Isn't it defined by the actual filter medium (paper) where a larger diameter (for a regular or stroked engine) wouldn't matter. Perhaps things change when an oiled/cotton filter is used and more air actually flows in? Would be nice for a tuner to actually measure pressure drop or intake velocity on some of these elbows with both paper and oiled cotton and share the results.
Originally Posted by Munit
These are one of those situations that if you simply step back and use common sense you will see that its really not sensible that bmw M would use a restrictive elbow if there was 10hp on the table from using a slightly larger diameter pipe. The diamter being smaller is not necessarily more restrictive. Its likely calculated based on desired intake air velocity whichi requires a certain diameter. Larger diameter means slower moving air. So while it may move "more" air, the air is slower and would take more time. This intake does not have a restriction for air in totally stock form.
Evidence is in the GTS 4.4 stroked engine which uses the identical intake box as the stock s65 m3. Clearly a 4.4L if anything requires more air and the stock setup provides enough air for that so our stocker engine has more than enough air.
I think a dyno gain is for the filter only but to each his own. M would not leave 10hp on the table if it were a matter of tweaking the intake elbow diameter. Just makes zero sense