Sustained RPM for long durations.

[90LX]

I don't think my truck will be able to do what I would like it to do. I have a 40' 5er that will weigh about 10,000 when loaded. Truck will be used to haul the trailer to races on weekends. Typical trip is 6 or 7 hours away if driven at 70-75 mph. I have 215/85 R16 tires (30") and 4.10s with a TH400. At 70 mph I would be turning 3214 rpm plus what ever torque convertor slip is present. I think this rpm is asking for trouble for that long of a drive. Reading about all the main web cracks and dampner problems does not help either. What kind of sustained rpm experience do others have here?

[britannic]

YMMV, but I have driven my truck (4.56/TH400 & 285/75-16 tires) at 3500-3800rpm, with peaks of 4100rpm (bouncing off the governor), for over 200 miles straight at a time with no problems.

My engine has over 153,000 miles (maybe more, after discovering over an inch of timing chain play :eek: ) and was previously operated by the Army, where the 55mph top mission speed was regularly ignored. It still has excellent compression and power and revs freely without vibration.

Accessories will wear a bit quicker at higher rpms, but WRT the engine, it's more important to avoid sustained lugging in too tall a gear ratio under load, since that will gall the crankshaft and raise the coolant/block temperatures.

The engine was designed for higher revs than most diesels (a Detroit Diesel trait) and will be happy at 80-85% of max governed rpms (about 3000-3500rpm assuming max governed speed of 3800-4100rpm).

Before a long run, do the usual checks and make absolutely sure that your harmonic balancer and crankshaft pulley damper is up to snuff - failure in one or both could lead to crank damage.

[NH2112]

I have to agree, running close to governed RPM won't hurt anything as long as the engine's in good shape. We used to run humvees well over 60mph on the autobahn in Germany for 4-5 hours at a time, and they had 36" tires and 5.24:1 gearing.

[dieselhumvee]

with 5.24 gearing, TH-400, and 37 inch tires on my hummer, I run it against the governor every single time I get on the interstate, I have driven 500 mile trips several times like this, with no problems. I did burst a heater hose that was dry rotted, and let me tell you at 3600 rpm with a 110 GPM peninsular water pump, I lost half my coolant in less than half a mile. The only thing I reccommend is a close eye on the guages, otherwise, let her scream!!!!

[90LX]

Thanks for the first hand info. I'll check my dampners over closely before I turn it lose like that. Last thing I want is to find myself rolling over engine parts. That should be reserved for the race car. :D

[britannic]

Originally posted by dieselhumvee:
with 5.24 gearing, TH-400, and 37 inch tires on my hummer, I run it against the governor every single time I get on the interstate, I have driven 500 mile trips several times like this, with no problems. I did burst a heater hose that was dry rotted, and let me tell you at 3600 rpm with a 110 GPM peninsular water pump, I lost half my coolant in less than half a mile. The only thing I reccommend is a close eye on the guages, otherwise, let her scream!!!! My upper radiator hose burst on i80 at 4000rpm (I was in the fast lane at the time) - very scary as the coolant was pumped all over the windshield in an "orange out" and I couldn't see - took less than 300 yards to lose everything the pump could get to whilst I emergency shutdown to protect the heads etc.

[JG62]

Penninsular diesel went from the the standard 21:1 to 19:1 compression ratio for their 6.2 marine motors.I believe this was because the marine motors experienced some failure when tested at top speed for extended periods.I would look for the dp article regarding it.

[britannic]

Originally posted by JG62:
Penninsular diesel went from the the standard 21:1 to 19:1 compression ratio for their 6.2 marine motors.I believe this was because the marine motors experienced some failure when tested at top speed for extended periods.I would look for the dp article regarding it. Under full load testing at 21:1 compression ratio, Peninsular Diesel found the pistons started load scuffing after just 3 hours. However, a marine engine has a very different life when compared to a truck, since we have gears to lighten the loads on the engine. If you're going to work the truck at full load for extended periods of time, then a lower compression ratio is recommended, otherwise, "drop a cog" (downshift) to lighten the load and enjoy extended mileage at 21:1 as others here have.