Dave,

 

With the super charger I typically cruise with engine rpm’s between 4850 and 5400 set with the prop controller.

I have a fixed pitch prop so RPM varies much more widely.  I see 5200 or so on climb out and 5400 to 6000 depending on how fast I want to cruise.  I saw 7500 when racing at Reno.  

It is important to note that variation of mass flow will help keep away from the surge line.  If you look at compressor map, as the pressure ratio goes up (the vertical axis) the mass flow through the turbo needs to go up to keep away from the surge line.  With your C/S prop and lower RPM, you are using higher boost (higher A/R) without increasing the mass flow through the compressor as much as if the RPM were increasing.

So, for a given horse-power output of the engine the mass flow is relatively fixed (i.e. air mass=oxygen mass=fuel mass burned=h.p).  However, the pressure ratio will be lower if the engine is turning faster at a lower MP to achieve the same power output.  This is more likely to prevent surge as you have noted.

Cruise climb around 5800-6200. Throttle Body is wide open and MP controlled with a vernier cable to the super charger bypass. Applying these settings to the published turbo compressor maps puts me outside the surge line at low engine rpm and high altitude.

Yup.  It is OK to run your rotary much faster than that.  Maybe the prop cant turn that fast?  Higher alititude always moves toward the surge issue because the pressure ratio is going up while the mass flow is not.  Automotive turbochargers were built for sea level.  The aircraft turbocharger makers dont seem to give up the data as readily.

Several things could be done to move the pressure ratio lower. Run higher rpm at altitudes above 15K,

Yes, as above, I recommend that.

use a ram air inlet to eliminate and boost air pressure at the turbo inlet or use an intercooler with a lower pressure drop.

You are likely to get minimal improvement from either of those because it is so hard to change things much (how much lower do you think you can get pressure drop?), but every little bit helps.

 I have not been able to find compressor maps that compare different turbine A/R’s for the same compressor components.

Again, the compressor map is INDEPENDENT of the the turbine A/R.  And by turbine, I mean the part that is in the exhaust.  The part that is in the intake is the compressor (even in a supercharger even though it is a turbine too, and the only turbine in a supercharger system)

My concern would be using a .96 or 1.0 A/R and moving further toward the surge line. That may not be the case.

If we have our terms right, increasing the size of the turbine A/R has no effect on surge whatsoever.  We want large a/r's in aircraft use to minimize back pressure and reduce risk of compressor over speed at altitude (which did in my first turbocharger).

My compressor is a T04H while my turbine is a T04 trimmed down and stuffed inside a stock turbine housing with a miserably small A/R and a stock waste gate.  I happens to works very for my particular goals, though there is much improvement to be had if loftier goals were in mind.

So for you to figure out which compressor you need, look not to A/R, but the mass flow A/R caluclator the input your goals and find a compressor that works.  I am sorry that I have not looked at the one you sent out, but here is the one I sent out years ago.

Hope it helps,

Dave Leonard

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