Re: [Amc-list] rear mount turbo

[David Crooks <david.crooks@xxxxxxxxxxxx>]

> The energy that makes the engine work is in the heat and expansion of the gases.  
> But when the exhaust valve opens it's the differential of combustion chamber 
> pressure to atmospheric pressure that evacuates the gases.  
> It's the size of the exhaust tubing (diameter and length) that would reduce 
> the "speed" of the exhaust gases.  Isn't this correct?

Sorry no, it's not quite correct...
You're right that the speed of the flow depends on the pressure differential, 
but you're wrong in assuming that the speed is constant through the entire 
exhaust system.  That would only hold true if the temperature were constant 
from the engine to the exhaust tip.  

Break out the old physic's textbook, and recall Boyle's law:
(Pressure * Volume)/Temperature is a constant.

What this means is that near the engine, where the temperature is high,
the pressure is also high.  But near the exhaust tip, where the temperature 
is low, the pressure is also lower (assuming a constant diameter exhaust
pipe).  So near the engine you've got a high pressure stream blowing at a
high speed, but near the tail pipe, you've got a low pressure stream blowing
at a lower speed.

As I said before, you can partially compensate for this by choosing different
turbines for a rear mount application, but for the best efficiency, the ideal
place to mount a turbo is right on the exhaust manifold.....

You could also insulate the exhaust pipe, to keep the heat in, all the
way to the turbo, but one of the main advantages claimed for the rear mount
system is the fact that the turbo never gets hot....

Cheers,

Dave

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