AHU *EXHAUST* port matching

[ChopSauce]

Hi,

There is an awesome gap between the head and the gasket/manifold.

It looks like the port matching job would almost just consist in displacing the (opposite of)restriction closer to the valves.

Anybody ever did the job / molded the inside of the head /

Thanks,

[jtanguay]

with exhaust you want more velocity (with a turbocharger) so better to leave them alone.

[ChopSauce]

Errr ... sorry. I am not 100% certain I understood/explained it well. Let me rephrase and please let me know if/where I am wrong:

 - do you really mean I should leave the exhaust ducts to their restricted - original - states or was my question poorly explained?

Well, a least section would mean a greatest speed - the flow being imposed by the piston - but when it comes to the manifold, the flow will ressemble something of a "backward facing step problem" and I am pretty sure this is far from optimal there (possibly with stagnation points.) At least, the flow will slow down before the turbo with duct section increase in the manifold.

Sorry I have no polymer at hand to mold the inside of the head ...

[gatti500]

i,d go with the flow.
lower resistance will result in a quicker response.

[Freidenker]

Sorry, I'm new to a lot of this stuff, but doesn't the turbo pressure overcome a lot of restriction problems by brute force pressure?

[subsonic]

For my clarification of the picture in my head, on what part, the head or the manifold,  is the opening bigger and on what part is the opening smaller?  It is OK for the exhaust port to go to an exhaust manifold that is slightly bigger.  A small step up in I.D if you will.  There is a concept out there that this will help with preventing reversion.

If the exhaust manifold I.D is SMALLER than the cylinder head exhaust port, then you will want to port match.    The key thing we are after is velocity in the exhaust as it heads for the turbo.  Opening up the exhaust ports or the manifold too much may decrease velocity.  

Is this what you were asking?

[ChopSauce]

Thanks for the reply. It's OK for me too that the manifold has bigger ducts than the head. However, there is a noticeable difference. The manifold almost matches the gaskets while the head not at all!

Picture taken :oops: from TDI-Club[1]



Ideally it would have been possible to rework the whole duct. However, it seems[2]



 that there will remain a restriction close to the valve - but by changing exhaust valve diameter, which is far beyond my abilities.

So it seems there is nothing an amateur can do but maybe smoothing the sharp angle close to the valve seat and dispatching the diameter difference along the final - straight - portion of the duct, by making it more conical?

 .. and yes - probably [3] - this won't make a huge difference on the final horsepower amount but ...
 _

References

[1] http://forums.tdiclub.com/showthread.php?t=203679

[2] http://forums.tdiclub.com/showthread.php?t=236364

[3] http://forums.tdiclub.com/showthread.php?t=157337

[ChopSauce]

Sorry, I'm new to a lot of this stuff, but doesn't the turbo pressure overcome a lot of restriction problems by brute force pressure?

I was just thinking of the exhaust, which is what drives the turbo. But you're partly true - I guess - since this issue seems to be of much less relevance than for NA engines.

[jtanguay]

i,d go with the flow.
lower resistance will result in a quicker response.

apparently its velocity that spools the turbo, and not flow... maybe polishing the exhaust port to remove casting flaws would be a good idea though... but i've heard that making it bigger isn't always best. however for a naturally aspirated engine, it is a really good idea!

[ChopSauce]

A few words for completion ...

First: it seems that I mistakingly used the word "flow" as the english word for "débit", which is the amount of mass flowing though a section per unit time. The "débit" indeed accounts for velocity but it might be more relevant as a quantity because the velocity varies even through a single section.

Secondly: I missed much of the importance to have only growing duct diameter sizes at the transitions. The sketching below was taken from flat4ever.com. The sketching is not a plot of flow computations. It is not quantitative at all, but it illustrates the importance of increasing duct diameters in order to keep the "débit" to a maximum.



Thanks again for making this more clear to me.

[chromejunks]

food for thought. turbos are driven by heat not air flow and as you expand air you lose heat. you may want to look at exit velocity / seat velocities as well.