Gas, Diesel, Gears and Towing
Help me to understand the differences. Why can three trucks that are identical other than engines and transmissions (understanding that a diesel truck will have a higher GVWR and a lower payload becau...
Forum Discussion
Torque at the engine crank shaft is modified by the gearing...but not just the rear diff but also the transmission gear ratio. Depending on which gear you are in, this translates to a torque at the wheels. A lower transmission gear will result in higher torque at the wheels.
Diesel engines operate at roughly twice the compression ratio, so they can generate a lot more torque at the crank shaft...especially at low RPM. Therefore, even though truck C has a higher geared rear end (lower numerically), it can still get more torque to the wheels (assuming the same transmission gearing).
With modern 8-10 speed transmissions, the rear diff has less impact unless you are pushing the truck to the limits, as there are more options for the transmission to pick the best gear to generate the required torque at the wheels (so you are seeing fewer rear diff options and a higher gear ratio more often because the transmission can adjust to what is needed)
Now torque just determines if you can climb the hill. Put a low enough gear in and you could drag the QEII up Pike's Peak...just not very fast because RPM has a maximum value.
HP is what determines how fast you can climb the hill (assuming you have enough torque in the right gear to keep it moving). This creates a myth that a 300hp diesel will out tow a 400hp gas engine. HP = Torque * RPM.
- With the diesel, it generates a lot of torque but has a much lower peak RPM.
- With the gas, it's happy to turn much higher RPM. You might hit peak HP around 4000rpm. People used to cruising around in trucks with empty beds aren't used to running at such high RPM and assuming (falsely) that it will damage the engine. As long as there isn't a preexisting problem, modern gas engines will not be harmed. It will tend to be a bit noisy, so if you frequently climb long steep grades, aesthetically it's displeasing.
One deviation from this for climbing mountains is naturally aspirated engines (diesel or gas) are at a sever disadvantage compared to turbo engines (diesel or gas). You lose about 3-4% of HP for every 1000ft of altitude due to thinner air. So at 10000ft, you are down 30-40% on power with a NA engine. A turbo engine on the other hand force feeds air resulting in minimal power loss at altitude.
The GVWR will generally be the same for an otherwise identically spec'd truck. But since the diesel weighs so much more, the PAYLOAD will typically be lower. Often, it is the payload that limits the trailer size, particularly for 5th wheels. 20% of a 15000lb 5er, is 3000lb pin weight...add in passengers, fire wood and other gear and a lot of 3/4ton trucks are over payload even if the tow rating is OK.
As far as stopping and handling, really doesn't make a lot of difference but make sure you are comparing apples to apples (Base 3/4 ton 4 door vs max payload 1 ton dually standard cab are not comparable). Once otherwise identical, it's about brakes and suspension. There is also a registration fee aspect that sometimes comes into play. Above certain GVWR, may result in different registration fees.
Now if you start looking at which option provides the ultimate highest CGVWR (combined total truck and trailer), it will usually be a diesel with a low geared (high numberically) rear diff as that gets the most torque to the drive wheels.
Diesel engines operate at roughly twice the compression ratio, so they can generate a lot more torque at the crank shaft...especially at low RPM. Therefore, even though truck C has a higher geared rear end (lower numerically), it can still get more torque to the wheels (assuming the same transmission gearing).
With modern 8-10 speed transmissions, the rear diff has less impact unless you are pushing the truck to the limits, as there are more options for the transmission to pick the best gear to generate the required torque at the wheels (so you are seeing fewer rear diff options and a higher gear ratio more often because the transmission can adjust to what is needed)
Now torque just determines if you can climb the hill. Put a low enough gear in and you could drag the QEII up Pike's Peak...just not very fast because RPM has a maximum value.
HP is what determines how fast you can climb the hill (assuming you have enough torque in the right gear to keep it moving). This creates a myth that a 300hp diesel will out tow a 400hp gas engine. HP = Torque * RPM.
- With the diesel, it generates a lot of torque but has a much lower peak RPM.
- With the gas, it's happy to turn much higher RPM. You might hit peak HP around 4000rpm. People used to cruising around in trucks with empty beds aren't used to running at such high RPM and assuming (falsely) that it will damage the engine. As long as there isn't a preexisting problem, modern gas engines will not be harmed. It will tend to be a bit noisy, so if you frequently climb long steep grades, aesthetically it's displeasing.
One deviation from this for climbing mountains is naturally aspirated engines (diesel or gas) are at a sever disadvantage compared to turbo engines (diesel or gas). You lose about 3-4% of HP for every 1000ft of altitude due to thinner air. So at 10000ft, you are down 30-40% on power with a NA engine. A turbo engine on the other hand force feeds air resulting in minimal power loss at altitude.
The GVWR will generally be the same for an otherwise identically spec'd truck. But since the diesel weighs so much more, the PAYLOAD will typically be lower. Often, it is the payload that limits the trailer size, particularly for 5th wheels. 20% of a 15000lb 5er, is 3000lb pin weight...add in passengers, fire wood and other gear and a lot of 3/4ton trucks are over payload even if the tow rating is OK.
As far as stopping and handling, really doesn't make a lot of difference but make sure you are comparing apples to apples (Base 3/4 ton 4 door vs max payload 1 ton dually standard cab are not comparable). Once otherwise identical, it's about brakes and suspension. There is also a registration fee aspect that sometimes comes into play. Above certain GVWR, may result in different registration fees.
Now if you start looking at which option provides the ultimate highest CGVWR (combined total truck and trailer), it will usually be a diesel with a low geared (high numberically) rear diff as that gets the most torque to the drive wheels.
