>Sstones replies:
>>But... Once the turbocharged intake strok is over... You get a 
>>compression stroke at a much higher pressure, there in lies the problem.

This is correct

>
>I reply:
>Ah-hah!  That makes sense.  With more gas (& pressure) in the cylinder at the
>beginning of the compression stroke, there will be higher ultimate pressure
>at the end of the stroke.  This must be why turbo engines generally run a
>lower compression ratio - to allow for the additional compression effect of
>the turbo.

afraid not.  It's not the little extra compression you get from the
turbo booster, it's the fact that while being compressed, gases heat up.

 The lower compression ratio is used to prevent knocking (premature
ignition).  as the turbo stuffs more air into the cylinder (so that you
get more Oxygen to burn the extra fuel with) the fuel-air mix heats up due
to compression, even before the piston starts to compress the mixture.
The same fuel-air mixture will have different 'ignition pressures' at different
temperatures -- the higher the temp, the lower the pressure it will ignite
under.

This (and low-octane gasoline) makes for very good conditions for knocking.
One way to deal with is to change the compression ratio.  Another is to put
intercoolers.  SAAB had the best solution early on, I do not know if they still
use the same system:  They had a knock-sensor (a microphone) attached
to the engine, which, when sensed the engine knocking, would reduce the turbo
boost ratio. 

Cetin

Cetin Seren				Direct:	412-635-3479
Software Development Engineer         	Main:	412-772-6600
Outbound Technology Group		fax:	412-635-3350
FORE Systems, Inc.			email:  cseren@fore.com	
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