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Old 09-21-2013, 02:19 PM   #1
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How little engine power is just barely enough?

The opposite is easy; you can never have too much power, or so they say. I'm interested in other extreme.

My question is more about how little power is actually necessary. Looking at Ford large RV chassis specs, it appears they rate the 362 HP V10 up to a maximum GCWR of 30,000 pounds. That's only about 121 HP for every 10,000 pounds it's moving around.

Likewise, the new Sprinter 4-cylinder 161 HP diesel is rated at a GCWR of up to 15,250 pounds. That works out to about 106 HP per 10,000 pounds when loaded and towing to maximum capacity.

Even if viewed based on Gross Vehicle Weight Rating these work out to 139 and 146 HP per 10,000 pounds of vehicle weight. That would be fully loaded vehicle and no towing.

What's been your experience when driving and /or towing when you felt available power was just not enough.


Let's assume the vehicle has enough gears that "torque" issues are not a factor and that driver is OK with letting engine rev as necessary to produce power and torque at driven wheels.
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Old 09-21-2013, 03:06 PM   #2
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Re: How little engine power is just barely enough?

First you probably need to answer some questions about:

- How fast do you want to drive?

- How quickly do you want to get to that speed?

- What kind of hills/mountains do you climb?

- Do you mind going 10-30mph up those hills?

- How aerodynamic is your vehicle?

Mike
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Old 09-21-2013, 03:48 PM   #3
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Re: How little engine power is just barely enough?

Mike, when Ford sells an RV chassis, it's meant to be an all-around vehicle. It's what it is. Presume slow, very slow. Same for Sprinter when loaded down.

Theoretical needed power based on conditions is OK I guess, but I'd like to hear actual real-world experiences. I've only found myself once while towing a trailer thinking what the hell am I doing because of inadequate engine power. By then I was committed. That was roughly at about 125 HP per 10,000 pounds. At times it felt dangerous.

I'm wondering if this happens occasionally. And if so, at what level.
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Old 09-21-2013, 05:20 PM   #4
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Re: How little engine power is just barely enough?

I have base response on OTR trucks. Generally rated for 80,000# and 3-600 horsepower. At full load and 600HP that is 75HP/10,000#. And that seems to get the job done. For me, that would be near the bottom of acceptable. I could probably accept less if everyone was in the same situation. I don't know if that is acceptable to you.

Being able to go is more important than how quickly I can go uphill. Years ago I drove a VW Westy across country pulling a trailer. It worked and it was a great trip.

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Old 09-23-2013, 10:33 AM   #5
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Re: How little engine power is just barely enough?

Quote:
Originally Posted by Ford_6L_E350
I have base response on OTR trucks. Generally rated for 80,000# and 3-600 horsepower. At full load and 600HP that is 75HP/10,000#. And that seems to get the job done. For me, that would be near the bottom of acceptable. I could probably accept less if everyone was in the same situation. I don't know if that is acceptable to you.

....cut.....
Mike, thatís a great way to look at a lower-end of power limit that works for many vehicles on a regular basis. It's hard to argue with success. We know semi-trucks go coast to coast regularly and cross the Rockies with few problems. Even if trucks had much more power I doubt they wouldnít save that much time overall traversing the US. Iím guessing that as long as a vehicle can hold cruising speed most of the time having to slow for occasional steep grade isnít going to affect total drive time that much.

Fortunately, I think that downsizing on a vanís power (more like engine size than power) in order to maximize fuel economy doesnít have to go anywhere near the extreme of a semi-truck. A lot of discussions around here and other forums center on whether to go 6.8 V10 or 5.4 V8, but it seems to me that even a 4.6 V8 (assuming it could improve on fuel economy) would be sufficient for a van as long as it wasnít going to be towing a large trailer regularly.
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Old 09-23-2013, 10:38 AM   #6
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Re: How little engine power is just barely enough?

Of possible interest: objective test data based on "approximately" 10,000 pounds per 100 HP

Doing research on what level of performance one could expect in real-world driving when specific engine power is grossly limited compared to what we are used to, I ran across data for a school bus certification that had a Ford 6.8-liter V10 and 6R140 transmission.

33K test weight
5.29 axle
517 rev/mile tires
85 square feet frontal area
.55 Cd

90 MPH top speed at 0% grade
66 MPH top speed at 3% grade
35 MPH top speed at 7% grade
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Old 09-23-2013, 10:46 AM   #7
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Re: How little engine power is just barely enough?

A couple of random thoughts.

1) I'm not sure a straight-line estimation is correct here. Seems to me there's a minimum threshold, and then a non-linear curve might be more appropriate.

2) Something that may be at least equally appropriate to look at is torque, and not just HP. Not just maximum torque, but also at what RPM that max torque is achieved.


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Old 09-23-2013, 11:57 AM   #8
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Re: How little engine power is just barely enough?

Quote:
Originally Posted by BroncoHauler
A couple of random thoughts.

1) I'm not sure a straight-line estimation is correct here. Seems to me there's a minimum threshold, and then a non-linear curve might be more appropriate.

....cut.....

Herb
Herb, I agree itís not linear. However, I expect that most recreational vehicles can reach an adequate cruising speed and that itís acceleration and climbing of steep grades (at slower speeds) that most drivers will find lacking to the point of being an objection. And since at slow speeds aerodynamic drag is a small part of total required power, then using power-to-weight gives a good indication of how a vehicle will accelerate in city traffic and/or climb the steepest mountain passes.

In the example I listed in the OP, this non-linear experience happened to me. On a level Interstate going into a head wind trucks passed me with ease, but when we were crossing the Continental Divide I was able to pass some of them back. I had more power per weight, but also had more aero drag per unit mass and power than they did. That was an extreme case but it shows that you are right; it doesnít have to be linear.
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Old 09-23-2013, 12:08 PM   #9
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Re: How little engine power is just barely enough?

Quote:
Originally Posted by BroncoHauler
A couple of random thoughts.

....cut.....

2) Something that may be at least equally appropriate to look at is torque, and not just HP. Not just maximum torque, but also at what RPM that max torque is achieved.


Herb
Power takes torque into account; and more.

Actually, the ďmoreĒ is RPM. As long as the vehicle has enough gears and the driver is able and willing to use them to get the most out of the engine, then power (which accounts for both torque and RPM) is a better indicator than a torque number alone. And if we include RPM along with torque, then thatís called power. We canít get around the physics of it.

I'm aware it's a controversial subject with many on both sides of the "torque vs. power" issue. What I hate is hearing commercials like the Ford truck one that states "torque is power". No, actually, torque is torque and power is power. Commericals like that one should be banned.
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Old 09-23-2013, 11:28 PM   #10
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Re: How little engine power is just barely enough?

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.
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