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I am interested in the real world applicability of vehicles' real world cruising RPMs. Consider two cars' Power Curves:

  1. 1.5L MPFI NA Petrol ~ 30hp @ 2000rpm @ 6th gear ~ 1150kg kerb wt ~ runs at 80kph 25% throttle
  2. 1.5L Turbo GDI ~ 60hp @ 2000rpm @ 6th gear ~ 1250kg kerb wt ~ runs at 95kph 20% throttle Both cars are variants of the same car with minor differences in suspension and no change in gear ratios, wheel & tyre size and same cylinder compression 10.5:1.

Shouldn't the 2nd car run significantly faster at this power or conversely shouldn't the rpms be significantly lower? The fuel consumption is quite similar at around 22km/l and 21km/l respectively, I conclude accounting for the turbo's fuel dumping to prevent knock/LSPI - the turbo is a VGT that achieves peak torque ~185ft-lb/250Nm at 1500 rpm.

What am I missing here? Is the power difference solely due to insignificant power difference at part throttle? Is the T-GDI only significantly powerful at full throttle?

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  • $\begingroup$ vehicle power curves are set at wide open throttle. It is almost impossible to compare two engines at different throttle positions. $\endgroup$
    – Tiger Guy
    Commented May 26 at 1:27
  • $\begingroup$ Worse than that, the throttle on a diesel is doing something entirely different (fuel rate) to the throttle on a petrol car (butterfly flap angle) $\endgroup$
    – Greg Locock
    Commented May 26 at 4:46

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