Back pressure reqd

[MarkCR750]

[ Guests cannot view attachments ] I just fitted a scorpion can to my scrambler, you have a few options with baffling, it has a baffle that fits in the front of the silencer where it joins the exhaust pipe and one in the usual position at the rear of the can, with no baffles at all it's stupidly loud and the performance feels flat, with just the rear baffle it is acceptable noise wise but loses power up to 3k revs, with just the front baffle it's loud but loses power around 6k revs, bit of a flat spot, with both baffles it's as good as the OE set up but just a bit deeper in tone and slightly louder, spot on really, it's 2.6kg lighter than OE as well so that's a bonus, just thought I'd mention as reminder that when we're building specials back pressure counts! 👍

[hairygit]

Less or more than the stock system often needs messing with to run as well as stock across the rev range. Prime example I have had issues with are Motad systems fitted to 750F1's, they strangle the top end quite badly, so we drill a series of 6mm holes around the exit of the silencer. Barely any increase in noise, but back to stock performance. Odd thing is, it only seems to be 750F1's that suffer this issue with Motads, all the other fours seem to run fine with them!

[florence]

my 500 was flat with Motad so I drilled holes in it which made a vast improvement.  Best system for performance I have experienced is the Laser I now have.  The only problem is it's too loud.

[MarkCR750]

[ Guests cannot view attachments ] I've got laser cans on my 900ss, it goes like stink but as you say a bit loud, maybe laser actually do a bit of research into flow etc ?, [ Guests cannot view attachments ] the generic silencer I've got on my CR750 definitely stifles it according to the dyno tester, but there are no flat spots so it will do until I make the 4 into 4 system.

[K2-K6]

It's a conflicting area of design as commonly viewed and spoken about.

Plain backpressure with a pipe that's too restrictive is exactly that,  too little flow capacity to allow the exhaust gasses to clear in sufficient volume and time which stops the engine from reach peak performance. Obviously undesirable.

The "other" backpressure usually lumped into the discussion is nothing like the above at all. My understanding is that it should be called a negative pressure wave, and this is desirable. 
It's derived from acoustically tuning the exhaust system. When an exhaust pulse changes in speed / volume we hear it because it has an acoustic signature,  to make that sound, for example as it exits the pipe, we get the pressure wave hit our eardrum which is accompanied inside the exhaust by a mini vacuum which has to be filled, it pulls volume into the pipe from the engine combustion chamber and if timed correctly occurs while both the inlet and exhaust valve are open together ( the overlap period of the camshaft) which in turn enhances cylinder filling of new mixture. If this all happens in the right cadence performance / efficiency will increase and produce more torque.  Bingo,  you'll go faster.
Different exhaust locations will affect the time the negative pulse takes to get back to the combustion chamber and so will affect different rpm points. Also affected by exhaust bore size and changes in system length.
A good system can make a big difference to how the engine performs.
Just being noisy doesn't necessarily mean it goes faster. As your scrambler above Mark,  bringing those different pulse points into use can better utilise the exhaust energy to fill in the torque curve rather than make noise.

[MarkCR750]

Yes.