Timing thread

[robof16]

**** Disclaimer: I am not claiming to be an expert and am only posting this to generate discussion.****


WRT diesel timing; wondered what it would look like if you took a pure RPM-adjusted approach as a starting point. Essentially keep the beginning of the pulse the same time before TDC through the RPM range. Example: 9 degrees at 1500 RPM is 1000us. Corrected for increased RPM, the lead time of the pulse increases; converted to degrees BTDC, and looks something like this:

RPM – Timing BTDC
1500 – 9.0
1600 – 9.6
1700 – 10.2
1800 – 10.8
1900 – 11.4
2000 – 12.0
2100 – 12.6
2400 – 14.4
2500 – 15.0
2750 – 16.5
2800 – 16.8
2900 – 17.4
3000 – 18.0
3200 – 19.2
3300 – 19.8
3400 – 20.4
4800 – 28.8

Using the simple spread sheet you could start with any number you want so, to be conservative, you could program the majority of the table for 6 degrees and just bump up the cruise area to what you know works. This is not designed to give you the entire table, only to provide a starting point for timing values that run diagonally from upper left to lower right in the timing tables. If you’ve done any logging at all you know this is by far the most-used area of the tables. The other areas of the tables (upper right, and lower left) I would leave stock if you don’t have a preference.

Is this corrected for heat, load, etc.? Nope. It simply starts the pulse the same number of microseconds before TDC throughout the RPM range. If I understand these diesels correctly, the pulse is likely burning as it is injected into the combustion chamber under normal circumstances so I can’t imagine increased heat or load requiring significantly altered timing. That still wouldn’t keep me from having a more conservative timing map ready to select on my DSP5 for those heavy uphill tows on a hot day.

Is this similar to the 50/50 method? No. This method addresses the beginning of the pulse and it doesn’t matter how long the pulse is. The 50/50 method and this may agreed at some point in the table but it would just be a coincidence.

Why is there such a large dip in stock timing in the middle of the table(s)? I’ve never heard an explanation that makes sense to me. My guess is that it is all about reducing emissions and noise, not performance or economy. I understand that retarded timing helps reduce NOx.

I am already running timing similar to this and haven’t found any negatives but I certainly don’t have the equipment or know how to prove its worth either.

Standing by for the spears…

[asmithIII]

Timing needs to vary with amount of fuel. There is little curve to your timing table. I've seen pilot tables that are similar to this. Small amount of fuel can take a while to burn because of small amount of heat during combustion. moderate amount of fuel will light quickly and burn quickly. This is the dip in tables. larger amounts of fuel take longer to ignite because of the cooling effect of the spray. fuel must change from a liquid to a vapor before combustion can take place.

i have not the equipment to nail down the optimum timing. Best I can do is reduce the boost, set timing by ear and then return boost to prior level. The reduced boost allows the conditions described above to become more pronounced. An increase in boost seems to soften the rattle caused by timing.

No spears from me

Appreciate discussion

We should compare and contrast tunes some day

[robof16]

Timing needs to vary with amount of fuel.

I really wonder how much though. With the way these pulses are timed, fuel isn't present until the conditions are right (theoretically), and it should start burning even as it is being injected, right? I would think especially at the higher boost levels you see where long pulse widths are common that the spray wouldn't have very much time to cool the chamber due to the immediate burn process. Maybe some cooling, but in the big scheme of things I wonder if it is significant. I still think most, if not all of the dip in the tables can be attributed to considerations other than performance/economy. Have you looked at Cummings or other common rail diesel timing map? That would be an interesting comparison.

Best I can do is reduce the boost, set timing by ear and then return boost to prior level. The reduced boost allows the conditions described above to become more pronounced. An increase in boost seems to soften the rattle caused by timing.

Interesting.

Apples and oranges but; I don’t understand why gas engine timing is so different. Take a SBC for example that may start off with over 12 degrees of timing at idle and end up with say 35 degrees of total advance! I would think that gasoline, being more volatile, would be less tolerant to timing advance. Maybe there are delays that I’m not considering with gas engines.

[Mitco39]

The biggest difference in a gasoline engine is that the air fuel mixture occurs due to turbulence. This mixture introduces rich and lean spots that change the flame propagation during the time the spark fires. Also in a gasoline engine your ignition source is the size of a pinhead, in a diesel the fuel auto ignites from the sheer heat due to the compression ratio of the diesel.

Also more boost = more heat = less timing as the burn will happen quicker. So setting your timing without boost and then adding boost in could hurt you if you are riding the line between pre detonation before boost only to add it in later.

[asmithIII]

Have you ever reduced the air and experienced knock in a Diesel? I'll do this a a final check of the timing.

Have you ever tried cutting the duration in half and leaving the timing the same? This would be similar to timing 1000us before TDC without regard to the amount of fuel. The engines that I've tried that on knock. The extra fuel did something to help with the noise of combustion.

[robof16]

I've seen pilot tables that are similar to this

Looked at a stock LLY tune a couple of days ago and noticed what you are talking about.

I thought that if I went back far enough in the Duramax evolution I might find timing values less concerned with emissions. I need to know more about LLY pilot injection logic to draw any conclusions though.

[THEFERMANATOR]

The LLY from what I recall actually had more aggressive timing since it had a cat and EGR valve on it. The LB7 federal trucks have a HORRIBLE timing table in stock form since they used tuning alone to meet emissions. The CALI LB7's had some differences in the pilot areas for emissions as well as fuel pressure and a few other tables. I tend to be rather conservative with timing myself, but I have found 55-60% timing using the V3 calcualtor to be pretty good for the top end. As for cruise, I run around 6-8 degrees for my LB7, but later trucks can run more timing in the cruise areas because of the smaller injectors.

[robof16]

The LLY from what I recall actually had more aggressive timing since it had a cat and EGR valve on it. The LB7 federal trucks have a HORRIBLE timing table in stock form since they used tuning alone to meet emissions. The CALI LB7's had some differences in the pilot areas for emissions as well as fuel pressure and a few other tables. I tend to be rather conservative with timing myself, but I have found 55-60% timing using the V3 calcualtor to be pretty good for the top end. As for cruise, I run around 6-8 degrees for my LB7, but later trucks can run more timing in the cruise areas because of the smaller injectors.

Do you know what under what conditions the LLY uses pilot injection?

I won't argue if it works or not, just don't understand the V3 calculator method. It seems to me what matters most is how far forward you can move the pulse and if the pulse is really long you've go no choice but to let it chase the piston down. Most of us are running cruise timing with 100% of the pulse BTDC and like you said, 55-60% in the top end. It seems to me if you change injector size the percentage of before/after TDC would change but the degrees BTDC should stay about the same. Now, if it were to change I would predict that you would need to move timing slightly later and now that I think of it, maybe the 50-60% method would still be close. At any rate, seems like the V3 method would only be good for one area of the map.
Good stuff...

[THEFERMANATOR]

Do you know what under what conditions the LLY uses pilot injection?

I won't argue if it works or not, just don't understand the V3 calculator method. It seems to me what matters most is how far forward you can move the pulse and if the pulse is really long you've go no choice but to let it chase the piston down. Most of us are running cruise timing with 100% of the pulse BTDC and like you said, 55-60% in the top end. It seems to me if you change injector size the percentage of before/after TDC would change but the degrees BTDC should stay about the same. Now, if it were to change I would predict that you would need to move timing slightly later and now that I think of it, maybe the 50-60% method would still be close. At any rate, seems like the V3 method would only be good for one area of the map.
Good stuff...

Pilot is active for most everything under 75Mm3 of fuel, and 2700 RPM's for LB7 and LLY. As to timing, I stick with the timing calculator to keep the percentage around 50-60%. I have tried going higher, but ended up with some rattle in some spots, and backed it back down. Another thing to keep in mind that some play with is ignition delay. You can use the calculator to get your split percentage, but add in some to offset for ignition delay/burn response. I believe it is roughly 4-5 degrees at 3000RPM's. Personally though, I have yet to see much if any gain going beyond 55-60% of timing using the calculator. I try to stay around 50-55% to keep smoke to a minumum. I tried going up to 60-65% before up top, and it didn't rattle, but it smoekd noticeably more with the timing advanced to those areas.

[asmithIII]

Good thread. Can't remember hardly any open discussion about timing/tuning. Sorry, but I must throw another variable into the discussion. That variable is rate of fuel delivery.

Using the timing calculator is helpful for me. Especially in full load (90mm - 120mm) section of the timing tables. Gives me some confidence and 55% BTDC seems to work well in this section. Timing calculator is not as useful in lower fuel delivery sections(0mm - 50mm). Cruise section is mostly over 100% timing but some sections of cruise can be much lower when the rate of fuel delivery is increased.

Rate and amount are different. Increased rate allows more fuel in a shorter time. Similar to simulating spark ignition in a Diesel engine but only up to a certain point. If fuel is delivered too rapidly, then stacking occurs. Fuel gathers before being burned and this is useful but again, only to a certain point. So most of my tuning is trying to allow the shortest burn time possible without stacking the fuel and timing the burn for best power/economy.

On an LMM see table B1059 for rate of fuel delivery.

Be aware that 1000mm/second is 3600 liters/hour or about 1000gallons/hour!!! Stock LMM used later timing but increased the rate of injection. Using the stock rate of injection, timing advance while tuning is limited. So far a little less rate and more advance is useful for economy at very light throttle settings.