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6.2L Ignition Timing - My Results

Discussion in '1982-Present GM Diesel' started by AgDieseler, Aug 6, 2004.

  1. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    I've been playing around with the timing on the blue machine with my new Snap-On MT480 diesel timing meter.

    It uses a luminosity probe in a glowplug hole, and a photoelectric sensor actually sees the combustion. There is an inductive sensor looking for an indentation of the harmonic balancer. Two to readings are compared at the box, and a timing result is displayed.

    To ensure a stable reading, one must wait for the needle to settle at least two seconds before loggin it in as accurate.

    Here are some of my results:

    Round 1:
    I went to 3* BTDC @ 1400 RPM. The engine clack is noticabely louder at idle to about 1500. Beyond that, the volume and quality of the clack sounds normal. My coworker's 96 Powersmoke is still ten time louder though.

    As predicted, turbo lag increased slightly, and boost levels were down by about 1/2 psi. There was a substantial increase in smoke throughout the rpm range, but cruising EGTs were lower by 50-75 degrees. It also feels like I picked up just a little bit of power; nothing much though.

    Round 2:
    I turned the timing down to 2* ATDC @ 1300. There is nearly zero black smoke unless you're really hammering down on it. Boost is very responsive throughout the rpm range, but power is down just slightly. All this at the cost of EGTs. They are much higher than desired at around 750* cruising at 60 mph. They rise quickly on any hill too.

    Round 3:
    It looks like we have a winner here.

    I bumped the mark forward to 2* BTDC @ 1300 rpm. There's hardly any smoke, power is good, and turbo lag is almost non-exsistent. I think this is a good setup for an engine like mine - a conservative fuel rate and low pressure injectors like the 6.2 long style.

    One thing that was of note:
    I tested three different positions one day to find the right one, and though I moved the pump each time, the marks settled to the same place. There is no strong correlation between the position of the timing marks and the timing figure. Depending on how I jiggled the pump, I could get two timing results that had as much as a 5* difference, but at the same physical location according to the timing marks. This just confirms that the factory-provided timing marks are a great ballpark figure for when one replaces their IP, but adjusting based on the "dime method" is hit and miss for tuning.

    I'm going to get some highway miles on it, and see if the economy was affected at all.

    Next up is getting some new 6.5TD injectors. I figure that the better atomization of fuel will really help out.

    I have a running record of my timing adjustments:
    http://www.oliverdiesel.com/images/mt480/6.2_Engine_Timing_Figures.xls
     
  2. jarheadk5

    jarheadk5 1/2 ton status

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    Interesting...
    Is there a more precise way of adjusting the timing than the factory marks? IMO that's pretty sloppy for something that seems to be pretty critical.
     
  3. BlueBlazer

    BlueBlazer 1/2 ton status

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    Why are you not setting the timing at idle? I would think the timing advance would start coming into play just above idle, though I do not know for sure. This would obviously explain your varying results with different pump locations. When you say BTDC, are you talking actual timing or timing off of factory spec? If that is actual timing, I had no idea it was so retarded, interesting to say the least.
     
  4. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    I'm using a Snap-On MT480 diesel timing meter.

    It uses a luminosity probe in a glowplug hole, and a photoelectric sensor actually sees the combustion. There is an inductive sensor looking for an indentation of the harmonic balancer. Two to readings are compared at the box, and a timing result is displayed.

    To ensure a stable reading, one must wait for the needle to settle at least two seconds before loggin it in as accurate.

    This is called dynamic timing. The figure did change at idle, but based on factory spec of TDC @ 1400 rpm and the military spec of 4* BTDC @ 1300 rpm, that's where I'm taking my readings.

    This kind of work is trial and error. Loosen the pump; bump the marks; check with the meter - repeat as needed.

    As noted, the marks are great for when you're doing an R&R on the IP, but less than reliable for tuning. There variation in performance from as little as 1.5* shows the need for precise measurement. That said, when I installed my pump originally and lined the marks up, TDC was at 1400 rpm - perfect for factory.

    Sorry, I left this out of my original post, but I've edited it to match. /forums/images/graemlins/thumb.gif
     
  5. BlueBlazer

    BlueBlazer 1/2 ton status

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    So that method of timing actually gets start of combustion instead of start of injection, wonder how long the ignition delay is in degrees. Do you have access to a piezoelectric timing device that clamps on the line and senses the pressure spike during injection? It would be interesting to see what the difference is. Is the factory and military spec to be set with a luminosity probe or a piezoelectric device?
     
  6. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    Most of the meters that get used are luminosity types. The miltary manual specifies ignition timing and not injection timing, so that leads me to believe that they used a luminosity probe. I know of only a couple of guys that use the Ferret pulse type sensor, but they haven't post any results lately. I think they're still piecing their meters together.

    It would be very interesing to see the difference in readings. I doubt that they could be compared quantitatively - apples and oranges probably.
     
  7. jarheadk5

    jarheadk5 1/2 ton status

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    [ QUOTE ]
    I'm using a Snap-On MT480 diesel timing meter

    [/ QUOTE ]

    If you don't mind me asking... how much does one of these run? Is it within the realm of the hobbyist, or is it a professional service tool?
     
  8. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    The MT480 & it's digital brother the MT1480 don't come cheap that often. Both are old part numbers, and no longer in production, so getting one used is the only option.

    I got mine off ebay for $150, but that didn't include the quartz lens and magnetic pickup, and while the lens is still a good Snap-On part number, the pickup is obsolete. I made many phone calls tracking down parts. Ford Rotunda has a similar digital meter, but like the Snap-On timers it's obsolete and only available used.

    To be honest, the cost benefit ratio is far too high for a hobbyist since the meter will be used once or twice in the engine's life. For the enthusiast or professional, it is worth the money since one particular engine may have two or three different injector/IP/turbo configurations over its life.
     
  9. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    Okay, so I went on a short 200 mile round trip yesterday, and here's what I found. Ambient temp was about 95 F, and I averaged 70 mph @ 2000 rpm. Cruising boost was 3.5 psi, and EGTs hovered at 600 F.

    I found that with the newest timing of 2* BTDC @ 1300 rpm I got an average of 1 mpg better economy. One 100 mile leg has some decent hills, and economy was just under 17 mpg whereas it had previously been 16 even. On the second 100 leg, the road was nearly perfectly flat, and economy was 17.5 mpg.

    /forums/images/graemlins/thumb.gif
     
  10. FWP

    FWP CRS

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    [ QUOTE ]
    I tested three different positions one day to find the right one, and though I moved the pump each time, the marks settled to the same place. There is no strong correlation between the position of the timing marks and the timing figure. Depending on how I jiggled the pump, I could get two timing results that had as much as a 5* difference, but at the same physical location according to the timing marks.

    [/ QUOTE ] Awesome info! Thank you! I've been having some high EGTs accompanied with some turbo lag. I haven't been able to locate the cause yet. I may play with the pump a little and see what happens.
     

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