Sizing a turbocharger installation by careful selection of a/r (area/radius ratio of the turbine scroll) is certainly a valid way to control your boost as long as you are aware of the limitations and your flight operation regime.

 

Like most things in this hobby it’s about compromise.  You can have greater simplicity or perhaps less complexity with certain approaches, but you frequently trade off some other feature or benefit. 

 

You can fly a turbo without a wastegate or other explicit type of boost control by carefully selecting the a/r size of your turbine housing and/or controlling your engine rpm.  However, as mentioned above you will give up some features. 

 

First if you use max engine power for take off at sea level then that will create the maximum air mass flow through your installation.  IF you size the a/r of your turbo to control boost (instead of using some other type of operated boost controller)  then you will need to size it so you don’t blow your engine at max power take off at sea level.  So that tells you you’re a/r is going to have to be considerably larger than a turbo which uses some sort of boost control to prevent boosting beyond your limit.  As you increase your altitude, the turbine will need to spend the compressor faster to maintain the same gauge boost – with a large a/r that permits your engine to survive a sea level max power take off  - it simply won’t be able to have full utilization of the exhaust gas energy  to give you as much boost at altitude.    

 

 

 Now the advantage of a large a/r (in my opinion) is that instead of getting lots of boost at lower rpm and then having to release exhaust gas through a gate (or some other method of control) at higher rpm to prevent blowing the engine, you make your power and boost at higher engine rpm.  So instead of a kick in the seat of the pants on take off, you will simply find you produce more power at altitude and the high end of your rpm range.

 

 

There are other advantages of a large a/r – one is that there is less obstruction to exhaust flow when you are not under boost, there is less exhaust back pressure because the flow is less restricted and of course it can be lighter without a wastegate.

 

So it is certainly possible, but you are going to have to experimental determine what size a/r turbine housing will do the job for you.  So I would buy my turbo from a company which would allow me to exchange the turbine housing (for different a/r) either free of charge or a modest exchange fee – because unless somebody has already done this, you will probably have to try two or three different a/r turbine housings to get the correct one.

 

 

However, you indicated you wanted great short field performance on take off and similar performance at 18,000 feet – I just don’t believe you are going to find any single a/r that will fit those conditions.  You can choose one or the other and get what you want, but it doubtful you’ll get what you want at both ends of the spectrum without some sort of boost controller.

 

I have no experience turbocharging aircraft although I have studied it quite a bit – but, I have scratch build turbocharger systems for cars. 

 

Good luck on your decision.

 

Ed