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Old 05-21-2008, 09:18 AM   #1
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HP, do we really need so much?

I've been thinking a lot about this lately. I recently purchased a 1963 Dodge D100 for trips to the dump and lumberyard. The truck has a 198 cubic inch
Slant-6 rated at 127 Gross hp. It weighs just a tad under 4000 lbs with me in it. Since modern cars are rated in net vs. gross hp, which is generally about 80% of the gross number. That would put my truck at right around 100 hp. Last time I took it to the dump I dropped off 1/2 ton of garbage. While I wouldn't be winning any drag races, I had no problem keeping up with traffic or mantaining the speed limit. That's at 50 lbs/hp (albiet only around 35lbs/ft-lb). Compare that to a Honda Fit economy car, which weighs probably around 2650 with a driver and tank of gas, and has 109 hp and 105 ft-lbs. That's 24 lbs/hp and 25 lbs/ft-lb. To match the numbers for my loaded truck it would only need 53 hp and/or 75 ft-lbs. To match my unloaded truck it would only need 66 hp and/or 94 ft-lbs Still, you could easily make that with a properly tuned 1.2L engine, maybe even a 1.0L.

I think Americans have been fooled into thinking they "need" hp, when in reality, a slow-turning, torquey engine would be far more efficient while still producing acceptable acceleration. A 1.5L Atkinson cycle engine in that Honda would likely net a massive increase in mpg albeit at the expense of unneeded acceleration potential. Sounds like a fair trade to me as gas approaches $4/gallon.
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Old 05-21-2008, 09:32 AM   #2
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High horsepower from a small engine does indeed come at the expense of overall efficiency.. To make horsepower numbers with a small naturally aspirated engine you must spin it up pretty hard.. Engine friction increases rapidly with rpm's, probably at about the square of the engine speed..

I understand that engines generally are at their highest efficiency near their torque peak, lower the peak torque rpm and it seems logical that overall efficiency would increase since engine friction would be lower at peak torque rpm.
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Old 05-21-2008, 09:55 AM   #3
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I understand that engines generally are at their highest efficiency near their torque peak
Efficiency tends to be high near the torque peak, but efficiency is also high at larger throttle openings. See here:

http://www.autospeed.com.au/cms/gall...0&a=110216&i=6

So samandw is exactly right. Our engines are excessively powerful, and this is a problem because it encourages us to use low throttle settings, which are inherently inefficient. P&G works because it gives us a way to use large throttle settings without accumulating excessive speed.

A small motor running at full throttle (which doesn't necessarily mean hi rpm) is going to be more efficient than a big motor operating at partial throttle, producing the same amount of power.
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Old 05-21-2008, 10:19 AM   #4
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"Horsepower sells cars, torque wins races". HP is sexy and can bring in money, but the fact is that torque is what most people are after. And yes, they have more than they need.

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Old 05-21-2008, 10:31 AM   #5
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Horsepower is a numbers game. An engine can make 200+ hp and only 50ft lbs if it revs high enough. My old nitro truck made 2.7HP at 35,000 rpm(yes, 35k) that's only .375Ft-Lbs.

GM has one of the lowest torque peak V6 engines in production from what I've been able to tell. The G8 3.6L makes 256hp at 6300 rpm and 248Ft/Lbs of torque at 2100 rpm. Nice, low torque in a HUGE power band.

The engine in your old truck is a 3.2L inline 6 but if you were to compare that with a newer inline 6 the power differences for the same displacement are pretty gross. My old 1988 Cressida with a 2.8L L6 made 156HP @ 5200 and 161Ft-Lbs @ 4400(figures that engine kept since its intro in 1984) and I averaged about 27mpg when I sold the car. What kind of mileage do you get? As the engines get more efficient it doesn't mean the displacement drops to make the HP numbers match.

An 88 corolla with the 1.6L got 23/31 making 90hp @ 6000 rpm and 95ft-lbs @ 3600 rpm and a new one has the 1.8 and gets 30/38 making 132 hp @ 6000 rpm and 128 lb.-ft. @ 4400 rpm both with a 4-speed auto.

2007 Silverado vs 1988 Silverado. 315hp @ 5200 and 338Ft-Lbs at 4400 in the new 5.3L while getting 16/21 while a 1988 Silverado with the 5.7L made 210hp at 4000 and 300 ft lbs at 2100 while only getting 14/18.

Take a 2007 Chevrolet Silverado for example again. The official EPA numbers for the 4.3L V6 automatic is 16/21 and for the 5.3L V8 automatic is 16/21 both with a 4-speed. Which one would you rather have? They get the same fuel economy but that V8 will have more power and probably last longer than the V6.

More HP doesn't always mean worse fuel economy. The Silverado here has 105 more HP(50% more) than it used to have 20 years ago but gets better economy with .4L less displacement.
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Old 05-21-2008, 10:40 AM   #6
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long stroke engines

I see this as one of the simplest ways to increase efficiency - increase the stroke length and reduce the piston diameter, extracting more work per F/A charge. A discussion along these lines could be very informative. This is 1940's technology! Many moons ago I recall riding in a 1965ish Ford van that could seat around 10+. It had a long stroke 6 in it. Owner reported getting around 20+mpg with it as long as he kept the speed within reason. Does it make sense to sleeve a current engine block and hassle with new pistons/rods/camshaft to get this effect?
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Old 05-21-2008, 10:57 AM   #7
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My old 1988 Cressida
Cool, another Cressida fan.

I had a '78 wagon, an '83, and an '85 wagon. And a family member had an '89. My favorite of the bunch was the '78.

I drove the '83 for over 10 years. It came to an untimely end when a kid ran a light and totaled it. A fairly severe accident, and I almost got hurt. The car had almost 300k at the time.

Sorry for the commercial interruption.
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Old 05-21-2008, 11:00 AM   #8
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I see this as one of the simplest ways to increase efficiency - increase the stroke length and reduce the piston diameter, extracting more work per F/A charge. A discussion along these lines could be very informative. This is 1940's technology! Many moons ago I recall riding in a 1965ish Ford van that could seat around 10+. It had a long stroke 6 in it. Owner reported getting around 20+mpg with it as long as he kept the speed within reason. Does it make sense to sleeve a current engine block and hassle with new pistons/rods/camshaft to get this effect?
Might be easier to get a lower displacement motor that will fit what you want to drive then get a stroker kit for it.

However... this can lead to poor top end (if you ever need it) due to increased piston speed. And done to excess can put you in that area at midrange.

But! Mods to increase burn speed will win some of this back and will increase thermal efficiency... to a point...

Myself, I'm looking at a shorter rod and getting a 2mm offset crank grind on Marvin's "new" motor for 4mm more stroke... it's not much really, but will keep torque nice and strong where I need it. Anything making peak torque much over 3500 is a useless POS IMO. Well I guess I should quantify that, there's some motors with decent low end, that have the peak shoved over by variable length induction systems and other tricks. Let's say anything that isn't making 80% of it's peak torque by 2K or so is a worthless POS.
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Old 05-21-2008, 11:15 AM   #9
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Originally Posted by dkjones96 View Post

The engine in your old truck is a 3.2L inline 6 but if you were to compare that with a newer inline 6 the power differences for the same displacement are pretty gross. My old 1988 Cressida with a 2.8L L6 made 156HP @ 5200 and 161Ft-Lbs @ 4400(figures that engine kept since its intro in 1984) and I averaged about 27mpg when I sold the car. What kind of mileage do you get? As the engines get more efficient it doesn't mean the displacement drops to make the HP numbers match.
I completely agree, my point was that we don't need nearly as much hp to be "functional" as people think. Obviously my ancient Slant 6 isn't a good example of an efficient engine. But if all I really NEED is 100 hp, that Cressida engine would only need to be 1.8L, which if it made good torque, should see significantly better mpg.

Quote:
Originally Posted by dkjones96 View Post
More HP doesn't always mean worse fuel economy. The Silverado here has 105 more HP(50% more) than it used to have 20 years ago but gets better economy with .4L less displacement.
Of course, one could argue that with 105 less hp, a modern technology Silverado could get better mpg than one using 20 year old technology.
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Old 05-21-2008, 11:23 AM   #10
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Efficiency tends to be high near the torque peak, but efficiency is also high at larger throttle openings. See here:
I'm aware of that, what I'm saying is that, all else being equal, an engine tends to be most efficient near its torque peak..

Make the the torque peak lower in rpm and you will most likely raise overall engine efficiency.

Which is one reason that diesels are more efficient than otto engines, the torque peak is almost always at a lower rpm on a diesel.
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89 Yamaha FZR400 Crotch rocket, semi naked with only the bikini fairing, no lowers, 60 plus mpg

87 Ranger 2.3 5spd.. Does not currently run..
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