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.
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).
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.
Things worthy of note: