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Timing and altitude change - WHAT'S THE THEORY BEHIND IT???

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Can Can, May 23, 2002.

  1. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    I live at an altitude of about 4000 ft. Whenever I travel and get to areas of 3000 ft and less, I get lots of pinging out of the engine. Obviously this is a timing issue, but how does a change in altitude affect the timing?

    I understand that air gets thicker as you drop towards sea-level, but how can this cause the engine to be over-timed? Can somebody please explain it to me? I've disabled the Electronic Spark Control, so I think I'm gonna have to start bringing a timing light along when I travel to my new property so I can bump the timing down manually. When I get there, the ol' 305 pings like a son of a gun every time I get to mid-throttle......../forums/images/icons/frown.gif

    Here's your chance, timing guru dudes!!!!/forums/images/icons/wink.gif
     
  2. mike reeh

    mike reeh 1/2 ton status

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    Ill take a stab at it.. at higher altitude the air is thinner resulting in less compression letting you run more advanced timing? when the air thickens, its bumps up the c/r and probably leans out the air:fuel mixture making it even more prone to pinging...

    mike
     
  3. 4x4k20

    4x4k20 1/2 ton status

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    it has nothing to do with your timing what happens when you change altitude the air going to your carb changes it chages the air /fuel ratio in your case the air gets thicker causing a lean condition that will cause your engine to ping i aint no expert but thats what i have always been told these newer ones have computer controls to compensate for air changes but with a carb. you will need to do it your self .some of the boys in colorado should know more about this .
     
  4. HarryH3

    HarryH3 1 ton status Author

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    The ambient air pressure rises as you get closer to sea level. This extra pressure causes more air to rush into the cylinders when the intake valve opens. This increases the pressure in the combustion chamber, so the effective compression ratio increases. Higher compression increases the tendency towards pre-ignition. You hear it as pinging.

    To make matters worse, if your location is anything like Colorado, as the altitude increases, the octane of "regular" unleaded gas that you can buy decreases. This lower octane pi$$-water works OK at high altitude (due to the lower effective compression ratio), but if it's still in the tank at lower altitudes, then again, the engine is more likely to ping. In Colorado Springs, they call 85 octane "regular" (and charge us a premium for the 87 octane stuff that is considered "regular" in most of the rest of the US). /forums/images/icons/mad.gif

    The lower air pressure as the altitude increases is also what makes your engine feel like it's losing power as you head up to high altitudes. It costs you about 3.5% of power per 1,000 feet increase above sea level. I live at 7,000 feet, so I lose almost 25% of what an engine would make at sea level. /forums/images/icons/shocked.gif Drive up to the top of nearby Pike's Peak, at 14,110 feet and you've lost nearly half of the power your engine could make at sea level! /forums/images/icons/shocked.gif
     
  5. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    Ok, then......I kind of understand the problem. So what will I have to do to fix it when I head to my property? Like I said before, my 84 pings like crazy after a certain point on the highway.
     
  6. HarryH3

    HarryH3 1 ton status Author

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    If you have an MSD ignition, then the gadget at the bottom of this page http://www.msdignition.com/timing_comp.htm will make it very easy to adjust your timing. /forums/images/icons/cool.gif Another option is to tweak the timing so that it doesn't ping at the lower altitudes and just leave it there. /forums/images/icons/frown.gif
     
  7. Thunder

    Thunder 3/4 ton status

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    Use preimum gas at lower altitudes.
    Also if you disconnected the ESC there is nothing to controll pinging. Thats its job. To retard the timing when the knock sensor detects pinging.. With out the ESC the thming stays advanced all the time.
     
  8. 70~K5

    70~K5 1/2 ton status

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    Before you head down lower fill up with HiTest gas and see if that helps.
     
  9. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    Actually it does have something to do with timing (inadvertantly). The timing can be set-up fine for high altitude (no pinging) due to the air charge being thinner and the optimum air to fuel ratio of 14:1 (IIRC), but as the air gets thicker that number jumps toward 15+:1 which is a lean condition. You can compensate for this by retarding the timing a little. You could leave it retarded and run at higher elevations, but you wouldn't be getting optimum power and MPG. Ignition, carburation/FI, cam specs, fuel octane rating, air density, air temp, static Compression Ratio, Spark plug temp range, radiator size, water pumps circulation, thermostat, exhaust efficiency, etc. etc, etc. all work together to give you optimum performance. You adjust it to be the most powerful at a certain temp and pressure (STP), but if you change one item (such as density of incoming air charge) it will affect the rest of the system and the other items need to be tweaked to compensate. Changing octane rating essentially changes the flash-point of the fuel and can compensate for some change, but without tweaking the rest of the system you are not running on optimum performance.
    As much as some hate FI for being too complicated, it can adjust several of these factors simultaneously and instantly to compensate for varying conditions much better than any carb. That is why they can be so efficient and powerful. There is a limit as to how much they can compensate, but when they do it's automatically. As systems get more advanced you will probably see some amazing HP and Torque numbers as well as MPG from fuel injection that will never be matched by carb. Just look at what some High Performance chips due for stock trucks. They allow more compensation than a stock computer and boost HP/Torque with very little effect on MPG.

    Now back to the post. It is ignition timing (as it is not set for the changed conditions), as you stated it is different density of air too, but there are many more conditions that apply to the pinging not just one or two things. Hope this helps.
     
  10. m j

    m j 1/2 ton status

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    a good fuel injection setup will compensate for altitude
    BAP and MAP sensors do that job
     
  11. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    No shite, brother!!! Got 1500 clams to give me????/forums/images/icons/wink.gif

    I just bought a chunk of land in your province and had to pay an additional 1% to your government for some reason./forums/images/icons/tongue.gif It ended up being about the same as what a used injection setup would have cost....
     
  12. m j

    m j 1/2 ton status

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    BC = bring cash
    it is to keep you flatlanders on that side of the fence
     
  13. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Re: Timing and altitude change - WHAT\'S THE THEORY BEHIND IT???

    Yeah, but the sad thing is that the only people who can afford property in the Kootenays is Albertans. And until the Eastern Slopes warm up a little more, more of us will be coming. We should just join forces and unite BC and AB.....
     

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