On the topic of bell mouths, I have a friend who is seriously into tuning his wrx. It has a fully custom and immaculately presented turbo steup, which includes a pair of plenum chambers he had made himself, after exhaustive flow bench testing.
For his setup, he has found offsetting the inlet runners from the chamber to give an angled oval mouth was very effective, not in increasing flow,but in making even airflow from one runner to the next. He claims about 3-4 percent variance in the volume which he can flow from the first runner to the second.
Secondly, he has exhaustively tested trumpet designs, with the idea he would include them in his plenums, and made a very good point about the bell mouth design. His tests indicated that the rim of the trumpet is one of the most restrictive areas. What his tests have shown is that a large share of the air drawn in to the opening has come from around the lip. So it effectively turns the corner around the bellmouth, up to around 180 degrees and accelerates as it does so.
I did some basic vacuum tests for myself and was able to observe this.
There was a lot of air drawing around the lip and very little observed to be straight in and down the funnel.
To me this means that two things are important.
- You need an excellent heat shield under the trumpets to allow cold air to be drawn from this location, closer to the heat of the engine, hence my massive tray.
- You need a smoothly machined lip with no hollow cast underside. i'd like to ee someone machine a teardrop shape on the lip and see some footage of how it flows, I think it would be very very good.
Once I truly considered why this might happen, Ive come up with a basic theory, and it relates to the pressure difference to the inlet runner and engine bay and surrounding air, of course.
Air doesn't have to be drawn in a linear sense, when there is a pressure difference between two areas, you will see all the nearby air within the surrounding area attempting to rush to the low pressure area, despite the barriers and obstructions in the way.
In this case the suction is created by the engines thirst for air.
Just as with our weather, wind is created by pressure differences and the resulting movement of air in a never ending attempt to equalise pressure.
I therefore think that even though the air around the trumpet itself and under the bellmouth is seemingly facing a difficult path, its point to point distance may only be a few mm from the low pressure area, therefore its being pulled up around and into the bellmouth by the pressure difference. This air is attempting to make its way into the inlet runner in the same amount of time as the air with an easy path above the the mouth, therefore it accelerates to do so. Air is such a flexible medium that it can be pulled around corners, and with some decent velocity too. Try it for yourself!!
I know my explanations aren't super scientific, but I though that this was an interesting point which hadnt been made yet, with the discussion focussing on inlet runner length primarily. Feel free to critique my theorys, as I'm always after a new perspective.
My inlet at the moment...
