Re: compression V efficiency - Saab Performance Bulletin Board - Saabnet.com
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Re: compression V efficiency
Posted by Dean (more from Dean) on Tue, 25 Sep 2001 10:06:29
In Reply to: compression V efficiency, Camm
[Profile/Gallery]
, Tue, 25 Sep 2001 00:18:11
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Increasing the compression ratio increases the efficiency. Yes bearing loads increase. But a turbo engine already has the compression ratio reduced to allow for the boost pressures and street fuel.
When on boost, the ingition timing is reduced and peak pressures occurs quite late in the cycle compared to NA high compression engines. In some cases the turbo engines under boost can produce lower peak pressures than the NA variant of the engine. So it is complex issue that you are addressing. When on boost at high RPM's the boost pressure on the pistons helps reduce kinetic tensile forces. Exhaust backpressure from the turbine has the same effect. So peak bearing loads can be reduced, but the pressures will be sustained across a larger crank angle.
When not on boost, which is most of the time, the overall efficiency is reduced compared to a NA variant at a higher compression ratio. But the engine is more efficient than a 6 cylinder NA engine with similar peak HP or torque.
Lowering restriction of the intake and exhaust systems reduces valve and pition temperatures and reduces charge air temperatures which reduces the tendancy to knock. So have no worries about improving intake, IC and exhaust.
With a turbo, the engine is often used at higher RPM's and with taller gears so there is less mechanical thrashing about compared to a NA variant.
posted by 207.43.19...
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