dezl-andy

will a 4:10 geared truck last as long as a truck with 3:55s?
seems like it would to me, but some people argue it doesnt. im just courious becase im thinking about changing to 3:55s

peobryant

Yes, they will last just as long.

92'1stGen

No it won't matter, these things are built to work and be run 24/7 near redline.

dezl-andy

well it just seems like to me that an engine that runs 2200 all the time would have alot less life in it, than one that only turns 1700 all the time.

carl48

there are trade off's the higher torque on bearings at 1700 offset extra revolutions. both will last well over a million miles with no difference in signs of wear. i have drivers and trucks to prove it

Hvytrkmech

Depends on how it is driven. It is well known that rpm's are key to longevity. The faster the engine spins the shorter the lttd,(life to tear down). The slower it spins the longer the lttd. Most diesel engines are made to operate right around their peak torque curve, not peak horse power. The ISB in our Dodge trucks will turn 3200 rpm at wot. A medium duty ISB will only turn 2,750 at wot.

96_12V

This begs a question I've had for some time (not to steal the thread). I had a 1995 4wd AT for six months prior to 96_12V which has the NV4500. With the 47RE AT I noted that at 70 mph it turned around 1925 rpm or so. Whereas with the 5-speed my truck always turns around 2050 to 2100 rpm at 70. Do I have different rear ends (and where is that info found) or is that the gearing in the transmission?

Thanks for the help...

dezl-andy

no the overdrive ratio is different in the 2 trannys. took me awile to figure that out also.

D2 Cat

Unless it's been removed, there should be a tag on one of the bolts on the differential stating the gear ratio.

You can also jack the rear end up, mark the driveshaft and a tire and turn and count and determine the ratio.

Could be tire size has been changed or tach not reading correctly to cause a mis-reading also.

rich

no its the gearing in the tranny, unless your gonna run giant tires or haul loads up and down steep grades get the 3:55 you will be happier, had one with 4:10' and will necer have another geared that low

Dartmouth 12V

The driver has much more to do with it than the gearing. Yes, higher rpms mean more fatigue cycles but it also means less stress per cycle. If the two engines ran on the dyno, at partial load the lower rpm engine will last marginally longer. It is important to realize that lugging is worse for the engine than revving it up because your stresses greatly increase.

The engine will most likely outlast the truck anyways.

rich

yep 5 out of my 7 the truck gave out before the motor 03 and 07 were motor issues

supr

There are trade offs to each approach. My rig with 4.10's will run 1700 RPM in 6th gear, at 55. This is fairly optimal for pulling my fiver. If I had 3.55, I would be running something like 62-65, and proabaly taking more fuel and higher exhast temps to do it.
I am sure their are diffrrent points of view, but for heavy hauling,, especiall Gen 2's, the 4.10's , I think, are OK.

kingofdodge7131

How do higher rpM reduce torque stress? Seems to me that givin your in the same gear. Its gonna take same torque to move load. Basically. That if your pullin 5k in 5th gear the cyl has to push just as hard at 1500 as 1900

Dartmouth 12V

Higher rpm puts less stress on the engine because there are more revolutions of the engine to do a given amount of work. This means that each combustion cycle needs to do less work so less fuel is injected and the stress is lower. However, because you do more cycles, that affects fatigue.

The same thing is true with exhaust gas temperatures. When you downshift, you need to inject less fuel per event to get the same rear wheel torque(it isn't engine torque that matters) so you run leaner and cooler.

A lot of people do not understand the difference between rear wheel torque and engine torque. Because we have transmissions, there is no direct correlation between these two things. For a simplified example, compare an engine turning 2k rpms and outputting 400ft-lbs of torque with a 2:1 reduction to another engine turning 1k delivering 500ft-lbs direct drive. The engine turning 2k rpms is actually outputting 8/5(2*400/500) as much torque as the one outputing more torque at the crank because of the gearing even though each vehicle is running down the road at the same speed.

If you want to get the rear wheel torque, you need to take the crank torque, multiply by your transmission ratio, differential ratio, and the radius of your tires. If you do this, you will get a number in units of pounds which can be thought of as the force pushing the truck ahead. Torque is a very misleading number on its own without knowing gearing and tire size.