GUFB/A3M1 decel fuel leanout - running log

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Chucko
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GUFB/A3M1 decel fuel leanout - running log

Post by Chucko » Sun Sep 15, 2019 3:19 pm

Today for the first time I tried decel fuel leanout (non-zero values in FN374). I've been running with DFSO for years, but got tired of the clunking. I was never brave enough to try shift DFSO.

So I started out with .9 as the leanout multiplier, then cutting it to .88. It seems to be working, looking at the injector pulsewidths, but the DFSO flag (GUFB doc calls it DFSFLG) never turns on.

Looking at the A9L disassembly, it turns out DFSO only turns on when the fuel is actually shut off. But there's another flag that indicates when DFSO is active, whether or not fuel is cut to 0. It's location 0xCA, bit 1. It's not mentioned in the GUFB document. I've put this in my modified strategy spreadsheet and will be trying it out soon.

I'm not hearing any misfire on decel. I want to push the multiplier lower without actually cutting off fuel. How would I detect a lean misfire under closed-throttle decel? Anyone have experience with this?
'89 LX 5.0, Crane 2040, Pro-M 75mm Bullet, 65 mm TB, ported stock intake & E7TE heads, 24 lb injs., JBA shortys, cat X-pipe, A3M1 (GUFB strategy) w/Moates Quarterhorse, BE & EA, close ratio T5Z, 3.55 rear

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Re: GUFB/A3M1 decel fuel leanout - running log

Post by cgrey8 » Sun Sep 15, 2019 6:29 pm

You've got to be pretty deep in the lean before you actually start getting misfires. When I run lean-cruise MFA, I often run in the 1.05-1.10 range however I've seen it well into the 1.16 range (lambda values). But I don't start noticing the obvious loss of power from misfires until the mix gets up closer to the 1.18+ range. Once it gets that high, certain cylinders start falling out while others are hanging on. Increase the load while still this lean, and you feel power, but it's sluggish. Although if you are boosted or at high RPMs, you probably don't want to be anywhere near those numbers. At cruising RPM/Loads, it's harmless. There's simply not enough fuel going into the cylinders to be dangerous at those conditions.

I don't know that this helps, but there's some info...
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89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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Chucko
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Re: GUFB/A3M1 decel fuel leanout - running log

Post by Chucko » Sun Sep 15, 2019 11:12 pm

That's good info Chris, thanks. I'm in no danger of blowing anything up because this is N/A at closed throttle.

The fuel multiplier is effectively 1/lambda, so your experience suggests multipliers of about 0.85 are on the border of lean misfire.

Since I have an idea what no fuel feels like, I think I'll just keep trying leaner until it starts to drag.
'89 LX 5.0, Crane 2040, Pro-M 75mm Bullet, 65 mm TB, ported stock intake & E7TE heads, 24 lb injs., JBA shortys, cat X-pipe, A3M1 (GUFB strategy) w/Moates Quarterhorse, BE & EA, close ratio T5Z, 3.55 rear

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Re: GUFB/A3M1 decel fuel leanout - running log

Post by sailorbob » Mon Sep 16, 2019 3:02 am

The DFSFLG flag is only used to disable the transient fuel compensation so that fuel is not added when the PW is meant to be zero.

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Re: GUFB/A3M1 decel fuel leanout - running log

Post by cgrey8 » Mon Sep 16, 2019 5:16 am

At absolute decel, I don't know that you'll be able to tell much difference between mild lean and heavy lean. However the moment combustion gets on the edge, it'll feel like the engine is surging while engine-braking as some cylinders will effectively be full engine-braking as they misfire and others are firing and not dragging the vehicle quite as much. My guess is that would not be ideal. You are either going to want full engine-braking or all cylinders firing under a decel condition.

I also suspect that compression will affect how lean you can get before the cylinders fall off. My guess is that lower compression engines will be less tolerant of lean and higher compression engines will tolerate higher levels of "lean-ness" (if that's a word). I could be completely wrong on that, but that's my take based on my very limited 2-engine experience of a stock Explorer 302 and a high compression 331. Obviously there are lots of other variables that would explain why one engine would be more tolerant than the other. But if I had to guess, the difference in compression is the dominant factor. Others may have a different opinion, and they might be right.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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