can

We have simmilar. I'd not put that on your bike as it will sound terrible, look terrible & the bike won't perform welll at all.

https://www.ebay.co.uk/itm/Derbi-cross-city-125-cc-07-breaking-for-spares-Rear-light-Amazingbikes-/321519451677?pt=UK_Motorcycle_Parts_13&hash=item4adc0d7e1d

I'd contact this guy and see if the exhaust is still availible.
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[JayWizz]

Well aint that just bloody typical....been searching for months with no success and one goes up the very same day the new can is delivered and I've modified it.

[Copycat73]

[JayWizz]

Good tip cheers fella

[Copycat73]

[salem1987]

my personal experience with those very silencers fitted too a cd200 is that you need to upjet 1 size and raise the needle one notch.

I pretty much found they make the bike run a little leaner, but this was without any wadding.

[Kickstart]

Hi

It is the pressure waves that are important (which pretty much comes down to the length of the exhaust, with the exhaust being small enough bore for the pressure waves to not just dissipate) rather than anything regarded as back pressure.

But change anything and there is a fair chance that the jetting will be affected.

All the best

Keith
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[stevo as b4]

Very general rule I would say that would work ok, is a straight through perforated silencer with the same diameter baffle tube as the header pipe diameter.

some engines will use a tapered pipe, but again in that case i'd match the size of the pipework before the silencer.

You need to do some fairly serious engine tuning, capacity increase or use forced induction before you need to really alter the size of the header pipe/system to maintain/increase performance.

You should be re-jetting to suit gain a little bit of something somewhere with all std pipework diameter's and a straight through absorbstion type silencer.

Single cylinder motors have less to think about exhaust system wise as your not trying to use one pot to help scavenge another etc.

Some bikes/engines respond well to changes in header pipe diameter. TDM900's have a narrow bore set of downpipes, good for low-midrange torque but big bore downpipes have seen more top end power on dyno tests. The same goes for some car's with close coupled catalysts and narrow bore tubular exhaust manifolds with very short primary tubes that are almost crimped together where they join the cat.

Fit a nice bigger bore tuned manifold and good gains can be seen with the right fueling changes to suit.

[Kickstart]

[stevo as b4]

Could well be, as I think the TDM has a cat too, and as you say increasing power at the top end was probably not a priority when they developed the 900 model?

But don't race bike engines have bigger bore shorter manifolds/collectors generally speaking? I've never really looked at sportsbike header's in relation of exhaust port size to header tube diameter. Maybe in a high revving bike engine small bore or equal to port size header pipes are more important for gas velocity and thus scavenging?

However a lot of biggish capacity/lower revving performance car engines (American V8's definitely) often use bigger bore header pipe diameter's than the exhaust port, as an anti reversion promoter. I think a lot of the old A-series performance manifolds were made with big bore header pipes I expect for when they are used with long duration and high overlap cam profiles.

I've always been told that bike engines tend to use high lift shorter duration cams than bigger cc per cylinder car engines, possibly for allowing higher compression ratio's? and possibly because a lot of the power in bike motors is made by running higher rpms?

[Rogerborg]

Valid and interesting question.

Speaking strictly from experience, when I de-catted my 125 exhausts (two of them on a single cylinder engine ) and drilled some Speed Holes in the end plates, I did have trouble starting and idling.

If you get that, try putting your hand over the exhaust. If it starts up nicely, then you need more back pressure.

However, if it starts and runs fine as-is, then don't sweat it. Bikes will run pretty well without being perfectly tuned. Just check the plug occasionally to see how it's getting on.
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[Kickstart]

Hi

Shorter manifolds for race bikes, but not necessarily bigger. Bigger means a lower exhaust gas velocity for the same gas flow, and whether the engine flows enough gas to maintain that gas speed is another question.

An interesting and fairly easy to understand article on exhaust design is here:-

https://www.hayabusa.org/forum/engine-performance-mods/25875-exhaust-myths-vs-reality.html

All the best

Keith
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[JayWizz]

That's exactly the kind of page I've been looking for, thanks Kieth.

Gotta say guys, I'm astounded by the volume and quality of response.

Just waiting for the reducer to turn up and I can crack on.

There is another consideration to be made as it goes. This bike has an emissions valve on it, returning spent gas from the exhaust port to the carb and airbox, I've read about people blocking it at the air box and gaining a bit of power, but I'm wondering if it should just come off completely....thoughts?

[_Iain_]