Mileage and IP timing

[fastcat800]

My truck has been running great and every time I filled up on my last 600 mile trip my mileage was better. The first tank was 12.2 mpg driving at 69MPH. Next was 12.8 mpg driving at about 60 average on a 2 lane road without cruise (first tank was freeway on cruise). Next tank was 13.4 mpg on the freeway on cruise at 65mph. This was pulling about 3000 lbs of snowmobiles. Yesterday I cut a hole in the top of the air box and I am hoping to see even better this weekend on about a 200 mile trip. Next week I am going to have my IP timming set. I have heard anything from stock at -2.5 to -5.0 to the best is -1.94 TDC. What should I set it at? No mods except air box and best mileage is most important to me. In a couple of months I plan to go with a 3 1/2" exhaust.

[ 03-04-2005, 10:37 AM: Message edited by: Dave Schumacher ]

[GMC Hauler]

you are talking about two different things when it pertains to your injection pump and timing:

TDC Offset, or the amount of difference between crankshaft timing and injection pump timing, is due mainly to timing chain slack. The best value for power seems to be a setting of -1.94. Now if i have gleaned information from this board correctly, a TDC offset slightle less negative (0 to -1.5 will yield better has mileage. How much, I dont know.

Your injection pump base timing is set at 3.5 degrees, or will have to be. There isnt much you can do for that except a custom computer chip, but then you will always have your foot in the pedal, which would negate any gains.

The best things you can do, and search for mileage on this board, there is lots of threads on it, is to probably change your gear ratios down to a 3.23. That is not conducive to heavy towing, though.

You could put a Gear Vendors (http://www.gearvendors.com/) or a US Gear (http://www.usgear.com/) overdrive unit on. They are precey, but at the rate diesel prices are, the payoff time is getting shorter and shorter. I may put one on in the next two years.

A low restriction exhast will nab a little better gas mileage.

[Barry Nave]

94-95 won't take more (less) than a TDC of -1.95.
Try going more on the Neg. TDC and you'll set a code. I have for years ran a -1.75 when I installed the DSG timing gear,cooling up grade.
Couples weeks ago had it reset to -1.95 but have not seen any improvement in MPG. A little more power and the best is the truck smokes less.
Being a 3500, loaded with tools I do see 12mpg with a mix of city and highway.
If it were'nt so much work,I'd like to see what I would get in the mpg, after removing the bed rail tool box,ladder rack (no airo flow there) and sliding bed tool box.
If it was'nt for the air bags I would be running on the over load springs 24-7.
You can aways tell when fuel price goes up.
As a DP member for many years(let my membership lasp once,"forgive me farther for I have sined,Amen") we were getting a handle on fuel cost at $1.00-1.35. I'm going into relapse :eek:

[ace58]

I replaced my engine, turbo, injectors, IP,computer, and added JK's 3.5 exhaust, big intercooler and a SRA combination turbo/EGT gauge with offset at -1.75, #9 resistor and with about 1500 miles am getting 14 MPG with additive. It should improve as engine breaks in, or we finally are done with winter blend fuel, which ever comes first.

[Barry Nave]

ace58
I'll get 14mpg in the summer,winter fuel has less BTU. You'll see better when we get back to #2. ;)

[kowsoc]

I remember some posts about this subject and I recall a couple members testing various degrees of offset. -1.50 was said to give the best fuel economy and -1.94 slightly improved performance. A search of the site may bring you that post and one of the members involved in testing, I believe he Charlie Peterson may have more details for you. smile.gif

[fastcat800]

Sounds like somewhere between -1.50 and -1.95 would be best for me. Don't know what the air box mod did as I was in a hurry and drove 70-75 mph and got 12.2 mpg. What is the stock timing supposed to be? Changing gears is not an option as the payback in fuel savings would be many years. I may go to taller tires next time I need them. Adding 2" in height would get me close to 3.5 gear ratio. Is there a search feature or do I have to page through all the posts to look for a subject?