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[sofiaspin]

Knapdog, to answer your question......I have the 50s silencer and K&N filter on my Euro 4 classic. Not the Brit downpipe. That alone caused the bike to ultimately fail, ie literally break down with fuel spilling out and the engine management system light on full time. I needed to get a power commander and it is now fine. The Euro 4 is highly sensitive to any tweaks to the air and fuelling set up in my experience. Others have found it less so - Enfields .....

[RoSy]

I'm going to say that back pressure is needed in a four stroke exhaust, because the outgoing gases tend to suck out the incoming fuel/air from the carb to some extent so some of this mixture is lost, back pressure slightly reduces this from happening. I think it's more to do with the sound wave travelling back up the pipe and reaching the chamber at the right time. Exhaust pipe length is also a critical factor for all this to happen. Ok now you can shoot me down, but that's what I learnt and practiced many years ago, and my 500 singles were never slow.

[knapdog]

Knapdog, to answer your question......I have the 50s silencer and K&N filter on my Euro 4 classic. Not the Brit downpipe. That alone caused the bike to ultimately fail, ie literally break down with fuel spilling out and the engine management system light on full time. I needed to get a power commander and it is now fine. The Euro 4 is highly sensitive to any tweaks to the air and fuelling set up in my experience. Others have found it less so - Enfields .....

Thank you. That's very helpful.
I shall leave the downpipe as it is with the inly change being my new Goldstar silencer.

[Count Johnny]

Some empirical information for you.

(Too) Big a pipe and no silencing:

https://www.youtube.com/watch?v=owIQcjBfDAQ

[windmill john]

In that video, back pressure could be a factor, but it could also just be a little too weak and the silencer corrected it; or a combination of factors.

But.... just correcting myself! If it were a little too weak, I’d expect it to race a little!

Ahhh mixtures eh...

[Wheaters]

I'm going to say that back pressure is needed in a four stroke exhaust, because the outgoing gases tend to suck out the incoming fuel/air from the carb to some extent so some of this mixture is lost, back pressure slightly reduces this from happening. I think it's more to do with the sound wave travelling back up the pipe and reaching the chamber at the right time. Exhaust pipe length is also a critical factor for all this to happen. Ok now you can shoot me down, but that's what I learnt and practiced many years ago, and my 500 singles were never slow.

RoSy,

I think that refers more to 2 stroke engine theory where a correctly "tuned" exhaust can be used in effect to "supercharge" the cylinder. There's a "slug" of exhaust gases which, by good pipe design /length, can help pull the new charge into the cylinder because there's a depression behind it (this also happens to some extent on a 4 stroke). But because the ports on a 2 stroke are controlled by the fast moving piston, rather than by a cam, they open very rapidly, causing a "sonic" wave which can be reflected back towards the exhaust port and can be used to help keep the new charge in the cylinder. On 2 strokes, unburned gases can enter the cylinder and straight out of the exhaust port because the incoming new gases have to push out the old on the same stroke (which is why they tend to pollute more). The reverse cone on a 2 stroke exhaust is designed to make this sonic wave reflect "backwards" at the correct time.

F1 cars and high performance 4 stroke aero engines definitely don't have exhaust systems designed to provide back pressure. Some of the fastest piston engine powered aircraft hardly had a manifold, only a pipe stub or collector to get the gases out of the cowling and away.

The expression "pumping losses" is the clue. If an exhaust system "pumps up" (causes back pressure), the engine uses energy to do it and this has to come from the fuel, which tends to rob performance and economy. Aircraft piston engines become notably more fuel efficient at high altitude and one of the reasons is that the atmosphere is thinner and so pumping losses are less. (I used to teach piston engine theory to RAF aircrew but the same principles apply to all four strokes).

[Chris H.]

I was reading recently on an F1 Tech site that thin exhaust headers cause the gas to flow at a higher velocity, which is apparently desirable.