Engine load vs rpm? (Climbing: hold top gear at very high load, or downshift?)
 

Which is better? I live in California, and I have to drive up the Cajon pass everyday (I think it's a 3% grade maybe more idk) and I can do about 70 @ 3krpm with really high engine load, my guess is somewhere around 80%, or 3.8k rpm with around 35% ish engine load. My question is, which one is better on fuel economy? Low rpm with high engine load or high rpm with low engine load?


I bought a cold air intake for my car a while back and it honestly has helped me a ton with judging engine load, I gained a 2 mpg increase once I put it on. I don't think it was the air intake itself, but me being able to adjust my throttle foot more sensitively to keep engine load down.

I have a lot more questions but I'm just starting with this one.

There's no way the load will drop to 35% going from 3000 to 3800. Unless that "load" isn't actually the correct load. Higher load is better. 100% load with engine running rich is often no less efficient than highway cruise at low load. If you don't need to downshift, don't unless the rpm is like 1000 and the engine is struggling.

Exactly the answer I was looking for, thanks. And maybe it's not 35%, I don't have the smarts to install a mpguino (at least i don't think I do) and my car is 1 year off from being able to use scangauge. It's the reason I bought my cold air intake, so I could hear the engine more and kinda try and guess where my engine load is at.

Going less than 70 would help a lot. If those people that come barreling up behind you can pass and don't like it, that's their problem.

If you can keep it in closed loop at highest load would be the best.

Here's some good discussion on the subject: http://ecomodder.com/forum/showthrea...ear-23957.html

I remember driving up that grade in an overloaded six cylinder 1955 Ford 1/2 ton at WOT in first gear, while watching the speed slowly decrease. No choice of gears on that trip. Barely made it.

I did practically the same thing up Afton Mountain in my 170 CI 63 Valient. Had to downshift to second gear, then back to third until speed dropped to 35 MPH, second to 50 then third again, rinse and repeat.

Got 28 MPG in that old Valient. Probably had the carb set a little lean.

Oh yeah, bias ply recaps :eek:.

regards
Mech

Any idea what kind of increase in mpg I will get between 70 and 75 mph?

Seventy and sixty-five? How soon do you plan on investing in an MPGuino?

You will get a negative increase. Try taking it down a few MPH.

Gas Mileage Tips - Driving More Efficiently

http://www.fueleconomy.gov/feg/image...pg_2012_sm.jpg

Mpg For Speed - Fuel Efficiency Vs. Speed says: "You'd be surprised to learn that a slight decrease in your highway driving speed can significantly reduce your gas consumption, while only adding a few minutes to your travel time. "

They have a calculator you may find enlightening. Plug in your MPGs, miles driven, and the price for a gallon of gas, and it will show you the difference, in dollars and minutes spent, across a range of average speeds.

good question
 

See lots of differing opinions too.
Looks like one for Myth busters or an rockett scientist.
But I go both directions here and say it depends upon the vehicle, speed, grade, and length.
Ive traveled that same section plenty in the past with two distinctly different cars; first 84 SVO mustang with turbo and 5 speed.
It would climb effortlessly most of the way with no change in the R foot @70but there was a banking LH turn that always sent Turbo boost higher thus more fuel required or I could travel slower and get same reaction before that point and have more loaded engine time, diddnt matter which really never noticed true economy difference.
Second car was 71 Coupe Deville with HIPO built 500 CI and turbo 400 trans. This vehicle could cruise up 1/2 way @65 mph with a feather on the go pedal.
But still chug to top @55 when it dropped a gear still virtually unloaded.
But let someone stick that Caddi behing thier R/v and put it at 45 mph and It would Roar sucking the hood down in second the whole way up.:mad:

Unloaded rpms do best IMHO most cases.
Use your pedal to tell ya how much fuel your using I guess.

I meant going from 75 to 70. To answer your other question, I would buy it right now if I had even the slightest idea how to hook the darn thing up. I want one really badly to just give me that last edge but like I said, not the slightest clue where to hook it up to....

vehicles are usually most efficient @ peak torque. find out which rpm that represents and
drive close to it. while climbing i would prefer going a bit higher especially in a vehicle of older technology. of course you want to keep your engine from melting so higher rpms are favorable to keep your water pump moving. once you get that hill mastered try the "tejon pass" ;~>