Originally Posted by willwren
Ram Air Myth: (try google for more info)
Alex, three good rules to follow:
1. Front to back. (flow direction) also ties in to #3.
2. Bottom to top. Convection. Heat rises.
3. What comes in must come out. Provide an inlet? Provide an outlet.
To add to this:
Ram air has been used in piston driven aircraft. Mooney aircraft developed their 'Executive' into the '201'. This was a 20 mph increase in speed without adding a bigger engine. Ram air was used as one of the things that made a difference.
The link that willwren provides, gives us a key to what is really happening in that case too. Although the airplane is going 200 mph, the actual "ram air" affect is negligible. The ducting provided by the ram air intake is shorter, straight, and therefore more efficient than the other. It is these ducting losses (or lack thereof) that allowed for the increase in manifold pressure from 29.5" to 30".
The dynamic pressure of a fluid changes by the SQUARE of its velocity, so at 200 mph it is four times what it is at 100 mph. Since our cars barely get to 100 mph and the effect of ram air at 200 mph is negligible, ram air in our cars is even more negligible, so effectively doesn't exist.
The link above is inaccurate in its explanation where it mentions compressibility since technically, ram air isn't compressing the fluid, it is about RECOVERING the dynamic pressure that is already there. None the less, the effect is the same - NO RAM AIR.
The three rules are brief and may be a little ambiguous; the "front to back" rule will cause the ducting to be as simple as possible, the "what goes in must come out" is also about the ducting.
The biggest key in all of this is to do whatever you can to avoid unnecessary complication in the duct. You want to get as much of the atmospheric pressure as possible.
2001 Bonneville SE - Dark Bronzemist Metallic - Mods: 180* thermostat, iPod adapter, Digital OnStar Upgrade
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