More grill block = better MPG

[RedDevil]

Originally Posted by Nautilus

If the engine bay temps are high enough to damage the battery, the airflow from the front of the car is too little to improve anything, even on highway, leave alone city traffic.

The best way to pump air through the battery box, if the car has a closed box: use the engine intake. Drill a hole about 1 inch through battery box and another in the air filter box, and fit a pipe (either plastic, or a wire-reinforced hose between them.)

A 1.6-liter engine draws thousands of liters of air per minute even at slow rpm, enough to bring quickly the inside of the battery box at outside temperatures.



The upper grill block may stay there all the time, if the air is properly ducted through radiator core.

The logic of ducting is like this:

1. Most of the drag at the front of the car is due to the airflow through engine bay, not the shape of the car's front;

2. The airflow through radiator is mostly due to pressure differential at the front and rear sides of the radiator.

So, if the intake grill is left open at the highest pressure point on the car's nose and the exit of the air from the engine bay is at the lowest pressure point, the grill opening may be very small and still achieve good cooling.

Drag improvement is not due to smaller opening, but due to leaving more air to flow over the nose of the car.

Reasonable points, but I disagree with point 1. There's a high pressure area of more or less stagnant air ahead of the car that helps move the air around and over the car like an airliner's nosecone, but without the skin friction. The less air can bleed away though the grill and whatnot, the larger the stagnant bubble is and the smoother the air flows over the car.

The effect of blocking also raises the air intake temperature, which makes the air less dense, so the throttle valve pulls less vacuum and there's less pumping loss on the intake stroke of the engine. At least, as long as that heat does not cause other problems.

Apart from damage to the battery, too much heat can decrease economy too.
My car has an EGR valve and I suspect that's the reason MPG drops when the intake air gets above 35 Celsius / 95 Fahrenheit. When it's too warm it will reduce EGR use.

But this varies per car. You need an ODBII reader (like my UltraGauge), set it to check intake temp and other relevant parameters and do lots of testing with more and less blocking on a set route and comparable conditions.