Engine/Transmission (1994 - 1998) Timing & Mileage... I Ain't Seeing It!

[MStearns]

Finally got around to bumping my timing last fall. Set it between 14. 5 and 15 looking for a little help with my mileage, expecting the price of fuel to take off again. However, instantly I saw a drop in mileage not an increase. From every thing I had heard and read, I was expecting a 1-2 mpg increase. Instead I saw a 2-3 decrease. Now my EGTs are maybe running 25-50 degrees cooler but my main concern was increasing my mileage. What I have found is the exact opposite. I went from running 19-20 mpg on winter fuel to 17... .



I did find a fuel leak a little later and thought maybe that is where my mileage was going... Nope. Thought it just might be my new snow tires... . But am back on my summer skins and still no mileage.



Before I bumped the timing I would get 22-23 on the 300 mile round trip to our family farm and back, running 60 with the cruise control. The best I've seen since has been 19. 5 (that's on summer tires & summer fuel). So what's going on?? Am I missing something here??? Thought I would ask the question before I go to the trouble of setting it back where it was.

 

[hsmith]

Not sure what the problem is but you should have not have negative results from bumping the timing. Mine has never gotten 20 mpg before or after the adjustment but the egt's did drop a bit while towing.

 

[Joseph Donnelly]

The timing setting on a 12-valve engine will be optimum at only one rpm. Stock timing is best at around 1400-1600 rpm. The often recommended 15. 5 degree setting is good at about 2000 rpm. If you operate at low rpm, you get more vibrations transmitted to the drivetrain. You are usually best off at 1800-2000 rpm for a combination of mileage and drivetrain longevity. At 60 mph you are probably at too low rpm to benefit from increasing timing over the stock setting.

 

[hsmith]

I think Joe's explanation says it all in a few words. With my increased timing, along with a few other mods, my truck tows best if I keep it at or above 2k. I can run with less boost and keep the egt's in check there.

 

[MStearns]

I've never had issues towing. If fact the truck runs like a scalded cat, always has. I was simply looking for a little mpg boost I tow heavy alot and my only complaint has ever been the 5 spd. That step between 3rd and 4th can be a real bear if you stuck behind someone on a mountain pass.



I haven't taken any trips on the interstate wtih the timing advanced to see if there was any increase there. I know, from extensive tracking, that 75 is as fast as I can run unloaded and still get 20mpg. The truck is just pushing to much wind at that point. But I don't think I want to optimize my truck for a set of circumstances I only see once or twice a year. 95% of my driving is around 60-65.



So I think I'll set it back to stock in the next week or two. That way if my mileage doesn't return then I will at least know there is something else amiss.

 

[BigPapa]

How did you set your timing? How much lift did you use?



Scott

 

[Bill Lins]

I'm running a 3. 54 axle & hit 2k rpm at 65 mph, my normal cruising speed. Dr. Joe set my truck at 15. 5 degrees & I picked up about 1. 5 mpg. A bit less if I keep my foot in it, but it feels so good!

 

[Vaughn MacKenzie]

Of all the engines Cummins has put in the Ram the P-pump 12-valve has the narrowest "optimal" RPM band, thanks to the fixed timing. The Older VE engines had a flatter BSFC curve due to a few degrees of built in dynamic timing. They're torquey and great for getting good towing mileage running at say 16-1700 RPM, yet won't drink the fuel as bad if you had low gearing and ran the revs up higher like at 2300 on the highway.

 

[LVJUNIOR]

Of all the engines Cummins has put in the Ram the P-pump 12-valve has the narrowest "optimal" RPM band, thanks to the fixed timing. The Older VE engines had a flatter BSFC curve due to a few degrees of built in dynamic timing. They're torquey and great for getting good towing mileage running at say 16-1700 RPM, yet won't drink the fuel as bad if you had low gearing and ran the revs up higher like at 2300 on the highway.

so if I want to run 75 can I move the timing up to say 17 and get optimal fuel mileage at 2100rpm or is the motor only efficient at 1600-1700rpm

 

[MStearns]

How did you set your timing? How much lift did you use?



Scott

I bumped the timing while I had the front cover off changing the front seal that was leaking. I wanted to make sure I got the pump shaft good and clean so the gear didn't slip. I set it to ~. 209 inches of lift.

 

[MStearns]

Ok, reset the timing this morning. It's setting between 13 and 13. 5 degrees @ . 198 inches of lift. So we'll see if my mileage returns. I didnt' go all the way back to 12. 5, which is where it was initially. I probably should have to completely iliminate the timing as a variable but the TSB said to set it at 13. 5 so I thought I might try it there first.

 

[MStearns]

Just filled up after 397 miles (300 "highway" at 60-65 and 97 in town) with only 18 gals.

That's 22mpg by my math. the tank before that I got 19mpg all in town. Only thing that has changed beside the timing is we took on a quick trip out to Montana pulling our camp trailer so the truck got worked good and hard. (75 up mountain passes in overdrive... Man I love my truck!!) So maybe that freed up an injector that wasn't functioning fully... I don't know. In my mileage log, the loss in mileage very clearly correlates to the advanced timing. Now that it is retimed, my mileage is back as well.



I think I'll leave well enough alone for now. I shouldn't be greedy. 22mpg is Ok I guess.

 

[Rogan]

Last tank run was 606mi and I ran out on the way to refueling. 85% of that tank was pulling a 9000lb trailer. Equated out to 18mpg as it took about 35 gal to fill it up.

Well satisfied am I.

 

[Rogan]

my fuel economy is just all over the place, lately. . Since theat tank ^^^, my next was 440, then 568, and now this one, with 1/4 tank left, has tripped 288mi. Usually, I'm in the low-mid 400mi range at that level. .



I'm so confused LOL

 

[charlessmith]

When you had the timing set to 14. 5* to 15* - are you real sure where it was to start with? It may have been at 16* or 16. 5* before you "retimed" it.

Plus, the factory figure is different for different engines / FI pumps.

For other interest, you can run it as far as about 16. 5* without a worry about the head gasket. More than that you ought to at least install APR studs to clamp the head on tighter. I think 17* or more, without studs or "o" ringing the head, is just asking for a head gasket problem.

 

[MStearns]

I double and triple checked. When I reset it, it was still set at ~. 209 inches of lift which for my engine and pump should have translated to between 14. 5 and 15 degrees. It hadn't slipped at all.



I didn't go all the way back to stock (which would have been 12. 5 degrees) due to the TSB that was out there. At . 198 inches of lift I'm between 13 and 13. 5. I'm back to getting 18-19 in town and low 20s (21-23) on trips around 60-65. I haven't had much of chance for much interstate travel yet. Historically 75 is as fast as I can run and still reliably get 20mpg. I'll test it this fall when I run out to Montana on a quick trip.