I don't believe cubes is the biggest factor. Hell, how is it an EVO can make 600+ whp with 2.0 & 2.3 liters(aftermarket) with a GT-35R which are the same wheels in the PT61? That may not be 800 hp, but that's over 700 at the engine with a 1/3 less cubes. I have far too many dyno pulls on my computer from these 4 cylinders making ridiculous power with similar size turbos and 1/3 less cubes. I believe it's more breathing related.