mythical numbers to avoid

[Christ]

I'm just far south of it right now... there's no way it takes even close to 265 lbft to move that truck at 60 MPH. Not even close, considering that it takes less than 20% throttle on a flat to maintain my speed. I can pull a 4-6% (According to the last land map I saw) grade at 20-25MPH in 5th gear without downshifting, as long as I don't let the RPM's drop much below ~800. I usually pull that hill in 4th, though, just over 1,000 RPM.

For some reason, I feel most comfortable with the truck at ~1,000 RPM.

I think I could get a pretty nice gain by halving my cruise RPM, though, especially if I use the splitter to accelerate and skip gears. I can keep the engine loaded better if I skip gears, and having the splitter will keep the engine loaded better under acceleration (not that I do that much of it).

My peak BSFC (theoretically) is probably going to occur somewhere around 80% load at or slightly above 2,000RPM, provided everything is working correctly. I can guarantee that at less than 20% throttle, I'm so deep into the area under the curve that I'm pretty close to running rich, despite the OBD-II system installed, and the EEC-V ignition control system.

Under the average circumstances, I think you're pretty well correct, though.

[JeepNmpg2]

I mostly agree, although I think I have had some marginal improvement with having slightly larger tires than OEM (1.7 inches larger diameter) which lets me cruise a bit faster and be at the engines peak range for FE. So far everything is good to 68, but as soon as I cross over to 70 MPH, FE starts diving dramatically... guess it's just time to move to the aeromod side.

[Christ]

I've got to do some improvements to my truck, both in aero terms, but only light modifications there, and also in terms of things that load the engine.

I may have a manual steering box sourced for it, which will eliminate the PS pump, I may have an electric fan w/ a thermo control sourced as well, which eliminates the fan from engine drag, and allows a faster warm up time. I'm going to be leaving the AC installed, though, on this one. It works perfectly, and there have been days where I could use it, especially if I have to head any distance south or into a city for any length of time during the hotter days.

Those things should help out a bit, especially when I have to accelerate.

[Christ]

I just happened across something by COMP (of course, I can't find the URL again) that claims that their "RV" grinds increase low-end torque and improve MPG over the OE ratings, as well... Would be great to get the best of both worlds, and since my HP/TQ peaks are so close together, I should see an improvement in HP as well.

How many times can you say you got more power, torque, and MPG?

[JeepNmpg2]

What in the world are "RV grinds"?

[comptiger5000]

The key for those of us with big engines is to make the engine itself more efficient. By doing this, it can develop more power when needed, and also get better mpg.

An RV grind is meant to give more low-end torque, for moving a heavy vehicle off the line.

Also, after thinking about it, with a properly geared transmission (not CVT), you would get maximum acceleration by shifting at, or just past, the horsepower peak, and having the RPMs fall back to the torque peak after the shift.

[Christ]

Originally Posted by comptiger5000

The key for those of us with big engines is to make the engine itself more efficient. By doing this, it can develop more power when needed, and also get better mpg.

An RV grind is meant to give more low-end torque, for moving a heavy vehicle off the line.

Also, after thinking about it, with a properly geared transmission (not CVT), you would get maximum acceleration by shifting at, or just past, the horsepower peak, and having the RPMs fall back to the torque peak after the shift.

Maximum acceleration occurs when the area under the curve between the shift point and the pickup point (new gear engagement) are nearly equal on both sides of the peak.

Dropping back to your torque peak won't get you maximum acceleration, unless your HP and torque peaks are within 1,000 or so RPM of each other. Gears designed for quickest acceleration seldom are desirable for high maintained speeds, and the same goes for the other way around... high maintained speed will require gearing that isn't conducive to the quickest acceleration. That has nothing to do with torque and HP peaks, though... it's only got to do with redline and RPM shift between gears.

[dcb]

yes, that is basically how you accelerate quickly, but not how you accelerate efficiently.

To accelerate efficiently (in a general sense), you have to know your engines bsfc map, and keep the engine load right (about 80% very roughly) and center your shifts on bsfc peak rpm, say 2500 rpm but do your own homework (and post a pic and some details if you find a map for your vehicle) Brake Specific Fuel Consumption (BSFC) Maps - EcoModder

Cruise is the same philosophy, you want to be at the bsfc peak, but most vehicles have engines that are waay to big for optimal cruise, so various detuning strategies might help (i.e. advance the intake cam)

[comptiger5000]

Technically, that's correct. However, in stock form, most of these larger engines have pretty close peaks. My torque peak is at 3200, HP at 4000, with plenty of guts down low as well.

Regarding the gear thing, I'll agree that while low gearing helps acceleration, it's brutal on the top end. My Jeep rips 0-60 in 6.8 seconds, but runs a 1/4 mile in 15.2 at about 90. Once you break 70 or so, and it's out of 2nd at WOT, it's done as far as moving fast. Fortunately, gas mileage prevents me from going that fast anyway.

[Christ]

Yeah, I'm able to get Gerald up to speed pretty quickly, if not for the crappy, notchy gear changes from the M5R2 tranny. They eat up at least 2 seconds from 0-60 where I'm not really accelerating at all.

Someday, I may install a smoother, quicker shifting transmission. This one's even a PITA to shift w/o the clutch, it's so finicky... wants exactly the correct RPM for the gear, or you get a "nick nick nick" noise on the way into engagement. Never tried it after revving over 3k, but I assume it'd be the same, only worse.

My shifting method is about as quick as it gets with a manual transmission, too. Out of gear when you let off, clutch as soon as you're off the accelerator completely, and by the time the clutch has released, you're already in gear and letting the pedal back out.