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  #21  
Old 09-24-2014, 03:29 PM
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I agree with the above post. Having driven diesels, turbodiesels, petrol, and turbocharged petrol engined cars (and trucks) I can, without a doubt, tell you that the turbodiesel engine is better in 95% of the situations in which you may find yourself in any given day. The only situation in which a petrol engine is better is the "emergency acceleration" situation, where you might need to pour the proverbial coals on to escape a collision with a tractor-trailer that is merging your direction on a multi-lane interstate. In that situation, where diesel engines have a tendency for long turbocharger spool, the more instantaneous thrust of a downshifted petrol engine is probably better. That, and redline-hunting freeway merging on short onramps. So basically, petrol engines are better when screaming at WOT toward redline.

Diesels and turbodiesels require lower RPMs to achieve excellent daily driveability. Due to the fact that most diesel engines redline in the 4K-4.5K range, much of the broad, flat torque curve of a diesel engine is at considerably lower RPMs. As a point of reference, my 6.0 Powerstroke in my Ford Excursion hits maximum torque (560 ft/lb) at right around 2,000 RPM. Remember, also, that the most important factor of what determines excellent driveability and performance is the so-called "area under the curve" of a basic horsepower and torque chart. Peak HP and peak TQ don't mean a lot if it is a narrow band at the highest RPM, with sharp dropoffs on either side. Again to cite my Powerstroke, it makes 400 ft/lb at 1,000 RPM (idles at about 700 RPM), makes 500 ft/lb at 1,500 RPM, and doesn't drop below 500 ft/lb again (after peaking right around 2,000 RPM) until the engine hits about 3,500 RPM. Given that it redlines at 4,000 RPM, that's a pretty broad torque curve, as roughly 50% of the useable engine rev range is at or above 500 ft/lb.

As anecdotal evidence, on my Excursion, at 55 MPH, I can toe in more throttle and rather comfortably accelerate to 70 MPH quickly without downshifting to a lower (4th) gear. On a 5-speed transmission, that's a considerable accomplishment. I can only wonder at what my truck would drive like with a 7-speed or 8-speed transmission as one finds in most luxury automakers cars now.

More anecdotal evidence: I loaded up the wife's X5 (2007 X5 4.8i) with a full load of camping gear and drove it almost 400 miles to my camping destination. It was the two of us, so there was a LOT of stuff (wife can't camp light LOL), and the X5 did OKAY, but there was a considerable dropoff in performance, especially going up any grade at all. The car lugged a lot, and downshifted frequently. My Excursion did the same trip, this time with 2 adults, two children, and two infants, with an even larger amount of stuff (thanks to the presence of children), and the truck barely seemed to "notice" the additional weight. Granted, there is a marked difference in curb weight between a 4,000 lb X5 and an 8,000 lb Excursion, but the added few hundred pounds of gear and food didn't slow me down at all. In fact, I still averaged 23+ MPG on highway and rural roads during both legs of the trip in my Excursion.

The point to the discussion is, if you want a racecar, or something that drives more like a sports car, a factory tuned diesel isn't for you. Stick with petrol. But if you want something that can pull loads, trailers, full cabins, or just give you civilized street manners without requiring lots of throttle to get you around town, the diesel is the way to go. Now that I drive a diesel full-time, I can't imagine driving anything else.
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  #22  
Old 09-24-2014, 03:53 PM
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My E53 gasser six with a 6 spd stick lacks the torque to be an ideal towing vehicle. I have towed approximately 4500-4800 lbs and have to do a lot of downshifting to 4th [even 3d] on rolling terrain. My experience with Ford trucks, supposedly 'comparably' powered [V-10 gasser vis. 7.3L V-8 diesel] was no comparison: the diesel's torque makes all the difference. I would 'hitch up' the diesel and be happy .....
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  #23  
Old 09-24-2014, 03:58 PM
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Quote:
Originally Posted by seattle View Post
Another analogy, better one.
Let's say you have a skinny fast guy on a 8-speed bike....
Sorry, but that is not a better analogy. We can use sumo wrestlers, or cyclists, whatever you like, but we can't mix up units of power, speed, and torque. The engineers among us have those definitions down.

The racing cyclist can spin at 7000. He has a certain torque, and so can push a given gear. The heavier cyclist can spin at 5000, but likes a lower cadence. He has a narrower range of efficient cadences. He can push a bigger gear at those lower cadences. But at the end of the day, the racing cyclist can do more work per unit of time. That is all hp is, work over time. A given weight, raised a given distance, in a given time.

Quote:
A lower torque vehicle cannot maintain speed under load
It can if it has more hp. Your statement is true if you eliminate the transmission, or the ability to shift. No frantic hunting for gears required, just drop it one gear or so.

Quote:
P.S. This is getting nerdy
Mechanical engineering. 24 years in the diesel business. We can get a lot nerdier than this
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  #24  
Old 09-24-2014, 04:26 PM
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Thought it may help to use a truer analogy.

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Originally Posted by seattle View Post
Another analogy, better one.
Let's say you have a skinny fast guy on a 8-speed bike. Fast pedaling = more work done = high horsepower. He can pedal and go fast all day in any gear with relative ease. Sounds good, right?

Now, let's hook a small trailer with to that bike and load 200lbs on it.
Will the skinny fast guy be able to pull the trailer? Yes, he will switch to lower gears. Coming up to a hill, change to even lower gear and just pedal fast (high RPM). Will sweat a lot and need lots of Gatorade.

Let's say this skinny guy can push 300lbs with his legs in the gim, and pedal 300 times a minute. That's X5 35i with 300 hp /300lbf torque rating. A very good athlete compared to others, no doubt about it.

Then you have another guy, who can't pedal as fast, but got muscular legs that can overcome more resistance (torque). This guy can pedal "only" 265 times per minute, but can press 425 lbs at the gym. 35d rating.
Skinny guy on an eight speed bike. He can pedal up to 7000, but develops peak torque at 1300. He can leg press 300 in the gym. Divide cadence by 50 to make it more believable.

Another guy, on a six speed bike. He can pedal up to 5000, but develops peak torque at 1700. He can leg press 425 at the gym.

The two of them are put on identical weight bikes with trailers that can carry variable weights. They are asked to ride up the same hill, to see what is the maximum total weight they can get to the top in one minute, using whatever gear they like. The skinny guy gets 300 lbs to the top. The other guy gets 265 lbs to the top. They are equally tired and sweaty. That is a better analogy for power, and it matches the engine ratings.

And the difference continues with more weight, ie larger trailers. Heavy guy is proportionally slower.
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  #25  
Old 09-24-2014, 04:51 PM
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Quote:
Originally Posted by slowlanemcvane View Post
Remember, also, that the most important factor of what determines excellent driveability and performance is the so-called "area under the curve" of a basic horsepower and torque chart. Peak HP and peak TQ don't mean a lot if it is a narrow band at the highest RPM, with sharp dropoffs on either side.
Agreed.

It is important to compare turbocharged engines with other turbocharged engines. The N57 diesel is listed as having full torque available from 1750 to 3000 rpm, a 1250 rpm band. The N55 gasoline engine is listed as having full torque available from 1200 to 5000 rpm, a 3800 rpm band, ie 3 times as wide. While the N57 has greater peak torque, which would you describe as having more drivability/flexibility?

With my 535i with a 6MT, I could pull away in 1st and then shift 3-5, or pull away in second and shift 4-6. Or just go 1-4. It would have out towed my former diesels if I could have put a hitch on it.
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  #26  
Old 09-24-2014, 04:58 PM
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Another interesting fact.
Take a look at the effect that added weight and wind resistance take on gasoline and diesel engines when it comes to acceleration:

2011 335i: 4.9s 0-60mph. Curb weight: 3814 lbs
2011 X5 35i: 6.5s 0-60 mph. Curb weight: 4960 lbs (+1,146 lbs, or +23%)
That's 25% reduction in acceleration at 23% increase in curb weight. A straight line in a chart.

2011 335d: 5.7s 0-60mph. Curb weight: 3825 lbs.
2011 X5 35d: 6.9s 0-60mph. Curb weight: 5192 (+1,367 lbs, or 26 %)
That's only 17% reduction in acceleration at 26% increase in curb weight.

Source: Car&Driver, Edmunds.

Shall we see projected acceleration given additional 6000 towed lbs?
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  #27  
Old 09-24-2014, 05:20 PM
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Quote:
Originally Posted by JCL View Post
The N55 gasoline engine is listed as having full torque available from 1200 to 5000 rpm, a 3800 rpm band, ie 3 times as wide.
It would be interesting to actually see the HP/TQ curve printout, and what BMW constitutes as "full torque available."

I don't really know if I've ever seen a petrol engine with 80+% torque available for that much of the rev range. If it is, indeed, the case with the N55, that's quite an impressive accomplishment, and BMW deserves some accolades for designing such an easily daily driver-friendly engine.
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  #28  
Old 09-24-2014, 05:23 PM
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Quote:
Originally Posted by JCL View Post
With my 535i with a 6MT, I could pull away in 1st and then shift 3-5, or pull away in second and shift 4-6. Or just go 1-4. It would have out towed my former diesels if I could have put a hitch on it.
I accomplished similar feats in my old 130 HP Ford Focus with the MTX-75 5-speed manual, due largely to the relatively broad, flat torque curve and the 2,400 LB curb weight of the car. I would routinely 2nd gear launch, then skip-shift straight to 5th, unless I was driving aggressively.

I did enjoy the mental image of a 535i towing a diesel, that made me chuckle.
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  #29  
Old 09-24-2014, 05:26 PM
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Quote:
Originally Posted by JCL View Post
............
What the diesel owners notice is that the vehicle doesn't need to shift as often. They describe that as feeling effortless. No argument. But it is still slower. And with the narrower power band of the diesel, and fewer gears in the transmission, it is a good thing it does have more flywheel torque.

Quote:
Originally Posted by JCL View Post
The N57 diesel is listed as having full torque available from 1750 to 3000 rpm, a 1250 rpm band. The N55 gasoline engine is listed as having full torque available from 1200 to 5000 rpm, a 3800 rpm band, ie 3 times as wide. While the N57 has greater peak torque, which would you describe as having more drivability/flexibility?
So why the engine with a narrower powerband shifts less
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  #30  
Old 09-24-2014, 05:32 PM
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Quote:
Originally Posted by slowlanemcvane View Post
It would be interesting to actually see the HP/TQ curve printout, and what BMW constitutes as "full torque available."

I don't really know if I've ever seen a petrol engine with 80+% torque available for that much of the rev range. If it is, indeed, the case with the N55, that's quite an impressive accomplishment, and BMW deserves some accolades for designing such an easily daily driver-friendly engine.
Realize that with twin scroll turbocharger, Valvetronic, and engine management software that limits torque to rated figures, they have a lot of tools in the toolbox. An impressive graph, nonetheless.



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