I'm a BMC importer -
In the testing comparing the stock, K&N, BMC STREET and the BMC RACE -
The stock and BMC Street flow very similarly - I don't recall a significant difference in power (under .5% generally). I didn't test the K&N.
The BMC RACE leaned out the engine just a teeny bit and made 1- 1.5 more TRUE HP than the stock filter.
We used the TEKA (to avoid ignition scatter power loss) and richened up the 100% throttle and power was a bit better - but - I suspect that I should have taken out the injectors and cleaned and blueprinted them at our other shop -
The amount of air actually flowed through the filter depends on how hard the engine is trying to get air ito itself -
One of the bigger misconceptions is that an air filter "will flow" 15% more air. The results of air filter marketers selling product.
If the engine actually took in 15% more air, you'd make, for simplicity sake, 15% more power.... and that's not the case :-)
So - the filter will flow "easier" and allow the engine to pull in a bit more air - If you gained 2% more power - the intake system is pulling in 2% more air.
That's how it works. 10% more air in = 10% more power.
Of course, fuel has to be adjusted to make Best Power with the new amount of air - Tuning to an a/f ratio simply gives goofy readings when comparing products.
All the test HP results were done on an EC997 dyno, tuned to the a/f ratio that made best power - so they are fully loaded tests and not subject to tire slippage or inertial mass errors of dealership level dynos.
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