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Old 09-24-2013, 02:30 AM   #11
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Re: How little engine power is just barely enough?

At some point it's not safe either. Having driven a G20 with a failing engine in Texas- I could stay at speed, but couldn't accelerate to merge and got forced off the Interstate more than a few times because of that.

I've also driven up the Rockies a fair amount, and while you can go up at 10mph, I'm not sure it's something you actually want to do in practice.

You can never really get more than a certain amount of power, you can always lift your foot to save mpg.
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Old 09-24-2013, 10:29 AM   #12
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Re: How little engine power is just barely enough?

Quote:
Originally Posted by Silver350
It's all about the torque which is in turn, multiplied by the gears and tire size. If you have the right gearing and enough gears you can do a whole lot with very little. If you have the patience, which most people seem to lack.

More HP just let's you do the same work faster, which is why Euro style diesels likely wouldn't sell very well here.

It's not the 550 HP that makes a C15 Cat such a joy to drive, it's the 2150 Ft. Lbs. of torque at 1100 RPM that's so addictive.
I have to agree with Chance. It is horsepower, not torque. Imagine the same 2150 lb-ft of torque at 2200 rpm. The torque is the same, but you could gear it down 2:1 and get 4300 lb-ft at 1100 rpm. Horsepower accounts for the torque and the ability to gear it and get more torque.

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Old 09-24-2013, 10:52 AM   #13
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Re: How little engine power is just barely enough?

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Originally Posted by Silver350
It's all about the torque which is in turn, multiplied by the gears and tire size. If you have the right gearing and enough gears you can do a whole lot with very little. If you have the patience, which most people seem to lack.

More HP just let's you do the same work faster, which is why Euro style diesels likely wouldn't sell very well here.

It's not the 550 HP that makes a C15 Cat such a joy to drive, it's the 2150 Ft. Lbs. of torque at 1100 RPM that's so addictive.
How a vehicle and engine combination feels to the driver is very subjective. It canít be measured with numbers. Iíll give you that.

On the other hand, power and torque are very objective by comparison and can not only be measured, they can be related to each other if only the engineís RPM is given as a point of reference. When a CAT C15 produces 2150 lb-ft or torque at 1100 RPM, it is exactly the same as saying 450 HP at 1100 RPM. Likewise, 550 HP at 1800 RPM is exactly the same as saying 1600 lb-ft of torque at 1800 RPM.

You are correct that more HP allows you to do the same work faster. And for driving on real-world highways, that means climbing the same hill at a faster speed. More horsepower might allow a van to climb a steep hill at 50 MPH whereas if it had less power it might only be able to go up at 30 MPH.

If we were to focus solely on torque, comparing an engine with 400 lb-ft to another with 800 lb-ft doesnít give us a clue as to which one might pull up a hill faster. Without rated RPM itís not enough information. And if we include RPMs in addition to torque, then it becomes power.
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Old 09-24-2013, 10:59 AM   #14
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Re: How little engine power is just barely enough?

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At some point it's not safe either. Having driven a G20 with a failing engine in Texas- I could stay at speed, but couldn't accelerate to merge and got forced off the Interstate more than a few times because of that.

I've also driven up the Rockies a fair amount, and while you can go up at 10mph, I'm not sure it's something you actually want to do in practice.

You can never really get more than a certain amount of power, you can always lift your foot to save mpg.
The highlighted part interest me the most. It relates directly to my question.

If I need only 50 HP to cruise at 70 MPH, I can lift off the throttle on my 305 HP V10 but that makes the engine run less efficient. By comparison, if I started out with a smaller engine like the 4.6 V8, I wouldn't have to lift off the throttle as much, thereby making the engine more efficient and saving fuel (assuming 50 HP at about 2200 RPM).

If we downsize the engine until it runs at the "most" efficient point on the load curve when only using 50 HP or less, would it still be enough for the occassions we need a lot more power? That's the balance I'm trying to find.
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Old 09-24-2013, 10:59 AM   #15
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Re: How little engine power is just barely enough?

I guess it depends on how one defines performance. To some its a sled pull, to others it's a drag race.

Torque and HP curves aren't linear and are at opposite ends of the RPM range. Just as TQ falls off, HP and therefore fuel consumption are still relatively low. That's the "sweet spot"

HP and TQ don't always go hand in hand.

Nobody that knows what they're doing drives at maximum rated HP but everyone drives at maximum TQ.

My 5.4 pulling a 6500# trailer up an 8% just knows to sit at 2500 RPM. right at maximum TQ.
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Old 09-24-2013, 11:12 AM   #16
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Re: How little engine power is just barely enough?

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.....cut......

Nobody that knows what they're doing drives at maximum rated HP but everyone drives at maximum TQ.

My 5.4 pulling a 6500# trailer up an 8% just knows to sit at 2500 RPM. right at maximum TQ.
Agree that it feels right, and engine sounds better, but if you wanted to go up that hill faster you could use a lower gear and let the engine go up close to rated power.

Ford rates new 5.4 V8 at 255 HP at 4500 RPM. That's same as 298 lb-ft of torque at 4500 RPM.

They also rate same engine at 350 lb-ft at 2500 RPM. That's same as 167 HP at 2500 RPM.

I also wouldn't do it myself on a regular basis, but you can see from above numbers that you could climb much faster at 255 HP than at 167 HP. We would just need to use a lower gear and let it rip.

As I said, I wouldn't do it to my own vehicle either; but the numbers don't lie.
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Old 09-24-2013, 11:33 AM   #17
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Re: How little engine power is just barely enough?

It's hard to tell with an automatic because the torque converter is capable of making its own gear ratio. A lower gear is also multiplying available TQ at a higher rate and on a 4 speed that difference can be 30% between gears.

But I can tell you that my big truck will not go up a big hill any measurable amount faster in a lower gear at higher RPM. The difference will be equal to a diff ratio or tire size change.
It will however use more fuel.
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Old 09-24-2013, 12:06 PM   #18
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Re: How little engine power is just barely enough?

Also, when changing to a lower gear with the intent of going up a hill faster....one has to assume that momentum lost during the gear change will be recovered and that the lower gear ratio will be such that the intended increased MPH will even be attainable at the intended RPM.
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Old 09-24-2013, 01:38 PM   #19
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Re: How little engine power is just barely enough?

I have onlly had an experience of too little power one time. I was about 15 years old and working on my uncles farm for the summer. He let me drive the loaded hay truck back home. It was down a steep hill. The truck was an old Studebaker, maybe from the 40's or 50's.

My uncle told me to put it in 1st gear and keep it there. Being 15, once I got down the steep part I decided to upshift to 2nd. It was a non-synchronized manual transmission. Of course I immediately started up a slight hill and needed to downshift. Of course I missed the shift and had to stop. The brakes were so bad I had to slide down on the seat and hold the pedal to the floor to stop from rolling backwards, until I was rescued.

I have vowed to never be underpowered again!
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Old 09-24-2013, 02:40 PM   #20
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Re: How little engine power is just barely enough?

Our Peterbilt with the "big-cam 400" motor was underpowered. The truck weighed about 33,000 empty. When you hooked a truck pulling doubles or triples behind that, that adds another 105,500 pounds. On one 7% grade the truck barely had enough power to launch in LOW-LOW, and could only manage pulling the hill in LOW-HI. It couldn't even hit 1st-LOW because it lost all its speed during the shift (going from LOW to HIGH only required flipping a switch and and a brief ease off the throttle). That meant top speed climbing that hill was a brisk walk. Not even a jog. That truck has now been replaced with a 550 hp powered Pete, but more importantly the new engine has a much broader torque band, so it doesn't fall on its face if too much speed is lost during a shift.
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