The effect of horsepower on gas mileage

[Matt Timion]

I've heard this many times, and I can't confirm it. Please help.

When I first started planning to increase the fuel efficiency of my car, some people said that race builds and economy builds are almost identical. Their theory behind this was that more horsepower means the engine works less to get to a certain speed, and this in turn saves on gas.

Made sense to me.... sort of.

I mean, if I threw a turbo in my car, or made some other modification that gave me an extra 30 HP, would that mean I could travel the same speed with lower RPMs? It seems to me that is the same way the long geared transmissions work, which give very high gas mileage.

So it makes sense, if it's true, except for the fact that the guys with super "fast and furious" cars only get like 20mpg. It can't entirely be due to the transmission.

Some people have claimed that higher HP equals higher gas mileage for this very reason. it is the exact thinking behind advertisements on Cold Air Intakes, racing headers (exhaust manifolds), and other performance modifications.

So, is this true?

If so, what is to stop me from getting a junked up CRX (very light) and doing the Individual Throttle Body conversion? (the ITB conversion will use a throttle body from a Honda motorcycle and use it for each cylinder, allowing for more percise airflow and claims to increase HP by 30). Will such a modification give me better gas mileage?

And lastly, if more HP means better mileage, why do all of the gas mileage beasts (prius, insight, CRX HF, Geo Metro, etc.) all have tiny little engines with no power?

[SVOboy]

Yessir, I have heard this also. It's silly.

Think about it this way:

For a crx with a b16a in it, it takes a light touch to get it to go 0-60 in 10 seconds. But for a crx with a 62 hp hf motor, you'll have to floor it to redline and still not make that time.

This is the theory of those who say more hp is equal to better mileage. You won't have to beat the hell out of your b16 to get up to speed as fast as you want like you would an hf. But if you go easy on the hf and don't try to accelerate like you've got thrice the hp you really do you'll get better mileage. The DOHC have a hell of a lot more moving parts from the added cams and blah from what I understand, and they should be losing a lot of potentional energy to heat.

Then you have to get at this idea: It only take our cars like 10 hp to overcome wind and rolling resistance to stay at 65 mph. If my car is putting out 62 max and only using 10, I'll be wasting a lot less than if I was putting 200 hp to the wheels and I only really need 10 hp.

I think the exhaust widenening might help us for mileage, but I'm not sure. CAI shouldn't, but more flow should, and more flow hot also. And from what I understand, most turboed cars use longer gearing also so they can give their turbo time to spool up before shifting, especially if it is a larger turbo.

I think that's it. More hp doesn't help, only people who have got a lead foot and don't won't to change their driving habits. That being said, your vtec-e has 50% more hp than a hf motor, but prolly will get better mileage. Technology can mean a lot, but a d16 versus a d15 with like a 20 hp difference the d15 will win in the mpgs.

[Matt Timion]

let's say we're not comparing apples and oranges.

For the sake of arguement, let's not compare a d15 with a b16, as they are entirely different. let's compare a d15b2 that is stock with a d15b2 that is modified to get more HP. Let's say both engines are MPFI and the only modification is the ITB on one engine.

It would take less work for the engine to get the same speed (when compared to the other engine). Wouldn't this equate to better gas mileage?

Or does this line of thinking only work for older non-computerized non-fuel injected engines?

[SVOboy]

I think the ITB deal works by allowing more efficient burn of the gas, and if so, better gas mileage I should think. The same would be true if we both put big *** exhausts on our b2s. But turbocharging will not help, because when you actually are boosting you're throwing a lot more gas in there, and the rest of the time you're just using your exhaust to push around a turbine. I think that preformance mods that do not increase gas flow through the injectors should be the rule for good mpg.

[Matt Timion]

At least that's what I just read on the internet.

Torque helps gas mileage. Horsepower does not. This actually makes sense considering diesel engines get much better gas mileage than gasoline engines.

Now, can anyone give me the physics telling WHY torque increases gas mileage and horsepower does not?

Are there any tricks to increase torque?

[SVOboy]

That's very interesting, torque. I'll look up some stuff and see what I can find.

By the way, where'd you hear that? D-series?

[Flatland2D]

This might be the connection I was looking for with torque and mpg.

Lets say you have two identical cars traveling at 65 mph in a vacuum except one is producing little torque and one is producing a lot of torque. Now suddenly turn on wind drag and the car with less torque will slow down more than the car with more torque. This is assuming a fixed throttle position throughout the experiment. However, the car producing more torque might be using more gas in the process, so it's not necessarily true all the time.

But think about this. Wind drag is the only thing preventing you from getting insane fuel economy. In a vacuum the only time you'd ever even need to burn gas would be to accelerate. Torque is all that's required to overcome this force. It's a good thing.

[dfoxengr]

hp is a function of torque.

[Flatland2D]

Horsepower is also a function of rpm, so it's possible to have high torque and low horsepower.

[SVOboy]

Originally Posted by Flatland2D

Horsepower is also a function of rpm, so it's possible to have high torque and low horsepower.

The new tdi engine is something like 92 hp, 100+ torque, and gets 48 mpgs. So really, I think torque must be important to getting the car going without burning so much gas, but it's hard for me to think how to improve torque very much in the 2300- rpm range where I usually am.