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This article covers a vent which I installed near the front of the hood on my E30 M3. The hood is a composite unit manufactured by MA Shaw in Soutern California. ( www.mashaw.com ) The hood weighs about 12 pounds and was installed with the factory "hinges" at the rear and a pair of hood-pins at the front. The MA Shaw hood includes underhood stiffening ribs, and although it could be lighter, it is very stiff and rigid. The vent was bonded onto the hood with epoxy prior to painting. So, why the vent? Well, the primary reason was to reduce front end lift. "German" John points to wind tunnel tests done in Germany which showed that the Evo III components could attain negative lift, i.e. true downforce. The hood vent would be pushing lift further into the negative direction at the front end. The secondary reason to add the hood vent was to improve cooling to the radiator. This idea of negative lift first, cooling second, is counter to what many folks would think. The anti-lift effect of a hood vent is only recently becoming widely known. The theory of this effect will be detailed in another article. Suffice it to say that the air which passes through the radiator is at high pressure (due to the stagnation point at the front of the vehicle). The air looses some pressure as it squeezes through the air passages of the radiator, but not that much. After that this high pressure air is "trapped" inside the engine compartment, with no where to go except down and out around the engine. This is bad from an aerodynamic point of view. Meanwhile the air which does not pass through the radiator mostly flows up and over the front edge of the hood. It must speed up to make this journey (and still conserve overall mass) thus the local pressure goes down. Add to this the fact that the air possibly separates into a local eddy as it turns the corner over the front edge of the hood and you have that there exists a local low pressure zone above the front portion of the hood. |
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The net result of these flow patterns is a large plan area (the hood), with high pressure underneath it (pushing upwards), and low pressure above it (sucking upwards). Multiply the difference in pressure by the area and you get an upwards force. With such a large area it does not take much pressure differential to create a rather large lift. Solution: vent the two pressure regions together. Let the high pressure air vent out to mix with the low pressure air thus reducing or even eliminating the pressure delta, and therefore also reducing the lift force. If one takes time to examine photos of any contemporary front engine race car it is quickly apparent that radiator (and oil cooler) outlet air is always vented out to a low pressure region in order to reduce under-hood lift. Sometimes it is out the top of the hood, sometimes out the sides of the front spoiler, or even out the lower portion of the car behind the front wheels (i.e. modern DTM cars). All of these exit regions are generally low pressure. |
This photo shows how the location of the vent was chosen to try to coincide with the area just behind
the radiator. This is a little farther back then might be originally supposed due to the thickness
of the front part of the chassis (i.e. the radiator core).
In an ideal world there would be ducting to specifically direct the air exiting the radiator upwards and through the vent. There is not a lot of room for such a duct with the factory E30 M3 engine layout. One could rightly ask if adding front downforce without a matching change to rear downforce could make the car unstable in high speed turns (high speed oversteer). This is indeed a possibility, and if so, would require some modification to the rear wing. Either moving the rear wing into better airflow (by spacing it up or back) or by substituting a more efficient wing altogether (or even a high drag DTM flap). At the time of this writing there has not been enough test time to determine which way to go regarding aero balance. |
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Remember that the DTM cars of the 1992 season also had a lowered front spoiler to compliment that huge flap on the rear wing. The bumper covers were molded so that the splitter and undertray venturi sat about 1" closer to the pavement. Combine that with the extremely low ride heights that these cars ran at, and you have a recipe for pretty substantial front downforce. My car runs at a higher ride height (out of necessity so that it can still be driven on the street), thus the rear DTM flap might shift the center of pressure too far to the rear, even with the hood vent. |
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The next page includes a few photos depicting the installation of the vent into the hood. |