What do you think is "not so logical" about the design?

 

A diesel ignites fuel with heat. An aluminum head conducts heat away from where you want it, in the block and head. So, by using a material that is good at conducting heat away and dispersing it, the Dmax uses more fuel to heat the engine, since it loses heat with each cycle. This could also affect power levels for any given design in a side by side aluminum vs iron head comparison.

Second, an aluminum head on an iron block inherently is difficult to seal. Pick any given engine that had this design, from the Escort, to the Dodge 2.2/2.5, this mating surface is tough to seal. The Duramax is no stranger to this. In fact, the compression ratio was lowered in an effort to correct this issue. The fact is, had this been considered, iron heads would have been used. Had iron heads been used, the Duramax might actually compete with the Cummins as a top competitor.

Lastly, in the world of hauling, all the diesel companies know that an inline six is far more efficient. For any given displacement, an inline six will put out more low end torque than the V8 of the same displacement. Proof of this can be found onder the hoods of better than 80% of the OTR trucks sold in the country. Inline six configuration also offers the smoothest delivery of power, which directly relates to the efficiency.

http://www.autozine.org/technical_sc...ne/smooth3.htm

However, GM and Ford seem to think that people want quick acceleration from their trucks. That is something that V8 can do fairly well. How well it does it with a load may be something entirely different.

 

MAX,

I'd like to see your first theory backed up with some evidence or mechanical design data. I'm no fan of aluminum heads, but conducting better is not the reason, it's the different expansion rate, lower strength and lower melting point. Aluminum heads have nothing to do with poor starting or lower efficiency that I know of. Please show us what you have.

Also, Ford and GM must consider many factors when deciding on an engine design. It's not just about towing efficiency at all. Being able to sell a V8 to the uneducated masses is easier than advertising a 6 against the proven Cummins. And fitting a fast reving and relatively short V8 into an already existing body style could be easier than redesigning the whole truck to fit a long six. Plus they already have had V8s for many years like the 6.9, 7.3, 6.2, 6.5 etc. They care about sales first. Outdoing Cummins with a six would be a tall order and a change in direction. Ford and Chevy are married to the V8s in full size pickups. Further, the Duramax has been developed into a good, albiet complicated engine. There is no way I'd ever own one, but they seem to be pretty good in an aluminum V8 sort of way.


John

 

The different expansion rate pertains to the faulty head gasket problems. Conducting heat away from the combustion chamber is what an aluminum head does; its why hotrodders and racers use them on gasoline engines where cooling the intake charge is a necessity to make power. Using aluminum in a diesel head is counter productive. As to what I have, try elementary physics, wherein you'll find that aluminum conducts heat far more quickly than cast iron or steel. Certainly the argument could be made that the aluminum will heat up more quickly as well. This is a delicate balance to achieve, between heating up more quickly, and losing that heat just as quickly. Given that the Cummins and 7.3 PSD work just fine without aluminum, and the inherent problems of aluminum, it seems a poor choice in the design of the Duramax.


"Pretty good in an aluminum V8 sort of way" isn't good enough when I'm laying $35,000 on the table. Secondly, while your points about the reasoning in using V8 diesels are all valid, (and I could add to the list, as well as prove some points wrong), the lack of design consideration will come to bite them in the backside, as it has already. Want proof? Take a look at the Toyota concept with the inline six Hino under the hood. If there is one company that seems to consistantly get things right, its Toyota. Ford has failed miserably since they did away with the 7.3PSD, and the Duramax is substantially better than it was, but still fails the basic design criteria of efficiency and power with smooth operation.

 

You bring up some interesting points. However, while not trying to start a flame war or argument I have to disagree with some of your points.
1. I frequent my gm forum and have heard no such complaints about head gasket failures ( althought I may not be seeing them in the sections for older trucks)
2. Everybody thinks that because rigs mostly use inline sixes that it's because they're the best and v8's stink. One big reason they mostly all use inline 6's is because you can resleeve the cylinders without pulling the engine and they are generally easier to work on because of the the narrower packaging. This contributes to reduced down time.
3. I read your link about engine smoothness and it's interesting, but practical experience can sometimes do more than reading. I own both and the d-max is way smoother than the cummins. Maybe it's harder to acheive but they've done it. The long stroke that we all love so much on the cummins creates alot of the vibrations that resonate through the whole truck.
4. I average 16.8-17 mpg with 50/50 city highway with some towing of a loaded dump trailer and hauling during that period. So although some d-maxes don't get great mileage most do and so your statement about the economy of a v8 is also erroneous.
5. My truck's peak torque is at 1600 rpm, (650 ft. lbs). What is the rpm for peak torque on a cummins... also 1600 rpm. So what's your point there?
6. On the d-max not being competition for the cummins, they actually are if you see any drag racing or sledpulling. Consider this, the cummins has been around alot longer than the d-max so of course more performance parts are available for them. They are also alot cheaper to bomb than a d-max when you reach a certain level so they are the economical choice for enthusiasts.

I love the cummins and sometimes wish I had a newer one but I don't wish I had a dodge. There will always be those that think they have the best and everything else is terrible but my truck does what I need it to do the way I want it to do it. Everybody can spend their money the way they want to. I chose to sacrifice the inline 6 for warranty and fit and finish but that's my discision and a pretty common one.

 

Regardless of seeing complaints or not, the inherent problem of sealing aluminum head to iron block exists. Head gasket failures under such conditions are a matter of when, not if.

Given that the big diesel producers, such as Cat, Cummins and Detroit all have a line up of far more inline engines than V8 engines, the evidence stacks up against the V8 fairly quickly. Again, more evidence to the contrary is the fact that low profile, low center of gravity is better in marine applications, yet anyone making a marine diesel has a lineup dominated by inline sixes.

Its not hard to achive if you use lighter weight parts and spin the crank at higher speeds. This too is a detriment to the Dmax, pulling down longevity figures. I'm not sure what year or which version of the Cummins you have, but my 2000 ISB Ram doesn't have any resonation problems. I have heard complaints of "booming" exhaust, and the early five speeds were a victim of resonation, but this was in the whole drivetrain.


My statment about the economy of a V8 versus an inline six is on the mark. Your single example is hardly enough to claim I'm wrong. Further, I've clocked 22MPG towing 5000lbs at 60MPH. I've also run trips from central PA to Maryland, cruising at 75MPH most of the way, and had an average of 19.8MPG. Interestingly, when I load the truck down, I find my MPG increases. I attribute this to the fact that I accelerate at a lesser rate. However, cruise speed remains above any posted limit encountered. In the end, MPG is a poor measure side by side in the real world, as conditions are too variable. I'll stick with the research conclusions about inline six engines being more efficient than V8 configurations.

The Duramax can only achieve this through the marvel of electronic fueling. The Cummins did this when it was first designed, before electronics were even common, let alone applied to the diesel. The further point is that the Cummins lives at that RPM, while the Dmax needs more RPM than the Cummins to make its hp rating. Thus, while the peak torque on both engines is 1600RPM, the Dmax does it by fueling, the Cummins does it by inherent physical design. This allows the Cummins to stay in its RPM range, make the hp rating, and put forth a flat torque "curve" across the RPM range.

The Cummins truck was not designed to drag race. Thus, for this discussion, I don't give a rats hind quarters what a Duramax does when modified, much like I don't for a modified Cummins. If you want an "apples to apples" comparison, stock is the only fair method. Straight off the assembly line, in a workday comparison, the Cummins does the job better than the Duramax. It will be interesting to see how the Toyota entry does.


Given what I've seen of all three engines and the platforms they come in, my 2000 Dodge has better fit and finish than any of the new trucks, including the Dodge. The seats are far and away better than the others, including the Dodge. But thats got nothing to do with the engine design. If your truck does what you want it to do, thats great... but it has nothing to do with which engine is a better design. How common the decision is to go with a GM truck, or a Ford truck, I don't know. But it is only partially to do with engine design.

In the end, the irrefutable facts remain:

Aluminum/iron mating surfaces have problems.

All else being equal, Inline six vs V8 will show the six as more efficient.

 

Max,

The heat conduction rate of aluminum vs iron has nothing to do with starting a diesel. It is true that aluminum conducts far better than iron but the compression heat in the cylinder is nothing compared to the combustion heat. It's not the material of the chamber wall that makes the engine easy or hard to start.

Some of the factors that most affect starting are 1. Combustion chamber volume with it's associated surface area. Volume increases faster than area. As volume increases there is a longer heat path to the chamber wall and more heat per surface area so the temp at the center can more easily reach combustion temp. This is why precombustion chamber engines don't start as well as direct injection engines. And the difference in material of the chamber wall, aluminum vs iron, has little affect on the cooling of this starting volume. 2. combustion chamber pressure. Not affected by the material. 3. Shape of the chamber. Again, not affected by the material. 4. Atomization. And again, not affected by the chamber wall material.

Of course there are other factors too, like altitude, fuel, cranking speed, ambient temp, injection timing, injection volume, etc.

You can reinforce your claims with the statement that's it's all just "elementary physics", but there is a lot to diesel design that you might not see at first glance.

Aluminum heads are good at getting the combustion heat to the water jacket, but they are not as durable in an overheat condition. They are also lighter and that's an advantage when weight counts. Transferring the combustion heat is way different than starting, where there is only enough energy to heat the air for a short time during compression and the chamber wall is usually cold. Cold iron and cold aluminum have a similar cooling affect on the compressed air.

As I said before, I'm not a big fan of the D-max but I'm glad they are out there. I'm happy to watch them from a distance and see how they do. I don't think they are having a lot of problems with head gasket sealing. Are they? My big deal is how simple, robust and proven the Cummins is. How easy it is to work on, and the torquey power curve. 90 degree V8s are very smooth, and so are inline sixes.

Oh, by the way, it sounds like you need a nap.


John

 

I worked in a high volume chevy/gmc dealership shop as a line tech, and in that time i saw a total of maybe 5 duramax's with head gasket failures. The old aluminum head on an iron block equals head gasket failure is a product of the 80's. The metallurgy technology of today (not to mention gasket and fastener tech) has improved 10 fold. There are more engines being made today with aluminum heads and iron blocks than at any point in history with a very small failure rate now. It was not done in any effort to increase power in the dmax, but to save weight in a 1000lbs. engine. All in all the duramax is a good, reliable engine, (once the injector problems were resolved, but that was a bosch issue not a gm or isuzu issue.) Don't get me wrong i love the CTD, but if i were to buy a new truck it'd be a dmax mostly because the GM trucks are much nicer and drive/ride far better with better fit and finish and build quality than the dodges (IMO) and if you are going to spend that kind of money might as well get something that you like to drive.

 

Right with ya....no problems with this dmax as you know.

 

Given that I never mentioned hard or easy starting, I'm not sure what you are rambling on about. I mentioned simple physics.... the engine runs on heat, the aluminum takes that heat away faster than iron does. Thus, more energy from combustion is spent on warming the mass of the head than would be if the head were iron. Its that simple.



May I just say, who cares? I never mentioned it, and you seem obsessed by it.


Except that the point I made was simple physics, and thats all the further I went. Feel free to ramble on about a problem which I never mentioned.

Weight counts. As in, more of it is better. Unless your truck is a 2x4, then you might want less. I'm fine with an 1100lb engine.

The Dmax had serious head gasket problems when it was introduced. GM lowered the compression ratio to combat the problem. Now, it seems they need some $50,000,000 (yeah, thats millions) or so to redesign the engine to meet emissions regs. Does this seem like a well designed, logically planned engine to you?

And you need a reading comprehension course.

 

I think you need to settle down and be more respectful to the people here.

Don't talk to others about being "obsessed" when you are obviously fixated on the cast vs aluminum head issue. Which, as has already been stated, is an obsolete concern. I have other things to do than talk about something that doesn't matter.

Here's the bottom line; Isuzu started in 1916 (although not adopting the name Isuzu until 1949). In 2003 they produced 16'000'000 engines for use world wide. In 2005 they were the world's largest producer of medium-heavy duty trucks. When you begin producing your own brand of diesel engines please let me know because I'll certainly have to get one since you know so much more than they do about building engines.

 

um. no they didnt.



Ive run 45-50psi for 83,000 miles on stock original head bolts, no studs.

 

thanx for all the interesting info Max!

 

I was being plenty respectful. If you feel otherwise, check you own behavior.


Its not an "obsolete concern", as GM found out when they had to lower compression to avoid making the problem worse. If you have other things to do, why ae you busy reprimanding me for no apparent reason?

No problem. Fact is, if you dared read what I said to begin with, I stated EXACTLY this in my post replying to those who felt Isuzu was somehow washed up in this country. IMO, Isuzu was what was right with the Duramax. GM, obviously, couldn't build a light truck diesel to save their own rectums.

The bottom line is, you don't happen to like my opinion. Despite this, you've offered nothing but conjecture in reply. I am slightly amused that you think science has somehow made aluminum and iron expand at the same rate, so there is no longer a problem. Further, perhaps you could respect my opinion and its supporting facts, much as I have acknowledged others opinion and their supporting facts. If not, consider doing the following

 

GM did in fact lower the Duramax compression ratio after the first couple years of production.

Compression ratio and boost are two entirely different things.