Is the rpm at which the engine has the most power also the rpm that gets the best fuel efficiency? In other words, if I was driving a car at a constant speed in 3rd gear and the engine was at peak power rpm, would i get better, worse, or the same fuel efficiency if I shifted up to fourth but kept the car moving at the same speed?
YACT: optimal rpm
- Thread starter Sureshot324
- Start date
depends on your engine, parts, conditions etc.....different for everybody..usually you can get a somewhat acceptable figure by driving the car a lot....mine "loves" ~2-2.2k rpm
No...
Generally, the fuel usage curve looks like an upside down C.
It's high at idle, but as you come off idle, efficiency goes up.. and peaks at around ~2500RPM. Above that, fuel usage starts to climb and goes sky high by redline.
Basically, you don't need to think about it too much. Keep the RPM as low as possible. Use 4th, that's what it is there for.
you'd get better fuel efficiency if you shifted to fourth in that situation. Because it takes a given horsepower value to keep the car moving at your speed, and the engine will still generate that power at a lower RPM. If a car engine was a reversible heat engine, I don't think the fuel consumption would change a bit between the two RPMs at the same power, but as it is the engine has more irreversibilities at a higher RPM (thus higher fuel consumption).
However, if the engine was at 100% load at both RPMs (pedal to the metal, but no acceleration), you'd get better efficiency (in terms of fuel required per HP) at the peak horsepower RPM.
However, if the engine was at 100% load at both RPMs (pedal to the metal, but no acceleration), you'd get better efficiency (in terms of fuel required per HP) at the peak horsepower RPM.
someone correct me if i'm wrong here.
for most driving, look at it this way: the engine needs to put out X amount of power to keep you moving at a steady speed. the power you need is basically what you need to overcome all the friction: air, rolling, mechanical. in 4th gear, the engine is spinning slower, so there's less mechanical friction. so you'll require less power in 4th than in 3rd. That's for maintaining a speed that your car can do without bogging down in 4th or redlining in 3rd, obviously. less power needed = less gas needed, so you'll get better mileage.
If you needed to maintain your speed going up a hill (increasing the power requirement), you could conceivably reach the point at which your power needs outstrip what the engine is capable of at that RPM. you just can't feed it enough gas to make the power because you're not exploding it fast enough. so you downshift to increase the RPM, which increases the rate at which you can explode the fuel, so power increases. but EFFICIENCY goes down because you also increased the friction. so basically downshifting just allowed you to use more gas.
for most driving, look at it this way: the engine needs to put out X amount of power to keep you moving at a steady speed. the power you need is basically what you need to overcome all the friction: air, rolling, mechanical. in 4th gear, the engine is spinning slower, so there's less mechanical friction. so you'll require less power in 4th than in 3rd. That's for maintaining a speed that your car can do without bogging down in 4th or redlining in 3rd, obviously. less power needed = less gas needed, so you'll get better mileage.
If you needed to maintain your speed going up a hill (increasing the power requirement), you could conceivably reach the point at which your power needs outstrip what the engine is capable of at that RPM. you just can't feed it enough gas to make the power because you're not exploding it fast enough. so you downshift to increase the RPM, which increases the rate at which you can explode the fuel, so power increases. but EFFICIENCY goes down because you also increased the friction. so basically downshifting just allowed you to use more gas.
But to overcome that gravitational force when you're going uphill, downshifting from 4th to 3rd will give you that extra kick. But when you shift it down to 3rd, is there a point at which exceeding a certain RPM will provide you with negative returns and thus inefficiency?
Say for example my prelude's max torque is at 5500rpm in 5th gear. Will going beyond that yield any gains in that uphill climbing battle?
On daily driving if I'm just cruising I will use 4th gear at 2000rpm instead of 3rd gear doing 3000rpm. The engine going at a faster rate requires more fuel. So for the same road speed I'd rather use 4th gear unless I anticipate that need in power.
Say for example my prelude's max torque is at 5500rpm in 5th gear. Will going beyond that yield any gains in that uphill climbing battle?
On daily driving if I'm just cruising I will use 4th gear at 2000rpm instead of 3rd gear doing 3000rpm. The engine going at a faster rate requires more fuel. So for the same road speed I'd rather use 4th gear unless I anticipate that need in power.
Internal engine friction is just a fraction of the "wasted" energy. Most of it goes out the exhaust pipe in the form of heat.
Say your car's power peak is 150hp at 4000rpm, and you get 100hp at 2000rpm. If you need 50hp to keep the car moving at a certain speed, that would be 50% of your power at 2000rpm and 33% of your power at 4000rpm, so you'd need to give it more gas at 2000rpm wouldn't you?
AgaBoogaBoo
Lifer
- Feb 16, 2003
- 26,108
- 5
- 81
Just give the least amount of gas needed to keep your speed
Remember, the gas mileage is ultimately determined by how far pressed you keep the gas pedal
Remember, the gas mileage is ultimately determined by how far pressed you keep the gas pedal
depends on many factors, engine compression, fuel (octane/type), spark(if needed), timing, valves (size, clean/dirty) fuel injectors (clean/dirty), transmission gear ratios, axle gear ratios, weight, size of tires (hight/width), type of tires (wet/dry/snow/etc. small role but important none the less) airodynamics (including undercarriage), and even to a small degree oil (conventional/synthetic). (and 5W-30 has been proven to average .5-3mpg improvement over 10W-30 of equal brand)
Toastedlightly
Diamond Member
- Aug 7, 2004
- 7,215
- 6
- 81
The viscocity of the 5w-30 of most brands is lower than thier 10w-30., resulting in less friction, less power is lost due to this and your engine becomes ever so slightly more efficient. Valvoline used to have a nice paper on this subject (and a lot more in depth that what I dug up at Penzoil), but they don't seem to have it up on thier web site anymore.
edit: I do remember that the difference also depends on how much you drive your vehicle with the engine warmed up to standard operating temperatures.
2500rpm?
Bummer, that's too hyper for me to cruise at.
Encore Puzzler Answer, 9/15/97: Do Speedometers lie?
Bummer, that's too hyper for me to cruise at.
Encore Puzzler Answer, 9/15/97: Do Speedometers lie?
- Here's the answer: Because when I looked down at the dashboard, I not only looked at the speedometer, but also the tachometer. Both of these cars redline at about 6,000 rpms. Anything over 6,000 you're in danger of having the pistons become part of the environment. At 60 mph, the BMW was doing 3100 rpms. I knew that at 120 mph it would be beyond the redline and incapable of doing 160 mph. 6200 rpms only got me to 120. So I knew I was going to be well into the red zone and probably decorating the highway with pistons. The Mustang Cobra at 60 mph was doing less than 2000 rpms -- it was doing about 1750 rpms. It turns out that we called the Ford Special Vehicle Team and they said it wouldn't do 160 mph. I was able to get it to 155...
So if 2000 to 2500 rpms is best for fuel efficiency, what do auto transmitions do? Compromise between fuel efficiency and power?
The shorter the gear ratio (IE: 3:1 is shorter than 2:1), the more torque multiplication, but also the more engine RPM required for a given vehicle speed.
The torque multiplication is why your car accelerates faster throguh first than it does through second, faster through 4th than through 5th, etc, over the same engine RPM range.
Your Prelude's max torque to the wheels (which is what accelerates the car) actually happens in first gear at your engine's torque peak of 5500rpm. This is because of the greater torque multiplication through gearing in first gear as opposed to 5th gear. This is why 90% of the cars out there are better off redlining through each gear to achieve maximum acceleration as opposed to shifting before redline just to enter the torque peak in a lower gear while losing the great benefit of torque multiplication through gearing.
As an example: the reason why an Acura RSX Type S is only as fast as it is because it has a high revving engien (8krpm) and a well matched 6 speed gearbox. I mean, my car puts out more torque to the wheels than the S does to the crank, but because it can rev to 8krpm and has better gearing (and hence, more torque at the wheels), it owns me after first gear. For reference, I'm putting out 145lb/ft
I won't go into it anymore unless you want me to, but that's the gist of it. It's always better to be able to rev higher because you can take advantage of your gearing better. A Big Block 454 Chevy revving to 8k rpm would be an absolute screamer in any car you toss it into.
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