BertP
Edmonton, Alberta, Canada
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Quote:As I said, many factors about the drivetrain are considered, for GCWR rating. Longevity of service is one of those factors, and is why the diesel has the higher GCWR rating. It (diesel) can handle the higher load, without wearing out as fast. Yes, the gasser can generate more horepower, and could handle more GCWR load than the diesel, since it has more horsepower. But, the gasser would NOT LAST AS LONG as the diesel, since the gasser would have to turn 5,000 RPMs all day in order to do it.
I believe that the 8.1 produces its max HP at around 4000 rpm, not 5000, but in any event, both the 8.1 and the DMax are rated as 200,000 miles engines. Since they have the same GCVW ratings in all except the 3500 with the Allison, it would seem that GM at least believes that the 8.1 gasser will las just as long as their DMax diesel doing exactly the same amount of work.
Quote:But, as many have said, it gets into economics - a big block V8 gasser that could turn 5,000 RPMs all day long and still last 200,000 miles, would be a VERY expensive engine to build, and would cost too much to put in regular production trucks. Much cheaper to just give the gasser a lower GCWR rating and gear it to not turn so many RPMs. Then, offer a diesel engine with a higher GCWR rating, since even though it has less maximum horsepower, it can still generate enough horsepower to move the bigger load, and do it without twisting as hard.
See above. GM says that the 8.1 will do just that and it is quite a bit cheaper than the DMax.
Bert
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AZDodge4Sand
Cave Creek, AZ, USA
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Diesel or gas? Diesel wins on engine efficiency. If gassers were the best choice to a TV, then all the commercial trucks would use gas motors - but they don't.
I'll add the following with a qualification first - I'm a degreed mechanical engineer. This topic is the stuff I studied in school and it's what a lot of BSME's do for a day job.
Any vehicle is going to reach its top speed when the engine is creating its peak hp number - as long as the gearing allows the motor to spin at that speed. Along with that statement goes this - the vehicles top speed is NOT achieved at the engines peak torque rpm. I am confident that no one here can state factually that their TV reaches max speed on a hill at the rpm the motor makes its peak torque. Plain and simple.
Torque is what gives a vehicle acceleration. Big trucks need lots of torque multiplied by low gearing so they can accelerate the load. Once they start upshifting, the rear wheel torque starts to drop and eventually acceleration stops. This is the maximum velocity the rig can manage. The rpm WILL be at the engines peak hp number.
In sum, an old friend explained it best when he said "Torque is what gets you moving, but hp is what determines how fast you'll end up going".
Chuck in AZ- It's not hot here, hehehe.
2005 Silverado Crew Cab 1500 5.3L
2004.5 Dodge CTD - gone but not forgotten
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Aquaduct
Winchester, VA USA
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Quote:I'll add the following with a qualification first - I'm a degreed mechanical engineer. This topic is the stuff I studied in school and it's what a lot of BSME's do for a day job.
What makes you think qualifications count? (Just funnin' ya. I'm a powertrain engineer and most people don't listen to me either).
Just to refresh your memory and put to bed some minor details;
Torque is a force. Like any force, it can be present without actually accomplishing anything. I can push on the side of a building with my hand and still apply a force without moving anything.
A force that moves something over a distance creates work. Work done in a certain amount of time is power.
Power and torque are mathematically related. Hp = torque x rpm/5252 in non-metric units.
The significant difference is that power is consistent throughout the engine/drivetrain system. Torque changes with gearing.
A moving vehicle requires a certain amount of power to move it at a certain speed (distance per unit time). It's a relatively simple formula that starts low at slow speeds when you're only overcoming rolling resistance and increases rapidly over about 30-40 mph as wind resistance builds. Any grades will tend to increase those requirements pretty substantially.
Gearing also forces the engine to operate at a particular rpm for a specific speed. Given some corrections for drivetrain efficiency, if the engine can deliver the required power at the given speed, you'll continue at that speed. Acceleration is actually a function of how much more power is available than is needed.
But remember, torque and power and mathematically joined at the hip. All these calculations can be done using engine torque, but the math ends up being more of a PITA. If you actually sit down with engine and drivetrain system data, you'll realize that anytime someone talks about great performance due to torque or power, similar math will get you to the same realization with the other.
Torque's real significance is at the only place where power doesn't exist (at the wheels anyway) which is when the vehicle is not in motion. Torque is significant in getting the wheels rolling or "startability" as it's called.
The efficiency and durability advantage of diesel doesn't tell the entire reason for thier popularity in commercial trucks. The fact is that diesels have yet to play by the same emissions rules as gas. That fact will potentially shake the heavy duty industry in the next decade as "fuel neutral" emission rules and low sulfur diesel fuel rattle the commonly held assumptions of diesel towing superiority.
Diesels really gained a heavy duty advantage over gas with the advent of turbochargers, allowing them to get substanial power to go along with low end torque (a byproduct of extra rotating masses and lower rated rpms). They were no longer "all grunt and no go" and commercial trucks could get fuel efficiency and speed making them economically superior.
2002 brought substantial emission reduction requirements met mostly with EGR and CAT's Miller Cycle operation. Fuel economy and price have taken a non-negligible hit and system durability has been affected somewhat.
2007 will virtually eliminate smoke emissions and will introduce aftertreatment systems for the first time, again impacting cost, economy, and system durability negatively.
2010 will virtually eliminate NOx, again with predictible results.
And all of that doesn't take into account the shift in fuel prices that can occur if diesel cars become as popular as they are in Europe once the emissions get cleaned up. I mean, just look at how much howling goes on when the heating oil season kicks in.
This is not to say that diesels will disappear, but it will even the field for gasoline which will continue to improve. I've seen knowledgable industry predicitions that diesel will lose its effective economic advantage by the middle of the next decade. Do I agree with the predictions? I do heavy diesels and I don't think things are that dire, but the preposterous notion that either cycle has a God-given superiority would prove to be economically fatal.
Yep. Right now diesels give better fuel efficiency that can offset the extra cost with solid performance. But I can buy gas at no less than 3 stations within 4 blocks of my house. I haven't found an in-town diesel station yet. And my wife doesn't like diesel. And I don't tow that much that I going to recognize much savings, if any, long term.
At the end of the day, buy what you like and what suits your job. It's not a simple one dimensional question and simple platitudes serve no purpose.
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BertP
Edmonton, Alberta, Canada
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Quote:Torque is what gives a vehicle acceleration. Big trucks need lots of torque multiplied by low gearing so they can accelerate the load. Once they start upshifting, the rear wheel torque starts to drop and eventually acceleration stops. This is the maximum velocity the rig can manage. The rpm WILL be at the engines peak hp number.
Chuck, I'm not sure if I am reading this correctly, but a big truck does not require lots of torque from the engine. As long as the gearing will allow the torque produced to be multiplied to the level that is required at the axle, you would be good to go. A case in point would be the turbine. Very low torque, but with the correct gearing, you would get all kinds of torque at the drive axle. The maximum amount of torque that can be present at the drive axle for a given speed is limited by the amount of HP available from the engine, with the caveat as you mentioned "as long as the gearing allows the motor to spin at that speed".
Quote:In sum, an old friend explained it best when he said "Torque is what gets you moving, but hp is what determines how fast you'll end up going".
And that is a good rule of thumb, as long as it is put in the correct context. As I mentioned above, the torque being referred to is that present at the axle, not necessarily that ouput by the engine.
Bert
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sack1
Kent, WA
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Joined: 08/16/2002
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I would like to point out something again to people reading here. Our 8.1 (I use it as a familiar example, not because I think it's superior) tows our 12k gross 5er very easily. I have lots of power to spare. We just got back from a 300 mile trip to the ocean and back and I simply cruised down the road. I usually ran at about 62-65 mph. My average mileage for the trip worked out to be 9.4 mpg. This was shown on the computer as well as being backed up when I refueled. I topped it off to the top of the filler neck to be as accurate as I could be. While I consider that mileage to be very reasonable the diesels will get a few mpg more. We have RVing friends with a 7.3 PSD and similar size/weight 5er and he averages just over 11.5 for the same trip. If this were a contest in who gets the best mileage bar none then he'd be the winner. But you know, we both tow down the road at the same speed. I do so with little fuss and so does he. The 8.1 is rated for the same life as the Duramax diesel which is 200k. The Cummins CTD may see a longer life but that depends on how its maintained as it does with any engine. Then too, for many people 200K is many many years down the road, let alone 350k. Will they own that vehicle that long? I know I most likely will not.
And for you diesel fans, it's easy to see how one might compare their old small block or old BB to their new shiny turbo diesel but they most likely were not rated to tow the same loads. It is like comparing apples to oranges. To make an honest comparison you need to look at those rigs rated to tow in the same league. For that in gas engines it's the GM 8.1, the Ford V10 and perhaps the upcoming larger Dodge Hemi. By the way, I have recently read that GM will be replacing its BBs with a line of maybe three V10s. One may show in the Corvette. If I remember correctly the one slated for use in the trucks will be in the neighborhood of 7.7 liter. If diesel were so domineering then doesn't it seem unlikely to see millions spent on a new engine family?
I suggest too that anyone read the Diesel Deliberations post I shared on the forum a few weeks back. It's not blatantly pro or con diesel but offers up a pretty in-depth valuation of the issue.
To sum up my thoughts, when I can RV down the road in comfort and quiet and do so within economic boundaries and with power to spare where does the supposed diesel superiority come into play and why am I doing it wrong?
'03 Chevy 2500HD 4x4, LT, Ext. cab, LB
8.1/Allison, 4.10
'03 32' 2955 Montana 5er
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FF-Mike
Lannon, WI, USA
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Joined: 10/07/2004
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Gas vs Diesel.
I will skip the basics that keep getting rehashed and get on with it.
If the vehicle will be TV only, go diesel
If only a TV a few times annualy and a daily driver otherwise- go gas
Diesel is better for towing as the engine develope more torque at a lower rpm which is what you need to get going.
Gas is better for daily driving as there will be quicker throttle response when zipping in and out of traffic
I have also read about using diesel appliances only in RVs. Never sen anything about gas appliances in RVs. Not something I would convert to unless I used the trailer alot and had a diesel TV.
99 Suburban K1500
88 FlagStaff M-621
5 little girls growing up tooo fast
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Rogerg
WESTERN N.Y. U.S.A.
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Just one question. My truck and trailer have a GCW of 25,000#. I can drive down the interstate with the cruise control on,in overdrive, and not worry about it downshifting. It only has 250hp. about 100hp less than the GM 8.1 gas and the ford V-10 gas. Why?
Rogerg
2001 F-550 CC
1999 Coachman Prospera 365IKS
* This post was
edited 10/10/04 07:25pm by Rogerg *
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justJeff
Ocean Shores, Washington
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All of this talk about only getting 200k out of a gas engine is interesting. I'm a builder, and the majority of our light duty company trucks have always been gas. They work hard, but we maintain the heck out of them. We have no problem getting 200,000 miles out of any of our gas rigs. They're mostly GM products, and we will usually rebuild an auto trans at least once in 200k, and the assorted water pumps and radiators, but no heads off, etc. We run Valvoline 15/40 fleet oil and change oil and filter every 3,000 miles in the gas rigs. I think almost any modern V-8 should be capable of 200k miles if it's taken care of. Trans and coolings systems hold up much better with regular maint. too. We've stuck with mostly GM products because of the aggressive crate engine program. Economically though we're finding it better to not put the second engine in as problems with rear ends and such seem to start happening long before the second engine is tired.
I recently ordered a new super duty Ford Diesel crew cab dually for my new company truck. I spent the extra money mostly because the section 179 tax deduction lets Uncle Sam pay for fully half of the new truck. We bought new equipment this year for the same reason, it's almost crazy to not take advantage of it if you're self employed. The accelerated one time depreciation on up to 400k a year makes things much more attractive.
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Aquaduct
Winchester, VA USA
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Quote:Just one question. My truck and trailer have a GCW of 25,000#. I can drive down the interstate with the cruise control on,in overdrive, and not worry about it downshifting. It only has 250hp. about 100hp less than the GM 8.1 gas and the ford V-10 gas. Why?
Because it's not really that big a deal.
A loaded 80,000 lb. tractor/trailer rig takes only about 200 hp to run 55-60 mph. Of course, with that much wieght, the grade penalty can be substantial, but, depending on how fast you run, the rest of the system, and the shape of the power curve, 25K should be do-able with 250 horse.
Of course if you had more system data- power curve (or torque curve, it's easy to get one from the other), gearing, effective tire diameter, maximum rig height and width, cruising speed, rpm at cruise, automatic/manual- I could diagram it for you. I could also diagram it for any other powertrain that you'd like to compare it to.
In fact, I've given the calculations to other forum members, maybe they've got the time to share them.
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Rogerg
WESTERN N.Y. U.S.A.
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I don't have the background that you have, but it seems that if there is less hp and you can pull heavier loads, that torque plays an important role. What I would like to do is a side by side test between gas and diesel with the same trailer.
Rogerg
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