Posted by Charlie on Wednesday, August 08, 2001 at 0:58AM :
In Reply to: Diesel RPM Question posted by Tim on Tuesday, August 07, 2001 at 11:25PM :
The 4BT/BTAs go up to 25-2600 rpm. Perkins and Isuzu 3.9-4.0L 4s turn 25-2600. Since the Dodge 6BTA is turned up to peak hp at 2700 and governed all the way up to 3200 the 4BT could be too. But that's about the limit for these 900-1000cc/cyl motors. I even saw a Deutz 3.8L 4 on their site that puts out about 170 hp at 2500 and 450 ft-lb!
To get up to 3600 rpm you need to be dealing with motors with displacements in the range of 850cc/cyl or smaller. The Toyota 3B is 3431cc and governed at 3500. My Cummins 6AT3.4 develops peak hp (somewhere around 125 hp or more) at 3600 and is just about barely short enough to squeeze into a M37/PW engine bay. A Nissan SD33T would probably fit but only puts out 101 hp @ 4000 and about 170 ft-lb of torque, getting down there for a >6000 lb truck.
Proper gearing/tire size correlation is key.
The vehicle should be geared so top speed in top gear is around 75 if on the low end of the power spectrum, more if up on the power spectrum, but never greater than 90-95 mph since I can't see cruising over 75 no matter what kind of motor you have, for other reasons like safety, economy, noise, speed limits.
A high powered diesel can make do with a 5spd, a lower powered one can use a 6 spd or more. Examples of do-it-yourself multispeed gearboxes would be attaching an overdrive to a wide ratio 4 spd which gives 5 usable road gears (2nd, 3rd, 3rd-OD, 4th, 4th-OD). Or even to a 5spd with certain ratios. Like an Eaton Fuller FS4205C aka Clark CL557 with ratios 6.99, 3.78, 2.12, 1.17, 1.00. An overdrive means you can use for road driving 2nd, 3rd, 3rd-OD, 4th, 5th, 4th-OD, 5th-OD. That's SEVEN usable road speeds with 17% splits in the top 4 gears.
The way I'd do this is use an overdrive on the back of the transfer case if the transmission has an SAE#3 attachment (the FS4205C) or a Ranger overdrive if you find a Clark CL557 with Chevy pattern.
Charlie
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