Curious about Tachometer Indicator and Horse Power


#1

Hello all:

I am trying to understand the relationship between RPM and Horse Power (HP). In my '97 Lexus ES 300 (200 HP), while going at 70 my RPM showed 29/3000. On my '04 Acura MDX (265 HP), while going at 70 my RPM shows about 2000. While on my recent Infiniti EX 35 (297 HP), while going at 70 my RPM shows 26/2700.

What, if any, is the relationship between HP and the tachometer showing the RPM? I thought (based on my Lexus and Acura) that higher the HP lower the tachometer reading at higher speeds - but obviously I am wrong (based on the Infiniti EX 35!). Any light you can shed on the subject would be appreciated. Thanks.


#2

RPM at any given speed is based on the gear ratios driving the wheels. If you have high enough gearing, and maybe an overdrive on top of those, you can get the RPM down to somewhere just above idle at 56. If I’m not mistaken, for some bizarre reason, that was the speed chosen for long distance mileage testing (well, that WAS years ago, anyway). That was for the CAFE numbers, I’m pretty sure.

HP is…well…power, but the relation between speed and horsepower doesn’t really exist. The most fuel efficient speed you can drive is the one where the motor produces the most horsepower, at the lowest RPM.

Feel free to correct as appropriate. :slight_smile:

Chase


#3

Overall gear ratios is more determined by engine displacement than horsepower.
For example, A Yamaha R-6 sport motorcycle makes more horsepower with its .6 liter engine than a Harley Davidson does with its 96 cubic inch engine but the big Harley V-twin goes down the highway with the engine spinning around 2600 rpm while the little sport bike’s 600cc, (36.6 cubic inch) engine is likely spinning around 5000 rpm.

Power is torque X rotational speed

Torque (in ft-lb) X rotational speed (in RPM) / 5252 = horsepower

Torque (in newton-meters) X rotational speed (in radians per second) = watts


#4

Peak horsepower normally occurs just before the engine’s redline–the level of RPM’s where most tachometers change colors from white to red and the auto transmission shifts when you “floor it”. Typically near 6500 rpm for most vehicles.

I found this article by searching “horsepower and torque curve graphs”.


#5

Also, all cars’ engines when cruising at 70 mph are putting out a small fraction of the maximum power that engine can produce at its peak horsepower rpm, and a small fraction of the maximum power it can produce at the rpm you see when cruising at 70 mph. Only at wide-open throttle is there a definite relationship between rpm and horsepower.


#6

Thank you all. Very interesting information…while I am trying to digest this information and make sense of it I have another question that arises from what I read from all your inputs. Referring back to my original question, is my Acura MDX more efficient than my Infiniti EX35 based on my understanding of “chaissos” and “B.L.E.” s’ responses above.


#7

Nope, efficiency has to do with burning the least amount of gas to accomplish a given amount of work. A rough measure would be the mpgs you get with each, but that’s different than efficiency. A semi may be operating very efficiently while pulling a 20-ton trailer, but still get less mpgs than your MDX. MPGs work when comparing similar vehicles.


#8

Thank you texases…however, does a higher RPM not indicate greater gas consumption? A higher RPM means greater rotations (of the crankshaft?) which would essentially require gas…Or am I missing something? I do not mean to be argumentative…just trying to comprehend.


#9

You’re missing the fact that, except at wide open throttle, much less than the maximum amount of fuel and gas are pumped through the engine at a given rpm. So just rpm really has no direct relationship to efficiency, especially when comparing different engine designs.


#10

“What, if any, is the relationship between HP and the tachometer showing the RPM?”

No fixed relationship.

You can rev your engine to 3000 rpm in neutral and your engine will be delivering 0 hp,
not counting power to overcome engine/trans losses.
Just takes a small nudge of the throttle.

Operating between that and full load & full throttle the hp vs rpm is … whatever.


#11

For any given engine, its horsepower will vary with RPM, there being a “sweet” range where it’s operating at its best. Every engine has its own “power curve”. No two are the same. Where within that range a design group chooses to put 70 mph, that being a result of gearing, how many gears, and wheels and tires chosen, will vary depending on specific design goals.

In short, your engine would probably be comfortable maintaining 70 mph in your car anywhere between 1,500 and 4,000 rpm, with modest differences in longevity. But one choice of gear ranges will produce faster acceoeration with a lower top speed of 130 while a different set of choices will produce slower acceleration with a top speed of 200 mph.

And the designers also consider weight and slipperiness (coefficient of drag, Cd) which is your particular car’s “multiplier” used to calculate wind resistance.

In short, the relationship between HP and RPM just isn’t that simple. The good news is that it’s important for the designers but you need not worry about it whatsoever. Just maintain the vehicle properly, don’t abuse it, and it’ll live a long happy life…


#12

“Thank you texases…however, does a higher RPM not indicate greater gas consumption? A higher RPM means greater rotations (of the crankshaft?) which would essentially require gas…Or am I missing something? I do not mean to be argumentative…just trying to comprehend.”

What you’re missing is Manifold Absloute Pressure (MAP). The amount of fuel consumed is proportional to the amount of air pumped through the engine. The amount of air pumped through the engine is proportional to the displacement of the engine (cc or cu in), times the RPMs, times the MAP (in fractions of an atmosphere).

Now you can make 70HP at (say) 2500RPM in overdrive, or 3500 in 3rd gear. If you dropped it into 3rd, your MAP would decrease. Your overall fuel economy would (probably) lower, due to higher internal engine friction at the higher RPM.


#13

I’d like to correct myself:

HP is…well…power, but the relation between speed and horsepower doesn’t really exist. The most fuel efficient speed you can drive is the one where the motor produces the most horsepower, at the lowest RPM.

That should have been “Torque”, and it’s not really fuel efficient, it’s just regular efficient in terms of power produced.

This stuff hurts my brain. :slight_smile:


#14

The attached are the definitions of horsepower.

Each individual engine will produce a horsepower “curve” as clearly illustrated by mleich’s link. And every engine is different. As a matter of fact, some engines peak well below the maximum safe rpm of the engine known as the “redline”. Race engines’ curves typically continue to increase in horsepower beyond their redlines, which are generally far higher than yours engine’s redline.

In referring to an engine, horsepower is often referred to as “XXXhp at 5,000 rpm”. Or, if you’re reading a British magazine, you’ll see “bhp”. My link explains the differences.


#15

As another example of why RPM is not an indication of how much fuel you’re using, consider the following scenarios involving a 2 year old car:

  1. Tach says 4,000rpm. You’re in 5th gear, cruising at 80mph with a 30mph tailwind.

  2. Tach says 4,000rpm. You’re in 5th gear, cruising at 80mph with a 30mph headwind.

  3. Tach says 4,000rpm. You’re in 5th gear, cruising at 80mph while descending a hill that’s so steep you don’t even have to keep your foot on the accelerator.

In all of the situations, the RPM is the same because 5th gear at 80mph in our hypothetical car will ALWAYS result in 4,000rpm. That’s already been discussed.

In situation 1, the 30mph tailwind means you don’t have to use as much throttle to maintain 80mph, and so you use less gas than you do:

In situation 2, where the 30mph headwind means you have to press harder on the accelerator to stay at speed against that wind.

And in situation 3, since you’re in a newer car, the engine computer actually shuts off the fuel supply as you’re going down hill because the wheels keep the transmission turning, which keeps the engine turning, and so no fuel is needed to keep the engine running, which means your mpg is, until you get to the bottom of the hill, infinite.

The best way to tell if your Acura is more efficient than your Infiniti is to let the experts do the figuring for you. Check the EPA fuel ratings for each car. The MDX has a rating of 15 city/ 21 highway, while the Infiniti gets 16/23. So overall the Infiniti is a more efficient vehicle (but not by a whole lot).


#16

Horsepower depends on engine RPM…The more RPM, the more horsepower. Horsepower is what makes your car accelerate. Torque, a major component of horsepower, (RPM X Torque = horsepower) is what maintains your cars speed at 70 or any other figure. Think trucks Vs race cars…


#17

The more RPM, the more horsepower

Not exactly:

That’s a HP (and torque) curve for an engine. You’ll notice that once the engine goes over 7,000RPM the horsepower drops off a cliff. That “sweet spot” others have talked about is basically a rough bell curve. There’s an RPM point (differs for each engine design) where you have maximum horsepower. Anything to the right or left of that point is going to be less.


#18

The dyno graph that shadowfax put up was also measured with the throttle fully open and is only relevant for full throttle operation.

As you close the throttle, the torque and horsepower “sweet spot” tends to move to a lower rpm.

This chart doesn’t display torque, however, on the bottom you see something else, the engine’s specific fuel consumption in lbs of gasoline per horsepower hour. Notice how inefficient the engine becomes at high rpm/small throttle openings as the specific fuel consumption goes to the moon at quarter throttle above 3000 rpm. That’s what overdrive gearing is for, to keep the engine out of that zone when you are cruising along at a speed that only requires about 8 horsepower to maintain on a level road.


#19

Shadowfax has it right…higher RPM does NOT equal higher HP…It would if torque was linear…it’s NOT…But if you drive like MOST people SHOULD…where you drive at a reasonable speed and not red-lining the engine…then yes higher RPM should equal higher HP.


#20

You mean I’m not SUPPOSED to redline the engine? Go figure. I guess I’ll have to change my driving habits…