Horse Power and MPG

[Sky_Boss]

I saw an interesting statement that made me think about engine size. I know this may sound overly general but I'll ask it anyway.

All things being equal, does a larger engine have lower MPG than a smaller one?

Let's say we have a MH that weighs in at 31,000#. Would there be a difference in MPGs between the same unit with 350 hp vs 400 hp?

As I would see it, maybe.

Certainly the 400 HP should have more guts to it for hills and head winds. But, I always heard it takes torque to get you going and HP to keep you going. So...once both of those RVs were up to speed, theoretically they should have the same MPG?

Any thoughts?

[VanDiemen23]

larger, or higher HP? HP takes fuel, it's how you get there and where that makes the difference, as far as volumetric and thermal efficiency goes.

[Sky_Boss]

Originally Posted by VanDiemen23

larger, or higher HP? HP takes fuel, it's how you get there and where that makes the difference, as far as volumetric and thermal efficiency goes.

Maybe I'm mixing terms up a bit. Bottom line is if you have an engine that produces 350 HP and one that produces 400 HP and you have an RV of the same weight, size and aerodynamic qualities...Wouldn't they be using the same amount of fuel to generate the same HP to move the coach along at a steady state? Ya, I know, there are some other variables such as tranny design and such that also play a part in it all.

I guess my thinking is that given similar rigs with that kind of decision in front of you, does that make a difference. i.e. If I don't expect a lot of big hills in my use, then I don't need the bigger engine if I also burn more fuel on the flatland drives. OTOH, if I am expecting lots of climbs (like you folks out west have) just to get anywhere, then I might cheat towards the 400 HP range because it makes more sense.

[cb1000rider]

Originally Posted by athuddriver

Maybe I'm mixing terms up a bit. Bottom line is if you have an engine that produces 350 HP and one that produces 400 HP and you have an RV of the same weight, size and aerodynamic qualities...Wouldn't they be using the same amount of fuel to generate the same HP to move the coach along at a steady state? Ya, I know, there are some other variables such as tranny design and such that also play a part in it all. \

You're doing the same amount of "work" to move the coach at steady state. The amount of fuel you use doing that work has a component of motor efficiency.

Look at the Ford Eco-Boost motor. It's not very big. It puts out big power. How? Ford cheats. They stuff a large amount of air (and fuel) into a motor at above atmospheric pressure via a turbo. Basically the motor displacement isn't big, but they're forcing a volume of air and fuel normally reserved for a big motor.

That being said... When you put in a big motor you've made a larger rig. A larger rig takes a heavier frame, bigger brakes, and overall weighs more. All of a sudden it takes more work to accelerate that rig to steady state...

Personally, I don't worry about the difference of 350 and 400hp, unless there is a technology difference between the two motors. I'd go after 400hp, because when I need that power, I need it.

A 1970s carb'd 350-hp motor is a lot less efficient than a 2010 350 hp motor with port injection, an 02-sensor, and dynamic timing... Even though the base motor design is roughly the same.

[Abnmarine]

I would be more concerned with the torque ratings. It's torque that moves an object, HP is the measurement of how quickly that happens. In theory the larger engine would be more efficient due to less rpms needed to match the same output of the smaller engine.

Here is a good read between the two:

RV Tech Library - Horsepower Versus Torque

[SVTotem]

Great link abnmarine. Thanks.

[Sky_Boss]

Originally Posted by SVTotem

Great link abnmarine. Thanks.

Agreed.

[IMDSailor]

Originally Posted by SVTotem

Great link abnmarine. Thanks.

X2

[doc]

Wouldn't they be using the same amount of fuel to generate the same HP to move the coach along at a steady state? Ya, I know, there are some other variables such as tranny design and such that also play a part in it all.

I doubt that. I had a 1963 Chevy Impala with a 283 engine. My buddy had the same car with a 409. I got considerably better milage.
I think that is an apples to apples comparison.
I do know we had much better milage with the 36 foot Itasca Horizon with the 300 Cat than we get with the 40 foot Damon Essence with the 425 Cummins. Much heavier coach with the Essence. That is NOT an Apples to Apples comparison.

[dennis45]

Originally Posted by athuddriver

I saw an interesting statement that made me think about engine size. I know this may sound overly general but I'll ask it anyway.

All things being equal, does a larger engine have lower MPG than a smaller one?

Let's say we have a MH that weighs in at 31,000#. Would there be a difference in MPGs between the same unit with 350 hp vs 400 hp?

As I would see it, maybe.

Certainly the 400 HP should have more guts to it for hills and head winds. But, I always heard it takes torque to get you going and HP to keep you going. So...once both of those RVs were up to speed, theoretically they should have the same MPG?

Any thoughts?

Fuel economy will not improve with HP unless the engine is more efficient.
I installed the Banks Power Pak on my 350 HP ISC Cummins which increased the HP by 110. Yes, there is a noticeable difference in power going up the hills but I also went from an average MPG of 6.5 to an average of 8.5. On good terrain I will even see as much as 9.5-10 MPG average.
The reason is that the Banks kit uses a new turbo hot housing that supplys more air along with better fuel/air management system making the engine more efficient.
The old saying "If you want the horses you have to feed them" is true however, throwing in some steriods certainly helps.

[NFlcamper]

Efficiency is the key. EXAMPLE: I have 2 motorcycles SAME motor/tranny/gear ratio on BOTH! Bike #1 gets up to 46 MPG bike #2 may get 38 MPG on a good day. Differences between the 2 bikes - #1 has more compression and a better aftermarket intake with a fuel management fine tuner. The higher compression gets more power from the same volume of air going in. The more efficient intake allows more air to enter during the intake stroke.
SAME 2 BIKES fuel efficiency drops by as much as 4 MPG when intakes and throttle body system gets dirty. Idle rpm increases just by cleaning intake and throttle bodies of the fumes that come through the crank case vent and the minute particles that get past the filter. That little bit of film slows the velocity and reduces the air intake.
Cold air also helps efficiency, it expands more in the internal combustion process. That is why Turbo motors usually have an intercooler.
Driving habits can wipe out all gains here. By utilizing the higher HP, you can use more fuel than a lower HP motor. Driving faster because you can tell the motor isn't straining like a lower HP motor would drinks up all fuel savings and then some! I can tell the difference towing with the Z71 and on the bikes riding at speeds above normal cruising speeds. (Sorry, I don't admit to speeding, LOL! )

[Gary RVRoamer]

There are too many variable to make any meaningful comparisons with one coach vs another or even one engine vs another. Almost impossible to be "apple to apples".

At least in theory, a large displacement engine running at 50% of it s max horsepower could be just as fuel efficient as a smaller engine that has to use 100% of its capability to produce the same power. But theory kind of goes out the window once you introduce RPM ranges for that power, thermal efficiency of the block, rotating resistance in the engine components, more engine weight vs less, and on and on and on...

Then there is the tendency to use a bit more power at all times just because it is available. Accelerate a bit faster, climb the hill a bit quicker, etc.

[TLGPE]

The simpliest direct answer is that -as others have stated, the fuel consumed is proportunal to the power extracted. Identical coaches, identical conditions, identical fuel used. That's the easy answer. If you have a larger engine, the engine itlelf may have more mechanical friction and use more fuel to give the same power in those conditions. Some engines get modified to produce more power (bigger turbo, etc.) and still have the same friction, so fuel usage will not change. If you then USE that extra power to race around or pull bigger loads, you will use more fuel.

It has been said here that it complicated. That's right. More powerful engines need heavier supports, stronger trannies, all that stuff. Most of the time, if you get a higher powered rig, your milage will be lower. It's not like the old case of a Cadillic getting better milage than a Chevy because of better technology. Most of the newer available coaches already have good technology applied, so that's about all you're going to get.

Tom (Your milage may vary).

[CD]

I am not talking about a MH but a 1T dually. When I got it I wasn't real pleased with fuel milage. After I had it a while I bought a chip that I was told would increase the HP by about 55HP and more torque by changing the timing rather than just increasing fuel. It realy increased performance. I have found that if I get heavy foot the milage goes down but if keep my foot out of it I get better milage than I ever got before and with the extra HP I am moving out like I was before but it does it easier. So if I drive it so I get acceleration etc.that I got before I get better milage. If I push it more (because it will do it) the milage goes to pot. That is just seat of the pants info.