Diesel vs gas......................

azwildcat99 wrote:
HP = Torque * RPM/5252 So unless you are revving your engine up above 5200RPMS or racing F1 at 19k RPMS, HP is largely irrelevant. It's the torque that matters. Diesel or Gas your TV makes more torque than HP at the engine speeds we are pulling at, any of us. Given that, a diesel usually has superior figures (500+ vs 300-400).

Running below 5252 RPM only means the arbitrary units of HP are lower than the arbitrary units of ft-lbs. You can't compare those two numbers as is and it's only by coincidence that they are even similiar in an average engine. We can just as easily use kW instead of HP to describe engine output power. (1 HP = 0.745699872 kW).

Again, a torque spec is USELESS without including the RPM it can produce that at. Without the RPM component you cannot make anything move. If the RPM at which an engine produces all its torque is lower than needed to turn a driveshaft to propel a truck at 60 MPH, you need to gear it up.

Here's the thing plenty of you keep ignoring: When a powered shaft is geared up (output is faster than input), the RPM goes up but the torque goes down proportionally. The HP in equals the HP out. The same is true in reverse: When the output turns slower than the input, the torque goes up while the RPM goes down. Use the equation above to convince yourself if you don't believe me.

What this means is you generate more driveline torque (and therefore more wheel torque) when running in a lower gear than you do in a higher gear. This equalizes things at the wheel for the high-rev'ing gasser running in a lower gear than the diesel. In the end, both are running the same wheel RPM to acheive the same ground speed. The gas engine just uses more of the gearing to trade its high RPM for wheel torque. The ONLY constant is HP from crankshaft to wheel where it's needed (minus some losses due to friction.)

Sure, there is a cost in both RPM and fuel comsumption for the gasser but that does not have anything to do with whether it "can" do it.

Don't misread what I'm saying here. I'm not saying diesels aren't the better tow vehicle. I'm just trying to show that it isn't simply because they have higher torque. Their higher torque is a consequence of they way they operate at a lower RPM which translates to "working less" for most people. Most diesels also have turbochargers which keeps them making most of their HP at high elevation. They're also far more efficient at using their fuel.