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For giggles, we decided to first test the car with no airbox! While it sounds much better, with a top-end roar that’s addicting to say the least, the midrange torque totally tanked. Keep in mind you’re looking at a horsepower graph, because that’s up to 25 lb-ft of torque it’s losing in the midrange, too! Unfortunately we couldn’t get a tach signal that day to show you the torque in graphical form.
The fact that running with no airbox showed a little more flow at the very top at least gave us some hope of a net horsepower gain with the Castro Motorsport unit we’d test later, but we honestly expected more here. It looks like BMW supplied the E46 M3 with a very effective box!
You may be wondering how on earth an open, completely unrestricted air path can actually lose so much horsepower. Well, if you look closely at the graph, it did indeed gain a few ponies right at around 8000 RPM. However, the midrange is all about air velocity, and what better way to ensure maximum speed of the air entering the motor than velocity stacks? This is a shot of the stacks inside the factory plenum.
Looking inside–oh goody gumdrops–the Castro Motorsport intake plenum has velocity stacks in it too! If this weren't the case, we’d potentially be in horsepower trouble.
Here’s another look at the Castro Motorsport unit (bottom) versus our factory airbox (top). That’s a huge difference in volume.
Here’s another comparison, looking from the engine side of the plenum.
In just a moment we'll get to see how much power this Castro Motorsport intake plenum is worth. What do you think we'll get? In our last article we had people guessing anywhere from 5-8whp, to over 25whp in the comments section. Find out what the actual result is on Page 8.
