Quote:
Originally Posted by Xicor01
i want to know what the math would say when the displacement is increased one or twice over in the bore, how fast a spool time for the crank when knife-edged, or even a way of calculating optimum compression ratio.
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And because you are so specific, I will be also
For math involving rough horsepower estimates, use these rough equations:
CID = Cubic inch displacement;
(CID * RPM / 3456) = CFM
CFM * .069 = lb/min airflow
So, for an SR20DE engine,
(122 * 7000 / 3456) = 247CFM
247 * .069 = 17 lb/min (170 horsepower)
that equation simply says that an OEM sr20de engine with 100% volumetric efficiency should produce 170 horsepower at the flywheel on a BAD DAY. Since .069 takes into account the air temp rise of a turbocharger, this estimate is LOW. But since we are neglecting the fact that an OEM sr20de engine is not at 100% VE around 7,000rpm we are actually HIGH in our estimate. That is why this equation is very ROUGH.
So lets turn up the boost some, SR20DET engine @ 14.5PSI:
(122 * 7000 / 3456) = 247CFM * 2 (double the atmosphere) = 494CFM
494 * .069 = 34lb/min (340 horsepower at the flywheel)
Now subtract drivetrain loss, about 13%, (340 * .87) = 295 RW Horsepower.
Most SR20DET engines with sufficient compressor flow and cams will product approx 300 RWHP at 15psi at 7,000rpm so this is very accurate estimate.
Notice if we bump displacement slightly (due to overbore) there barely any increase to the overall flow capacity of the engine. If you want a larger displacement then you need a bigger engine, dont expect an overbore to give you back any spectacular results. Knife edge crankshaft will reduce drivetrain losses, and reduce rotating mass, which is a good thing for fuel economy and 4-cylinder engines in general but again nothing spectacular will happen to your power output. and OPTIMAL COMPRESSION RATIO is a misnomer since atmospheric conditions change from second to second, so there is no such thing for these engines. You need to modify the ignition timing and air/fuel ratio, for instance, as the temperature drops outside, or going uphill, or in a very high water vapor situation, to compensate. Therefore, you best case scenario tune is "worst case scenario", that is, very conservative. If you tune to the bleeding edge, and then go uphill on a hot day, you are past the bleeding edge and engine damage may result.