Same turbine section. Garrett naming convention is first two digits=turbine selection, second two digits=compressor exducer diameter.
GT4294r=gt42 frame, 94mm compressor wheel outer diameter at exducer.
GT4202r=gt42 frame, 102mm compressor wheel OD etc etc...
Fluid mechanics off of charts is easy, the key is getting units converted to
Match, and knowing when to convert between mass flow and volumetric flow. Turbos are matched by determining your goal power output, calculating fuel and air required for that power based on volumetric efficiency and a realistically less than stoichiometric combustion efficiency. The ammount of air required can be approximated in volume using your average atmospheric conditions (convenience property is "density altitude"), make sure it's a realistic volume to put through the given engine, and then you select the turbo you'd like and find what pressure ratio and efficiency it would be operating at for that hp level. That's 1 point on a chart. Now calculate for many rpm points and throttle positions for driving conditions other than "full throttle, 3500+ RPM" and see how the turbo really "fits" the engine for all your potential uses.
To match compounds take the output from the turbo above and select a pressure ratio like 2.5:1. Figure out what mass of air it flows under those conditions, then convert that mass back to volume at the new higher pressure and temp, then repeat the process to find a smaller turbo matching the larger turbo's mass flow but at a lower volumetric flow since air is already compressed.
It's like gypsy magic but nerdier.
The info from Garrett is generally tailored to crank hp in gassers, so take hp claims with a few grains of salt. That said Garretts historically continue performing at the bleeding edge of published compressor maps.
With your overall goals I'd go 4294. If you're not happy put a 4718 overtop and blow s*** up. If your wife is that cool tell her the Internet told you to do it. She should be understanding.
-Mark
yes I also realize those numbers are flywheel power #'s, I don't really ever plan or desire to achieve more than that, I see the benefit of being able to go bigger, but I am also considering the benefit of having a smaller turbine wheel to spin in daily driving duty, and possibly towing duty.
