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Author Topic: Engine HP v. mileage  (Read 4090 times)
BG6
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« on: June 14, 2010, 10:00:34 AM »

In another discussion, someone suggested that a big engine would get poor mileage.

This is not true -- at least, not for any engine you are going to see on a coach.

It takes a certain amount of power to move a particular mass at a particular velocity.  It takes a certain amount of fuel to get that horsepower.

Let's say that Engine A is a DD Series 60, set for 500 HP.  Engine B is a Cat 3176 (used in a lot of diesel pusher high-end S&S RVs) that gets 365 HP.

Let's also say that the loads are identical, the only difference being which engine is in which coach, going on I-80 between Salt Lake City and Sacramento, then turn around and go back.

Going across the salt flats -- no road in the world is flatter -- both will get the same mileage IN STILL AIR.  The engines have the same amount of work to do.  It doesn't matter how big the cylinders are that do that work.

However, the moment the load changes -- headwinds, hills, variable speeds for traffic, etc -- the extra horsepower starts saving you money.

This lesson was learned the hard way 10 - 15 years ago.  A lot of truck companies put in smaller engines after the Exxon Valdez accident made fuel prices jump.  That 3176 engine went into thousands of semis, owned by fleets nationwide.

What they discovered was that the majority of fuel is used to accelerate, and that the less time spent accelerating, the sooner the driver could get into the fuel-efficient RPM range.

They also discovered that an engine with enough power let the driver stay in the higher gears, thus lower RPM, while going up hills or getting back up to speed after slowing for traffic.

Think of the Apollo-Saturn launch system that took us to the Moon.  A massive amount of fuel was expended during launch, followed by days of coasting.  This burned a fraction of the fuel which would have been needed for a constant acceleration, turnover, then constant deceleration. 

Your coach is the same way.  Those two or three pedals are the keys to someone else's cash register.  Every time you touch one, you spend money, either for fuel or for brakes and clutch.  It used to be said that every time you push the clutch pedal cost a penny in worn lining and pressure plate.  I hate to think what it costs now, but I "float" my shifts whenever possible.

Fuel is the most expensive item over the life of a semi.  An owner may spend $500 PER WEEK on the truck payment, but if he's driving 3000 miles per week, he will burn over 400 gallons of fuel!  If having a big engine was a bad thing, you would see Freightliners using Briggs & Stratton pull-start engines instead of 500 and 600 HP Detroits! 
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bevans6
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« Reply #1 on: June 14, 2010, 10:11:31 AM »

I think you're right, for identical engines, but engine efficiency can make a big difference to fuel usage.  Only in theory does an identical amount of fuel produce an identical amount of energy in different engines.  Breathing efficiency, compression, cam timing, injector profiles and timing, turbo, intake and exhaust manifolds, and drivetrains all have big impacts on fuel usage.

The big downside to a high power engine is if you use the power you use more fuel.  The driver makes a huge difference to fuel usage with his foot and what he does with gears, and how he manages terrain.  If you never used the power advantage, a big engine wouldn't need more cooling that a little engine.

Brian
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« Reply #2 on: June 14, 2010, 10:21:43 AM »

The low rpm torque curve is where fuel saving come into play you can have a S60 without the right gearing to run in the 1400 rpm range and running at 1800 rpm will use as much fuel as 8v92 running the same rpm.
I know to many people that have Cummins, Cat and series 60 that I got better mileage with a 8v92 than they did because of gearing


good luck
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« Reply #3 on: June 14, 2010, 11:12:24 AM »

I think you're right, for identical engines, but engine efficiency can make a big difference to fuel usage.  

Not as great a difference as you might think.

The big iron diesels these days are within a couple of percent of each other, no matter the make or model, due to computerization.  This is down below the noise level, where your tires, drive train, etc are a greater factor.

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BG6
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« Reply #4 on: June 14, 2010, 11:14:42 AM »

The low rpm torque curve is where fuel saving come into play you can have a S60 without the right gearing to run in the 1400 rpm range and running at 1800 rpm will use as much fuel as 8v92 running the same rpm.
I know to many people that have Cummins, Cat and series 60 that I got better mileage with a 8v92 than they did because of gearing

Yes, gearing is a big issue, but if I have a DD S60 at 500 HP, I can stay in my higher gear when that Cat has had to drop to a lower gear at higher RPM.

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« Reply #5 on: June 14, 2010, 11:33:08 AM »

Install a C15 Cat it will dance with series 60 but a 3176 is not that big of a engine


good luck
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« Reply #6 on: June 14, 2010, 12:40:32 PM »

Install a C15 Cat it will dance with series 60 but a 3176 is not that big of a engine

Tell me about it.  I sat behind one for two years as an owner-operator.  When anyone asked what I had for an engine, I proudly said "Briggs & Stratton, 3-1/2 horsepower!"
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RoyJ
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« Reply #7 on: June 14, 2010, 02:45:49 PM »

Install a C15 Cat it will dance with series 60 but a 3176 is not that big of a engine


good luck

Actually, 9 times out of 10 the big Cat would murder a S60 when hauling heavy. By trucking standards the S60 is pretty gutless down low, and Cats (or Cummins) shine down in the RPM range. Now the DD15/16 might be a different story, but I've never driven one.


Regarding hp vs mileage, it gets a little more complicated than simply saying engine "size" doesn't matter. Yes, bottom line is two identical coaches would require the exact same hp going down the road, but the difference in mileage comes from engine efficiency, or else OTR trucks would use blown HEMIs...

In a typical coach (where hp requirement is fairly low), a smaller engine, such as an ISM, would have higher thermal efficiency, due to the smaller surface area. The pumping / reciprocating losses would also be less, from smaller displacement/moving parts. At cruising, you're also much closer to the engine's peak efficiency load/rpm.

Now, if you're talking about underpowering a coach/truck, then a bigger engine would get better mpg, as you're pushing the smaller engine past its ideal efficiency envelope.

There's also quite a difference between a high hp small engine (450hp ISM) vs low hp big engine (425hp ISX). Here, the worst case is the "fleet setup", where they detune a big engine down to ridiculous levels. Reason being, to have a big block producing only 425hp, you'll be driving it much like a small block - revving way up. Then, you have the power of a small engine, but ALL the parasitic losses of a big engine (thermal, pumping, reciprocating, accessory drives).

Further complicating the issue, a big engine / high hp setup can sometimes get very good mpg, IF driven correctly. Guys with hopped up Cats making 900 hp often get better mpg than fleet trucks. My guess is that by having excessive hp, you can pull the same load at a much lower rpm. A 900hp Cat probably has a healthy 500hp at only 1300 rpm (w/ 2020 lb-ft torque). Pulling hills at this rpm would have much lower parasitic friction/pumping losses.

There're probably a bunch of other factors I didn't touch on. But in conclusion, is fair to say that just because a coach has a fixed hp requirement, doesn't mean every engine can get the exact same efficiency making that amount of hp.
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cody
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« Reply #8 on: June 14, 2010, 03:01:55 PM »

900 hp in my iggle would boggle me, to see the look on the corvette down the street as I do a burnout would be priceless lol.  Yeah, I know I'm dreaming but it still would be priceless lol.
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« Reply #9 on: June 14, 2010, 03:40:29 PM »

LoL! Now Cody you you know as well as I do... all it takes is dough  Grin Grin but seeing the look on the drivers face of that Vette when he realizes your still on his bumper... that's Priceless!  Grin Grin Grin Wink
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« Reply #10 on: June 14, 2010, 03:58:07 PM »

 Grin
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BG6
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« Reply #11 on: June 14, 2010, 04:54:01 PM »

900 hp in my iggle would boggle me, to see the look on the corvette down the street as I do a burnout would be priceless lol.  Yeah, I know I'm dreaming but it still would be priceless lol.

You know, if you take out a couple of windows and add some duct, you could drop a jet engine with afterburner  . . .imagine the look on his face when you drive up to the stop sign, shut down the diesel, a set of doors in the back open and he hears the Pratt & Whitney spool up . . .
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« Reply #12 on: June 14, 2010, 05:10:03 PM »

Imagine the look on his face when you pass him and peel the paint off of that vet! Grin
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« Reply #13 on: June 14, 2010, 08:28:42 PM »

Lovely discussion, and fleetmen across the world play the variables in order to earn a few more pennies on the fuel budget...

But what busnut is installing a large engine and not dipping into it?

Blows it all out the window, for he with 500 HP will surely use more of it than is necessary for efficient movement down the road.

I'm buying a bottle to drink it, not put it on a shelf?

happy coaching!
buswarrior
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« Reply #14 on: June 14, 2010, 09:11:07 PM »

900 hp in my iggle would boggle me, to see the look on the corvette down the street as I do a burnout would be priceless lol.  Yeah, I know I'm dreaming but it still would be priceless lol.

Well, couple years back, a co-worker of mine had a 1100hp Dodge Ram 5.9 Cummins. Even on street trim he had close to 900 hp. When he goes fishing in the summer, he has a camper on the back, and pulls a 14' boat behind.

He got to see a LOT of priceless looks on Vette and Stang drivers up the Coquihalla pass here in BC!

But what busnut is installing a large engine and not dipping into it?

Who said you can't dip into it?  Grin

If I had a C15 with full marine goodies and Pittsburgh box, in my 24,000lbs bus, you can bet I'll let other drivers know about it! (and the tire shop...)

I like to buy the bottle, drink it, and let people know I've got the biggest bottle around  Cheesy
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« Reply #15 on: June 15, 2010, 06:01:48 AM »

I thought I posted this yesterday, but obviously I neglected to hit "post"...   Anyway, watching Top Gear and they are doing fuel mileage comparisons.  Of the supercars, the Ferrari lost with 1.8 mpg (at full chat on a race track) and the Audi R8 won with 5 mpg.  Then they had to do a comparo relevant to the common man...so they compared a Toyota Prius to a BMW M3 with a 400 hp V8 engine.  They went around the race track as fast as the Prius could go, all out.  But a walk in the park for the BMW to keep up.  Prius got 17 mpg and the 400 hp BMW got 19 mpg, proving for all time that the BMW M3 is a more ecologically sound choice than the Prius... Grin

Brian
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« Reply #16 on: June 15, 2010, 06:50:34 AM »

Most engines when run flat out will get fairly lousy mileage compared to regular highway speeds.  The BMW was not maxed out as the Prius was.  I bet the BMW running flat out would get worse mileage than the Prius.  The BMW can presumably go a lot faster than the Prius.

The original Dodge Dakota crew cab was available with a V6 or the 4.7L V8.  The V8 actually got better mileage as the V6 was pretty maxed out with the extra weight of the crew cab.  My Dina has the smaller 11.1L Series 60.  The newer Dinas with the 12.7L Series 60 seem to get slightly better mileage because the smaller engine is probably working pretty hard.
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« Reply #17 on: June 15, 2010, 08:59:30 AM »

But what busnut is installing a large engine and not dipping into it?

Blows it all out the window, for he with 500 HP will surely use more of it than is necessary for efficient movement down the road.

Give this some thought.

You come out of the hole at a stoplight, put your foot in it, then go up through the gears.  When you get to highway speed, you come off the pedal to get the right speed.

How will you burn more fuel if you get through the gears in 30 seconds than if it takes a minute and a half?

How will you burn more fuel at 65MPH using 50 HP from a 500 HP engine than using 50 HP from a 400 HP engine?

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« Reply #18 on: June 15, 2010, 09:58:15 AM »

Give this some thought.

You come out of the hole at a stoplight, put your foot in it, then go up through the gears.  When you get to highway speed, you come off the pedal to get the right speed.

How will you burn more fuel if you get through the gears in 30 seconds than if it takes a minute and a half?

I have a book produced by Porsche on 'Advanced Driving Techniques' which I think they gave away to their customers in the 1980s. It says unequivocally that accelerating quickly to a given speed uses no more fuel than accelerating slowly to the same speed. I do not believe this is correct though, because it's wrong to consider the 'time' aspect in that way (ie. "going through the gears in 30 seconds rather than a minute and a half" etc). You might just as well say driving quickly uses less fuel than driving slowly because if you drive quickly you reach your destination sooner, and therefore run the engine for less time.

Jeremy
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« Reply #19 on: June 15, 2010, 10:14:34 AM »

I think the only way that can work is if you are computing miles per gallon per hour.
Thus if you valued your time at $50.00 per hour and saved an hour by using $20.00 more fuel, it would be worthwhile.  Most of us are not in that position.
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« Reply #20 on: June 15, 2010, 12:06:48 PM »


How will you burn more fuel if you get through the gears in 30 seconds than if it takes a minute and a half?

How will you burn more fuel at 65MPH using 50 HP from a 500 HP engine than using 50 HP from a 400 HP engine?




Like Jeremy mentioned above, you can't simply use a time factor to judge fuel usage. If that was the case, top fuel dragsters at 1 gal/second would be the most fuel efficient car on earth.

It comes down to engine efficiency at converting fuel to work. If you run an engine to its redline, it's inefficient. It may be doing 500hp of work, but consuming 2500hp worth of fuel. Back it off a bit, and efficiency recovers. At 250hp, engine uses lower than 1/2 the amount of fuel, while vehicle does not, generally, take twice as long to gain speed.

At 50hp, a 400 and 500hp engine will likely have the same efficiency (crappy). But, a 300hp engine making 200 hp would be a lot more efficient than a 500hp engine making 200hp.
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« Reply #21 on: June 15, 2010, 01:24:49 PM »

The biggest factor in engine fuel mileage (and this has been proven with big truck fleets) is the loose nut behind the steering wheel and how much lead is in his shoes.  A bus cruising at a leisurely 55mph @ say 1400rpm is going to get a whole lot better fuel mileage then the same bus cruising at 75mph @ 1900rpm. Big horsepower engines use lots of fuel when you ask them to work-like uphill, into the wind, etc.

This discussion has been played over and over with owner/operator truck drivers for years.  Personally-I like to have big power loafing down the road, then a small engine screaming all the time.  Then if you need sudden power for passing, etc, the big engine can do it.  Just like my now turbocharged 8V-71-I can actually pass vehicles (even with my car in tow) now when before would just have to wait-much nicer.  Good Luck, TomC
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« Reply #22 on: June 15, 2010, 05:53:50 PM »

Relating to passenger car engine efficiencies (miles per gallon) and engine emissions, the federal gov has set the bar rather high for the car companies. The car companies, to achieve this, are producing engines that have their optimum efficiencies and lowest emissions in a rather narrow rpm range. To utilize the narrow rpm range, they are coupling the engines up with 5 and 6 speed automatic transmissions which under acceleration and at cruising speeds the additional transmission ratios attempt to keep the engine in this rpm range. All engines have their sweet spot when it come efficiency and as Tom said the driver has the most impact over this.
Kenny
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« Reply #23 on: June 16, 2010, 06:07:19 AM »

HEllo:     DOnt forget the effects of windage on the flat front of a bus.   ie the same or very similar to a truck.  The effect is to decrease fuel mileage by .1 mpg for every 1 mile increase over 55.   In other words at 65 mph you lose 1 mpg for wind resistance. THat is just to push the bus through the air in front of it.   Thats why usual practice is based on 55 mph because most studies were based on that number.  If you look up mileage and the trucking industry you can find all sorts of facts and figures about driving and mileage
    Regards and happpy bussin   mike



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« Reply #24 on: June 16, 2010, 07:09:10 AM »

Certainly the nut behind the wheel has a huge effect on MPG, but I still want a vehicle that gets the best overall MPG.  I would certainly rather get say 9 MPG at 55 MPH instead of 7 MPG with a different engine and bus.

If I had a goal in mind of getting say 8 MPG I would personally rather be able to go 65 MPH instead of 55 MPH to attain that goal.  My style of driving is interstates for long distances and others may be fine with 55 MPH if they take the back roads.  The extra 10 MPH can cut hours off of a long trip.
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« Reply #25 on: June 16, 2010, 12:35:52 PM »

It all depends on how you use your bus and what your priorities are.  If you make your living in the bus and getting there on time matters, then that is a major consideration.

Consider a bus that gets 5.5 MPG at 65 MPH and 7.5 MPG at 55 MPH. (Is that a reasonable assumption)?

If you drive 400 miles in a day at 55 MPH and fuel is $3.00/gallon, you will arrive in 7 hrs and 16 minutes at a cost of $160.00.

The same trip at 65 MPH will take only 6 hours and 9 minutes and cost $218.00

So, if my math is correct, you saved 1 hour and 7 minutes at a cost of $58.00
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« Reply #26 on: June 16, 2010, 04:34:24 PM »

Quote
If I had a goal in mind of getting say 8 MPG I would personally rather be able to go 65 MPH instead of 55 MPH to attain that goal.

Brian, You need my MC9, last trip 2400 miles, 20 ft enclosed trailer, approx 34,00lbs, 65 mph, got exactly 8 mpg (6V92)
Kenny
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« Reply #27 on: June 16, 2010, 04:43:15 PM »

I'm not really sure, but I don't think my bus has nearly a 2 MPG difference between 55 MPH and 65 MPH.  The main issue for me is time.  I have different bus use than many.  We drive straight through for 2,000 miles with five to six drivers.  An extra 5 hours on a trip that is already 31 hours before stops is a big deal.

55 MPH would be a safety hazard on interstates with posted speed limits mostly 75 MPH with some 70 MPH sections.  Yes, I could take 2 lane roads, but I can't stand them if I have a choice.  They generally are more taxing to drive with less margin for any errors.  The guys on the trip don't mind an extra $10 to $20 each to save 10 hours overall.

Kenny, I get almost exactly 8 MPG with my bus at 65 MPH.  I am doubting a little bit your numbers.  Most with a 6V92 report at least 1 MPG less  at 65 MPH without a 20 foot trailer.
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« Reply #28 on: June 16, 2010, 04:50:47 PM »

Brian I was shocked and didn't believe either but every fill up I did the math and even combined all the millage and fuel used at the end of the trip and it was exactly 8mpg. Maybe that's the reason my buddies call it the Magic Bus.
Kenny
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« Reply #29 on: June 17, 2010, 10:29:18 AM »

Tis a complicated thing this HP vs. MPG
If it was simple . . . . .

My two cents -

Fuel mileage is a result of the efficiency of the engine as it is being used. Some setups are inherently poor for efficiency while others are better. More available HP will allow you to accelerate back to an efficient rpm range when a low HP engine can't. In this case, if the driver of the low HP engine will usually have to drop a gear & then will ride the governor to maximize speed up the grade. In this case, if a little more speed was dropped, the engine would be in a more efficient rpm range & use less fuel.

I like having the choice of using more power or not - I don't much like feeling strapped.

If you look at the fuel curve of a given engine, you will see the rpm range that provides the most HP for a given amount of fuel. The higher HP engines will allow you to spend more time in this rpm band. You can also get there by having more gear ratios available in the transmission - but few of us are interested in constantly changing gears.

If your driving style is to wait until the bus looses speed on a hill then accelerate back up to desired cruise speed, then you are gonna have to feed those extra horses you used. However, if you 'float' your speed a little on hills, your mileage will be better.

I've used 2 suburbans to drag my 32' tin turd down the road. The 350ci would barely do 65mph on the flats & got only 9 mpg. The 454ci will accelerate up any hill I've encountered & gets about the same mileage, maybe a little better - but still in the single digits when the tin turd is attached.   Shocked

The 454 is so much easier to drive & it is amazing how much less frustrating traffic is when you can keep up if you want to.


If I get the chance, I'll install as much HP as I can - in the mean time I'll have to play the cards in my hand (hard to justify buying another engine when you have several running ones . . . . Sad )
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« Reply #30 on: June 17, 2010, 11:48:23 AM »

There are many good replies to this post and some which are in my opinion baseless.

I don't know much about birthing babies.

I do know that in my Eagle I had a 8-71N (318 hp) and replaced it with a Canadian Grizzly 8V92TA. (525 hp). I made several trips from Alaska to Key West, Mazatlan, Yuma and other winter destinations with both engines. I keep a mileage record of every time I fuel the coach (15 years and a LOT of miles). There is negligible difference in the fuel mileage. I use the same driving principals with the 92 that I did the 71. I also pulled the same trailer with both engines. The difference is I no longer have to endure 15mph climbs over 3000 miles of Rockies nor insulting remarks from the truckers over the CB like "You shoulda spent more on power and less on paint". Now they say "Bring my doors back".

If you install a hot rod engine and use that engine like a hot rod, fuel mileage will be your smallest problem. how much worse than 6.5mpg can you get?

If you install a big horse motor and use it when big horses are called for while considering the rest of your drive train you can never have too much hp and will be much happier.

You do not in most cases need more hp for speed, only climbs. Most coaches are designed to run hwy speeds. If you need more speed, change gearing.

You do not get an accurate estimate of mpg by taking a trip. You get a better picture after use of your coach over time and different conditions. I understand the principals of speed/drag/hp etc.

Most of us are different. The coach is a different brand, design, height, width, weight, rearend, transmission, is driven under different conditions (I now live in SD. Everywhere is uphill and against the wind), etc.

Some of us are compelled by nature to stretch the truth about mpg claims. That doesn't matter, one need only be concerned about their own.

In my opinion, if a mile or 2 of mpg is a great concern, The coach should be sold and a VW should be used.

Fleets of buses or trucks or any other vehicle need be concerned about these small differences due to the amount of fuel consumed spread across the fleet. That is one of the reasons you see generators in most trucks now that power the a/c and heat.

As someone on the boards says, "Your mileage may vary".
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« Reply #31 on: June 17, 2010, 01:41:45 PM »

    Horse power doesn't mount to a hill of beans if the torque is not there thats what moves the mass.
Two engines with the same horse power and diffrent ft lbs of torque is going to effect the fuel milage
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« Reply #32 on: June 17, 2010, 02:07:39 PM »

Unclewilly is right.
To further expand;

Torque is a measure of twisting effort independent of time - A pound-foot is the moment created by a force of 1 pound applied to the end of a lever arm 1 foot long.

HP is a measure of work - 1HP is the rate of work required to raise 33,000 pounds 1 foot in 1 minute.

HP can be related to torque if time is considered :
HP = (torque lb-ft) x rpm / 5252


So . . .
Torque moves the vehicle up the hill.
HP tells you how fast you make it to the top. (& gear ratios required)

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« Reply #33 on: June 17, 2010, 02:15:13 PM »

That was the nice thing about owning a 4104.  I never had to worry about such foolishness as how fast I was going to drive.  I simply drove as fast as it would go.  A lot like driving an old VW Beetle.
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« Reply #34 on: June 17, 2010, 04:05:03 PM »

Give this some thought.

You come out of the hole at a stoplight, put your foot in it, then go up through the gears.  When you get to highway speed, you come off the pedal to get the right speed.

How will you burn more fuel if you get through the gears in 30 seconds than if it takes a minute and a half?

I have a book produced by Porsche on 'Advanced Driving Techniques' which I think they gave away to their customers in the 1980s. It says unequivocally that accelerating quickly to a given speed uses no more fuel than accelerating slowly to the same speed. I do not believe this is correct though, because it's wrong to consider the 'time' aspect in that way (ie. "going through the gears in 30 seconds rather than a minute and a half" etc). You might just as well say driving quickly uses less fuel than driving slowly because if you drive quickly you reach your destination sooner, and therefore run the engine for less time.

Jeremy

Jeremy, as others have said; efficiency in acceleration versus cruise are 2 totally different games.  One depends more on weight and the other depends on drag (as I see it anyway). Power required for given speeds goes up exponentially because of drag.
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« Reply #35 on: June 17, 2010, 04:18:16 PM »

Jeremy, as others have said; efficiency in acceleration versus cruise are 2 totally different games.  One depends more on weight and the other depends on drag (as I see it anyway). Power required for given speeds goes up exponentially because of drag.

You might have meant to make this reply to someone else...I haven't said anything about this...

Jeremy
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