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.

[Matt Timion]

I just had a funny thought. What if someone marketed a fuel additive called "Torque Boost" or something like that?

"Guarenteed to increase your engine's torque by 10%!"

Just wait, it will happen now that I've said something about it.

[dfoxengr]

thats because most diesels are either matching or beating the HP numbers with Tq

[dfoxengr]

Originally Posted by Flatland2D

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

hp= f(rpm), f(tq)...at the same time....

HP = (RPM * TQ) / 5252

[Flatland2D]

That's what I said.

[Jonathan_ED3]

The TQ vs HP argument is completely proven when you look at the CRX HF.

If you look at its powerband, it has the powerband of a diesel for the most part.

Loads of TQ (relative) at a pretty low RPM.

[Matt Timion]

Is this something that can be done with ECU tuning?

[Jonathan_ED3]

I believe it's more a function of the individual characteristics of the HF engine.

I'm sure ECU tuning plays a large part, though.

[diamondlarry]

I think the camshaft is what basically determines the torque characteristics of an engine.

[Matt Timion]

Originally Posted by diamondlarry

I think the camshaft is what basically determines the torque characteristics of an engine.

you're right. A long time ago bagpipe goatee told me to swap out the camshaft from a civic STD if I wanted better gas mileage. This could be why.

(the STD was the fuel efficient hatch civic of the time)

I'm still reluctant to do any major modification to my engine, as it's build for economy as it is. This isn't to say, however, that other people cannot benefit from a camshaft swap.

[dfoxengr]

lots more than just a cam affects power, lol

[Ted Hart]

Originally Posted by Flatland2D

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

IF HP=(T x RPM) / 5252 ... let's assume we want the HP at 5252 RPM...
Shazam! The 5252 cancels out the 5252, and we have HP=TORQUE! This points out one concrete point : HP sells cars , torque moves cars!
Torque is from classical physics (a force applied through a lever arm), while horsepower is derived from a typical horse turning an imagined turnstyle at some constant speed (RPM) for a measured time. Quite unscientific! And crude!And quite old! Give me torque anyday! -whitevette

[maxc]

Originally Posted by Ted Hart

IF HP=(T x RPM) / 5252 ... let's assume we want the HP at 5252 RPM...
Shazam! The 5252 cancels out the 5252, and we have HP=TORQUE! This points out one concrete point : HP sells cars , torque moves cars!
Torque is from classical physics (a force applied through a lever arm), while horsepower is derived from a typical horse turning an imagined turnstyle at some constant speed (RPM) for a measured time. Quite unscientific! And crude!And quite old! Give me torque anyday! -whitevette

Given to cars. One with 400hp,500Ft torque. One with 450hp,450ft torque.They both weight the same traction best gearing. More
hp wins 1/4 mile. In theory.

[omgwtfbyobbq]

Torque is a function of how much fuel is being combusted and some efficiency, and horsepower is torque at some rpm. Gasoline engine efficiency is, among other things, a function of displacement. A corvette and Insight are not that far off in terms of CdA and weight, but the vette get's crap for mileage due to it's large displacement engine having much more in the way of pumping losses at some given output.

Generally, manufacturers have increased power by increasing displacement, which increases pumping losses at some given output and reduces efficiency. This is why GM adopted cylinder deactivation. It allows increased engine efficiency via decreased pumping losses at some output, while still having the other four cylinders available for peak power. At maximum output a gasoline is only 10-25% less efficient than a diesel, but since the large majority of use is at less than maximum output, efficiency suffers due to pumping losses.

This is why Pulse and Glide is so effective for the Prius. By accelerating up to some speed, the driver is effectively minimizing pumping losses by maximizing output. Faster acceleration implies more fuel, and more importantly, more air, which increases the pressure in the cylinder during the intake stroke and reduces the negative work done by the pressure difference between the cylinder and crankcase. Anyway, by breaking up driving into an acceleration phase, and engine off deceleration phase, with the high/low speed window centered on some average speed, the driver will see an increase in FE compared to using the cruise control at the same average speed.

Now, a question I have is. Why don't hybrids just have 30hp engines with battery packs that allow for ~40-50 miles all electric? They'd get something like 80-100mpg in normal driving, with all the kickass acceleration an electric motor can provide. Well, I think this has to do with the manufacturer's unfamiliarity with battery packs and electric motors, they know ICE motors, and they may have heard that electric motors and battery packs can last for so long, but they don't have the long term empirical data to back this up. Preformance concerns are also present, with only 30hp, the car wouldn't be able to cruise at 90-100mph. They also have to deal with contracts that are years in advance, so they can't up and cancel all those ICE engines they've already ordered, which is why GM is in such a tight pickle. Their ~7 year development cycle is still geared for SUV's in a world where gas will probably never go below $2 a gallon, so they have to wiggle around with their parts suppliers and manufacturing such that they can sell as much as possible, and loose as little as possible.

In any event, given Toyota's short ~3 year development cycle, they should be set to roll out a Prius with more power via supercharging, a bigger, plug-in li-ion battery pack, and ~100mpg in 2008. Now, the only problem with this Prius is that on gasoline, not counting plug-in, it won't show as much of a difference when utilizing Pulse and Glide, but for the average driver, the FE will be much better.

Now that I mentioned it, turbocharging allows for the efficiency of a small engine, and the power of a larger one. It does this by reducing pumping losses at some load via the energy present in the exhaust gasses. A CAI will theoretically increase pumping losses a little bit by bringing in colder air that will decrease cylinder pressure, which is why a WAI increases efficiency slightly. But these are very small changes. Diesels are efficient because they don't suffer from pumping losses since they're compression ignition. They rely on the overlean running condition, and have no throttle plate to restrict air into the cylinder. Otoh, if a diesel ever gets down to stoich, it would probably melt because there would be gobs of fuel the didn't totaly combust and would melt through the rings.

Anyway, **** I wrote a lot. Turbocharging your engine probably won't result in any increase in FE, however, replacing your current engine with a smaller displacement turbocharged version with the same peak power will. If you toss a turbo on, you'll still need Xhp to move at Nmph, and the engine will still bring in the same amount of air, so you won't see a decrease in pumping losses. ITBs can increase hp and FE by allowing you to lean out your mixture, which reduces pumping losses and improves efficiency, but a standalone ecu can do the same much more accurately. A taller transmission/taller tires/bigger rims results in the engine turning slower at the same speed, which means it'll need more air/fuel to make the same amount of power, so that can help a bit. If you toss a 200-250hp PDI diesel with different maps in an Insight, you can probably get something like 80-100mpg EPA highway with lots of hp/tq. It'd probably make a really nice swap in a rabbit too, but the Cd's too high and mpg would be in the mid 60's. Unless aero mods really helped a rabbit down to the Insight's .25, then you'd have sports car performance with great FE.

[white90crxhf]

Originally Posted by Jonathan_ED3

I believe it's more a function of the individual characteristics of the HF engine.

I'm sure ECU tuning plays a large part, though.

i'd say it's the engine, with my si transmission i cruse at 65mph with 3k on the tach, i'm getting rougly the same gas mileage as before and my car will actually move when i put my foot down.

[Randy]

Originally Posted by omgwtfbyobbq

A corvette and Insight are not that far off in terms of CdA and weight, but the vette get's crap for mileage due to it's large displacement engine having much more in the way of pumping losses at some given output.

Vettes are more than 3000 lbs; Insights are less than 2000. You can get 30mpg in a Vette... Chevy fans love comparing them to S2000's when someone says Hondas are fuel-efficient.

Diesel mileage isn't really fair because there's about 12% more energy per gallon. Fuel costs per mile is fair enough, and BTU per mile is fair too, but straight MPG with different fuels is silly. You couldn't compare things like CNG, and ethanol or LPG would be tragic. The lack of a throttle, high compression ratio, and the way those combine with a turbo do make diesels really efficient motors, just 12% less efficient than you think.

There seems to be a lot in common between efficiency and horsepower, but it's down to the individual thing. Like a rich mixture bumps up power, but obviously trashes mileage. Things like indexing plugs or high compression helps both.

I don't know if individual throttle bodies would help mileage or not, but I haven't heard of them used for mileage. They seem to be on things like motorcycles and performance v12's. The big thing being the great throttle response. Tuning is per-cylinder though, so it would be a big pain.

[omgwtfbyobbq]

Originally Posted by Randy

Vettes are more than 3000 lbs; Insights are less than 2000. You can get 30mpg in a Vette... Chevy fans love comparing them to S2000's when someone says Hondas are fuel-efficient.

Yes, but it can't get 50mpg, even though the vette glider is only marginally more inefficient than a insight glider. 1000lbs~500kg only adds ~10N of force to overcome rolling friction, whereas the force needed to overcome fluid friction at that speed is ~200N. Weight does not have that much of a bearing on FE, especially highway FE. A vette glider would be getting ~50mpg EPA highway compared to an insight glider if they had the same engine, but the 6.0L engine results in way too much energy lost when not using the ~400hp available. Go check out the FE ratings for the 2007 Malibu on fueleconomy.gov. They are all the same car, all have automatic 4sp transmissions, but as the engine size increases FE decreases because pumping losses increase.

Diesel mileage isn't really fair because there's about 12% more energy per gallon. Fuel costs per mile is fair enough, and BTU per mile is fair too, but straight MPG with different fuels is silly. You couldn't compare things like CNG, and ethanol or LPG would be tragic. The lack of a throttle, high compression ratio, and the way those combine with a turbo do make diesels really efficient motors, just 12% less efficient than you think.

I think the EPA factors this in because they test the carbon output through the exhaust, so when people buy diesels, they usually end up getting better than the EPA mpg rating. The most efficient gasoline engines are slightly less efficient as the most efficient diesel engines, bsfc of ~235g/kwh (gas) vs ~190g/kwh (diesel), however, the diesel engine is more or less always this efficient, while the gasoline engine is almost always not this efficient.

[tomauto]

Comparing s2000 gearing vs. corvette isn't fair at all. Corvette's 6sp is MADE for cruising on highway...The s2000 gearing is more related to a Indy car 6-7th gear change...super close. The s2000 is much more high strung than a corvette.........Those GM bastards...

[JanGeo]

A high torque engine running at a lower rpm to produce X hp will have less rotational losses than a low torque engine running at a high rpm to produce the same X hp. The higher rpm engine will require more gear reduction to maintain the same speed as the low torque engine which will result in the same torque at the wheel to make the cargo.

[Sludgy]

Originally Posted by 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?

I'm not sure about Japanese engines, but big american V8 engines can easily be altered to produce low RPM torque via a cam swap.

Racing engines need lots of valve lift and valve overlap for high speed operation. But big valve lift stresses the valvetrain and the added friction makes it waste more power. Valve overlap can allow part of the intake charge to pass through the cylinder (at low, FE rpms) without actually burning in the cylinder. But valve overlap makes the engine breathe much more ealisy and make a lot of power.

A cam swap with mild lift and no valve overlap can produce FE gains. V-8 cam swaps are cheap, too.

High compression pistons are another way to get better FE. The higher expansion ratio extracts more power during the power stroke. But high compression often causes pinging. New pistons are an expensive change, much more expensive than a cam.

If I were going to trick out a V8 for FE with an unlimited budget, I'd buy the highest compression pistons I could find, and find a custom cam grinding shop to make an Atkinson cycle cam. This would prevent pinging and still have the high compresion ratio.

[thisisntjared]

wow this thread has a lot of fuzzy logic.... i think im just going to sit this one out.