De-tuned hybrids in a PMR

[F-127]

Well I don't like making another thread, but I think that it may get enough discussion to get its own seperate from my other 'project' thread. With larger injectors going in one truck, the current 238/100 hybrids are going in another.

Just so happens to be a 02 SuperDuty with a PMR engine. Now with all the past talk of more HP or larger nozzles and the resultant window to wave "hi Mr. Crankshaft" on a PMR engine, it's got me a little worried, but I'm pretty confident I'll have it working. I have my single shot timing(fairly conservative) related maps copied/grafted in the SD file. Then grafted in the pulsewidth map I had working on the OBS. The PW map for the hybrids I started with has flow detuned to basically match output of an AA injector across the entire map.(that was because I was initially seeking a like-stock file when I started tuning the injectors for the OBS).

Now is where I started looking a little closer, and perhaps I should have done this when tuning for these injectors the first time. Being a little more cautious for the PMRs got me looking closer though. For the most part, my PW map for the 238/100 is fairly close to stock. Hybrids needing a little more PW here, little less there, but the big changer is up top, the PW map for AA stock and the hybrids starts deviate a lot more for me for MFDs >70. The hybrids can flow equally to stock AAs about .5ms quicker. I suppose I should pull some timing, unsure of how much. Took me a while to figure it out, but after staring at the ICP desired, SOI, and PW(namely the change in PW), I pulled some timing from all RPMs down to about a MFD of 70 using the rotational time calc that Bill has made for excel. At 3000 RPM and high MFD, with a shortened PW of .5ms, to have injection 'end' at the same time, 9* of timing would be pulled. When at 1000 RPMs that drops to 3*. (just so happens that PW difference was equal at that MFD over varying RPMs, it was not always like that so it wasn't always 1/3 less timing to pull at 1000RPMs).

Now I thought I found something that could likely have caused issues, and it was the biggest difference I could find when going with larger nozzles. If you raised the PW map in those areas MFD>70 and they match more like stock PW length, timing wouldn't really need to be pulled. For most, PW probably wouldn't be decreased to limit HP like I have, unless you were to say make an economy tune. My guesses are either what I seemed to have found or just too aggressive low RPMed fuelling, lotsa torque.

Anywho, thats my thoughts for the night, if anything else talking(writing) about it helped. Just thought I'd post it to get some additional opinions, hoping to avoid a windowed block myself, and if it helps others avoid one too even better!

[Power Hungry]

One of the big concerns with any larger nozzled injector (160/80 Stage 2, 200/100, 238/100, 250/200, etc) is not just the relationship of timing to injection flow (which is significant, of course) but also the significance of ICP to the entire combustion event.

If you follow any of the timing related maps, you can very easily see that as ICP increases the SOI decreases. This is due to the fact that increased ICP causes 1) an inherent change in timing do to the charge being introduced into the cylinder faster and 2) increased ICP causes a "hotter" spray and advances the SOC (start of combustion) point of the fuel.

The major concern with tuning for many of the windowed blocks that I've analyzed, was not pulse width... it was excessive timing and ICP at moderate RPMs. Ever wonder why folks would say "Never had a problem when I drove the truck hard, but threw a rod just driving down the street"? There are very specific reasons for that and we need to take a logical look at exactly why it happens.

In any engine, as RPM increases the SOC point (spark for gas engines or SOI for diesels) needs to be advanced as well so that the highest point of the cylinder pressure occurs shortly after TDC. This provides the most efficient use of the thermal expansion of the air in the cylinder. With gasoline engines, the combustion point is easy to define and the only significant influence to the combustion process is how fast the flame front propagates from the ignition point. Diesels are much harder to pinpoint since there are several factors that not only influence the combustion point, but also the propagation of the flame front. Some of these are:

1) Base Fuel Temperature
2) Engine Temperature
3) Air Temperature
4) Static Cylinder Pressure (PSI vs. Crank Angle)
5) Air Charge (Boost)
6) Injection Pressure (Droplet Size and Spray Pattern)
7) Injector Nozzle (Droplet Size)
8) SOI

Changes in any of these factors can significantly affect the SOC. Of course, the easiest to control are the SOI and ICP, but considerations must be made for Engine Temp and Boost.

One thing we continue to try to determine is when the SOC will occur, and that is a much harder thing to do than it sounds. Take, for example, you are running 2200 RPM with 1800 PSI ICP and that you have an SOI event that occurs at 30º BTC and the resulting SOC event occurs at 4º BTC. Raising the ICP to 2600 PSI could change the SOC to about 12º BTC, which in itself may not be a bad thing but when you consider that you are also increasing the fuel volume nearly 40%, you end up with a significantly higher Cylinder Pressure by the time you reach TDC. This is the phenomenon that causes engine failures... Achieving peak Cylinder Pressure BEFORE you reach TDC.

So what's the solution? The reality is that there is no clear-cut, singled answer for this. However, one important guideline to follow is to use the highest reasonable ICP possible for the given RPM range. The reason ICP is low at idle at because of the minimum pulsewidth limitations of the HEUI injectors. They don't respond well below about .75 ms (in fact, the minimum PW is .6 ms) so they ICP needs to drop to keep the fuel volume low enough to achieve idle. ICP is ramped up quickly off idle to help achieve both improved atomization and higher spray volumes while still maintaining PW in the 1.5 to 2.5 ms range. With this in mind, ICP has more to do with the fuel volume injected than the pulsewidth does. Again, higher ICP will have an inherent advance in SOC so always be sure to balance your SOI with your ICP curve.

I'm sure that this may seem like a bit of an oversimplification, but once you got your head around the full relationship of SOI and ICP, it really starts to make sense.

I hope this helps.

[Shaw]

Originally Posted by Power Hungry

The major concern with tuning for many of the windowed blocks that I've analyzed, was not pulse width... it was excessive timing and ICP at moderate RPMs.

Bill, any chance you could give a ball park number of what you found?

[F-127]

Originally Posted by Power Hungry

The major concern with tuning for many of the windowed blocks that I've analyzed, was not pulse width... it was excessive timing and ICP at moderate RPMs

That's good to hear. I have basically left timing and ICP quite like a massaged stock, as well as most other maps.

By me trying to match the 238/100 flow output to stock injectors(AA), I figured it would work with the other stock single shot tables quite well. The main reason I thought I should be worried about PW was about the only map that was no longer stock. The only correction to make it fit with the other tables seemed to be trying to make a correction for injection duration.

I pulled some timing when injection duration is .5ms(or 9 degrees@3000RPMs) quicker and injecting the same amount of fuel because I thought that peak cylinder pressure is bound to happen quicker. Pulling the majority of that time had me feeling I was err-ing on the cautious side.

Now when I add .5 PW back where it is now short to start adding some power I imagine I could add a majority of the timing back, but possibly not all in effort to balance cylinder pressure, since there will now be more fuel injected during that duration.

Originally Posted by Power Hungry

Take, for example, you are running 2200 RPM with 1800 PSI ICP and that you have an SOI event that occurs at 30º BTC and the resulting SOC event occurs at 4º BTC. Raising the ICP to 2600 PSI could change the SOC to about 12º BTC, which in itself may not be a bad thing but when you consider that you are also increasing the fuel volume nearly 40%, you end up with a significantly higher Cylinder Pressure by the time you reach TDC.

Intersting, I was thinking that a commanding a higher ICP, resulting in a shortened PW was attempting to inject the same amount of fuel(I guess it also might depend on how the PW map is set up). I also had the thought that the majority of the 'advance' nature of the ICP was because of a reduced PW.

Originally Posted by Power Hungry

I'm sure that this may seem like a bit of an oversimplification, but once you got your head around the full relationship of SOI and ICP, it really starts to make sense.

I think my head is starting to grasp that relationship a little better now, especially after a well written post as you have made.

Originally Posted by Power Hungry

I hope this helps.

Absolutely! It's always great to read one of your posts and get some insight.

[F-127]

Well, update time... did it the swap happen?? Didn't it?? Were you all wondering? The 238/100s got installed last weekend along with the H2E which were both from my truck. I left after just completing the swap to finish off my last week of class with only a single tune really ready to go, and that was about HP output of a stock SD. Finished up my last week of classes and prepped a new tune during that time which should have bumped up the HP a good deal. Burned the new file on Thursday and took some test drives. My brother seems very happy, I drove it as well and it really drove great. I only noticed some minor shift point changes that could possibly be made for mid-throttle. He said he'd be happy with just that tune. He must have liked it. Will likely try bumping up the HP some more but it does have nearly all stock components, fuel system, headbolts, PMRs, you name it, besides the turbo and injector set up.