fuel econ/mpg

[Sri Ram]

in another thread dvalentine said

Guys have found that the factory setting (degrees before top dead center) that begins the injection event can be a little behind (late) and that an advanced setting can positively affect the running of the engine. Those looking at things from a emissions standpoint will disagree. The advancing or retarding of the timing changes the emissions output.

How do advanced/retarded times affect mileage?

 

[dpuckett]

SriRam-



AFC= Air Fuel Control. The aneroid on top of the injector pump. I believe Fritz's Ram Tech page has detailed diagrams. Basically, it is a smoke control device which is adjustable, given the VE is mechanically controlled. It holds back the fuel til there is air to burn it, hence the name.



The timing affects MPGs based on the fact that advanced timing will equal more power on a given amount of fuel, to a point. The reason timing is retarded from optimal is mainly due to emissions concerns (gotta love the Egotistical Pinhead Admin)- details of which I wont go into, since this is a fuel economy thread- but also to a point, to help longevity, since advanced timing increases internal cylinder pressures. But the Cummins is so over engineered in stock for that it wont become an issue til about 850K.



So, more power from "x" amount of fuel can also equal the same power on less fuel, or better MPGs. Simply put, it has to do with combustion, volumetric efficeincy, and the ever important timing. And, unlike a gasser, you dont have to worry about preignition in the same way you do in a gasser, since the fuel self ignites anyway. Does this clear stuff up a little? or did I just muddy the water?



But, gearing also comes into play, due to the BSFC (Brake Specific Fuel Consumption) curve. What that is, is a graph that displays the fuel needed per horsepower at a given RPM. Higher RPMs require more fuel, and more power requires more fuel. So, all else being equal, you should be able to go farther on a gallon of fuel while turning 1800RPM than you would at 2400RPM. Like timing, there is a peak of efficiency, where lower RPMs require more fuel. I think the peak efficiency in stock form is around 1750-1800 RPM. Different turbo configurations, head porting, etc, can affect eficiency at various RPMs, so that is something to consider. If you alter the air or fuel flow curve, you alter the BSFC.



Daniel

 

[Sri Ram]

dpuckett: all of a sudden I'm having flashbacks to Lyle Cummins' Diesel's Engine, Vol. 1! I'm no engineer, but what you said does clear things up a lot. I remeber Cummins' book was lacking in an explanation of bsfc for the novice. I'll have to search here for tips on fiddling with the afc settings. As for peak efficiency and rpms, I think I've noticed--even before I put a tach in--that that window of 1750-1800 was the place to be. thanks for the specifics. I'd be interested to know what emissions legislation(?) led to the timing retardation.

 

[btoscano]

What M. P. H. is 1700-1800 RPM with a G360 and 3. 54's and 235/85R16 stock tires?



-brian

 

[91W350]

We just made a 2400 mile run with the W250, the first tank was 16. 7, a lot of cross wind and I was pushing 80. The next few tanks were all in the 21 range, which I think is excellent for the hills and number of stops we made. It was a pleasure to drive, I think I need a Borgeson though. Glen

 

[Sri Ram]

What M. P. H. is 1700-1800 RPM with a G360 and 3. 54's and 235/85R16 stock tires?

That would depend on what gear you were in, wouldn't it?

 

[TKingsbury]

What M. P. H. is 1700-1800 RPM with a G360 and 3. 54's and 235/85R16 stock tires?



-brian

65ish if memory serves.

Travis. .

 

[Stomp]

Well I am a little jealous. I usually get 13-15 in town and 18. 5 on the highway at 70mph. I did get 20 on my way up to Piers's for the First Gen day with my 14mm head/rotor. I get about the same with the 12mm vs. 14mm. I just got another 14mm pump built and I got 18. 4 on the way back home.



When I go home to visit my parents I usually get 16 on the way up and 18 on the way back. There are allot of mountain passes along the way. I guess it still isn't that bad.



Now if I go and play on the dyno or do allot of Stomping on the go petal well it goes way :--) down. :-laf



Stomp

 

[Sri Ram]

Well, Stomp, I guess there's a price to all that power…But, that's not gonna stop anyone!

 

[TKingsbury]

65ish if memory serves.

Travis. .

I drove my truck to work today and made a point to check the mph. I show 62 mph @ 1800rpm in 5th gear. I have stock tires and 3. 54 gears.

Travis. .

 

[Stomp]

Well, Stomp, I guess there's a price to all that power…But, that's not gonna stop anyone!

Yeah I wouldn't trade the power for mileage any day. :-laf



I noticed a big drop in mileage is when I went from a H1C/12 to a HX40/16. I no longer had that boost off of idle. I lost about 2-3mpg in town. On the highway I lost a little also, but not much. It could have been that I was playing with the skinny petal a little too much.



Stomp