For keeping the turbo up to speed, you must either double clutch or keep your foot into it on shifting -- this is where an auto beats a manual, since you don't lift your foot for shifting. Otherwise, live with it, this isn't really a performance car anyway.

If you add enough propane to get an ignitable mixture, it will ignite long before TDC on a turbo -- this will rapidly either burn holes in the pistons or over-pressure the rings and ream out the cylinders (or both). Not pretty. If you want more power on a 617, adjust the boost control spring to give 13.1 psi, no more. In the words of my mechanic friend "more and you will greatly reduce the life of the engine".

I always cool the turbo down if I have not been driving at light throttle near idle for several minutes. I live in the country where the speed limit is 40 mph, so don't bother at home, but any time I'm on the highway, I let it idle a couple minutes before shutdown.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!