Quote:
Originally Posted by twin turbo
All of the threads I read were on e90post and N54tech. There are literally a couple of thousand 335's with the same or similar intakes. It's funny that there would be that many people using hot air intakes and all losing power. Most state that there are no real performance gains until 5,000 rpm and 14 psi of boost or higher. Many have proven these intakes reduce the work load on the turbos which offsets the slightly higher intake temps.
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Unfortunately, a lot of stuff people put on their cars doesn't really make it run better, although it might seem cool.
The issue is a really a matter of simple physics that's undeniable (although some will try). A gasoline engine needs an air to fuel mixture of around 14.7:1. That is 14.7 parts air to 1 part fuel. This may change a bit over time and with different engine setups, but the ratio is pretty much the same.
In the old days, some of us had to use manual carburetor chokes on a cold day. The cold air had a higher density and a cold engine couldn’t immediately warm the air, so a choke had to be used to restrict the amount of air. In effect an engine setup to run at normal operating temperature would get
too much cold dense air. That's the physics part: air with a greater density (cold air) must be "choked" so the 14.7:1 remains. Essentially, the air molecules are closer together, or more compact, when the air is cold. The inverse is true as well warm or hot air is less compact.
Said another way, a volume of cold air measuring 1 cubic foot is heavier than the same volume of warm air.
A trick racers used to do was put ice on their intake manifolds just before the race started resulting in a denser air charge being pulled into the engine. The carburetor was setup to provide a richer mixture of gas accordingly and would produce more horsepower - sort of like a supercharger/turbocharger dose by packing pressurized air into the engine.
The heat generated by the engine causes the air in the engine bay to be
far less dense that air outside you engine compartment. That's the reason for hood scopes and air ducts that pull in outside air. When a car moves at just about any speed a pressure dam of air builds up in front of the radiator which is cooler and denser than the engine compartment, so a duct is often used in this location.
A turbocharger works because it pressurizes available air and shoves it into the combustion chamber, increasing the dynamic compression ratio. Even though the engine's static compression ratio (e.g., cylinder volume vs combustion chamber volume) doesn't change, the dynamic compression ratio does change based on air density in the cylinder.
A higher dynamic compression ratio = more air/fuel to burn = more horsepower.
A lower dynamic compression ratio = less air/fuel to burn equals less horsepower.
Air expanding as it goes through a hot turbocharger tends to offset its function: the turbocharger wants to increase air density (compress) whereas the heat from the turbocharger wants to decrease air density (decompress), so an intercooler is required to remove heat, thus restoring air density. If hot air from the engine compartment is drawn into the turbocharger, the compressed air exiting the turbocharger is less dense than it could be if cooler air was used to begin with. Staring off with hot air in effect nullifies than the intercooler's function and power is lost because the intercooler's ability to extract heat is limited.
BMW's car computer is smart enough to sense the factors that contribute to the dynamic compression calculation, and alters the amount of fuel, ignition timing, valve opening duration and timing, etc. Such factors include octane of the gas, engine temp, outside air temp, air density, etc. Again, if the car's computer senses the air coming into the engine is less dense it reduces the amount of fuel injected into the combustion chamber because it has to maintain the 14.7:1 ratio. Less fuel means less horsepower.