Originally Posted by
MetroMPG
Actually, one of the major (maybe even the major) efficiency advantage to grille blocking is reducing aerodynamic drag. Diverting excess air flow around the car's body instead of through the engine compartment has a measurable effect on MPG. That's why more and more new cars (especially efficiency focused ones) are coming with automated grille shutters. (Expect to see it on the next Prius.)
But sizing a grille block is highly dependent on driving style, terrain, traffic and accessory use. So yes, that's the tricky part! You need proper instrumentation and you need to monitor it.
I drive with my grille over 90% blocked, year round, and the cooling fan rarely comes on even in the summer. I also watch coolant temp and know what the e-fan threshold is.
Well I was camping on St Laurence river on 4th of July a couple years back, and I was freezing! everyone else acted like it was a heat wave and I think it got up to 80F in afternoon. This year has been cool but a few years back MD had 3 consequent weeks of temps exceeding 100F, big difference, esp if it doesn't go below 78-76 overnight.
I doubt Gen4 Prius will have active shatters. Toyota tested and found aerodynamic improvements negligible, MPG improvements were something like 0.1MPG or so. Need to find an interview with Toyota engineer.
Blocking does improve MPG in cold weather as it shortens warm up cycle. If it gets too cold it would also keep Prius from restarting engine just to keep it warm.
But aerodrag improvements are minor, you probably get more by removing wipers or installing plastic shielding under engine compartment.
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View my fuel log 2015 Mirage DE 1.2 manual: 46.4 mpg (US) ... 19.7 km/L ... 5.1 L/100 km ... 55.7 mpg (Imp)