compensating for a lack of back pressure

**R6_kid:Me** · 28th July 2006, 20:47

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** · 28th July 2006, 20:53

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** · 28th July 2006, 20:54

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

**R6_kid:Me** · 28th July 2006, 21:01

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:Me** · 28th July 2006, 21:02

Originally Posted by Patrick

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

how did you know what i had for dinner

**Motu** · 28th July 2006, 21:18

Originally Posted by bugjuice

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:Me** · 28th July 2006, 21:30

so not worth worrying about then?

**Motu** · 28th July 2006, 21:41

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:Me** · 28th July 2006, 22:26

sweet, i guess google is my friend then.

**cowpoos** · 29th July 2006, 11:05

Originally Posted by gareth_d

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** · 29th July 2006, 11:41

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

**sAsLEX** · 29th July 2006, 11:49

Originally Posted by Motu

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** · 29th July 2006, 12:03

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** · 29th July 2006, 14:23

Originally Posted by sAsLEX

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.