Diesel misconception

[Southern Gorilla]

I'm no mechanic. But I have been driving the big diesels for right around fifteen years now. I'd like to call on that experience to dispel what seems to be the most pervasive myth about diesels. Going with a smaller diesel will NOT improve your fuel mileage compared to a bigger diesel.

This may seem counterintuitive. But diesels do not operate like gas engines. Gas engines restrict airflow to control engine RPM. Diesels restrict fuel flow. There is nothing in the intake at all to slow down the air coming into a diesel.

What this means is that if you need 127 HP to push your rig through the wind your engine will drink 127 HP worth of fuel regardless whether it's a 3.9, 5.9, 6.5, 7.3, or whatever.

The difference between a smaller diesel and a bigger one is how they produce this power. Yes, a 4bt can be made to produce 300 or more HP and keep up pretty well with a stock 6bt. But the 4bt will be pushing the limits of what it can handle. It will have to rev higher and use more boost to produce the same power as the 6bt. So reliability will suffer.

Also, engines are like turbos in that they have an RPM range in which they are most efficient and their performance suffers outside of that range. If you build a small diesel to rev higher for more power you will push it outside its optimum range and it will use more fuel than a bigger diesel running in its optimum range at the same output.

How you intend to use your rig plays a major role in deciding which engine to go with. A trail rig that never sees highway speeds can easily get by with a small diesel. All the power will come from the gearing. And at very low RPMs a small diesel will be more efficient than a big diesel since it takes less power to move the smaller rotating bits inside.

But if you expect to run down the interstate, pull a trailer, or haul a busload of kids and gear to the ballgame you are better off with a bigger engine. For the same load the bigger engine will be less stressed, burn less fuel, and last longer.

I say this from experience. I've driven for too many companies who think they are going to save a fortune by running dinky 12L engines in their rigs. Then I show up with my 15L and fall right in the middle of the fleet MPG average. I traded my old rig for a newer one with just a 14L engine and I get exactly the same mileage.

Diesel mileage is mostly about what you haul and where you haul it. If you pull trailers through the hills your mileage is going to suck regardless what engine you have. Trying to compensate by swapping in a smaller engine will only leave you disappointed. The fuel you thought you would save will still get burned. Then you'll burn additional fuel shifting gears to get back up to speed. But if you run mostly flat land and never hit the interstate you can probably live quite happily with an SD33 under the hood.

I'm not dissing small diesels or anybody who wants one. I'm just advising people to reconsider if they are downsizing their plans in order to save fuel. I had thought about a 4bt for my future conversion. But I want to be able to pull a trailer at freeway speeds. A mild 6bt will do that job much more reliably without hurting my wallet.

For the curious among you, my 14L Cummins manages 8-12 MPG when I'm running empty on flat land. That's at about 1600 RPM, 68 MPH. Which is identical to the 15L Caterpillar in my previous rig. Both rigs have 13-speed Eatons and 3-something gears in the axles.

On a semi-related note-- if anybody ever makes a 13-speed for light-duty rigs I will be all over it. I wish I could duplicate that effect with junkyard parts. Oh... and if I can find a Cat engine in the right size I will be all over it. Just not sure about having a 1,500 pound lump under the hood. If I could even stretch the hood far enough. Might just have to paint a 6bt yellow and pretend.

[altrocker1]

i agree with you 100%...personally im doing a 4 cyl isuzu for a lot of reasons. for me it was easy when comparing side by side with the stock 4.2l. the isuzu is actually rated for more hp than the stock motor. im also not going up to a crazy tire size with my truck. i have a small boat that i will tow 5-10 miles to the lake and maybe 1 time i year i will use it to pull a car or jeep. im hoping to do mostly highway with this truck...i also won't try to squeeze crazy hp out of the little engine. thought the idea of having a sleeved motor makes me less leery about pushing the engine just a little bit the way i look at it, if an isuzu npr box truck can run down the highway at 75 for well over 200k, my j10 isn't going to stress it too much. if i ever start pulling heavier loads, or stepping up in tire sizes i will absolutely want a bigger motor.

[Southern Gorilla]

Is that a 4bd1t? I just started reading about those tonight. Interesting little engine. A sleeved engine is a definite advantage. On a previous build a buddy and I were contemplating we had panned an International DT466 for exactly that reason. Then we decided on an old Detroit just because the noise made us giggle. Probably not the best criteria to use when choosing an engine.

I'm planning a 6bt just because I do plan to get silly with tire size. I may not end up with the tallest tires on this board. But I bet I end up with the heaviest. And the axles to spin them. Then I still want to pull a big trailer to haul whatever I may need to haul.

My philosophy has always been to build/buy bigger than I think I'll need. I used an F-350 crewcab dually flatbed as a daily driver just in case I needed to go to Lowe's once in a while.

[altrocker1]

yep...its a 4bd1t. in fact, once i get it im actually hoping to buy a second to rebuild. it may not work out, but i've been working on the owner for a while and the truck is down the street from my house.

in our big truck we have a dt466...she's a real thumper, but is still set completely factory from the trucking company that used to own it. i've been reading a lot on how to juice it up a bit since that truck hauls a ton of stuff. its also still on a box truck that would benefit incredibly from a 13 speed tranny...but right now it has an allison 4 speed. its kind of a dog, but its the right motor for the job...just needs to be tuned right. it has a light knock when its cold, but it still runs well even when its a bazillion below zero. what were you trying to put that engine in. that sucker is a monster!

[Southern Gorilla]

Originally Posted by altrocker1

...what were you trying to put that engine in. that sucker is a monster!

A Cherokee.


Do you have any information posted around here on the 4bd1t? I've read a little about it on other forums. What I gather is that it's pretty much interchangeable with the 4bt. Not parts-wise. I just mean that either engine will do basically the same job. I'm just curious what the advantages are of the Isuzu. Besides being cheap.

[altrocker1]

Originally Posted by Southern Gorilla

A Cherokee.


Do you have any information posted around here on the 4bd1t? I've read a little about it on other forums. What I gather is that it's pretty much interchangeable with the 4bt. Not parts-wise. I just mean that either engine will do basically the same job. I'm just curious what the advantages are of the Isuzu. Besides being cheap.

there is a whole section of info for the isuzu motors on 4btswaps.com, thats where i do most of my reading. they are very similar to the 4bt...both are 3.9L. the p pumps on the 4bt are probably a little better, but the 4bd1t is direct injection and capable of decent power if you know what you are doing. im not going to go crazy, but im really excited to see what the motor can do in a j truck. the biggest disadvantage is that the aftermarket support isn't as strong...especially in the adapter arena. however, as more guys start to use the isuzus, the parts are becoming more available. i would actually love to put one in my xj cherokee someday!

the 466 in a cherokee...man, will that even fit? aren't they around 460 cubic inches???!!! Great Googley MoogleyGreat Googley MoogleyGreat Googley MoogleyGreat Googley Moogley. i know they are pigs. i look at ours a lot and try to figure out what it might fit in. i would love to put one in a deuce sometime and turn it up. supposedly the 466's are a lot easier to deal with than the 466e's i haven't messed with it all, but have been contemplating it since i have started working on the truck a bit.

[csuengr]

Originally Posted by Southern Gorilla

What this means is that if you need 127 HP to push your rig through the wind your engine will drink 127 HP worth of fuel regardless whether it's a 3.9, 5.9, 6.5, 7.3, or whatever.

Not exactly true. Today's engines can produce 127 hp using less fuel than engines 20 years ago could. So the 6.7 Cummins in a new Dodge will use less fuel to make the 127 hp than a 4bt. It's a thermal efficiency issue, not displacement.

[rocklaurence]

Originally Posted by csuengr

Not exactly true. Today's engines can produce 127 hp using less fuel than engines 20 years ago could. So the 6.7 Cummins in a new Dodge will use less fuel to make the 127 hp than a 4bt. It's a thermal efficiency issue, not displacement.

IDK, 33% more moving parts and weight (6 cylinder vs 4) is a large disadvantage. A large portion of the power is lost through mechanical drag--Most loss is in heat.

[csuengr]

It be all thermal efficiency, which includes mechanical efficiency. Larger engines don't have to work as hard, to a point anyway. At some point, engine displacement gets large enough where you can't do an apples to apples comparison, unless you use thermal efficiency. The extremely large two stroke marine Diesels have a thermal efficiency in the low 50s.

[Southern Gorilla]

Originally Posted by csuengr

Not exactly true. Today's engines can produce 127 hp using less fuel than engines 20 years ago could. So the 6.7 Cummins in a new Dodge will use less fuel to make the 127 hp than a 4bt. It's a thermal efficiency issue, not displacement.

It's true that modern engines are more efficient than older engines. But that just shows you have to consider other factors when looking for fuel economy. Injector design, port efficiency, injector timing, compression ratio... there are scores of things that impact both mileage and power. I should have clarified that *all else being equal* two engines with different displacement will use the same amount of fuel to produce the same amount of power.

Rocklaurence, nobody has ever proven to me that mechanical drag is a significant factor. Everybody talks about it. But I've never seen hard numbers from anybody actually measuring it. Since an engine in good condition can easily be turned over by hand I'm going to go on record as saying that the drag is irrelevant. It may be measurable under lab conditions, but by the time you put an engine in a vehicle and put it under a load the internal mechanical drag is completely overshadowed by the load. Not to mention the fact that drag increases with RPM. A smaller engine has to use more RPM to produce power. So it isn't even certain that a smaller engine would have less drag at a given power level than a bigger engine.
And I'm sure you could take a 1.6L VW turbodiesel and boost it enough to propel an FSJ if you wanted to. It might have low mechanical drag. But I guarantee it would get lousy fuel mileage because it would always be near its peak output. It would be unreliable and unpleasant to drive.

I'm not saying everybody should build an 800lb-ft monster to drive around town. I wouldn't build such an engine for my rig. What I said was that dropping down to a smaller engine in an attempt to save fuel will not necessarily have the desired effect. If most of your driving is of the stop & go variety, a smaller engine may pay dividends since it will burn less while idling. A trail-only rig could probably make some small gains from a smaller engine for the same reason. But folks like me who want to get down the interstate without holding up traffic will not see gains from the smaller engine. This isn't just some random thought I pulled out of the aether. Fifteen years behind the wheel of a rig has taught me that a smaller engine does not automatically mean better mileage. I've seen with my own eyes engines 20% smaller than mine averaging the same MPG.

Even the route you take to work can impact your mileage. UPS has a policy of routing their trucks to make mostly righthand turns. Why? Because you don't have to sit and wait as long to make a right as you do to make a left. So they burn less fuel per day per truck because they aren't waiting for lights to turn green or waiting for a gap in oncoming traffic so they can turn left. That will make a bigger difference than internal mechanical drag. Taking a less congested route or a route with fewer traffic signals will improve your efficiency. What I'm getting at here is that fuel economy has much more to do with driving style, terrain, and load than it does with engine size. At least in a diesel.

Or you can tune to about the 4-minute mark of this video and watch as a 400 hp BMW M3 gets better mileage that a Toyota Prius; http://www.youtube.com/watch?v=JmxUsGiGp3w&hd=1

[FSJunkie]

I heard somewhere that when Dodge first started putting Cummins in their trucks, they had to detune it because they were tristing the truck frames.

I like the DT466E in alot of IC School busses. Some of those suckers had half a million miles on them withou a problem.

[Southern Gorilla]

Originally Posted by FSJunkie

I heard somewhere that when Dodge first started putting Cummins in their trucks, they had to detune it because they were tristing the truck frames.

I've heard that same tale. I strongly suspect it is a slight exaggeration.
The 5.9 Cummins is a strong engine though. Those "little" yard trucks companies use to shuttle trailers around are almost always powered by a 5.9 Cummins with an automatic behind it. I drove one for a year and a half. Pretty impressive power. Of course, it was geared so low it would never get up to highway speeds.

[altrocker1]

http://desmoines.craigslist.org/pts/3107654048.html

i wish i could pick this sucker up and drop out 466 in it...that would make a sweet tow rig

[Carnuck]

Originally Posted by FSJunkie

I heard somewhere that when Dodge first started putting Cummins in their trucks, they had to detune it because they were tristing the truck frames.

I like the DT466E in alot of IC School busses. Some of those suckers had half a million miles on them withou a problem.

They were cracking the frames from the torque and vibrations. Ford did too with the IH 7.3 and 6.9 until they started making Super Duty models.

[HerrKooled]

Interesting they are cracking frames. I have a chevelle with a 640 horse BBC, along with 640 ft lbs of tQ, and a chevelle frame is nothing like a 1 ton truck frame, yet it's not twisted or cracked?