The effect of horsepower on gas mileage
 

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?

^_^
 

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.

okay
 

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?

Hmm
 

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.

Torque helps gas mileage, not horsepower
 

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?

Hmm
 

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?

This might be the connection
 

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.

hp is a function of torque.
 

hp is a function of torque.

Horsepower is also a
 

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

Re: Horsepower is also a
 

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.

Torque Boost!
 

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.

thats because most diesels
 

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

Re: Horsepower is also a
 

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

HP = (RPM * TQ) / 5252

That's what I said.
 

That's what I said.

The TQ vs HP argument is
 

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.

Is this something that can
 

Is this something that can be done with ECU tuning?

I believe it's more a
 

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 think the camshaft is what
 

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

Re: I think the camshaft is what
 

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.

lots more than just a cam
 

lots more than just a cam affects power, lol

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 :D

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.

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. :thumbup:

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.

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. :eek:

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.

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.

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.

s2000 gearing vs. corvette
 

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...

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.

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.

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

The Insight has a CdA of 5.125 square feet. This is from a published Cd of .25 and published frontal area of 20.5 square feet.

The Corvette has a CdA of 6.132 square feet, from published .28 Cd and estimated 21.9 square foot frontal area(Car and Driver).

Not only is the Insight 1,100 pounds lighter, it also uses LRR tires with a Cr around .006. The Corvette's tires are very sticky, Cr around .012.

The difference in forces to overcome for the Vette versus the Insight are actually going to be quite large. If you go to fueleconomy.gov, and note the differences in fuel economy between a V6 and V8 Mustang, or the L4 and V6 Camry, the differences for combined mpg are around 20%. The highway mpg difference between the V6 Camry and the hybrid Camry is only 18%!


Were that Vette to have the same CdA as an Insight, same weight, same tires, and keep the V8, it would probably do around 40 mpg highway. As is, the Vette does 29 mpg highway. Likewise, would the Insight have the same CdA as the Vette, were 1,100 pounds heavier, and had sticky tires with the same hybrid drivetrain, it would probably get around 45 mpg highway.



By adressing aero drag and rolling losses, we could have 40-50 mpg hwy V8 musclecars. Say, take a GM Precept concept car with a .16 Cd, and hypothetically shove a Corvette engine into it as opposed to the hybrid-diesel drive. I'd lay down money on the table that it would get better than 35 combined MPG. Having the Corvette V8 engine with a 'mild hybrid' powertrain like Citroen uses to kill idling at a stop would probably bring that number over 40 mpg combined.

Run your numbers yourself, sum up the rolling and fluid friction @48mph for each vehicle. There is ~27% difference in energy required to move the Vette @48mph EPA highway, but the difference in mpg is ~136%! Where does this extra inefficiency come from? Primarily, the fact that the Vette has an engine six times the size of the Insight's, and pumping losses at 50mph are huge!

Hybrids aside, since we're talking about pumping losses, so any tech like electric motors, idle shut-off, mds, that minimizes those, and masks the impact of pumping losses should probably not be looked at... ;)
The Mustang you mentioned (mt) has a difference in displacement of 4.6L/4L=~15%, and a difference in highway efficiency of 28mpg/25mpg=~12%. The Camry you mentioned, not the hybrid version, get's 28mpg for the the 3.3L 4sp auto, and 34mpg for the 2.4L 4sp auto. 3.3L/2.4L=~38% difference in displacement, and 34mpg/28mpg=~21% difference in mpg. So, the drop in efficiency is not linear, probably because friction losses are greater in smaller displacement engines. But, the point I'm making is that pumping losses are a big part of gasoline FE! And the Vette would still get ~48~50mpg (maybe more, maybe less, ballpark) if it had the Insight's engine, since there is only a ~27% difference in glider energy requirements. The only way to get the vette to 50mpg would be to drop the force needed at 48mph by ~67%, LRR tires only give about 8%, so the remaining must be done via aero, and like you said, if we improve that by 50%, we'll be there.
Here I go off topic again! :thumbdown:
Anyway, pumping losses>friction losses. *EV motors>ICE engines! :D

*I've been thinking about a li-ion powered velomobile. ;)

You also have to account for the fact that the Corvette's alignment isn't tuned for efficiency but for traction. Same with its brakes, which are likely the cause of much drag. For that matter, there's also its transmission.

When it comes down to it, the difference between a V8 and a L3/electric drive would be more closer to 50% than 100%. Lets run some numbers, a set for the Corvette, and a set for the Hybrid.

2007 Chevrolet Corvette Coupe:

Mass(W): 1,442 kilograms
Drag Coefficient(Cd): .28
Frontal Area(A): 2.08 square meters
Rolling Resistance Coefficient(Cr): .012
Transmission Efficiency(TE): .90

Velocity(V): expressed in meters per second
Force Drag(FD): expressed in newtons
Force Rolling(FR): expressed in newtons
Stray Force(SF): 40 newtons
Wheel Power(WP): expressed in watts
Engine Power(EP): expressed in watts

Air Density(Rho): 1.25 kg/m^3
Gravitational Constant(G): 9.8 N/kg


Equations used:

FD = .5 * Rho * Cd * A * V^2
FR = Cr * W * G

WP = (FD + FR + FS) * V
MP = WP / TE

Results:

At 21.6 m/s(48 mph):

FD = 170
FR = 170
SF = 40
WP = 8,208
MP = 9,120

At 26.8 m/s(60 mph):

FD = 261
FR = 170
SF = 40
WP = 12,623
MP = 14,025

2006 Honda Insight:

Mass(W): 840 kilograms
Drag Coefficient(Cd): .25
Frontal Area(A): 1.9 square meters
Rolling Resistance Coefficient(Cr): .006
Transmission Efficiency(TE): .94

Velocity(V): expressed in meters per second
Force Drag(FD): expressed in newtons
Force Rolling(FR): expressed in newtons
Stray Force(SF): 20 newtons
Wheel Power(WP): expressed in watts
Engine Power(EP): expressed in watts

Results:

At 21.6 m/s(48 mph):

FD = 139
FR = 49
SF = 20
WP = 4,493
MP = 4,780

At 26.8 m/s(60 mph):

FD = 213
FR = 49
SF = 20
WP = 7,558
MP = 8,040



At 60 mph, the power at the motor of the Honda Insight would be about 57% that of the Vette. At 48 mph, the power required by the Insight is about 55% that of the Vette!


That's where the bulk of the mileage differences are at. Put the V8 in the Insight, and fuel economy would probably drop around 40% or so. And it would still embarass most anything else on the road when it comes to fuel economy.

I chose a higher amount of stray friction for the Vette over the Insight due to the Insight being designed with efficiency in mind, as opposed to maximizing traction and other performance aspects. The Insight also has a more efficient transmission and the numbers were chosen accordingly.

I think your FR's are way, way high because the manufacturer curb weight already includes gravity, since they weight them on scales. If the vette weighs 1.442kg than we're already including gravity, and in this context weight is in the N=kg as opposed to the actual mass kg. If you divide your FR's by 9.8, than I think you're at the force needed to overcome rolling resistance. As per the rolling resistance expression, Crr is dimensionless, so the W must be in N already.

Kg is a unit of mass, not weight. At the Earth's level of gravity, there are ~2.2035 pounds of force for every kilogram of mass. In space, mass and weight would basically have no relation, as mass would stay the same as it was on Earth and weight would basically be 0 pounds.

To find the total force gravity in Newtons, you take the mass in kilograms and multiply by the gravitational constant.

With pounds as opposed to kilograms, the gravitational constant is already included and it's a simple conversion. Newtons and pounds measure the same thing: weight. Kilograms is not a unit of weight.

It is not unusual for a car to have 30 pounds of rolling force. That equates to a 3,000 pound car with a .010 Cr. 1 pound is equal to 4.45 Newtons. Convert accordingly.

I hate standard units. :o
This sucks beause I was seriously underestimating rolling friction, but it's very very cool because if my tires have a Crr=.012, getting LRR tires Crr=.006 and aligning everything 0,0,0 should bring my mileage up to almost 70mpg@50mph! Anyway, as we've seen before, I hate units! :eek:
Not that pumping losses aren't significant, there just not as significant as my ignorance of the imperial units, crazy *** things. :p

Quote:

Engineering Challenges: Adding another technology to a vehicle does not necessarily improve fuel efficiency. For example, variable valve timing and cylinder deactivation both target the energy ?pumping loss? that occurs as the engine ?breathes? (that is, operates at a lower intensity). Once the first technology is applied, that pumping loss has been largely reduced so the second technology will provide less benefit.

If I had my way, the auto industry would be bringing on the 40+ mpg musclecars, 80+ mpg biodiesel-powered family sedans, and 250 mile range pure electric cars.

My design philosophy is that performance and fuel economy need not be mutually exclusive. Both can be had in a properly designed car that retains much of the amenities of the cars seen today.

I think I see where you and I both went wrong! If like you stated, with pounds, the gravitational constant (~32ft/s^2) is already included, to convert to the mass in kilograms, we need to first divide the weight by the acceleration due to gravity to arrive at the mass in pounds, then we convert that mass in pounds to mass in kilograms, then multiply that by 9.8m/s^2, and finally multiply by the dimensionless Cr.

In other words, if the weight in pounds is 3174pdl, then we divide by 32ft/s^2 to arrive at a mass of 99lbs. Converting to kg we get 45kg, and the normal force is 45kg multiplied by 9.8m/s^2, which is 441N, so the rolling friction is the product of this and .012, which is roughly 5N, which converts to ~38pdl, which is what you get if you multiply 3174pdl by .012.

So, it must be that rolling friction is much small than either one of us though, or, in fact, the weight in pounds is actually the mass in lbs and we must multiply by 32ft/s^2, then multiply by the Cr to get the rolling friction, and the same goes for the mass in kg. So, the question is, provided that the units convert correctly in both cases, is weight generally a measurement of mass, or of force? :confused:

Hi, again!
"Tricks to increase torque?" Now you're on the right track! It's pressure on each piston which produces the twist on the crank journal. This pressure (from the burning gases above each piston) is called the BMEP ( brake mean effective pressure). Increase this, and we have increased the torque (maybe not the HP!). How can we do this? Lots of ways! Change camshaft profiles, change CR (pistons? Mill the head? Both?), change this, change that. But be wary of the FED! Lots of changes create emission rate changes! Some good, most bad, some illegal! Welcome to the world of the "snake-oil" salesman! "He" will lead you down every primrose path known to man...to get your money!
I will now tell you a perfect way to change (increase) your BMEP; change the gasoline! Today's gas is slop! I've read "horsewhizz", I've read " Tiger piss"....But, the fact remains- until someone figures out a way to run a gasoline engine on diesel fuel- pump gas ain't real gas! Too much oil in it! If we can't get the oil out, we can do something to burn this "gass" better!
Interested?

You don't divide the weight of an object in pounds over 32ft/s^2 unless you want to measure mass. What we're concerned with is force, not mass. Pounds are already a measure of force. Kg is mass, and we must multiply by 9.8 N/kg to figure out the force given Earth's gravity.

For a 99 pound object with a .012 Cr, 5N of rolling force sounds about right(Actually 5.287). Scale that up for a 3,200 pound car using the decimal value, and you get 171 N of rolling force.

171N of rolling force is equal to ~38 pounds of rolling force.


Go to google.com, type in Newtons per pound. 1 newtons = 0.224808943 pounds force Both Newtons and pounds measure force. Force is essentially weight.