Adjusting Pilot Timing when Base Timing is changed?

[koolky]

Adjusting Pilot Timing when Base Timing is changed?
I would like to know how important it is to scale back pilot timing as you increase base timing. One of the first thing I experimented with on my tunes was to use a timing calculator to set my timing tables with a minimum of 50% injection event BTDC while retaining any stock value that was above 50% BTDC. After reading about piston crack and large PW’s causing fuel to be injected above the piston bowl I realized that when base timing is increased so is pilot injection timing. So here’s where my thought go crazy. I was think that I could recalculate the Pilot 1 Timing Base (B1201) and reduce the amount of microseconds before the main injection event so that even with a modified Main Timing (B0909) I could time the Pilot injection event so that it would take place at the same time as a stock tune. Would this work? Would PW of Pilot overlap the Main injections?

I did a little bit of work calculating this but I am not sure if this is worth pursuing.
So far I made an excel spread sheet that converts main timing base from degrees to microseconds. Here's what I have for notes:

Pilot Injection Timing/PW Calculations
(Alt 0181 µ)
Convert rpm to deg/µs (*/µs)
((rpm/60seconds)/1000000microseconds)x360*= degrees per µs. Simplify equation: (3608/100000µs=0.00036) 60/0.00036=166666.666
Therefore rpm divided by 166666.666 = */µs (degrees per microsecond)

Example:
-0.5* of timing from the table divided by */µs at say 800rpm (800rpm = 0.0048*/µs)
(do not divide by -0.5. divide by 0.5 and then convert it to negative).
0.5 / 0.0048= -104.166µs ATDC

Now look at Pilot 1 timing base. At the same rpm/mm3 it calls for pilot injection to start 900µs BMI before main injection.
So, if main timing it happening at -104.166µs BTDC (in this case ATDC) add 900µs to that to find out when the pilot injection event starts. In this case 795.834µs BTDC.
Take the above µs BTDC and multiply that by */µs for the given rpm and you will get the pilot injection timing in degrees
795.834 x 0.0048= 3.82* BTDC

So what im thinking is that I can calculate the stock timing in *BTDC of the pilot injection event and keep it constant while adjusting base timing. Im sure this could affect some other things so I would appreciate any input.

[Wheelz]

Do some google searches for effects of pilot timing on combustion. There are some very interesting technical articles out there. They may take a couple times to read before you understand it. At least it took me a few to fully grasp the concepts.

But in short, pilot timing and amount is just as crucial as main timing. Also, from the factory it most likely isn't optimal. It will be a good average to achieve their emissions goals. But pilot injection (timing and amount) effects the ignition delay and burn rate, which in turn effects rattle, mpgs, and emissions output. It is well worth looking into and should be changed along with main timing. Also, pilot injection events should always be referenced Before Main Injection Pulse. It's timing should remain consistent relative to the start of the injection event, not TDC.

Also, the pilot injection is such a small amount you don't have to worry about it damaging a piston or gasket.

[robof16]

The articles I’ve found seem to be most concerned with emissions (NOx IIRC) and that can be at odds with efficiency/performance. This serves to make it more difficult to discern from the reading what adjustments to make.
I’ve seen what I believe to be a noticeable improvement at low RPM, low load conditions (light acceleration and cruise) with pilot timing set to 15-20 CAD before the main injection pulse. This is in addition to my already advanced-over-stock timing BTW. Supposedly a short-duration pilot delivered with higher pressure and a longer dwell before the main pulse will yield better fuel economy.

[Wheelz]

That seems on par with what I remember, but the article I read only had data at a given rpm, which I would think the ignition delay would be about the same, which would require advancing pilot timing for increasing rpm.

But 15-20 CAD prior is about right for cruise rpms. I know it's weird to think of timing in terms of ms advance, but with pilot timing it will make the math easier and the map less complex.

[koolky]

Thanks for the posts guys. The thing that gets me about the stock pilot injection timing B1201 is that it is not consistent in the way that makes sense to me. Example: At 300rpm and 1000µs BMI (Before Main Injection) the result is PI taking place 1.8* BMI. At 1400rpm and the same timing (1000µs) the result is PI taking place 8.4* BMI. At 2000rpm it is 12* BMI. That is if my calculator is set up properly. Can anyone confirm these numbers?

[Wheelz]

Thanks for the posts guys. The thing that gets me about the stock pilot injection timing B1201 is that it is not consistent in the way that makes sense to me. Example: At 300rpm and 1000µs BMI (Before Main Injection) the result is PI taking place 1.8* BMI. At 1400rpm and the same timing (1000µs) the result is PI taking place 8.4* BMI. At 2000rpm it is 12* BMI. That is if my calculator is set up properly. Can anyone confirm these numbers?

Why does that not make sense to you? The math makes a logical progression that for the same ms advance, with increasing RPM, PI CAD timing must also advance. The theory is your pilot event will have a consistent ms delay before ignition, which is what you are trying to control so that the combustion chamber is primed to the same point, at any given rpm

[koolky]

After coming back to it this morning with a clear mind I see what you’re saying and don’t really know what I was getting at. Maybe it was that B1201 seems stair step when I thought it would make more sense if it looked smoother like a main timing table. When I look at B1201 now that it’s been converted to degrees I can see that it is doing exactly what I expected it to. Low numbers at low rpm/mm3 increasing at a fairly even rate as rpm/mm3 increase.

[Wheelz]

I do agree, the stair stepped nature of the factory PI timing makes zero sense to me as well.

[koolky]

That seems on par with what I remember, but the article I read only had data at a given rpm, which I would think the ignition delay would be about the same, which would require advancing pilot timing for increasing rpm.

But 15-20 CAD prior is about right for cruise rpms. I know it's weird to think of timing in terms of ms advance, but with pilot timing it will make the math easier and the map less complex.

Do you ever play with PI timing? Stock seems to be 8-14 CAD BTDC in the cruising range. I am experimenting with upping PI timing to 1100 µs for 0-30 mm3 up to 1900 RPM. I know it’s not a lot but I can’t find much information about LBZ PI timing. I might try 15-20 like you mentioned. I’m assuming that there is good potential for improved efficiency by modifying PI timing since stock is likely kept retarded for emission reasons.

[Wheelz]

I've changed it up quite a bit. Ill post up some of the tunes I've been working on when I get home tonight, and try to find the tech paper I had been working from. I'm working late today so it'll be around midnight before I get a chance.

I've found that keeping a short duration between PI and MI makes the engine run quieter. My dads truck, which is the LBZ I've been tuning, will get around 18-19 mpg highway running 33" BFG ATs and a bone stock motor/exhaust.

Also, what I've been discovering, is that for a given engine load, the ms duration between your pulse events should remain close to the same throughout the RPM range of the motor. CAD advance really isn't a good working unit because ms/CAD is constantly changing with RPM. I used to try to think of PI timing in terms of CAD and it's just not necessary. I haven't figured out the best way to extrapolate the PI timing for increased engine loads. I need to do some more experimenting to understand how they are related to each other so the MI burn is as consistent as possible throughout the load range.