Engine Specific Info and Questions > TDI - VE Pump TDI Engines

AHU Optimum Cruise RPM

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dave.arthur:
Doing the groundwork on a T3 AHU conversion. Looking for thoughts on where the rpm should be  for 60 to 65 mph cruising (100kph) with an AHU engine. What is the 'sweet spot' for the engine?
I have a variety of top gear and final drive ratios to pick from to match that rpm.

Chuck1978:
Better late than never, the ideal cruising RPM of the 1Z, AHU, & ALH are all around 1950rpm, with around 2150 still being pretty highly optimal.

libbydiesel:
While that might be true for the engine (cruising at max torque will result in best efficiency), gearing for that rpm at that speed might be (will be) unmanageable in a T3 (Vanagon) because it is quite heavy, not particularly aerodynamic, and has very large frontal area.  Because of those characteristics, it requires a fair amount more horsepower to overcome wind resistance and maintain highway cruising speeds than was needed for the cars the engine originally came in.  Because Power = Torque X RPM, trying to get rpms so low at cruising speeds can easily result in the engine not developing enough power to go the cruising speed you were targeting - if you cut the rpms in half you need 2X the torque to produce the same power.  The Vanagon in North America also came almost exclusively with a 4 speed manual or 3 speed automatic (with the '83 diesel vanagon being the only exception).  Trying to get decent spacing between the 4 gears and low rpms at cruising speeds is also quite challenging.  All of that said, I would not try to go taller than 3,000 rpms at your desired high cruising speed.   

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