V8Astro Captain

iTrader: (1)

I want to try the 7747 lean hiway cruise patch. I'm not not 100% sure what a few of these commands do...

I'm not too clear on what LD026 thru LD02A does. Are these transition SA modifiers?

thanks

RBob

iTrader: (1)

D026 - D02A are for PE spark.

RBob.

escort_ford84

I dont see any type of leaning there, just spark advance

Dominic Sorresso

That's how Hway Cruz mode works. It adds spark. That leans out the mixture and it overrides C/L for the duration of HWY mode. It'll go in and out of that mode. Won't run constantly that way in order to protect the cat.

escort_ford84

So running high spark advance at high vacuum will lean the AFR right? Like 40-48 degrees in highway mode? i run about 35-38 wihtout any highway spark advance. my engine cruises on the hiway at 10" of vacuum in high gear.would 48 degrees be ok for highway mode?

RBob

iTrader: (1)

Lean cruise highway mode is used for better fuel mileage. The AFR is commanded leaner, as high as 16.6:1. Since a lean mix burns slower SA is also added. Doing this regains lost power as the mix now completes the burn at the proper time.

In some GM code they have separate lean modes and highway SA modes. Why, I don't know. Doesn't make sense that they would lean the mix without adding timing.

Other GM code has the 2 items tied together. This works out and can improve highway mileage 10% or more.

RBob.

escort_ford84

Is there a way to tell when the ECU goes into Hiway Mode? Would the ecu go into open loop (Learn Disable)?

DENN_SHAH

iTrader: (1)

i don't know how the 747 does it, but in 730 8D the ECM stays in closed loop & learn control is disabled during hiway mode, also the O2 sensor voltage drops low & target AFR goes above 14.73.

Cflick

Hwy mode in the 7747 was of great interest to me, since my milage sucks on a good day.
From what I can see, and having stepped through the hac looking for it, the 7747 ONLY advances spark. It's still chasing 14.7 AFR. Someone, please correct me if I'm wrong, but watching it as I run down the road, the only thing I see, is an instant advance as it goes in, and an instant retard as it goes out. The BLM, INT, and O2 still chase just as they do otherwise. Initially, I added 1-2 degrees. Got as far as adding 8. Saw no improvement in milage. Did see knock counts. ( 49 degrees total is a lot of spark, even for this ancient slow-burn big block )
Lean cruise is THE reason my truck now sports an 8746. That one has a fuel table I can play with. No real progress, yet, but.....
As RBob suggests, my in-cylinder preassure readings suggest that hiway mode and lean burn isn't a result of advanced spark. Lean burn requires more advance. Not the other way around.