Ace996,
I see what you are saying but I doubt if your idea will really benefit performance.
-I have my doubts, too...it will only make a difference is there is indeed negative pressure effect (or a loss of dynamic pressure from not having sufficint positive pressure compared to the front two intakes) from the hood vent at speed, at high load, at high rpm. Thus far, with my caveman-scientific deduction of watching water drops fly out and leaves poke out through the vent, there is without doubt air pouring out the vent at speed. I do not know if this happens at high load/high rpm, though. But I can deduce that the hood vent is overpowered by the two front intakes.
I think the selection of graphs and data that I've posted actually show how an engine can and does benefit from a ram air-intake which is not an entirely closed system. As I said earlier, "The VE can only ever be at max when at WOT". The whole point of the throttle is that it acts as a REGULATOR to the amount of air that enters the cylinders on each stroke, to maintain a steady cruising speed below Vmax, the charge of air/fuel entering the cylinder on each stroke only needs to be a percentage less than Max. As I stated before: "When my MC is travelling at a steady 30mph under cruise control, the VE is between 9%-20%, only. At 38mph under cruise control, VE is between 24% -34%, only. At Idle the VE is between 12% -13%".
I agree with all of that...no issues.- If ram pressure in the air-intake is forcing a greater volume of air into the cylinders with each stroke, then to maintain your steady speed you need a lighter touch on the throttle otherwise the car will always accelerate with any throttle opening.
-surge...yes, but we would need a significantly greater amount of air pressure to have that happen...like a turbo car that's blow-off/bypass valve is faulty. At less than WOT, an effective ram-air intake is just making engine breathing easier and consequently more efficient.
yes, which should have a minor effect on MPG...in theory.
In all driving scenarios which require less than WOT, even at 99% WOT, it is the throttle which is acting as the regulator to air intake into the engine and any build up of excess air under ram pressure before the throttle which cannot be used will be eventually vented in some way or other, even if only by overflowing out of the air intakes; like a bath which is overflowing when the taps are left running.
-great point here and let me expand on that. I believe/surmise/guess that is happening somewhat in the M3 airbox: The front two intakes receive a much stronger dynamic pressure, from their location, than the hood vent. This is like three hoses/pumps blowing into a container. If two pumps are pushing significantly harder than the third, pushing much more fluid (air) into the container, and that pressure eventually is higher than the pumping force of the third pump (the hood vent), then there will be a "backflow" situation, and the excess pressure will bleed out the vent.
I am not a strong believer that the hood vent is a very positive-pressure zone. most other fabricators/manufacturers that place hood intakes will employ an additional design component to maximize the positive-pressure potential of that location.... NACA duct, scoop...like the drag cars, or a blended scoop like Ferarri or, to a much greater extent, the Subaru STI. I think that without an additional component like those, the positive pressure at that location is quite weak...at least it is during 50-60mph cruising when water droplets fly out.
This can easily be demonstrated with a Magnehelic pressure gauge with the end of the pilot tube placed inside the air filter box: when you lift your foot off the accelerator the pressure inside the system goes up to almost the theoretical value for ram pressure at the vehicle speed as shown in the graph I've posted above entitled "
Pressure increase versus forwards velocity".
Looking at the diagram of the M3's air intake as posted by Swamp2,
ALL the air intakes are sited in zones of positive pressure on a moving vehicle. See this article here:
http://www.autospeed.com/cms/A_2162/article.html - again, I agree but it appears to be a weak positive pressure zone...easily overcome by the pressure of the front two inlets (especially the one in the driver's foglight area).
Therefore on your M3, when ask for WOT, the engine will be receiving air from zones of positive air pressure. What more can you ask for? -
a real ram-air effect, without the pressure loss out of the hood. Think about it...a balloon with a hole in it will not build pressure as effectively as one that is sealed...even if there is a light pressure on the hole, it will be better than nothing, but not as effective as if it was sealed.
Personally, I think your OEM air intake is an outstanding design.
-it appears so, some even think that the vent is to circulate air so that the airbox is "cooled" by the excess air that is not ingested by the motor....makes sense, it could be... I'd love to be able to take some pressure readings and datalogging from one!
-yes, that would be optimal...then we'd have a better picture of what's happening. I bet it achieves well over 100% VE.
it stands to reason that it does, but it could also be better...again, just my opinion.