This is a continuation between me and Mr. mudpuddle from a different thread:

I'm stating that % of HP used does not affect fuel economy, he's saying it does.

Anyways, onto my argument:

Take to of the same engine(say a c15) one 475 hp and one 600 hp, and have them both cruising along a nice hill at 50 mph both using 475 hp.

They'll both get the exact same fuel mileage. The fact that the second has 600 potential hp is irrelevant.

They're injecting the same amount of fuel, the same amount of time, and BSFC is exactly the same for both.

 

Again i will ask you to read the articles. The 600 hp engine can be geared differently due to increased hp. The fuel pressure required to maintain 1300rpm in a six hundred hp engine (to produce 475hp) will be considerably lower than the fuel pressure required to turn the 2100 rpm required to produce 475hp from the 475hp engine. BSFC is DIRECTLY related to volumetric efficiency. Lower compression ratios and higher boost pressures (the setup for high HP engines) produce better BSFC than high compression and lower boost (the setup for lower hp).

I don't want to get into the same pissing contest as the last thread on this subject, I do think you could open your mind a little and do some more investigating on this. Gearing, setup, tires, driving style all go into fuel mileage. Just stating as fact that any driver in any conditions will get two mpg better with a m11 than with a 15 litre cat is wrong.

Load a m11 powered truck with a 79,000 lb load and load a 550 cat with the same load and head west on I 80 from coast to coast and compare the mileage figures you will be surprised at the similarity in the average mileage.

 

Sure, but is that a good thing? I've driven a few different trucks on the same exact run, and it was always the 11 litre engines spinning higher that got the best fuel mileage. All driven exactly the same.

only if BSFC has gone up at 2100 rpms. It probably has, but I'll tell you right now an m11 is more efficient at 2000 rpms then a cat is at 1300 rpms. If they're both making the same HP at their respective rpms, which engine is the winner?

Yes it's true, increasing volumetric efficiency lowers BSFC, but that doesn't take into account many factors. You can have two of the same engines, exactly the same volumetric efficiency, but one is more thermally efficient.

I also know a guy who states that acert c15's tend to do the worst on his dyno when you consider BSFC. As far as I'm concerned, dyno testing BSFC is the only way to go.

OK, I'm definitely willing to listen to that. I read a few of his articles, but I don't see where he said that. I'd love to have cold hard #'s as well. The problem is he might have changed the exhaust, the turbo, etc.. So it's not a fair comparison.

Also, I'd like to compare stock engines. Modified engines flow better at lower HP then stock engines, so of course they're going to be more efficient. It is *NOT* due to the extra available HP.

My mind is more then open, trust me. I don't consider this a pissing contest at all! Maybe I'm coming across the wrong way...

Yes, but they can all be equated to "required horsepower" and "required fuel to make said horsepower". Of course I'm talking about cruising down the highway at a steady state. Since most of us cruise along the interstate 90%+ of the time it's a good way to look at things.

It's a very simple equation:

Required HP(steady state, or average) * BSFC = amount of fuel you burn in an hour

If you're using 250 hp, and your engine uses .35 gallons of fuel per horsepower-hour, you're burning 87 lbs of fuel an hour. 87 / 7 = 12.5 gallons in an hour. 70 mph / 12.5 gallons = 5.6 mpg.

everything relates to BSFC!

I was *NOT* stating that as fact, but rather using it as an example of how important it is to get engine choices RIGHT. It could cost you $10,000+ a year!

I really doubt it, to be honest.

BTW, so far this month I've averaged a little above 8.5 MPG. Calculated at the tank, of course.

 

I found that insulating the turbo itself didn't help, but insulating the manifold helped tremendously.

 

Did this lead to higher indicated EGT on the pyrometer?

 

Correct me if I'm wrong!
So, if I'll turn up my 475 C15 to 550, running the same way i do now, considering the torque will stay the same 1850lb, I'll get a better mileage?

 

I have to put my 2 cents in,

1 gallon of diesel = 147,000 btu’s
it takes 2544.044 btu’s to make 1HP for 1 hour.
That is the minimum of btu’s to make that 1hp for 1 hour if it take more btu’s to get that 1hp to the flyweel then the efficiency of the engine is now the difference in fuel vs horsepower.

Example a 8.3 liter engine modified to produce 450Hp vs a 15 liter engine that produces 450 HP They both have to burn the minimum fuel to get to that horsepower. But the combustion process and it efficiency determines how much actual fuel will each engine really need to produce that 450Hp. Now if both engines are producing 200 HP @ the flywheel, here again there is a minimum of btu’s of energy required to produce this 200 HP over a given amount of time. How well each engine and convert that fuel into work is the difference. I understand what both of your are saying. Allan is saying it takes x amount of fuel to make x amount of horsepower which is right. Mud puddle is say that in the process of creating that horsepower other factors come into play, which I agree.

 

Solo, if you turn your engine up it would change the power curve slightly and if you optimized your set up for the new power curve you could see a slight difference in mileage. The changes would be very small though because you make such a small change.
My point to allan though is that a small displacement low horsepower engine operating at near max power output will not be a great deal more fuel efficient than a larger engine working at a much lower percentage of its power potential.

Allan can answer to his driving style. Solo you have said in other threads you get around 6.7mpg with your cat. I assume to do this you run moderate speeds, don't accelerate up to speed quickly, and coast down hills and to stops etc. To get the 8+ mpg he claims Allan must be driving in a similar way and probably using a fairly low average speed (what is your average speed Allan).

Allan makes the case that a m11 will always get much better fuel mileage. I don't dispute that the m11 will kill the cat on flat land with 65,000 gross. My point is put the same two trucks in the rockies loaded to 79,000 and the mileage difference will be much closer.

To get the maximum efficiency from your m11 Allan rebuild the engine lower the compression increase the turbo size reset the fuel pump and injectors then regear the truck to match the new power curve and you would see a nice increase in average speeds (better climbing power) while keeping your mileage the same or even climbing a little. Efficiency with this new setup would be better because you will use lower fuel pressures to maintain a given road speed. Lower fuel pressures combined with your smaller displacement would the give you the fuel mileage advantage back over the larger displacement engine.

Split shifter. Bruce's customers at pittsburgh power get 900+ horsepower out of big cam cummins engines with egt readings in the 1100 to 1200 degree range. The large turbo and lower compression ratio keep the combustion temperatures well in control. Iron eagle ran one of these engines in the past and he could probably tell us from experience the temps he saw and the mpg he got.

I am in no way saying that a 15 litre engine will get better mileage than one of a much smaller displacement in all conditions as an average. What I am saying is the larger more powerful engine will dramatically close the gap when large amounts of power are required to maintain a given speed.

 

Yes exactly. The low compression high boost set up used on the engines built by pittsburgh power increase the efficiency of the combustion process. More horse power same or only slightly higher fuel use.

 

Yes, indicating that what I did worked.

I noticed it spooled up a touch quicker. I didn't notice a huge increase of boost at the top end, possibly indicating my turbo is a bit undersized.

I also noticed that it makes more boost at lower horsepower levels.

For example, cruising at 60 mph at 7 mpg(50% throttle) it used to make 10-11 lbs of boost. Now at 8 mpg I make the same boost.

It's also less affected by humidity and heat.