A/R Choice & You

[Nikuk]

Mods - If this goes somewhere useful, move it.

This has been the best “layman’s” explanation I’ve seen. What d’yah think?

@some old guy

The relationship between A/R ratio is not just about flow. It is about the “velocity triangles”. The A portion is easy to understand (small area, higher velocity). The R is a little harder, it basically governs the angle the exhaust gas contacts the turbine wheel, and this basically controls how much “work” the fluid is forced to do on the turbine. So this is why it is A/r and not just A. Decreaseing the area will have the same effect as increasing radius.
Most people do not wish to understand all this, and I admit I barely understand it (the r part, area is easy to understand), and that is only when I am staring at the equations.
Practically what this means is a small A/r number forces the exhaust contacting the turbine to do more work on it. Running a small A/r ratio spools quickly because you are utilizing more exhaust energy to drive the turbine, but this also raises the backpressure. Available energy to drive the turbo is basically a product of mass flow and pressure. Running a larger A/r ratio means to get the same amount of work out of the turbo, you must pass more mass through it (but at a lower pressure).
You are probably thinking why not run a small housing, and just bypass the turbine with the wastegate? The reason is the wastegate allows both mass and pressure by, meaning the backpressure the engine sees will still be higher than if all the mass passed through the turbine, but at a lower pressure to get the same work (out of the turbine.)
So you see, the small a/r housing isn’t really a matter of flow restriction (after all, the wastegate bypasses all of this when open), it is a different way of getting the same amount of work that drives up back pressure. You will make MUCH more power with a larger A/r ratio. Yes, there are 600 hp hondas with .48 housings, but the would be 680hp honda with .82’s. It is not going to put a ceiling on the power because of the flow, but it will definately make it harder to make power.