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jurhip jurhip is offline
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Join Date: Jul 2002
Location: SoCal
Posts: 643
I suppose on a theoretical level you could perhaps calculate effective pressure, but it will be different in reality, because, for example the K27 will flow more airflow at the same boost levels than say a K26. So in the case of VoitureLTD he will have a different mean and maximum cylinder pressure than that of a stock 930. His turbo will pump more airflow into the cylinder than a stock turbo.
Brett raised some really good points about how engine design can affect knock that skew straight octane vs compression ration/heat of compression. That I won't go into.
But, at full boost, turbocharging systems will be designed to flow enough air that excess must be vented out the wastegate (or bypass on newer engines). This means that no matter how much compressor "flow" available, the air density supplied to the cylinder is the same. 1.0 bar of charge results in the same amount of air no matter how much flow the turbocharger can supply.

Now, if your turbocharger went into choke at full throttle load, then the higher flowing unit will definitely provide more pressure. But, as I stated before, no manufacturer would design a turbocharger to hit its choke line.

A larger turbocharger combined with a correctly sized turbine wheel will likely hit a higher flow value at a lower turbine rpm (due to its compression curve), perhaps giving a higher boost pressure at a subsequent lower engine rpm. This is not guaranteed, but typically the K27 turbine and compressor wheels are designed for this. This means you may hit full boost pressure at a lower engine rpm, but if you have a 1 bar wastegate spring, the max pressure you will can get is still 1 bar.

Now, completely off topic but this reminded me of something someone said about turbochargers at high rpm. The statement was to increased boost pressure to account for the higher altitude. This is not true. Temperature being constant, the air density at seal level at 1 bar is equal to the density at whatever elevation. The only difference may be a slightly higher compressor discharge temp, due to higher delta P at max boost combined with lower cooling capacity from the intercooler, due to loss of air density all resulting in a higher intake temperature. At the same pressure, the higher temperature will result in a less dense mixture and lower O2 values.

I definitely wandered off topic. And I am pretty sure I had some really long, grammatically incorrect sentences
Old 03-08-2008, 09:45 PM
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