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

AHU Optimum Cruise RPM


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.

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.

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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