Dr.Jeff
True Classic
I've always had the impression that aerodynamics isn't a simple straightforward thing, but a rather complex science that does not necessarily follow common logic or simple thinking. Unfortunately I don't have any particular knowledge of it myself. Your second post is a perfect example; I agree, my suggestion of adding a air scoop to the roof to direct cool air into the engine bay may be completely wrong. I think without a windtunnel or lots of testing with various scientific methods it would be almost impossible to know if something is actually a benefit or not. I suppose for a street car it may not matter too much. As you say, at lower speeds most aero effects are very minimal. Frankly a lot of it is more for looks on street cars, which isn't necessarily a bad thing if that's what the owner likes.
Regarding the question of engine bay air traveling from the bottom up or top down. That is something I keep debating in my mind, at least for one of my cars that I am assembling a turbo package on. I got the largest intercooler that I could fit without making the connecting pipes any longer than necessary. The layout came out even better than I expected overall, with it laying flat (horizontal to the ground) just above the area between the trans/firewall/engine/strut tower, sort of where the coolant overflow used to sit. But like everything there are still compromises. The IC ended up just a tad too big to fit under the stock engine lid. So I've decided to replace the lid with a custom mesh one that is a little taller so it clears the IC. Hopefully that also allows hot engine bay air to escape more. But on the other hand, I am also adding a electric fan to the IC to increase its efficiency (vital for a turbo in a restrictive engine bay like these). The oil cooler is mounted under the IC to also get air flow from the fan. So do I wire the fan to push air upward or downward? I know the first thought is to go up along with what we assume the normal airflow to be. But that is moving hot air (from the engine bay) across the IC, significantly decreasing its efficiency. By going downward it not only pushes cool air across the IC and oil cooler, but also moves cooler air into the bay in general (it is a very powerful Spal fan that moves a LOT of air). That air is then being directed rearward, where the rear panels have been opened up to allow the hot engine bay to vent out the back of the car (the taillight panel is also open with a mesh grill). Another consideration for this is the fact the car's ride height is very low and there is a large air dam on the nose, so less air is moving under the car overall. That means less air moving upward through the bay from below. Especially when combined with the open mesh lid above. Going from the top downward was part of my thinking with a roof scoop to direct airflow to the bay from above, at least in this example if I run the fan flowing downward.
So which way will work better overall, up or down? Honestly I won't know until I finish it and can try to get a few temp measurements at various spots. Fortunately is will be easy to reverse the fan's direction to experiment with this.
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