idle tuning unusual air volume
12-11-2016, 12:14 AM
#1
idle tuning unusual air volume I am trying to nail down some idle quirks in a tune on a 4.8l. It has a sidemount whipple blowing through a large fmic. This means a huge volume of air is AFTER the throttle body. The current issues are going lean after blipping the throttle at idle, and cold weather, cold engine startup flare dropping down to almost killing the idle. I thought I had this under control during the summer, but with 60* lower temps I'm not so sure. I have "trial and error" tuned some of the problems, but I don't know how to properly tune for the delay between what the IAC is trying to do vs what is happening.
12-12-2016, 08:25 PM
#3
The throttle is on the blower inlet, as is customary with any PD blower, hence no need for a BOV. It goes air filter-TB-blower-piping-FMIC-piping-MAF-piping-intake manifold. The MAF is still there for little other than the built-in IAT. We had to remove any silicone elbows and use hard pipe, as the elbows would suck closed under vacuum. The idle tuning was never a problem before adding the FMIC, so the issue has to relate to the extra volume of air behind the TB. If (when) I get this figured out I will post whatever fix I find.
12-12-2016, 09:20 PM
#5
I have adjusted the decay, delay, and just about every other IAC, fuel, or spark table involved. The problem is that I can get everything sorted except cold engine cold start. If I make enough changes to get a quick fire and good idle when dead cold, I get problems elsewhere. I'm starting to look into injector voltage offsets and potentially transient fueling. I just recently ran across what may or may not be legit voltage offset data on these 8.1 marine injectors, and the offsets at lower voltages are quite a bit higher than what I would have guessed based on other injectors I have tuned. None of this presented much of an issue until the weather dipped below 40F.
12-13-2016, 12:35 AM
#6
There could be a possibility of a fuel boiling/evap issue since the problem arose with a colder aircharge. A.k.a. transient as you proposed.. maybe fiddle the evap tables where the transient occurs?
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12-13-2016, 09:40 PM
#8
There are only a handful of people who have tried an A/A FMIC with a PD blower. One of the bigger shops that did it used a second throttle blade at the intake manifold to avoid some of these problems. I refuse to believe I can't figure this out. The problem with cold engine, cold start tuning is that you can't log anything of use in those first few seconds. If you key on the engine long enough for the scanner to connect, it gives time for the fuel pressure to change slightly from just jumping in and starting the vehicle like any normal person would do. I have several variations of the tune that work in other conditions, so if I can't get this figured out I will post whichever one works best otherwise.
12-16-2016, 09:14 PM
#9
It turns out that in weather under 40*F, the whippled 4.8 needs several of the tables related to cold start cranking to be much closer to what you would expect from a 5.7 than a 4.8. I used some of the cold values from my turboed GTO as a starting point instead of stock 4.8 values, and ended up with a starting point I could work with. I still have a bit of tweaking to do on the transient stuff, but it now fires up with little flare, and settles into a proper idle in 30* weather. I now need to log the rest of the tune to make sure the new voltage offsets I found for the injectors haven't skewed anything under load.
12-18-2016, 05:35 PM
#16
The auburn posi wore out its cones years ago. One wheel peels just aren't as cool. He keeps talking about replacing the diff, but doesn't want to lay out the cash. In another case of "if I knew then what I know now" I would have installed a rebuildable limited slip instead of the auburn way back when.