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-Diesel Timing-

Ignition timing, and you how you can do it.

When adding fuel and boost in the quest for better performance, a key component, timing, is often overlooked. Timing mechanically injected diesels like the General Motors (Detroit Diesel) 6.2L, 1993 6.5L, and the Ford (Navistar) 6.9L and 7.3L diesels (all of which use the Stanadyne DB2 rotary injection pump) is simple in practice, but understanding the why and where to time is what this article is all about.

Most light-truck diesels come off the line tuned for moderate power and little or no smoke, and as miles are added and power drops off, the general wisdom has been to advance the pump to reclaim some of those lost horses. We'll look deeper into the idea of diesel timing, and its practical application on the project 6.2L engine.

Diesel Timing: an overview

Conservative timing results in low nitrogen oxides (NOx) emissions. For our 6.2L project engine, that resulted in the timing marks perfectly aligned which was top dead center (TDC) at 1400 rpm. While lower timing figures like these are best for NOx levels, brake specific fuel consumption increases, as does carbon monoxide; hydrocarbon levels decrease. In addition, lowered timing decreases cylinder pressure and reduces peak flame temperature since the fuel charge is injected once the piston has past TDC and is already on its way back down the cylinder in the power stroke. This is why it is more difficult to get a complete burn, and it is this hot, unspent exhaust gas that gets past the valves and decreases turbo lag.

On the other hand, advancing the timing results in increased cylinder pressures and higher peak flame temperatures which leads to a more complete burn of the fuel injected. This is demonstrated by a drop in exhaust gas temperatures, and increased turbo lag. The effect on emissions is significant. The NOx level increases dramatically past just a couple of degrees of advance, while hydrocarbons increase and carbon monoxide decreases. Brake specific fuel consumption drops off quickly as well.

This is a graph that was lifted from a diesel emissions reference book. It is not specific to any light-truck diesel, but does give one a general idea of the effects that ignition timing has on emissions.

As always, there is a happy medium between fuel rate and ingnition timing. A general rule to follow, and a good starting point for tuning, is that the more fuel you're running, the more conservative the ingnition timing. It all goes back to cylinder pressure. Adding fuel increases the combustion pressure since there is now more gas trying to expand rapidly during the power stroke.

Using the Snap-On MT480 Diesel Timing Meter:

Since there are no spark plugs on a diesel engine, a spark-triggered timing meter or light cannot be used to read the ignition timing. A few diesel timing meters have been developed that employ a luminosity probe to detect ignition. The most popular meters are the Snap-On MT480 (what we'll be using), and the Ford Rotunda digital meter.

These meters compare readings from two sensors to achieve a timing result: 1) a quartz lens is placed in the appropriate glowplug hole; a photoelectric sensor attaches and actually views the combustion taking place, 2) an inductive sensor detects an indentation on the harmonic balancer which is placed a certain number of degrees ATDC (9.5° for older engines that don't conform to the new 20° ISO standard).

Be sure that you set up the magnetic probe to clear any accessories. Here one can easily see the magnetic probe holder and the indentation on the harmonic balancer. #3 glowplug is easily accessed, and the sensor hooks up snugly.

How to adjust the timing:

Fully warm the engine, and shut it off. Remove the appropriate glowplug (#3 for 6.2/6.5 diesels, #1 for 6.9/7.3 Ford truck diesels, and #4 on Ford vans), and install the MT161 luminosity probe. For Chevrolet diesel, #3 cylinder is chosen since combustion could start close to 9.5° ATDC thus making the first cylinder unusable. Another 90° of crankshaft rotation is available for signal processing. Install the magnetic probe in the holder on the front cover. Power the meter via one of the batteries. Make sure that there are no wires dangling in the way of the engine fan; zip-tie if necessary. Restart the engine, and check timing at selected intervals to establish a baseline.

To adjust the timing figure, rotate injection pump to increase or decrease the timing. To do so, Loosen the three bolts (15mm nuts on GM trucks, and 9/16 for Fords) and rotate appropriately. For 6.2/6.5 diesels, advance is towards the driver side; for 6.9/7.3 diesels, advance is toward the passenger side. NOTE: Do not adjust the timing with the engine running as this will cause severe damage to the injection pump geartrain.

Use the same RPM for each timing session to ensure the comparability of readings throughout the tuning process.

Our Results:

  • Round 1: experimenting with timing advance
    We went to 3.5° BTDC @ 1300 RPM. The engine clack is noticabely louder at idle to about 1500. Beyond that, the volume and quality of the clack sounds normal. 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.
  • Round 2: experimenting with timing retard
    We turned the timing down to 2° ATDC @ 1300. There is nearly zero black smoke unless you're really hammering down on it. Boost is more 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: trying to find the sweet spot
    We bumped the mark forward to 2° BTDC @ 1300 rpm. There's hardly any smoke, power is good, and turbo lag is almost non-existent. Fuel economy increased slightly by an average of .75 mpg. This is a goodl setup for a conservative fuel rate (less than 60 cu mm) and low pressure injectors like the 1800 psi 6.2L long style.

Things worthy of note:

  • The Snap-On manual specifies that the magnetic probe is to touch the harmonic balancer to ensure an accurate reading. Based on my conversations with Snap-On repair facilities, I believe this is a poor practice. The national repair facilities found themselves sending out a large number of magnetic probes since mechanics across the US had ground theirs down by allowing them to contact the harmonic balancer. It is necessary to get the probe close to the balancer to get an RPM reading, but employing a feeler gauge insured that we did not make any physical contact with the balancer. When dealing with an obsolete parts list, it is critical to take extra care and not damage the equipment. It is likely that a replacement will be difficult to find or wholly unavailable.
  • Some of the injection pump bolts are difficult to reach. There are special tools available from Snap-On that make the job easier. Once particlar tool is the rotating wrench. There are two holes on the passenger side of the DB2 pump that this wrench grabs on to, and a 3/8" drive attaches for leverage. Since the adjustments are so small, and the injection lines to provide some resistance to rotation, this wrench is particularly helpful. It should be noted that this wrench will also work on the Stanadyne DS4 series found on 1994-current 6.5L TD.
  • GM spec calls out TDC @ 1400 RPM, and Ford's NA specs vary depending on the elevation and your fuel cetane rating. Figures range from 4° to 7° ATDC @ 1400 RPM. There are many members on The Diesel Stop Forums, www.thedieselstop.com, have found that advancing as far as 8.5° BTDC @ 2000 RPM provides a great deal more power with the tradeoff being some increased smoke and a little more diesel rattle. It may not be the ticket for everybody, but it's worth a try if you have the time.
  • The needle on the timing figure tends to wander, especially around the peak torque RPM. Though it doesn't wiggle much, allow for the needle to settle on a reading for a minimum of two seconds before recording it.
  • In one sitting, three different positions were tested. Although the pump was moved each time, the timing marks on the pump and front cover stayed in relatively the same place. There is no strong correlation between the position of the timing marks and the timing figure. Depending on how the pump was jiggled, it was possible to get two timing results that were as much as a 5° apart, yet were 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 factory timing when one replaces their injection pump, but adjusting based on the "dime method" is hit and miss for tuning.

 

Sources:

Snap-On Tools