Should you make EFI super-lean at cruise?

[Caveman Tony]

or on decel?

and if you wanted to do that, what would the repercussions be?

is it even possible? (I'm guessing its easier with some systems than with others, etc..)

[Barry Donovan]

air bypass on decel. I love it.

lean at cruise, not a good idea.

[TheSilverBuick]

I go as lean as possible until it mis-fires. Those mis-fires kill mpg's though, I gained a bit when I richened it up and quit running the ragged edge. I'm hoping when I go to sequential injection that I might be able to balance the fuel charge to each cylinder better at low cruising rpm such that I can lean it out further and call it good.

For cruising I typically will run it lean until it mis-fires a bit, then advance timing until it quits, then lean it out a bit more until it mis-fires then advance the timing a bit. Eventually I have to back the timing down a hair and add a touch of fuel for it to be stable while cruising. I keep it simple, so say I cruise at 2500rpm, I make one of the rpm bins 2500rpm, so I'm only playing with one or two cells as vacuum goes up or down. Before I had MS2/extra code loaded with the fuel cut on deaccel (zero fuel when coasting), I had the bottom of my fuel map table zero'd to accomplish essentially the same thing.

[Caveman Tony]

What kind of *edit* improvement in mpg's were you seeing with that?

That was on your T-bird or on the Buick?

[TheSilverBuick]

The Buick. I haven't done any actual out of town driving with the T-bird since it's conversion. Just around town and to work. I am planning on taking it on the Car Craft Anti-Tour this year, so that'll be a ~700 mile trip.

On the Skylark, I haven't noticed anything really picked up since getting the overrun fuel cut feature set up, but that's likely because my fuel table had cuts already programmed into it. I did that from essentially day one so no day and night comparison. Through leaning the table out I gradually moved from 16.5mpg consistently to 19.5mpg consistently. I'm going to be making a ~1200+ mile road trip to Phoenix next weekend, and I have the fuel cut set more aggressive. So I'll see if I notice any gains. Since the engine went back together with a new cam, pistons and aluminum heads I've been at 17-17.5mpg, so I'm aiming to beat that.

[Caveman Tony]

17.5mpg from a 455.

NICE.

Just curious, what are your rear gears on that? Car weighs 3500-ish?

[TheSilverBuick]

Currently 3.42's, 26" tire, and a DEEP .64 overdrive. With the trunk empty, gas tank full and me not in it, it scaled 3650lbs at Drag Week. With me 3800lbs, and then probably another 150-200lbs in the trunk between the spare, jack, tools, etc.

The 17.5 was averaged from GPS miles AND over the 3,800+ miles I drove on Drag Week versus fuel used.

[dieselgeek]

lean at cruise, not a good idea.

Lean it out at cruise until the engine starts to misfire, then richen it back up just enough that it's not bucking. (you cannot hurt the motor running lean anywhere besides WOT/high load).

In my experience, most of the MPG comes from tricks like decel fuel cut and really working your TPS enrichments so they put in the minimum of extra fuel but don't cause misfire. Any lean misfire kills fuel economy too. It's a double edged sword.

[TheSilverBuick]

That reminds me, for my trip to AZ I think I'll put the TPS accel enrichment gauge up to watch for any "unnecessary" enrichments while cruising.

[JeffMcKC]

Lean it out at cruise until the engine starts to misfire, then richen it back up just enough that it's not bucking. (you cannot hurt the motor running lean anywhere besides WOT/high load).

In my experience, most of the MPG comes from tricks like decel fuel cut and really working your TPS enrichments so they put in the minimum of extra fuel but don't cause misfire. Any lean misfire kills fuel economy too. It's a double edged sword.

Make sure the misfire is not from ign break down also. lean mixtures are harder to fire.

[BKBridges]

All of the above!
Also dont forget the transient adjustments! Map AE Decay rate should be maxxed for your operating temp and motor build to pull the fuel out quickly as you hit the fixed decel table values. I was getting rich transient AF on decel until I jacked the decay rate up a lot. Now, no more black bumper! Your basic dmap corrections should be adjusted to "stun" for your motor as well to improve mileage during throttle changes. For steady cruise, dont rely on a "fixed" AF target (fixed in your mind that is). To adjust the fuel and spark table at cruise, listen to the motor... it knows! You may pop some header coating during the tuning process, but...
BKB

[BKBridges]

I guess I should admit Im running a BS3.. Not sure if other systems have that adjustment capability on Decel.
BKB

[CDMBill]

That can be managed in Megasquirt but that is the only software I have any experience with. In this sense EFI and traditional fuel mixing are the same, we want to give the engine what needs run best at all operating ranges, idle, light acceeration, decel WOT etc.

What makes EFI and the associaited sensors such as AFR so interesing is that we can directlly manage the inputs and measure and assess the outputs. I know that all the software adjustments have their analogs in the mechanical/fluid dynamics world of carbs and distributors and I often find that I'm thinking about the changes being made in those terms, accelerator pump cams and squirters, power valves, vacuum advance.

Those folks who were great at engine tuning in the analog world have a huge advantage in the digital version as those black arts like plug reading and being able to hear and smell what the engine is doing give them the ability to do the ongoing reality check. (this isn't me, I'm a hack)

So the original question of 'should you make EFI super-lean...' inevitably becomes one of to accomplish what? under what circumastances? and with what trade offs? No black and white answer here.

[JeffMcKC]

That can be managed in Megasquirt but that is the only software I have any experience with. In this sense EFI and traditional fuel mixing are the same, we want to give the engine what needs run best at all operating ranges, idle, light acceeration, decel WOT etc.

What makes EFI and the associaited sensors such as AFR so interesing is that we can directlly manage the inputs and measure and assess the outputs. I know that all the software adjustments have their analogs in the mechanical/fluid dynamics world of carbs and distributors and I often find that I'm thinking about the changes being made in those terms, accelerator pump cams and squirters, power valves, vacuum advance.

Those folks who were great at engine tuning in the analog world have a huge advantage in the digital version as those black arts like plug reading and being able to hear and smell what the engine is doing give them the ability to do the ongoing reality check. (this isn't me, I'm a hack)

So the original question of 'should you make EFI super-lean...' inevitably becomes one of to accomplish what? under what circumastances? and with what trade offs? No black and white answer here.

When I met Scott he was stuck on the AFR has to Read........ We got him away from that real quick, I would like to think him hanging out with us in the N/A world has made him a better tuner overall. Bill I agree make it run best, to the lean side for Cruise no surge.

[dieselgeek]

I definitely learned to use my ears and intuition over the AFRs working on Doc and Jeff's programs, especially for anything outside of WOT use.

BKB, I have to give you props if you have your BS3 showing good throttle response - that's one of the hardest things (for me) to tune on the BS3/FAST/DFI boxes. Way to go!