Hell, you all probably go to your graves denying it, but you guys are the ones who are so wrong its sad. Fluid dynamics covers everything that is not a solid. Air is a fluid with the ability to easily compress, all the same mathmatics still applies. Do you think aerospace engineers only design submarines?

What do you think computers are using to calculate and design flows through housings and turbines? Its called computational fluid dynamics for a reason. Before the extreme expense of creating a turbine and testing it is ever invested, 98% of the flow characteristics of a turbine are determined by CFD before it will ever leave the drawing board. The laws of physics are still the same even when we make computers calculate the finite details for us instead of using slide rules.

Force fed is saving himself some embarasment by stating that a way undersized turbine housing can restrict your exhaust flow to get you any crazy pressure ratio you want. Unless someone is looking to get high boost at very low RPMs and screw themselves at the high RPMs doing so is sheer stupidity.

Maybe some of the fucked up hack jobs you have come across are using way undersized turbines, but any properly set up radial turbine should never see more than a 2.5:1 pressure ratio across the turbine. For that reason in any factory setup you will never see a higher pressure ratio than that across the turbine.

If any of you all had bothered learning something more about how centripetal turbines and compressors work than what the other 'tuners' are saying who knows what kind of better numbers you could be squeezing out of your setups.

Just as there is a limit to the pressure an centripetal compressor can generate, there is also a practical limit to how much pressure you would want to run across a centripetal turbine.

So I can save a lot of time of explaning FD to you all, I will just give you a simple chart of typical automotive turbine performance characteristics to prove my point.



As you can see from both the mass flow and efficiency there is a sharp drop in centripetal turbine efficiency and mass flow after the 2:1 pressure ratio is exceeded. Since the power you get from any turbine is based on these two factors, your power output from a turbine tapers off just the same. If you really want to push the envelope then you can go as high as 2.5, but anything after that pressure ratio gains you NOTHING.

Concidering that the maximum pressure ratio you can get out of your centripetal compressor is also around 2.5:1 I hope you can all start to see the basic reason why there is no good reason why you should ever have much higher pressure ratio in the exhaust manifold than the intake.

Anyone who is doing so for any other reason than they can't afford the properly sized turbo for their engine air flows is just doing so because they are ignorant of the facts.

If any of you nay-sayers would have to take even a basic test on the fluid dynamics of centripetal turbines and compressors would get a big fat F.