Cruising Air fuel ratios.

[riche]

I appreciate yalls help with timing, and have made some progress. But I have a question about air fuel ratios.
In Holey's software you can set the air/fuel ratio table. Then it has a base fuel and a learn table. The air/fuel table then compares the O2 readings to the air/ fuel table. That in-turn applies an adjustment value to a learn table. Then the learn table modifies the base fuel table all in real time. For the most part it works really well with a few exceptions I learned about the hard way.
Just wanted to ask a few questions about cruising air/fuel ratio. I have been playing around with it leaning it out and was hopping to get some opinions of what is too lean foe a early model small block with vortec heads.
I leaned it out to 15.2:1 (in the "cruise" area of my table) and it seems like the temp is jumping up to 195 degrees cruising down the interstate. Just wondering if that is too much.

[steveo]

although 15.2 isnt so bad (if it actually IS 15.2, are you usign a wideband?)... i wouldn't be pushing it past stoich if i were you, not unless it has provisions for proper lean cruse that engages and disengages with very tight thresholds.

leaning it out seems to be one of those things where it gets way worse before it gets better. you can go super lean, and suddenly the combustion temperature drops again. but if you put any load on the engine while it's at that AFR, things go sideways really quickly.

[riche]

Ok. I appreciate the advice.
Yes it is a wide band O2. I did notice with the plugs that My o2 appears to be on the rich bank. So I added a few points across my table.
Then I started thinking about what you mentioned in my previous post about my cam and vacuum. I am wondering if I really can't get to a low load condition b/c of the weight/aerodynamics of my truck and the cam in my motor. I.E. if a car has my same motor and a better cam it could get to a lower area of the map at a certain mph lets just say that is 45 kpa @ 65 mph and my truck with it's cam and weight is 65 kpa @ 65 mph. So do I assume that my truck shouldn't go as lean and I shouldn't advance my timing as much as I would in a light car with a better cam and aerodynamics?

[trippyjoey]

I would agree with Steveo don't lean it out unless it has a Hiway lean cruise mode. I've gone down that path before leaned it out and my BLM's were in the high 130's low 140's it seemed to get better mileage but the engine temps were rising and the engine was always running at 80kpa map. I richened it up so my BLM's dropped to 126-128 and my fuel economy increased and my engine is now running at 50's Kpa map so the engine is under much less load now that it is making enough power to easily move my big Yukon.

[Fast355]

I would agree with Steveo don't lean it out unless it has a Hiway lean cruise mode. I've gone down that path before leaned it out and my BLM's were in the high 130's low 140's it seemed to get better mileage but the engine temps were rising and the engine was always running at 80kpa map. I richened it up so my BLM's dropped to 126-128 and my fuel economy increased and my engine is now running at 50's Kpa map so the engine is under much less load now that it is making enough power to easily move my big Yukon.

I religiously use lean cruise, even with the 305 I had pushing my 700r4, 3.08 geared G20 van back in the day. 1,800 rpm @ 70 mph, running about 65-70 KPA. As long as your air/fuel mixture richens as the MAP increases, it works well. At 30-50 KP I would run about 17:1, 50-70 KPA about 15:1, and 70-100 KPA tapered up to 14:1, then PE active would bring it down to 12.6:1. With E10 fuel knock .75 off those values.

[RobertISaar]

i love lean cruise....

with a light enough load, you're only really limited to how little fuel you can inject and not have a misfire. how well the air/fuel mixture actually mixes will play a large part in that, which will be RPM and presumably MAP specific.

there is a really noticable bell curve when it comes to AFR and combustion temps, in theory you'll get max burn temps at 14.7:1, but in practice it will be leaner than that. how much leaner will be engine dependant(mixing is big here too). i wouldn't want to operate in lean cruise right on that crest.... might be a bit more heat than what the headgasket or piston crown will allow. leaner than that though, to bring it down to where the 14.7:1 AFR mix produces temps at or leaner, that is where i would look. EGT probes are great for this.

there are some fairly conventional(not direct injected) gas engines that run some incredibly lean AFRs(22:1 for mid 90s honda lean-burn engines), but they are specifically built to do so. most take advantage of only operating 1 of 2 intake valves or only supplying fuel to 1 of 2 intake valves, all kinds of tricks that may or may not be usable depending on the engine you're playing with.