Re: 41 with cummins 4BT


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Posted by Charlie on July 09, 2000 at 16:02:50:

In Reply to: 41 with cummins 4BT posted by gary on July 09, 2000 at 11:40:37:

1) If the radiator is in good condition (you should pull it out and have a competent radiator specialist check it) it will be enough. My turbo0intercooled Hercules puts out 125 hp and is kept cool by my same sized M37 radiator. Unless you really soup up the 4BT from its' standard 130 or most likely 105 hp (up to 175 hp or more) your radiator will do OK. Even if you do soup it up, it still takes the same 65 hp to haul it down the road at 65 mph. So the heat rejection requirement will be the same. 65 hp generates about 65 hp of waste heat in a water cooled diesel; about 120 hp of waste heat in a water cooled gasoline motor.
2)The 4BTA has an integral air-to-water intercooler that is next to the valve cover. If it isn't there the motor is a 4BT, not 4BTA.
3)Someone correct me if I'm wrong. 11.00-18s are 41" tall, that is about 507.5 rev/mile. with 4.89s that will make 2482 rpm @ 60 mph. The 4BT is governed at 2500. That will be a bit fast if you expect to do some highway driving. It is not a "bolt-up" to attach your non-synchro 4spd to the Cummins. You have several good options:
a) get an easy to get SAE#3 flywheel and flywheel housing for the 4BT and use a Spicer 3053A 2.5 ton 5 spd. They can be found in good condition for about $450. That will drop your 60 mph rpm in top gear to either 1960 or 2110, depending on whether the 5th gear ratio is 0.85 or 0.79 (controversial!)
b) get a Dodge Ram bellhousing and NV4500, 2wd version (or 4wd version with NP205 and you'll have low range then) That will drop rpms to 1836 in 5th at 60.
c) get an Advance Adapters Ranger overdrive and either a NP420, SM420, or SM465 4spd. That will drop 60 mph rpms to 1812. With the SM465 you could get a GM adapter and a married NP205 as well.
The cheapest alternative is the 3053A. It also guarantees the best power in 5th gear. A lower rated 4BT could be a bit weak at 1800 rpm at 60, though something with 300-330 ft-lb should do fine, since the torque peak is at 1500. But it doesn't give you low range. that of course can be fixed by putting in a divorced NP200, which isn't too expensive either. For smaller tires the taller geared overdrives are better because you want to gear for about 2000 rpm at 60 mph, which gives a theoretical (and achievable) top speed of 75 mph, which allows an actual relaxed cruise speed of 65-70 mph, at least 150-300 rpm below governed speed. Diesel fuel efficiency is peak at peak torque, diminishes slowly at first as rpm rises, then drops more steeply near peak governed rpm. But too tall gearing can make a top gear unusable. I had a 77 FJ45 Landcruiser 1 ton pickup with 4.11s and a Perkins 4.236 (80 hp@2800, 193ft-lb @ 1400) and a NP540. With 7.50R16s it did fine in 5th (2217 rpm @ 60). With 9.00-16 NDs it didn't do fine (1998 rpm). Of course the motor (3.9L) was non-turbocharged. It weighed a bit less than a Dodge, about 5800 lbs with normal junk in it and on it.
Charlie


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