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      11-04-2008, 07:36 AM   #44
Second Lieutenant

Drives: Z3 M Coupe(S54) and Z4 M Coupe
Join Date: Apr 2008
Location: UK

iTrader: (0)


I see what you are saying but I doubt if your idea will really benefit performance.

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%". 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. At less than WOT, an effective ram-air intake is just making engine breathing easier and consequently more efficient.

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. 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:

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?

Personally, I think your OEM air intake is an outstanding design. I'd love to be able to take some pressure readings and datalogging from one! I bet it achieves well over 100% VE.