compensating for a lack of back pressure

[R6_kid]

When tuning a bike how would you go about compensating for a lack of back pressure?

Considering running a homemade shortcut motoGP growler type setup and im assuming its gonna reduce the back pressure in the exhaust considerably.

From what i know its gonna take away the low end but give me more up top, is there anyway to compensate for this in the tuning is it a simple matter of a loss for a gain?

[bugjuice]

depending on who you talk to, some say back pressure is a myth.. personally, i believe it's needed. What about welding a small calculated size disc somewhere in the pipe. that way, it'll create some pressure and you won't see it.. what size you'll need, i dunno. that'll be down to how much pressure you want. plus, you could even tune the pitch if you got smart.. or/and make one for higher revving 'track' use, then a larger pressure disc for lower rev use

[Patrick]

Eat more beans, that'll get the back pressure up a bit...

[R6_kid]

lol, bugjuice your always on the same wavelength as me. Im thinking it'll be mainly for track/ride use as opposed to commuting.

I know that the pitch/note will depend on the length, whether that is the length of the can or of the complete exhaust system i dont know, but i am interested to know what effect the perforated pipe has.

Have sent a PM off to the neptune guys, and if they are in auckland then im hoping they'll let me go have a look and let me in on a bit of how it all works so i can get something that works good.

[R6_kid]

Eat more beans, that'll get the back pressure up a bit...

how did you know what i had for dinner :gob:

[Motu]

back pressure is a myth..

You are right - why on earth would you try to restrict the evacuation of exhaust gases.I guess if you need to restrict the power of your bike for some reason you could try.Bloody crazy if you ask me.

[R6_kid]

so not worth worrying about then?

[Motu]

Very much worth worrying about,if you do it the correct way.Volumes have been written about exhaust systems - and I bet the internet is full of information too,in fact I know it is.

[R6_kid]

sweet, i guess google is my friend then.

[cowpoos]

sweet, i guess google is my friend then.
whatch out for myths and wives tails.....


Ps: big exhaust makers spend huge $$$$$$$$$ on R&D for a reason....buy one of there exhausts!

[Ixion]

What you need is a powerband. Available from all good motorcycle stores. Make sure you get a four stroke on though.

[sAsLEX]

You are right - why on earth would you try to restrict the evacuation of exhaust gases.I guess if you need to restrict the power of your bike for some reason you could try.Bloody crazy if you ask me.

SO thats why two strokes often just have a straight pipe out the back.......NOT!


They need back pressure to assist the lack of valve and to assist pulling the exhaust gases out of the cylinder.

But 4 strokes....

[Motu]

That is NOT backpressure! Like ''accident'' that word should be burned from our language.In no way does an expansion chamber restrict exhaust flow - it is bouncing back and intensifying pressure waves...because a smoker actualy has very low pressure waves,they have to be applified,4 strokes have strong enough pressure waves to be usefull.

[imdying]

SO thats why two strokes often just have a straight pipe out the back.......NOT!


They need back pressure to assist the lack of valve and to assist pulling the exhaust gases out of the cylinder.

But 4 strokes....That's not back pressure in either instance. And scavenging is as important on 4 strokes as it is on 2 strokes. A 2 strokes total exhaust length matters as much as does the header length on a 4 stroke.

[Firefight]

what is it that you are compensating for gareth ?

its not length that counts !


F/F

[sAsLEX]

That's not back pressure in either instance. And scavenging is as important on 4 strokes as it is on 2 strokes. A 2 strokes total exhaust length matters as much as does the header length on a 4 stroke.

Sets up a kind of standing wave dont it? And when you hit the right frequencies, Ie the powerband, this standing wave is its most effective?

[R6_kid]

the standing wave would be formed when the length of the pipe and length of the exhaust pulse 'wave' are equal in the sense that:

exhaust length = X. pulse-wavelength
where X is n =1,2,3....

thats off the top of my head though, and assuming that its open at both ends? - i failed that paper. Thats where the grey area is for me, i dont know whether to take it as a closed end pipe, or open at both ends. Will have to check my notes to make sure which formula is for which, but hopefully the interweb will divulge some information as to which is best to use AND how the hell i figure out the length of the exhaust pulse waves.

[Motu]

You should just put a gate valve on the end of your exhaust,then you can adjust it to the amount of back pressure required - there is always a simple (and cheap!!) answer...

[R6_kid]

like a butterfly valve? or more like those things they have on the top of truck exhausts?

[imdying]

Sets up a kind of standing wave dont it? And when you hit the right frequencies, Ie the powerband, this standing wave is its most effective?Most effective is a pretty loose term, which is why we have powervalves on strokers, and EXUP type valves on 4 strokes. There's an optimum tuning (length/diameter/whatever) for an exhaust pipe for a given set of engine conditions (throttle/load/revs/whatever), for every engine, that gives maximum torque. The cunning part of course is getting it all right... and of course once you've done that, you want to do the same with the intake (variable length/diameter runners).

[sAsLEX]

Most effective is a pretty loose term, which is why we have powervalves on strokers, and EXUP type valves on 4 strokes. There's an optimum tuning (length/diameter/whatever) for an exhaust pipe for a given set of engine conditions (throttle/load/revs/whatever), for every engine, that gives maximum torque. The cunning part of course is getting it all right... and of course once you've done that, you want to do the same with the intake (variable length/diameter runners).

hmmm I see varying the intake length as not that great a challange as its low temp and you could make something that varied constantly without too much difficulty, most systems I have seen jsut use two different lengths though.

The same with the exhaust most have two different configurations which they switch between. In a 4 stroke for example is the length that produces optimum at low revs compared to high very different? ie in the range of inches or feet?

[imdying]

You would have to calculate that for a given application.

[sAsLEX]

You would have to calculate that for a given application.

How did I know you would come back with that!


ok roughly for a 800 cc V4 4 stroke at a rough guess ballpark kinda figure?

[Motu]

The same with the exhaust most have two different configurations which they switch between. In a 4 stroke for example is the length that produces optimum at low revs compared to high very different? ie in the range of inches or feet?

With the Rickman Metisse you see in my avatar I made a set of tuned length TT pipes,they come together under the engine and then splay out either side of the rear wheel.They made the motor really camy,with a big power kick at 4500 rpm,then out to 10,500 if I wanted too....and a ropey idle,a raspy noisy thing.Then I fitted gutted VW tail pipes on the end,you can see those by the rear wheel - with these in it didn't come ''on cam'',almost the same power but spread over the whole rev range,but less revs,a nice idle and deep mellow tone,I would pull out the tailpipes for go fast stuff.So - 150mm say to transform a screaming chainsaw into a mellow grunter.

[merv]

Its the right length that counts.

[imdying]

How did I know you would come back with that!


ok roughly for a 800 cc V4 4 stroke at a rough guess ballpark kinda figure?Because it's obvious how important it is! It would depend entirely on workable range of rpm desired. Everything is a compromise in engine design, you're trying to minimze them is all. On an 800cc road bike, an acceptable compromise might be a change in length of 6-12 inches.