Altitude Adjustment

[VegasDMAX]

I recently determined that at cruising at altitudes of 6500 to 7500 feet, I get about 2 miles less per gallon than cruising below 5000 feet. Also, the soot really loads up and when you stomp on it, you get huge black clouds, much worse than at lower altitudes. I would like to improve this situation, but I've never played with the altitude adjustment tables before.

Looking at the fuel pressure adjustment, table B1006 and multiplier B1007, it appears it kicks in at about 6000 feet and maxes out at about 8000 feet, with some really drastic reductions in fuel pressure.

Then the injection timing adjustment is slightly more complicated. Table B0970 and multiplier B0971 start adding a little advance at about 1000 feet and max out at about 5000 feet. But table B0972 and multiplier B0973 start retarding the timing at about 5000 feet and rapidly subtract timing up to about 15,000 feet. I don't know of any roads in the US much over 10,000 feet, but still it looks like the retarding effect of B0972 and B0973 rapidly overtake the advance effect of table B0970 and B0971.

My thoughts on this are that at higher altitudes, the effect of lower fuel pressure causes injection pulses to be longer, and couple that with retarded injection timing and you get much less efficient engine operation.

Can anyone give some kind of engineering reason for reducing fuel pressure as you climb in altitude? Has anyone zeroed out this adjustment to see what happens? Unfortunately, testing this for me is going to be very difficult.

On timing, it seems to me as you climb in altitude, you only want to advance the injection timing, up to some limit of course. Does anyone know why GM has this two-way adjustment in timing and is there some rule of thumb as to how much timing should advance per 1000 feet for best engine operation?

Thanks for any suggestions...
Ed

[LBZoom]

I think that unfotunately this is more for emissions than efficiency in many cases. Your common sense is telling you the right things. At higher altitudes, thinner air, there's not as much air to gather and therfore you lose efficiency in general. You have to get more boost to achieve the same air volume...but then the IAT heats up potentially also and you're back to point A.

With less air entering the cylinders, you get slower combusition (depends on IAT also though) and less atomization, however you have to cut fuel because of the lack of airflow, which could explain some of the reduction in fuel pressure. The combination of less air and lower fuel pressure as you predicted, should mean an increase in the injection timing, but look at the boost multipliers, if boost increases at high altitude then that would explain why GM has reduced timing. (Idon't have a map in front of me to look at just this second) Again it all comes back to IAT.

My thoughts are that as you enter higher altitude and thinner air, your airflow entering the engine is going to get warmer. Having hotter air in the cylinders prior to injection means a higher likelyhood of pre-ignition which would explain the drop in fuel pressure and timing, it's a safety function.

[LBZoom]

If you want to maintain your fuel pressure you'll have to be very careful with your timing setting, and timing multipliers based on IAT, these multipliers can be very handy when trying to incorporate your own style of safety functions!

[VegasDMAX]

For emission control, the stock setup seems to make matters worse, for soot anyway, at higher altitudes. I understand the safety of the engine issue, but the logical way to do that would have been to use the IAT timing adjustment table. Looking at it, it is totally ineffective above 86* F and below that it advances timing, so they probably used it for warm up issues. In the LB7's I think they only advance the timing with altitude, through the use of the 3 timing tables. They also have a fuel pressure reduction table though.

I'm thinking the fuel pressure reduction may have something to do with the physics of pressure above higher altitudes, but maybe I'm wrong....

[LBZoom]

you're probably on to something with the physics of pressure, the person to ask in regards to this topic is "killerbee" he's pretty sharp...

[VegasDMAX]

Yes, I'm familiar with KB's posts - I was hoping he would read this thread and chime in....

[IdahoRob]

Have you logged boost, timing, and pressure at the two different elevations. MPG increases can be had with a bit of extra boost, more pressure, and bit more timing at any elevation. Really depends on what you have changed and where, or are you running all stock tables.

[VegasDMAX]

I'm running a modified main injection timing table B with all post injections and egr shut off. Everything else is stock. I haven't logged anything in some time, I've been happy with what the truck does. But I made a trip across Wyoming on I-80 where the elevation is consistently above 6000 feet for a long distance and that's where I noticed the reduced mileage and extra soot. I'm pretty sure this resulted from the reduced fuel pressure and retarded timing in the tables I mentioned in my original post. What I'm asking is if anyone knows a reason other than emissions control that GM would have set up those tables as they did, because they seem counter-intuitive. I suspect the timing adjustment is for emissions, but I'm wondering if there is some other reason behind the fuel pressure reduction at altitude.

[rcr1978]

I wonder if what you are noticing is different fuel blends, I would think that Vegas winter blend fuel is about the same as the colder areas summer fuel. Higher elevation areas with cold winters have a winter blend fuel to keep gelling problems down but the BTU content is lower and mileage suffers. May not be the issue in your case but could be a thought?

[VegasDMAX]

Nope, that's not it. I bought fuel at the same station in Evanston, coming and going, 2 days apart. That same fuel gave me about 2 1/2 more mpg coming down I-15 to Vegas than it did going across I-80 to Laramie. Actually, I got much better mileage overall than I had expected, with all the talk about how winter time blends can cost you up to 4 mpg. I would say the difference between the Vegas fuel and the Evanston fuel was maybe 1 mpg.