Towing: 35i vs 35d
 

So I am fortunate enough to have two E70 X5s: One is a 2013 35i and one is a 2011 35d. We ordered the 2013 new and will likely keep it a long, long time as it has all the options we wanted (including AD) and is in great condition since we have had it since new (my wife is the primary driver). I liked it so much I searched for a 35d and purchased a CPO at a decent price in the color combo I wanted.

So....I am planning on having an invisihitch installed that I am planning on towing about a 4500-5000 lb boat behind. I assume the 35d will be better at pulling the boat, but I also know I will likely have the 35i for much longer. I don't really feel like putting a hitch on the 35d, but then having to go to the hassle of taking the hitch off and getting it installed on the 35i (with all of the harness wiring, coding, etc) after a few years.

So, how much better would the 35d be at towing a 5000 lb boat. Any opinions? Any first hand experience with both?

I'll let you know how my 2011 35d trailers a 2100# U-haul trailer with my 2007 M6 on it from Mississippi to Wisconsin (1200 miles) in a few days. The X is rated to tow 6000# and I'll be at about 5800#. The diesel has 400+ Ft # of torque I believe which is much more than your gas X5. It took me less than 2 hours to install the hitch the other day and it's less time consuming than most people think.

Both will pull it fine. The 35d may do better on towing fuel economy if you are towing a lot and that matters in your case.

Engine torque doesn't much matter, for all the press it gets. It is hp that gets you up the hill at a given speed. While the diesel has good low end torque, so does the 35i, it is a characteristic of the turbos more than the combustion cycle. While the diesel has higher peak torque, you are fortunate to have a transmission so wheel torque will be higher on the 35i than the 35d, at a given speed.

Does your 35i have the 8 speed? That would matter more than the engine, to me at least.

Short story, whichever one makes the most sense for you.

The truth is,you will not know until you test both.
My '03 4.6is used to tow #6500 car on a trailer on occasions and never struggled even on a steep grades,usually getting around 14 mpg on a flats.But was really struggling with my 21" Sea Ray 210SD,which is around twice as high and wide as the X5.Should be around #5500 with the trailer and some stuff inside.I was getting 9-10 Mpg.The air resistance is the key factor here.
The '12 X5D pulls the boat in a very commanding matter,5th and 6th gear and 15-16 Mpg.That is where the diesel shines really and i hardly have to go over 1500 RPM.
I cant say how the 30i will be,my guess is it will be at least a few Mpg down and it may need more high RPM and more downshifting.Regardless of the 8 speed tranny you cant tow the boat on 8th gear with 1500 RPM with the gas.But if you decide to try i'll be glad to hear your impressions.

Sorry,but there is no way M6 on a heavy U-haul trailer could be under #6000.But that is fine,I did tow up to #6500 at occasions and the X5 feels always in command.

I think it will be over 6K and watch your tongue weight.

2007 BMW M6 Specs

The curb weight of a 2007 M6 coupe is 3909#. I've removed over 120# from the exhaust system, 12# with an aluminum flywheel, 4# of exhaust shielding (Yes, 2 Mufflers deleted at 44# each) + resonators deleted and other exhaust mods. The weight is around 3725#. Trailer 2153# with a total weight of around 5920#. Yes, the fuel tank is empty. When reserving the U-Haul trailer it requires you to input the towed vehicle and the tow vehicle to compute if it's acceptable and the combination does fall within U-haul weight standards. To make the M6 as light as possible I'm removing the battery, air pump, and tool kit from the trunk and putting that in the X. That'll be another 75#. Every # will help. You are correct bawareca, even if it was 6500# it would probably still be OK. The M6 is very aerodynamic so that'll help for less wind resistance down the highway also.

I'll note the RPM at various highway speeds when I begin the trip tomorrow.

You did good,adding some lightness to the M6 :thumbup: Still the actual weight could be a tad more than you expect.You will be amazed how competent towing machine the X5 is and the people laughing at it have no idea of it's qualities.My diesel have no SLS on the back and I load the cars with a rearwards bias,more often the rear end first.U-haul trailer is very nose heavy(for safety reasons,of course)and if your X5 has no SLS you may consider loading the M6 backwards.
Good luck with your trip.

I think it is completely opposite. It is the torque that sustains a given speed uphill. Horsepower is what accelerates.
I always use this analogy for horsepower vs. torque:
Horsepower = quick ninja. Torque = sumo wrestler.
A ninja is hella quick, but won't be as quick with a heavy backpack on it's back. A sumo wrestler is slow and steady empty handed, and won't even feel that backpack.

That's in line with how people who tow with 35ds describe it: acceleration is pretty much the same with or without a boat attached.

Diesel, no ifs and or buts about it.

This is a very true analogy.

Thanks for the feedback everyone.

Curious why you say this. What properties about the diesel make it that much better....or what am I going to miss if I use the 35i?

he meant that the diesel is just doing the job,no questions asked.The 35i may do the job Ok,may do it excellent or may do it poor.In my mind the closer comparison is between the 35D and 50i,with advantage for 50i on the power side and disadvantage on the efficiency side.Maintenance and repairs may be a wash.

Neither! Sell both and get a 50i and tow with that :nanana:
Only kidding…sounds like you already have 2 nice X5's. The diesel would probably be the better choice but your 35i is newer and sounds like you'll keep it longer. Given that, I'd put the hitch on that one and just go with it. Might not be the best towing vehicle, but it'll get the job done and you won't have to worry about moving the hitch from one vehicle to another.

It is torque that sustains a given speed, correct. But it is wheel torque, or rim pull, not flywheel torque. Transmissions multiply that engine torque. Just means you may be in a different gear between the two models.

And with the higher hp of the 35i, that given speed will be higher than that of the diesel. The 35i can simply do more work.

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.

If you're going to keep the 35i, I'd hitch that. The X5 is rated to tow the same regardless of the engine. I still think the diesel will tow more "effortlessly" and efficiently than the 35i, but ultimately they will be equally as capable.

Just towed the M6 home (6000# total weight with trailer & car) to Wisconsin with the 2011 diesel. 952 miles and averaged 17.2 MPG running mostly at 74 MPH. At 70 MPH it was running at 2000 RPM. It did seem effortless and I was very surprised at how well the towing went. I'm guessing there is no way a gas X5 would get close to the same MPG's.

Exactly.....
No way a gasser is doing that with the same ease (except if its the 5.0) ;)

So is this all I am gaining with the diesel...mpg? If so, mileage is not that big of a deal to me given the frequency and distance I will be towing.

No, not the only gain. Read everything above.
Seems like you really don't want to have to remove the hitch from the diesel in a couple years. Just go with putting on the 35i. You will be fine considering frequency and distance :)

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.

Put this guy on a 6-speed unloaded bike. He'll move slower than previous guy due to less frequent pedaling. A hill comes up. This guy just pedals at the same speed and is able to overcome resistance due to stronger legs. A trailer is hooked up. This guy is able to overcome that, too.

Right, higher speed. With unloaded vehicle. Let's hook up a boat and watch gearbox frantically search for gears an RPMs redline, all to maintain that speed. A lower torque vehicle cannot maintain speed under load.

Not necessarily slower when under load.

P.S. This is getting nerdy

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.

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 .....

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.

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.

Mechanical engineering. 24 years in the diesel business. We can get a lot nerdier than this :rofl:

Thought it may help to use a truer analogy.

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.

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.

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?

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.

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.

So why the engine with a narrower powerband shifts less :dunno:

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.



http://www.1addicts.com/forums/attac...1&d=1270074298

Because of the higher torque available at a specific rpm. The driver isn't using all available torque at any given moment.

Ah, I guess I never really thought about active computer management limiting torque the way it does and at the point that it does. That is pretty impressive. I guess what impresses me the most now is that there really isn't a measurable (seat-of-the-pants) improvement with valvetronic versus without.

In essence, they seem to have actually RESTRICTED the engine in order to improve driveability.

And now bring fuel efficiency into it. That's the real goal. Drivability is just a bonus.

The X5 35i rating is the lowest of four or so different ratings, IIRC, but is broader.

And that is, indeed, the truth. Broad, flat torque curves with lots of "area under" not only improve driveability, but improve fuel mileage, due to the reduction in time spent with large amounts of throttle input.

Hence, this is probably why the traditional "diesel versus petrol versus turbocharged petrol" seems to favor diesel until technology catches up on the turbocharged petrol side of the house.

Interesting. Thanks for bringing a engineer's perspective into the discussion. I know a fair amount about things like this, but it was good to hear another side of the story, rather than gas purists screaming at diesel enthusiasts, and vice versa.

Drove for the first time on an extended highway trip for 118 miles on a 2-lane highway through 9 small towns so had to slow down and accelerate often and got 31.7 MPG. Cruise set at 62 MPH outside of the towns. Sure is nice filling up much less often than with the M6 although diesel is still more expensive than the 93 octane premium.

That is true for all engines,gas and diesel.You mention to the the other guy that we have to look not at the engines alone,but with the tranny and diff,as a complete drivetrain.But you now compare both engines as a standalone units.You miss to point out that the shorter gears on the diesel completely make it up for the narrow powerband for the sake of less torque to the wheels.But really the biggest downside of the US diesel to the 3.5i is the 6 speed tranny.The diesel with it's narrow powerband is the one that needs more gears.

Yes, it is. But given the higher torque ratings of the diesel, at a given rpm and load, the diesel has more surplus torque, ie more headroom. So it can be programmed to shift less. That won't make it faster, but it provides that sense of power that makes owners think they have more hp than they do.

BMW could install a transmission with more ratios, and has done on newer models, but I suspect there is a diminishing return there.

Or CVT LOL

I am confident that the diesel will get 2-3 more MPG on a freeway with 8 gear tranny.At 75 mph it is cruising with 2100-2200 RPM which is way out of it's efficiency range.