Can someone explain exhaust back pressure to me?

[Mr2Mike]

A standard rear silencer is much more likely to have baffles and/or helmholtz resonators rather than absorbent wadding.

[JonnyVTEC]

Go too large and the now slower moving exhaust gas removes the 'column inertia' of the moving gasflow, this effect post exhaust valve closing reduces pressure in the system prior to the next valve opening. The pressure differential between the combustion chamber and exhaust is then slightly higher and you get more exhaust residuals out of the chamber and hence volumetric efficincy should increase and drive more power output.

Thats pretty much whats happens with the myth of 'too big and you lose back pressure'

[V8RX7]

texasjohn said:

If you have a bargain barge and just want to release a little more volume would removing a bit of baffle material from the tailpipe silencer be a good option to avoid mucking up the exhaust tuning too much?

Generally luxury barges haven't been tuned for ultimate power but are required to be silent which costs power so I wouldn't worry about removing a box or two - generally it will release more power.

Certainly removing all 4 rear silencers on my Soarer V8 seemed to liven it up a bit and it sounds 10x as fast

[texasjohn]

As it happens, I've release a bit of noise for the total cost of 15 pence today; purchased a tube connector to bypass the exhaust valve solenoid!

Bit of a burble on idle now the second tailpipe is functional below 3000 rpm

Still no TVR though lol

[DKS]

My Saab T16 makes a superb burble, most noticable at low RPM/ idle and apparently it's because the exhaust manifold is very simple, from Cyl 1-4 in a straight line, then turbo, so it's all sequential. Also with the cat removed (not me) it only has the rear box left. Makes a lovely, slow, dur-dur-dur noise on tickover.

[Mave]

texasjohn said:

As it happens, I've release a bit of noise for the total cost of 15 pence today; purchased a tube connector to bypass the exhaust valve solenoid!

Bit of a burble on idle now the second tailpipe is functional below 3000 rpm

I've heard you can do the same thing plugging the vacuum line with a golf tee :-)

[Huff]

texasjohn said:

In very simple terms...

Is it true to say that the further along the exhaust system you go (ie closer to the exit) the less impact there will be on the scavenging effect?

If you have a bargain barge and just want to release a little more volume would removing a bit of baffle material from the tailpipe silencer be a good option to avoid mucking up the exhaust tuning too much?

Chav consideration aside of course

1. Yes that's very true. Which is why, these days, manufacturers place the main 'silencing' effort as close to the back bumper as possible. But this is also because such a remote placement maximises the path length from the noise source (exhaust valve) so any acoustic treatment placed here has best value in terms of cost.

And it's related to answer
(2) In truth, the manufacturer's placement of catalyst and then resonator (if used - they all do these days) provides the 'pressure termination' i.e. acoustically separates the tailpipe and silencer from the engine pulse-tuning.

Once the exhaust pulse gets to the first large discontinuity (e.g. step-change in diameter) in the system - usually a resonator, a perforated pipe through an otherwise empty 'box' - the pulse tuning effects 'see' a large empty column like the atmosphere and everything downstream of that point really, really doesn't matter for power tuning; it only controls noise output (and the spectrum of that noise output)

So if you want to pull the stuffing out of the backbox for your flavour, I'd say do it and don't worry - but take any factory-fitted resonator as a 'good idea to keep' for tuning purposes. If nothing else, it'll suppress that horrible 'rip'/empty-drainpipe sound you hear in all too many crap aftermarket systems

[texasjohn]

Mave said:

I've heard you can do the same thing plugging the vacuum line with a golf tee :-)

That's for the 6 cylinder engines I believe. 8 cylinder works a bit differently, you connect the constant vacuum line from the engine to the exhaust flap actuation line (bypassing the solenoid).

See pics here

https://www.bimmerfest.com/forums/showthread.php?t...

[texasjohn]

Huff said:

texasjohn said:

In very simple terms...

Is it true to say that the further along the exhaust system you go (ie closer to the exit) the less impact there will be on the scavenging effect?

If you have a bargain barge and just want to release a little more volume would removing a bit of baffle material from the tailpipe silencer be a good option to avoid mucking up the exhaust tuning too much?

Chav consideration aside of course

1. Yes that's very true. Which is why, these days, manufacturers place the main 'silencing' effort as close to the back bumper as possible. But this is also because such a remote placement maximises the path length from the noise source (exhaust valve) so any acoustic treatment placed here has best value in terms of cost.

And it's related to answer
(2) In truth, the manufacturer's placement of catalyst and then resonator (if used - they all do these days) provides the 'pressure termination' i.e. acoustically separates the tailpipe and silencer from the engine pulse-tuning.

Once the exhaust pulse gets to the first large discontinuity (e.g. step-change in diameter) in the system - usually a resonator, a perforated pipe through an otherwise empty 'box' - the pulse tuning effects 'see' a large empty column like the atmosphere and everything downstream of that point really, really doesn't matter for power tuning; it only controls noise output (and the spectrum of that noise output)

So if you want to pull the stuffing out of the backbox for your flavour, I'd say do it and don't worry - but take any factory-fitted resonator as a 'good idea to keep' for tuning purposes. If nothing else, it'll suppress that horrible 'rip'/empty-drainpipe sound you hear in all too many crap aftermarket systems

Thanks, great answer!

[Mave]

texasjohn said:

That's for the 6 cylinder engines I believe. 8 cylinder works a bit differently,

Yep, for a 6. Didn't realise you had an 8, could only see a picture of a BMW rocker cover in your profile :-)

[rodgling]

Apart from scavenging, I think valve overlap is also a factor - if the engine is designed with an amount of valve overlap, back pressure may stop (some of) the intake charge blowing straight through, so back pressure and valve timing are related. This could well be bks though, interested to know if anyone can confirm or deny?

[Mr2Mike]

Huff said:

Once the exhaust pulse gets to the first large discontinuity (e.g. step-change in diameter) in the system - usually a resonator,

That would usually be the cat converter.