I've always though the same too, if the 625 hp motor is driven like a 435, and never use more then 435 hp, it should get exactly the same. Of course, you have to have exactly the same motor(except for hp ratings) since different motors have different efficiencies...

But if you do that, you might as well have a 435 hp motor. Since the only difference is the 625 hp motor injects more fuel(not always true, but most of the time it is, some engines have different "classes").

I've never understood the argument that more horsepower gets better fuel mileage, when it's always the 12 litre trucks getting the best fuel mileage.

horsepower = fuel over time

Another thing, torque isn't free either. Torque is created from cylinder pressure, which needs fuel. You guessed it:

torque = fuel in one injection sequence

 

Tracer, can't argue much since I don't know about your dyno-tuned motor. Must be nice :bow:!

I do know that my 435 C15 has the same 1850 lb-ft of torque that the 625 does. The 625 engine program takes that 1850 and moves it up the RPM range to get 625. 1700 RPMs at 1850 lb-ft is roughly 600 HP...

So, with a factory Cat program, the only way you'd notice increased power would be to rev it, since at the 1325 RPMs I'm turning in 8-hi, there'd be no difference.

Just curious, what does your torque curve look like?

 

I think the 625 has 2050 torque....

 

I believe, you are wrong, on a both counts...Allan already said about 625, and i never heard about 1850, option on 435...

 

You're right, Solo379 and Allan5oh;

http://ohe.cat.com/cda/files/448363/...e%208.9.07.pdf

Mine's the 435 multi-torque, which is 1550/1750, NOT 1850.

Perhaps I should have double-checked before opening my big mouth...

 

WE have many trucks in the company I drive for. For example we have Volvos with Ved 12s, D13s and D16s. The D16 gets better average fuel economy then the D13 and they both get better average fuel econeomy then the VED12s.

The larger cummins motors in the fleet get better economy then the smaller motors in the fleet.

We have a few cats left but not many now as the boss is quickly dumping them because no one can seem to get over 4 mpg unless they have them played with. But the stock MBN C15s 475 get around 3.3 mpg while some of the boosted ones get 3.8 for a round trip average.

Mind you that we pull some pretty draggy trailers. Trinity Eagle Bridge with 3 axles and wide spreads with big domes on them to keep the product from freezing in the winter time and that our average gross weights are in the 100,000 lb range.

But the larger engines just basically idle around at highway speeds with low boost pressures etc while the smaller engines are running around at the same speeds with higher boost pressures and egts. This means that the ecms are actually dumping more fuel into the smaller engines to keep the loads moving down the roads while the ecms on the larger engines are actually dumping less fuel to keep the loads moving.

A bigger engine can get a load up to highway speed much quicker and easier which means they spend more time at cruising speed were the smaller engines are using 100% of power for a longer period to get up to speed and spend far less time at cruising speed thus they tend to use more fuel per month.


Back a few years ago I had my own smaller truck. I pulled a smaller wedge trailer as a hotshot. When we got the truck it was putting out about 150 hp at the wheels. When pulling the trailer with three smaller vehicles on it I averaged about 5 mpg. I had larger injectors put in the motor, had the timing bumped and the fuel bumped on it. On the chassis dyno we set the power at about 300 rear wheel hp.

My economy shot way up. The smaller loades had me at 14 mpg while larger loads such as 3 Chev suburbans or equaly heavy Avalanches had me averaging between 10 and 12 mpg. These are all paper figures as I did my average mpg nightly when I fueled up.

I did not have to drive as agressively to do the jobs and that is why I was able to save fuel.

 

I find that aweful hard to believe.

You cannot compare boost pressures and EGT's of two different motors. Only the same motors.

But the larger motor has a higher fuel rate at 100% power. That's how it makes horsepower.

You were, because you had more horsepower.

Even in the middle of winter, in the Canadian prairies when my ole volvo d12 is chugging along HARD, I'm still getting 6.5+ mpg, while the other guys with the "big hp" motors pulling similar loads are getting 5 if they're lucky.

I have yet to talk to a guy that owns a d13 or d16 that even comes close to my d12.

 

I have driven a truck with a VED 13 and other trucks with the 13s and the 16s. I can get better milage in the higher powered trucks pulling the loads that we pull.


As far as how I drove my own truck before and after the upgrade in power. Well I had to run it harder with less power. had to spend alot of time in the turbo and injectors to keep it moving or get it going. After the power upgrade I spent alot less time in the turbo and injectors and was able to keep it rolling much easier in a higher gear thus lower rpms and little to no boost or exhaust temps.

It is all about torque and HP not just about HP. Peak torque can be easily reached on bigger engines with actualy less fuel numbers on a dyno.

Take an example.

Say you have a smaller motor that has to turn 1700 rpm to reach peak HP

The larger motor will reach the same hp as the smaller motor but at less rpm.

So instead of having to run 1700 rpm to reach 450 hp you are now running at 1500 rpm to reach the same hp. The engine is not struggling as hard to stay at the power.

Sure if you want to run at peak power then you are going to have to get into it and then yes your fuel econeomy will go down.

See that is the trick, you don't have to dog you truck and motor as hard to keep the same loads moving.

Take the truck I am driving now. It is an 04 pete with a MBN c15 475 motor. Before I did some tweeking it would struggle pulling the loads we pull. My fuel economy was down to low 3s. I did a few things and upped the power a little and now I am getting high 3s to sometimes low 4s for average. This is pulling the same exact trailer with the same loads. The truck is governed out to 68 mph so I can not reach the peak rpm in the top 2 gears thus I can not reach my peak power in them two gears. My power went up through out the rpm range and now have more power then before at 68 mph thus I don't have to dog the engine and truck so much.

We have a few cat 550s and they get mid to high 4s for economy pulling these loads. They are still not on par with the Volvos in the fleet which is why the boss has been replacing them at 5 to 6 a year with volvos with d13s in them.

I will not knock the volvo engines as they seem to be good engines as far as economy goes. I just don't really care for some of the other systems that the volvos have along with the rest of the truck. If they would make a truck that does not lean in a side wind and put some real steps on them and not get thrown around as much on the road then I would tell the boss to put me back in one.

 

all these different opinions are making my head spin.. all I know is my lil 11.1 ser60 det has always given me great m.p.g over 1.3 mil & stil over 6 ,min always. 7 is easy doing 62-65 mph with no headwind.

 

I'll let you know on Monday, after the Dyno. They usually give out a printout with lots of interesting info about the old/new hp, old/new torque, old/new fuel rate etc. My truck is ready to join the big horse power league: our shop has just finished putting in my rebuilt tranny RTLO-18913 and the new LIPE 2,000 lb-ft clutch. Should have 550 hp at 1,600 rpm by the end of Monday