Quote:
Originally Posted by comptiger5000
I'm sorry, but 33hp per 1000 pounds isn't necessarily enough. To achieve this AND gain efficiency, this would mean my 4000 pound Jeep would have a 2.5 or 3 liter engine. In other words, it would have way too little torque, and would be a nightmare to drive. The 4.0l 6 cylinder (195hp / 230 tq) is underpowered in these things on the highway. Around town, the power isn't a big deal. However, at 60, it has to downshift, so it's no longer turning 1800 rpm, but screaming 2800, to maintain speed on a hill. However, my 5.9L V8 (245hp, 345 tq) with the same gearing has no trouble churning up that hill (and even gaining speed if desired) at 1800 rpm in OD with the TC locked. I've pulled hills around town at 1100 rpm at 37-38 mph in OD with the TC locked. My instant mpg reads about 9-10 doing this. With the smaller engine (4.0), it reads about 8 instant mpg turning 2000 rpm in 3rd (TC unlocked unless you manually lock out OD, then it locks in 3rd) on those hills around town.
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Gearing makes more of a difference here, as well as the fact that you're using an auto trans. The trans downshifts way before it actually needs to, in almost all applications.
IMO, it shouldn't downshift until you're at ~80% throttle and still decelerating. I used to remove kickdown cables and levers and just downshift manually for this reason.
Anyway, climbing that hill has everything to do with torque production at the RPMs you're afforded.
I have 265 lbft of torque at 2,000 RPM in my F150, and in 5th gear at 1,000 RPM, I'm ~32 MPH, IIRC.
Today, I climbed a 4% grade for over 1,000 vertical feet in 5th gear at 70% throttle, vehicle speed ~28 MPH, which is = ~900 RPM.
At one specific corner, I slowed down to ~20 MPH, which is ~500-600 RPM. If I go any lower than that, I'm still not lugging, but oil pressure starts to drop off, and I don't like that. After the corner, I never downshifted, just put it back to 70% throttle and went back to ~30-32 MPH until I got to the top of the hill.
This is in my '96 F150, carrying about 800lbs of crap in the bed, including a motorcycle and set of 30x9.5-15LT rims and tires.
Vehicle speed vs RPM vs load vs scenario makes a huge difference in whether or not you can do certain things. Believe it or not, if I'd downshifted to 3rd, I'd likely have had to slow down, because at ~3k RPM, I have nearly no torque (comparatively).
This isn't particularly the case for your 4.0, but that auto isn't really helping your case.