Ceramic coating of downpipe

[the bruce]

Didn't you understand my tellings or didn't you even read it? If you didn't understand it you
better ask a specific question.

Many of the postings on this topic are based of half-whisdom and misunderstanding of physics,
some are commercial.

Hotter gasses improve flow? I explained why it's nonsense. It doesn't help reading retailers'
sites who want to sell their stuff. You better read a proper scientific book (and understand it).

 

[vRS_Pagey]

I love it!! 

[Janner_Sy]

Didn't you understand my tellings or didn't you even read it? If you didn't understand it you
better ask a specific question.

Many of the postings on this topic are based of half-whisdom and misunderstanding of physics,
some are commercial.

Hotter gasses improve flow? I explained why it's nonsense. It doesn't help reading retailers'
sites who want to sell their stuff. You better read a proper scientific book (and understand it).

 

I too have studied physics at degree level as part of an engineering degree.  I agree with everyone on here but you 

[craigy123]

Didn't you understand my tellings or didn't you even read it? If you didn't understand it you
better ask a specific question.

Many of the postings on this topic are based of half-whisdom and misunderstanding of physics,
some are commercial.

Hotter gasses improve flow? I explained why it's nonsense. It doesn't help reading retailers'
sites who want to sell their stuff. You better read a proper scientific book (and understand it).

 

None of the retailers or tuners in this topic have tried to sell anything. They have been kind enough to take there time to answer my question and give some advice.  

[94Luke]

Hmm I was always under the impression too that keeping heat in the exhaust is beneficial for increasing gas speed. Watching this thread with interest 

[the bruce]

I too have studied physics at degree level as part of an engineering degree.  I agree with everyone on here but you 

What's wrong?

[Janner_Sy]

I too have studied physics at degree level as part of an engineering degree.  I agree with everyone on here but you 

What's wrong?

you  

[94Luke]

The Bruce, what do you reckon to this statement: "If you imagine how hot the gases are as they pass the exhaust valve, they need to be kept as close to that temperature as possible for the length of the tuned header pipe in order to maintain laminar flow".

[Janner_Sy]

it would still have a laminar flow even if cooler unless the flow is disrupted by something else to some how to turn it turbulant.  That said there are specific velocities of certain fluids where the flow stays laminar.  Outside of this and it becomes turbulant.  No idea what that would be on an exhaust gas, I know what it is with fluids in H/CWS pipework

[the bruce]

"If you imagine how hot the gases are as they pass the exhaust valve, they need to
be kept as close to that temperature as possible for the length of the tuned header
pipe in order to maintain laminar flow".

If you look a page or so back I already answered this, Luke.  


The hotter the exhaust gas the more it is expanded (I guess there is no doubt and
no different opinion on this). The more it is expanded the more volume. The more gas
volume (the more expanded) a pipe has to transport in the same time the higher is
gas speed and so drag and the higher the back pressure.

Still a simple thing. So:

There is no advantage in keeping high temperatures. They even disturb gas flow as
hotter gases ( = expanded gases ) are harder to flow in a limited diameter pipe.

There is even no need to think about laminar vs turbulent flow etc.. All the turbulence
issues increase with more flowing gas volume because gas speed increases.


If someone would say hotter gases will flow better ( = lower drag ) he would also
say that a car has lower drag at 200 mph than at 50 mph.

What's wrong?

you 

Very interesting posting. 

[vRSAlex]

What would pour faster, cold custard or hot custard?  It's the same amount of custard therefore the same amount of molecules. With the hot custard your molecules are further apart (as previously agreed) and therefore "looser" and therefore easier to pour.
Same goes for exhaust gases. Everything IS further apart but its still the same volume coming through the engine. It doesn't magicaly suck extra exhaust gases out just because its hotter.

And whilst we are on the subject, can you tell me what's heavier...a ton of bricks or a ton of feathers?

Oh, and would you prefer to be attacked by one horse sized duck or ten duck sized horses? 

[fab5freddy]

^^^^^^^ 

[rich83]

10 duck sized horses every time.... 

[94Luke]

Awaiting Bruce's reply 

[alackofspeed]

My memory of reading exhaust design books years ago is that the post turbo exhaust design fundementals can be (relatively) easily simplified as there's not the same level of complication to the pulse tuning / scavenging issues that one has to contend with on a NA engine.

In gist, my memory of maximising on boost performance is:

- maximise differential pressure across the exhaust turbine

do this by:

- minimising pressure drop in post turbo pipework
- keep heat (within reason) in the exhaust pre turbo to assist working the gas as hard as possible across the turbo

so:

- don't fit a 1.5" pea shooter on a Veyron.....
- minimise pressure drop by getting heat out of the exhaust ASAP post turbo - lower the volumetric flow rate, and as a by-product lower the gas velocity.

Roughly, pressure drop for a body of area A moving in a fluid of density rho can be calculated by:



If you've got a high velocity fluid you have a high pressure drop per metre of pipe, if you have a high temperature fluid you have a higher molar volume which dictates a higher velocity flow. Post turbo, temperature is bad, high velocity is bad, high back pressure is bad.

It's not quite as simple as the ideal gas law, as there are simultaneous elements to calculations governing exhaust gas flow, but very crudely... double the temp, double the volume. Double the volume, double the velocity (for a given molar flow). Double the velocity, quadruple the pressure drop.

My 2 p worth.