yowps3

Will keeping my 120hp 1.8L 4-cyl car below 2000rpm guarantee good MPG?

cino

iTrader: (1)

Under 2K is kinda low, but you will definitely improve your MPG under 2K. I wouldn't drive at that low RPM though.

yowps3

The thing is that these 1.8L eceonoboxes are so underpowered that you feel guilty when you take them above 3000rpm. They're thrashy and can't pull ******#t.

This is the curve of the vehicle I'm talking about..



I'd imagine at 2000rpm or below it just uses fuel mists

My only concern is that driving below 2k rpm the torque converter may not lock up, and that certainly will not help MPG..

geko29

From the graph, looks like you should be shifting around 3500 for best mpg. Revs much below 3000 are lugging the engine, which is very bad for fuel economy.

Depends on the gearing, but if you get a 5-700 rpm drop on an upshift, doing so at 3500 would just take you down to where the torque peak starts. Obviously when cruising at constant speed you'd want as low an RPM as possible, but when accelerating you want to be in the meat of the torque curve.

SteVTEC

You'll definitely get better MPG keeping revs below 2000rpm. It's not the torque curve that you want to look at, but rather the BSFC curve, except that these are proprietary and rarely published except in SAE white papers. Peak BSFC on most engines (brake specific fuel consumption, by the way) typically peaks on most petrol engines from what I've seen at or below the 3000rpm range. The most amount of power that can be produced with the least amount of fuel possible. Of course, keeping it below 2000 means you're hardly ever touching the gas, which is the most efficient way to drive. Yeah, trickier to do with an automatic. It's counter-intuitive, but the best mileage I ever got in my old 1999 Maxima with a 5-spd manual was using heavy amounts of throttle, but short-shifting it at some stupidly low RPMs like just over 2000 or something. Heavy throttle plus low RPMs is exactly where peak efficiency is, as you're maximizing the power the engine can make at the lowest RPM possible where the losses from friction are also the lowest. Less fuel needed floating an engine at a higher RPM than necessary or to overcome higher amounts of friction means that fuel can go into creating power instead. They're trying to build this into modern cars with the latest automatic transmissions and electronic throttle controls.

You have less control with an automatic transmission so you pretty much have to drive within the mapping of the transmission. Like when the torque converter will lockup, and at what speeds it will actually go into top gear so you can coast the best.

TangoRed

Really? I find that hard to believe given automatic transmission programming that races to the top gear in most cars with an "eco" mode.

SteVTEC

Yeah. That's not true. Keeping the engine as loaded as possible at the lowest RPM it can in the highest gear possible is precisely the way to achieve best efficiency (while under load), and is how most automatic transmissions are mapped in their "normal" modes.

spwolf

lower the rpm and lower the acceleration, less fuel you use.

yowps3

Unfortunately the vehicle I'm talking about has a 4-speed auto.

But when I drive manual I know what you mean, short shift to keep RPMs as low as possible. So much control that you just don't get with auto.
On the hwy there's not much you can do to improve MPG other than not revving above 2500rpm or so.

But around town my theory is to keep it below 2000rpm all the time.

SteVTEC

Technically the peak efficiency point of the engine in terms of BSFC might be between 2000-3000rpm if not higher on some engines, but keeping it below 2000 means you're never burning almost any fuel which can lead to better efficiency too, even if it's not the most efficient point of the engine. Yes it's quite limiting with a 4-speed auto. One thing I've come to appreciate with CVTs or 8-speed autos is that they do do an excellent job of keeping the revs planted firmly at low RPMs. There's no need to rev the engine up such that when you upshift you're still making a reasonable amount of power. The modern automatics save you from having to rev up to 3k or more if you just need to stay above 2k to make the needed power. Now they can shift at 2200-2500 to stay above 2k with a lot more gears to play with. No power is lost due to extra friction at 3000 vs 2500 or less.

tex2670

I'd leave yourself a lot of extra time to get where you are going....

bitkahuna

iTrader: (20)

guilty? bwahaha when i recently drove a 1.2L 3cyl skoda, i enjoyed thrashing the engine. sounded like a 4 stroke golf cart (with less torque ) but fun...

LexFather

One of the reasons for cars with all these gears is so they churn under 2000 RPM on the highway for cruising. Its one of the reasons the new C7 Vette with 445hp makes I believe 28 MPG on the highway. When I drove it in 7th at 80mph it was under 2000 RPM and if you floored it, it took forever to get up to any speed.

At low speeds driving around under 2000 RPM could be dangerous as the car would not be responsive at all to engine inputs. There is no way a 4 or 5 speed automatic car will be geared for under 2000 RPM for city driving. I believe the new Honda Fit makes 120hp or so but at 80MPH its at over 3000 RPM which means its burning fuel.

If your car makes 120hp PEAK, that means most of the time it doesn't and your MPG is pretty much as good as it will get. I am unaware of any 120hp car or so with 7-9 gears where you would be burning no fuel at all lol

Whitigir

I think: Cruising = lowest rpm for better MPG, Accelerating = In the mid torque range for the best Accel and MPG. That is how I drive my manual anyways

bitkahuna

iTrader: (20)

my v8 explorer is at 1900rpm at 75mph.