rear axle gear ratio

Huntindog wrote:

4x4ord wrote:

brulaz wrote:

4x4ord wrote:


Therefore if the engine is capable of making the required 100 hp at 1300 rpm it will normally get significantly better fuel economy by gearing up and slowing the engine down.

But some folks claim that "lugging" the engine at low rpms below the 1700-1800 rpm peak is detrimental in some way. They prefer to generate their needed HP at higher rpms at or above the rpm peak (and with lower needed boost).

When towing on level ground, my CTD seems perfectly happy idling away at lower rpms (~1300-1500 rpm) and with higher boost as needed. It even seems to get better fuel efficiency like you say. And I suspect the higher exhaust temperatures from the extra boost might help with passive regen. But are there other potential engine issues I should be aware of?

I believe these new computerized engines can look after themselves. There is no way you're going to hurt a Cummins engine by letting it shift into a higher gear.I disagree. Think about what is going on inside the motor when lugging it under a load... The oil pressure for one will be less, as will the flow. Is it low enough to hurt the bearings over time???? Your motor, your call. I just don't get the warm fuzzies thinking about it.
Coupled with higher EGTs, which are detrimental to valves, pistons, rings, turbo, etc.... None of this will likely fail immeadiately (depending on just how demanding the conditions are), but none of it is good.
Your motor, your call. If you have a standard transmission and the truck seems perfectly happy in the higher gear, you are not lugging the engine. I don't know much about the passive regen but what you say sounds reasonable. The higher exhaust gas temperature is due to a lower volume of air flowing through the engine for the amount of fuel being injected....another reason why better fuel economy results from lower rpm.....the engine pumps less air at a lower rpm even though the boost pressure may be running a little higher. The more air you push through the engine, for a similar amount of fuel, the cooler the exhaust will be.

Not quite correct. The boost pressure is higher as the turbo is increasing the air flow at the lower RPMs... It does this by raising the pressure of course, which increases the temperture of the air.... So actually a similar volume of air for the fuel, but hotter air.
There is no free lunch.

Do you really think the engineers are not aware of the lubrication needs of the crankshaft?

High egt can be detrimental to an engine but what is high? Temperatures under 1200 degrees are not considered detrimental. Think of the diesel engine aftermarket programmers out there. They add more fuel and increase boost pressure until exhaust gas temperatures limit the amount of power the engine can make. What does the guy that still wants more power do? He gets a bigger turbo so he can increase boost pressure, push more air through and add more fuel and still have lower egt. If your egt's are too high you would make matters worse by degreasing the boost pressure without decreasing the fuel injected.