I just installed baffles in my TOGA Gold Star mufflers on my SuperHawk. The bike had become so ungodly loud that my friends refused to ride behind me, and I think it was actually damaging my hearing.
What I did notice after having installed them was that I seemed to get a bit of a boost in low and mid-range power (this is merely from a seat-of-the-pants perspective), but I lost some in the very top end (from 3/4 throttle to full throttle in 4th the power does not change, and maybe I need to go up on the main jet size). It does not 'nose-over' if I back off on the throttle, so that's why I'm thinking I need to go up in my main jet size (currently I have a 150 main in my AMAL 626's, and I was thinking of trying a 160 or 170).
So why is it that my low and mid-range may have been boosted by the addition of this seeming restriction in the exhaust? If it's because I've now introduced 'back pressure', what exactly does back pressure do? And why would I then lose some power in the top end? If I'm reducing air flow, why would I have to (maybe) increase my main jet size to add more fuel? Or do I have this all wrong?????
The Concept of Back Pressure...
Vince
MBell is on the right track. You have changed the characteristic of your exhaust system so that may require a change in carburation to fix. In the case of an engine, each pulse of air (sound) is caused by the valve opening. The 'organ pipe' effect of a pipe will have effects right through the rev range and depends mostly on the speed of the engine but also on the temperature and composition (fuel) of the air. A column of resonating air is just like any other vibrating system; there will be high pressure pulses at any location followed in a regular pattern by low pressure pulses. When these are in sympathy with the speed of the engine, bingo! You can actually help suck the spent gases from the cylinder, helping the engine breathe. Conversely, at other points of the rev range, the opposite can happen and the stale gases get pushed back into the head. The efficiency of scavenging (removing burned gases) is material to the efficiency of the engine as any remaining burned gas will reduce the amount of fresh charge you can admit on the next cycle. While this is going on the exhaust has to do its job of expelling gas into the open air. As the revs rise and the throttle opens, more gas is produced. You have put a restriction into the muffler so you have reduced the ability of the system to pass air which makes it harder for the engine to operate at it's peak. Absorbtion-type mufflers (glassfibre wrapped around a perforated tube) soak up the high pressure pulses (noise) without restricting the flow and are therefore popular in racing machines. Think of a gun silencer (muffler?) the bullet gets through but the noise of the shot (one giant pressure pulse) does not. '60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F I've seen smart, civil men almost end up in a fist fight discussing 'back pressure' in exhaust systems. I reduce this to common sense- you're trying to get spent exhaust gases out of the combustion chamber, so any 'back pressure' is working against that goal. There's a complicated pressure/sonic wave going through the exhaust- the pressure wave magically, to me anyway, transforms into a reverse, vacuum, wave running in the opposite direction, as it exits the pipe. This inverse wave helps to pull spent gases out of the combustion chamber. The increasing diameter of a megaphone somehow shapes or amplifies the waves in both directions. Makes sense to me so far. Common sense falls apart when I get to the 'reverse cone' megaphone- the reverse is supposed to induce a pressure wave timed to keep the fresh fuel/air mixture in the combustion chamber and not slipping right out the exhaust port- people with dynos swear it works, but I can't quite grasp it- it's part of the 'tuning to a target RPM' thing that doesn't make sense to me- the waves should vary with RPM, so why don't they 'tune' themselves?
The same arguments apply to intake manifold length. I haven't been able to reconcile theory and 'look at the dyno chart' empirical results to come up with an answer, so I look for a free flowing, unrestricted exhaust path. Rick
I thought that too, but I found that from 3/4 throttle to full throttle, there was no change in power. If I were to have backed off on the throttle and it picked up speed, then yes I'd think to reduce the main jet size. But that didn't happen, so we're going to try the bigger jet route. Sounds weird to me too, as you'd think that reducing the amount of airflow would result in a smaller jet! The plugs look nice and tan at the moment, so we'll see what happens later in the week when the jets arrive. If you think this thread has the potential to drive men insane, you should read some of teh twaddle on 2 stoke sites.
Back pressure is more or less a complete furphy. It has little or no relevance, but reflected waves do. In simple terms when the valve opens it send a positive pressure wave down the pipe. We're not talking the gas itself here, but a wave like a sound wave moving through the gas. When that wave reaches and open end, the wave is reflected back but the pressure has changed to negative. If that negative (sucking ) wave arrives when both valves are open it will suck out the old gas along with some fresh gas leaving a nice clean new mix in the combustion chamber. The timing of all that depends on cam timing, pipe length and gas temperature, plus engine speed. So that explains why teh effect only works over a limited rev range - not one exact perfect RPM, but a range. A megaphone creates a long steady small negative wave effect and greatly extends the useful rpm range. A solid end to a pipe reflects back a wave that did not change from positive to negative, so it acts as a stuffing wave. So if the muffler contains flat plates, it probably creates stuffing waves. At the gas flow level a restriction will slow down the gas and make it harder to get out of the way of the next pules of gas and tends to act as a stuffer which tends to be more useful at lower revs but restricts flow at higher revs. There's a whole lot more to pipe and muffler design, but that sorta covers the basics.
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