Which Diesel is Best?

In Class 8 trucks, and considering what I've driven in the past.

Caterpiller has the reputation among most as the strongest motor, and for a good reason. They usually are. I was reading a combine forum the other day, and in the trucking section some character claimed Detroit Diesels had the best low end torque.

Say What?

Lets look at displacements. The standard bearer for Cat is the C15, which is 928 cubic inches (15.2L). Cummins basic block for years was the 855 block, which - gee guess what - 855 cid(14L). The largest Series 60 Detroit is also 14L. Welp, cubic inch displacement generally means more low end torque and better horsepower, particularly if they all operate in the same rpm range. Yeah, Formula One cars get a lot of power out of small displacement motors, but they rev over 10k as well. The average diesel doesn't get much above 2100 these days, and if you want mileage, it's less.

Back when dirt was new, Detroits were two stroke motors. Most of the old Greyhound and Trailways buses had Detroit two stroke power. If you heard a big truck that sounded like a bus, it more than likely had a Detroit powering it. The truck motors were by and large V8s and V6s with individual cylinder displacement in the designation. A 6V71 was a V6 with six 71cid cylinders. The later motors had 92cid/cylinder, thus the big-un was an 8V92. Hot rodders loved these motors because since they were two strokes, in order to achieve proper air flow, required superchargers just to move the air. The dragsters would adapt the old "Jimmy" blowers to their fuelies for, heh heh, more power. The two stroke diesels couldn't naturally draw enough air to run. Motors that needed more power had turbochargers just like the rest of the diesels out there.

These puppies wound up tight and sang a good song, but whenever confronted with a slight pull, generally fell on their faces. No low end torque. If you could keep it "on the boil" by keeping the rpms high, you could pull a hill better. It was said to properly drive a Detroit, one should put one's hand in the doorjamb and slam the door on your hand. This would put you in the proper frame of mind to drive the thing - you had to abuse it to get any power. The fuel delivery system made the throttle response like a car - if you pushed down on the accelerator just a bit when it wasn't under load, it would wind right up to the shutoff. I've driven some of the newer Series 60 motors in the past three years, and while they are four stroke like the rest of the world, they still don't have any "guts." Drive it like you stole it, if you don't want to be crawling up the mountain.

Cummins motors have remained largely unchanged since the fifties - all on the same 855 block. There have been a slew of changes, but a lot of the parts could interchange. The Cummins motors I've driven all had a lot more low end torque than the Detroits. Their fuel delivery system is similar to the Detroits - bumping the accelerator will jump the rpms up in a hurry. The new ISX motors are twin overhead cams with 912cid(14.9L). I have not had the pleasure of driving one of these motors. We have one in our fleet rated at 550hp, the same as all our Cats, but it apparently actually outpulls all the newer low sulfur motors.

Then we come to the old time Cats. They were all larger displacement, and their injection system metered out a precise amount of fuel to each cylinder depending on the accelerator input and the rpms running. Unloaded, if you put your foot halfway in, the tach only went about halfway up. With the rpms tightly controlled, shifting is far easier. Plus, if you had your foot in it about 3/4 and started up a hill, it didn't matter if you mashed your foot to the firewall or just let the motor try to pull to 3/4 the rpms. It actually pulled better if you let it do the work.

Buuut, there were problems with the older Cats. They did not make the fuel mileage, and they were expensive as hell to repair and overhaul. The old V8 Cats were pulling monsters, but they'd "pass everything but a fuel pump."

Now, with electronics, Cats try to mimic the throttle response of the older models. They are different to drive, but most drivers prefer them more after getting used to one. The motor my hot rod has is an ACERT C15, so it has twin turbos and it won't turn more than 1900 rpms. I generally shift up at 1600 or 1700, where in a Detroit I'd push it to 2000 or 2100. In lower gears, I might let it drop to 1000 rpms and not bother shifting down - like pulling through a town. All the newer motors do not allow you to stomp on the fuel and really accelerate. The old motors would, and then they'd blow lots of black smoke. That is just not politically correct anymore, so we have to let the boost build up a bit before the fuel pump in allowed to pump fuel. However, the boost won't build until the fuel is pumped up. It's a slow process, and sometimes if you need power in a passing situation, it just isn't there. The ACERT Cats can also be distinguished by the twin turbos. The older C15s had one huge single turbo, but the ACERTs have small twin turbos that will supposedly spool up quicker. The older motors didn't have much in the way of feedback on dangerous temperatures - you could overheat one pretty easily, or get the exhaust temperature so high the turbo would burn up. Now, the ECM won't allow it. The motors will "derate" before they'll self destruct. That means there is enough power to limp along to the repair shop to determine what is wrong.

One of my favorite motors from my past was a Cummins Big Cam III that had been hopped up a bit. It had the latest Holset turbo and the new (for it's day) style exhaust manifold. The fuel pump had been worked over, too. When it was in a long pull, it was a good idea to watch the pyrometer (exhaust temperature gauge), or it could blow some rings, pop a head gasket or blow a head. The rpm limiter was set for a far higher figure than 2100rpms - it altered the fuel curve so the lower ranges had more fuel. Those older motors also had an "aneroid" valve - it was the mechanical equivalent of the electronic motor's self regulation when accelerating to prevent the dreaded black smoke. That valve was backed off all the way, baby. The fuel pump had a smaller number "button" in it that allowed greater fuel flow. So, for a Cummins in it's day, it pulled pretty dern good. I could stomp on it to pass and it would roll out some smoke and just go. The original rating was 400hp, but I'm sure it cranked out considerably more. Most of the "running" motors of that era had similar "upgrades" to their pumps as well. And, you'd think that the fuel mileage would be considerably worse, but someone slogging along with an unmodified 350 pulling it's guts out would make worse mileage than the hopped up motor loafing along. So, if the operator wasn't an idiot that would burn it up or drink up the fuel, the "breathed on" motors did pretty well.

Now, which motors do I prefer? I'm finally getting used to this ACERT motor, and while it's okay, I prefer the older style. I actually do like the electronic motors better than the old mechanical beasts. They might not wind up as quick, but the computer controlled motors actually do pull better and get better mileage doing it. Which motors are the most fuel efficient? Lately, it sure ain't Caterpillers. Detroits have a pretty good track record in that regard. The new Cummins ISX may stand that assumption on it's head. Cat isn't gonna make any more motors for the US market, so the question of the future is moot as far as the yellow motors go. Cat's are still more expensive to fix and overhaul. My neighbor bought an older truck with a Series 60 the other day, and I don't blame him. It will probably cost him less in the long term than a Cat. Personally, I'd prefer a Cummins for farm use, but that's just me. They all have their weak points.

I'm not sure just how well these newer electronic marvels will last after the truck is parked for the winter and the mice move into the wiring harnesses, either.

Also, I've never driven a Volvo powered Volvo truck, or a Mercedes powered Freightliner, nor a Mack truck. Macks have had a reputation for low end torque for years, but pretty much any time I've ever run with one while I was driving a Cat or a Cummins - I'd outpull it. Plus, most of the "fleet" trucks out there have smaller versions of the motors I've mentioned, particularly the Detroits.

Might makes right, I always say.