Poorly Z4M, Idle bounce and poor throttle response/misfire

[daz05]

Go and set up your thread and that put a poll up, let the viewers decide.

 

[exdos]

The biggest restriction in those backboxes will be the two 180 degree bends that you've removed. What you've effectively done is create a plenum which will pressurise before the exhaust gases can exit the backboxes and will create a restriction of sorts, but may well be less of a restriction than the two 180 degree bends.

The way that I see this is with an analogy. If I create a draught through a room (plenum) by leaving a window open (exhaust inlet) and then open the door into the room (exhaust outlet), the air rushes straight from the window directly through the doorway. There's no delay whilst the air entering the room needs to pressurise the air in the room first.

The optimum solution would have been to replace the whole back box section with a straight section of tube.

I hate that kind of exhaust with a passion, they are invariably horribly noisy. :thumbsdown:

 

[exdos]

On the exhaust "hack" front - i would like to know what the thing sounds like?

I've been out in the car to day with my wife for the first time since I fitted the modded silencers, and she made no comment at all, which means it's civilised in normal driving conditions. But it's a quite a bit louder and fruitier at full throttle.

 

[pilchardthecat]

The biggest restriction in those backboxes will be the two 180 degree bends that you've removed. What you've effectively done is create a plenum which will pressurise before the exhaust gases can exit the backboxes and will create a restriction of sorts, but may well be less of a restriction than the two 180 degree bends.

The way that I see this is with an analogy. If I create a draught through a room (plenum) by leaving a window open (exhaust inlet) and then open the door into the room (exhaust outlet), the air rushes straight from the window directly through the doorway. There's no delay whilst the air entering the room needs to pressurise the air in the room first.

The optimum solution would have been to replace the whole back box section with a straight section of tube.

I hate that kind of exhaust with a passion, they are invariably horribly noisy. :thumbsdown:

it's not quite as simple as your 'room' analogy in fact. It's been a few years since i did any fluid mechanics, but the advantage of pressurised flow through a constant cross section is that within most tolerances it's primarily laminar, which is much more predictable and more easily to model.

introducing the "room" changes the mode to turbulent flow, which is much harder (borderline impossible) to mathematically model and is generally a much less efficient way of moving a fluid from on place to another. However, you have reduced the distance the fluid as to move. If you'd asked me this question 20 years ago i could probably have had a decent stab at working out what the net effect was, but the text books are long gone..... although i might i still have the fluid modelling computer program i wrote somewhere ( written in FORTRAN77) it was designed to model turbulent flow around bridge piers......

 

[exdos]

it's not quite as simple as your 'room' analogy in fact. It's been a few years since i did any fluid mechanics, but the advantage of pressurised flow through a constant cross section is that within most tolerances it's primarily laminar, which is much more predictable and more easily to model.

introducing the "room" changes the mode to turbulent flow, which is much harder (borderline impossible) to mathematically model and is generally a much less efficient way of moving a fluid from on place to another. However, you have reduced the distance the fluid as to move. If you'd asked me this question 20 years ago i could probably have had a decent stab at working out what the net effect was, but the text books are long gone..... although i might i still have the fluid modelling computer program i wrote somewhere ( written in FORTRAN77) it was designed to model turbulent flow around bridge piers......

I fully release that things aren't as simple as my analogy but does it actually matter if it isn't a perfect model? I have limited resources, no CFD facility, no wind tunnel as well as limited specialist expertise, although I do have a science-based degree. I therefore rely on empirical science: i.e. suck it and see. Things either work or they don't and we don't always have an explanation for why they succeed or fail. I find that measuring can go a long way in this process.

 

[Shooter]

What can i say? Perhaps our gas is dodgier than yours.

Didn't intend to be rude, my question was genuine. Sorry for that. :wink:
I know my english is next to perfect, but it's still not my original language.

 

[ga41]

I didnt take offence. English is a second language to me as well.

 

[pilchardthecat]

it's not quite as simple as your 'room' analogy in fact. It's been a few years since i did any fluid mechanics, but the advantage of pressurised flow through a constant cross section is that within most tolerances it's primarily laminar, which is much more predictable and more easily to model.

introducing the "room" changes the mode to turbulent flow, which is much harder (borderline impossible) to mathematically model and is generally a much less efficient way of moving a fluid from on place to another. However, you have reduced the distance the fluid as to move. If you'd asked me this question 20 years ago i could probably have had a decent stab at working out what the net effect was, but the text books are long gone..... although i might i still have the fluid modelling computer program i wrote somewhere ( written in FORTRAN77) it was designed to model turbulent flow around bridge piers......

I fully release that things aren't as simple as my analogy but does it actually matter if it isn't a perfect model? I have limited resources, no CFD facility, no wind tunnel as well as limited specialist expertise, although I do have a science-based degree. I therefore rely on empirical science: i.e. suck it and see. Things either work or they don't and we don't always have an explanation for why they succeed or fail. I find that measuring can go a long way in this process.

Nothing wrong with empiricism. It's a valid scientific methodology.

Any time you want a peer review of your complete design, let me know

 

[Lower]

The way that I see this is with an analogy. If I create a draught through a room (plenum) by leaving a window open (exhaust inlet) and then open the door into the room (exhaust outlet), the air rushes straight from the window directly through the doorway. There's no delay whilst the air entering the room needs to pressurise the air in the room first.

The analogy isn't bad but doesn't allow for the flow rate and pressure of the exhaust gas which is proportionally much much higher than your draught through the room. Nor is it actually a constant flow like your draughty room. The exhaust gas flow is actually made up of a series of high pressure pulses. You will get rapid expansion of the gas pulse when it hits the plenum which will in turn slow it down and create a back pressure. You will also get turbulence which will also restrict the flow. However, it may well be that these restrictions are less that the restrictions caused by the two 180 degree bends in the original design.

The optimum solution would have been to replace the whole back box section with a straight section of tube.

I hate that kind of exhaust with a passion, they are invariably horribly noisy. :thumbsdown:

A straight through pipe made of perforated steel enclosed in mineral wool inside a casing would be the compromise.

 

[exdos]

The analogy isn't bad but doesn't allow for the flow rate and pressures of the exhaust gas which is proportionally much much higher than your draught through the room. You will get rapid expansion of the gas when it hits the plenum which will in turn slow it down and create a pressure. You will also get turbulence which will also restrict the flow. However, it may well be that the restrictions described above are less that the restrictions caused by the two 180 degree bends

I fully understood and realised before embarking on gutting the silencers that my "surgery" would pay no heed whatsoever to any understanding of fluid dynamics or be based upon any scientific calculations at all. Pure "suck it and see" methodology, but with a hunch of what would work based on the practical experience of doing something similar before. I've previously gutted about a dozen pairs of Z3MC silencers in various different ways and gone through the monitoring process of the performance impact of the different configurations and I've formed my own ideas of what might be happening to flow, and equally important, the sound of the exhaust. Once I was inside those Z4MC boxes and with plasma cutter in hand, what could remain and what had to go was based entirely on my own ideas of what I hoped might happen in practice once the silencers were welded up and on the car. If I were to gut a pair of Z4MC silencers again, now that I've heard this pair in action, next time I'd be choose to be more radical.

With regard to the effect of a plenum in the system; the cats and centre sliencer, if it's of "X" pipe configuration, also create similar smaller plenums en route too.

A straight through pipe made of perforated steel enclosed in mineral wool inside a casing would be the compromise.

I have one of those on my Citroen C2 VTS (runabout/lugger) and it was the most irritating and droning thing imaginable until I stuck a solid pipe up the exhaust outlet to restrict the bore and reduce the aperture (again after a series of trial and error experiments). I'd NEVER fit anything like that on any car again. I'm happy that the combined effect of all my mods to the intake/exhaust system of my Z4MC now show a gain over OEM, irrespective of the fact that my knowledge of fluid dynamics is woefully inadequate.

 

[sammyz]

More radical There was nothing left in there to cut out

I swapped out the x pipe which is flattened slightly and looked as though it might restrict flow. The centre resonator is a tiny thing compared to the bulk of the two rear boxes. So I binned that as well!

 

[exdos]

More radical There was nothing left in there to cut out

There's plenty more to take out if I were to copy what I've got on my Z3MC and they sound absolutely superb! Always better to take out too little than too much; I can always go back in them and have another go. 8)

 

[pilchardthecat]

Well i voted for your picture (and tried to persuade the site admin to rig it in your favour)

I suppose we will never know the end of this story now

 

[sammyz]

Not being very technically minded...chop out twisty bits in centre of exhaust boxes, chuck OEM air scoop away, replace it with an RPI style scoop add missing ingredient and we go from 325 to 365 flywheel b.h.p.?

That's some increase! What is the missing ingredient? :?

Can't be a K & N type filter as it costs money,
can't be a supercharger either

So maybe remove or modify something in the airbox? If we guess right will he tell us????

 

[ChawenHalo]

there is no cheap shortcut to get more Bhp out of the S54 and keeping it NA. 8)

 

[sammyz]

there is no cheap shortcut to get more Bhp out of the S54 and keeping it NA. 8)

there is according to his dyno runs!

 

[pilchardthecat]

since i don't think we are going to be told, i shall have no alternative but to conclude that he is a charlatan

which is a shame, as i have enjoyed this thread and have some admiration for his dedication to empiricism.

as a scientist, though, peer review is the only road to be trod.... and in the absence of proof

 

[Paza3]

since i don't think we are going to be told, i shall have no alternative but to conclude that he is a charlatan

which is a shame, as i have enjoyed this thread and have some admiration for his dedication to empiricism.

as a scientist, though, peer review is the only road to be trod.... and in the absence of proof

x 2 , shame the idea of these forums is to share knowledge

 

[exdos]

since i don't think we are going to be told, i shall have no alternative but to conclude that he is a charlatan

Thank you for your kind words!

I can assure you that all the information that I have posted has been genuine and unadulterated. Just because I haven't given you 1% of the information that you would like to have, you are now so ready to besmirch me. :thumbsdown: As you know, you have already sent me PMs last week requesting that I give you the little piece of information that I have thus far retained, and I was intending to do so by PM, but since you are so ready to publicly insult me, why would I now want to do so?

x 2 , shame the idea of these forums is to share knowledge

I totally agree and I'm normally very happy to share my information with fellow enthusiasts and I do so on other forums. However, it appears that sharing knowledge only applies on this forum if it is ME doing the sharing. I am still waiting for the replies to PMs that I've sent to 3 active posters asking for their opinions/information on unrelated issues.

 

[exdos]

there is no cheap shortcut to get more Bhp out of the S54 and keeping it NA. 8)

there is according to his dyno runs!

And according to the static dynos of Evolve and Powerstation, as witnessed by other respected individuals.