That sounds reasonable, if that's true then a ratio change of 20% should result in a mileage increase on the order of 6% (give or take a bit). Sound reasonable?

This is where I needed Duke to comment. If the engine, in neutral, is reduced in speed by 20%, what would be the reduction in fuel consumption? I'll bet it's more than 20%.......probably about 30%.

So, if we assume 30%, the formula is:

(.30 x 70%) + (.70 x 100%) = .91

So the mileage increase would be 9% in this scenario. It matches closely with the data from the fellows with the 2.47 swap.

It's nowhere near the claimed 20%................

That relationship would be hard to predict without test data, and would be dependent on the performance map for the specific engine model. I assume the internal friction power would be a function of the square of the speed, and the auxiliary load power would be at least linear. The efficiency of the combustion process would be more complicated. I assume that there is a maximum efficiency speed (based on IP timing, etc.) and that speeds both above and below will be less efficient. I suspect that the best efficiency point for SI engines is always near WOT (to eliminate intake loses), maximum load, and fairly high rpm; but I assume it would be at a lower speed for diesels (due to the lower combustion speed). In any event, I could believe either 6% or 9%, but not 20%.

It's very difficult to accurately characterize internal engine friction without motoring an engine on a dynamometer. There are no magic formulas.

The important relationship to remember is that engine friction power (not including pumping loss on SI engines) increases with the square of speed, and as a VERY rough approximation, engine friction at a given speed increases linearly with displacement.

However, engine interal friction is a function of many design parameters including bearing size, and, in particular, piston skirt and ring design, as this is the primarly source of internal friction. Dedicated racing engines have low tension rings, small piston skirts, and bearings as small as possible to minimize internal friction, but such designs would not be suitable for low revs and high load.

If you want to investigate this further you should get some IC engine textbooks and search the literature for technical papers. It's a very complex subject.

Based on my research of vintage Corvette engines a SHP 327 generates about 35 HP internal friction (not including pumping loss) at 3000 revs and a 427 is about 48 HP. With standard gearing 3000 revs is about 65 MPH and the road load is about 45 HP, so the big block is consuming as much fuel to overcome internal engine resistance as road load, and mechanical efficiency is only 50 percent. At higher load the mechanical efficiency would be closer to 80 percent since internal friction does not increase materially with load at constant revs and pumping loss is reduced at high load.

The bottom line is lowest BSFC is achieved at low revs and high loads, and with six speed transmissions, modern cars can achieve this without loss of performance.

Fuel consumption is a big issue with commercial engines, and if you look at most designs, they are typically designed to "cruise' at about 1800 FPM mean piston speed. As a point of reference the best street performance engines achieve peak power at over 4000 FPM, and F1
and NASCAR engine peak revs are at about 5200 FPM, which seems to be the current "limit".

Duke

Sorry all you arguers, but in my humble opinion, their are way way too many variables involved in everything discussed in order to reach any valid conclusions. Tire pressures, drivers, wind, humidity etc etc. It is difficult enough to get valid repeatable data in the laboratory.
Just my two cents worth.
John BSME, MIT (Automotive option)

yes, that is true.

and mentioning that 70% of the fuel consumed is used for road load and 30% is for internal friction. but since a diesel engine is only 40% effecient, these numbers are way off.

we can sit here and talk all about theories all we want and it won't prove anything.

the proof is in the on the road performance.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

These numbers are quite accurate. Nowhere did we introduce the constant loss due to rejected heat. Why add another variable to the equation? Effectively 70% of the fuel goes to road load (and waste heat) and 30% goes to internal friction (and waste heat). That would be the accurate statement.

Three degreed engineers have now stated that you are incorrect in your conclusions.

Believe what you wish.

so now take a 240 with a four speed. i think that it is aknowledged that 30 mpg is about the limit for normal highway travel. the guys with five speeds (that is about an 18.6% od) are getting about 35.

that is a lot more than a 6% increase. 6 would be about 32.

so if you want to believe that a 20% od will result in 6% increase in economy in a normally aspirated diesel, you too are entitled to believe that.

an engineering degree does not endow anyone with the power to predict the answer in a complex problem with a miraid of variables with total certainty. and it does not give them the right to ridicule other folks who have real world experiences to share.

as the epa says,"results will vary".

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Was a proper back to back test done, with the same driver, and the same engine, over the same roads? What you have provided, yet again, is a story that contains no useful data.

An engineering degree allows a person who has a grasp of the physics of the situation to predict the outcome with "reasonable" accuracy. We can be certain that achieving a 20% fuel economy increase from a 20% axle ratio change on a diesel engine is impossible.

Like many shade tree mechanics, your real world experiences are colored with your desired outcome.

But, like I told you before........you can believe what you want.......however, it's highly unlikely that anybody else will believe you when the physics of the problem are carefully detailed as has been done on this thread.

Actually, 6% was my initial guess and Brian's estimate of 9% is probably more accurate. Even if we round up to 10%, that would be a change from 30 mpg to 33 mpg, which I could believe. There is no doubt that increasing the ratio will increase mileage, but the physics will just not allow it to be a 1 to 1 ratio. As always, there's no free lunch in physics.

i never said equal i said near or close.

but hey engineers never color anything the way they want to. i know that cause brain said so.

i still havent figured out how a 40 % effecient diesel loses 30% to heat and friction and still manages to put 70% of the power burned into moving the car forward (and heat).

must have some serious powertrain gain instead of the usual loss.

i guess i would have thought that if it were 40% effecient that you automatically had 60% loss to heat and friction, and then you would have to make do with the forty % left and even lose some in the transmission ds and rear end, not to mention tire friction.

i would have thought that you might be lucky to have 25 or 30% left to actually apply forward motion to the car.

but i am obviously not an actual engineer so i wouldn't be able to understand anything so complicated.

hey, i am not even qualified to measure gas mileage.

oh well, i guess i will go watch a sit com.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I don't think there is anything more that I can possibly add.

well, i should have said nearly the same reduction in consumption.

you caught me there.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I have read this thread with interest, and I'll try to reconcile two positions which in my opinon are not divergent in concept but in number (10% or so vs. 20% or so). First off, after reading many threads here, I have learned to respect the tremendous body of knowledge of the participants as a whole. Some is technical, some experimental, some anecdotal (sp?) but most participants bring valuable ideas. This is true from this thread too.
Firs off, Tom W is very astute in pointing out that it is important to explain how that 40% efficiency of the diesel engine factors in with waste heat etc.
A couple of things come to mind. First that 40% is the product of a thermal efficiency and a mechanical efficiency that are by no means constant, as we drive that thermal efficiency changes constantly and, to a lesser extent so does the mechanical one (let's not forget tire deformation either). Clearly, changing the gear ratio will influence both, and everybody seems to agree that the efficiency can be increased. Can be increased by 20%? I would think that it may be possible in some cases (a car with a very innefficient set-up, lets say 10%, should be easily improved to 12%) while it may be impossible in others (if you are 40% efficient, it is tough to go to 48%).
Only data can resove that. And my personal experience tells me that anecdotal data may point the direction in which we move, but it is worhtless in determining how much. So everybody is right.
My $0.00002

JL

Just for the record, the thread starter made the following statement:

The entire discussion in the thread is based upon the accuracy or inaccuracy of this statement.

"Everybody" cannot be "right".

Either the statement is true.........or it's not.

You already know my position.