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.

[rcr1978]

I've drove that way south before to and always got great mileage heading south vs coming back north, you might be overlooking the fact that its mostly down hill back getting better mileage on the same fuel. Winter fuel takes me from 16.5-17 freeway cruising at 78mph in the summer to 13-13.5 in the winter on the exact same routs traveled hundreds of times. Elevation ranges from 5500-7500 feet and I have adjusted timing, boost, rail pressure on my tune. This is a 9500lb LMM with no dpf though. I'm not saying its the factory tuning at higher elevation thats hurting your mileage but I think your over thinking it, head winds, tail winds, rolling hills, steap passes, and a cake downhill cruise back to vegas.

[killerbee]

Can anyone give some kind of engineering reason for reducing fuel pressure as you climb in altitude?

Hi Ed. Welcome to the world of GM mystery mixtures. If that table is pulling that much pressure, you definately would observe the soot increase you mention. I honestly can't say if there is an elevation standard for emissions. I tend to doubt that there is. I am interested in seeing your log of highway pressure.

Your logic is spot on. If those tables actually do conspire (many don't necessarily work), then pressure would drop to well below 30 mPa during cruise. And that will create very long pulsewidth, reduced atomization, retarded ignition, and toileted economy. Nothing surprises me any more about GM tuning. Looking forward to your conclusions.

[killerbee]

One other suggestion, if you are comfortable with adding a custom calc.pid to your operation, a mixture pid, like equivalence ratio (mixture equation of pulse, pressure and air) will be very helpful in comparing your different runs. It can be hard to otherwise draw conclusions from several constantly changing variables. This standardizes the mixture challenge, as pulse, pressure and air relationship all vary.

Timing is another thing entirely, and as LBZoom points out, IAT has a lot to say about it, one of at least 5 important factors. But if cruising at 30mPa, then you would need at least 12 btdc to optimize. I prefer around 60mPa empty cruise. A good compromise of efficient drop size, noise emissions, and CP3 workload (drag). Combine that with around 10 btdc for a starting place. (empty highway cruise only)

[VegasDMAX]

Thanks for the suggestions Michael. I've never done calc.pid's, I'll have to look into it. Doing any real testing at higher altitudes is going to be tough. I have a couple of mountain recreation areas nearby where I can drive at 7000 feet or so, but the roads are such that you can't get any real "cruise" info, i.e., too much on-and-off on the throttle.

Are you suggesting that I attack this more with a timing adjustment and live with the stock fuel pressure reductions? Or could I try decreasing the reduction in fuel pressure at altitude? I'm just wondering if there are CP3 life or FPR life issues that come into play by making them produce more fuel pressure with less atmospheric pressure. Since GM has the same type of fuel pressure reduction mechanism in the LB7, I thought there might be something to that. My background is EE, and I think you're ME, so I thought you might have some better insight into that than me. BTW, I've read your papers on tuning - great job on those.

rcr1978, I appreciate your input, but I don't buy the downhill theory. The only real downhill stretch is from Cedar City to St. George, or maybe Mesquite at the outside, and that's only an 80 mile leg. Other than that, I think the ups and downs are pretty equivalent to what you see on the 80. But the 80 has a higher base altitude. Although I don't have any logged data, I've given this a lot of thought looking at the tables known to EFILive. Unless, as Michael suggests, some of those aren't really even active, I think that is where the problem lies.

Ed

[killerbee]

Are you suggesting that I attack this more with a timing adjustment and live with the stock fuel pressure reductions?

Not at all. And it doesn't have to be all "straight and level". I would log it, then zero out the pressure reduction, and log it again. Very often I will use the avg. of a half hour log to draw better conclusions on the changes I have made (and sometimes discover I have actually changed nothing, because of a non-active table). So logs are what is needed for this thread to develop. You should pursue it, because I have no doubt you may be on to something important, if those tables do work.

I use a pressure table I developed for earlier gen trucks, adjusted a bit for the injector differences. This is where, IMO a lot of refinement can be had. I notice that nearly nobody touches it, and that just may because it is not accepted that it matter that much. It does, especially for light throttle stop-go with the 01-05 tunes.

I am a CN, fwiw.

[rcr1978]

What kind of mileage did you get? If you have done some tuning/logging on your truck you will be quite surprised that the timing numbers in the B table are not even close to what you actualy get in your logs unless you zero multipliers and other related tables witch I never really liked for a daily driver.

[VegasDMAX]

Sorry guys, can't seem to make the "reply with quote" work with my browser. Anyway, for rcr, cruising at 70 to 75, I regularly get about 19.5 mpg, unloaded of course, and driving below 5000 feet for the most part. Can't seem to break the 20mpg barrier. As I said, I haven't logged anything for quite some time, and I have only logged near Las Vegas. But as I recall, my actual timing was pretty close to expected for table B plus the first altitude correction table/multiplier, B0970/B0971. At first, that wasn't the case, but then I did zero out the AAT adjustment table which seems to be backwards anyway, i.e., it advances timing as air temperature increases, then the timing came into what I expected. As long as I was at operating temperature and at my altitude of about 2200 feet, the timing agreed. If EFILive is correct on their table definitions, and I hope they are, the other timing adjustments only kick in if pilot2 is active (never seen that while logging, and Pilot3 is zero from the factory on mine), or if you're below regular engine coolant temperatures ( I think the ECT tables are only effective below 158*). The IAT adjustment only kicks in below 86* (never seen that while logging either). I'm kind of shooting from the hip here, just going from memory instead of reviewing the tables before I write this, so if I've mis-stated something, I apologize.

For Michael, I like the idea of zeroing out the fuel pressure adjustment for altitude if you don't think that will hurt anything. It (the stock adjustment) seems counter-intuitive for engine efficiency. I'll try it it at the next opportunity to see if the table even works, but this thread may die before I get that done. I take it you haven't fooled with the altitude adjustment in your fuel pressure refinements?