I put a thermometer in the engine compartment to gauge air flow for air intake purposes. So far (less than one week) I see that there is a lot of air exchange in a moving vehicle. The hottest the thermo read was 40-50 degrees higher. That was in stop and go traffic on a mid 70s day. Normally, the temp is only around 10-20 degrees higher than outside, and I haven't got a chance to see how it is on the highway.

A couple of things to note are, I removed the baffles in the grill area that hold the stock air intake. That opened up the grill area alot. Another thing to take note of is even tho the thermo reads higher than outside air, it is a large object that retains alot of heat, so it's readings are going to be higher than the air flowing in the compartment.

To sum it up, I think there is a good amount of air exchange (at least for me with the baffles removed) in the engine area. Hope this helps.