I came across this video and thought this group might find it interesting,

 

Lean burn cruise mode.

 

I already had mine running, but this video had me rethink and alter a bit. Thanks. I've found this hiway mode hidden in the 3800 code too.

 

I've been running highway mode for over 20 years now...

I can DEFINITELY attest to the mpg capability. I have mine tuned to about 16-16.5:1 via the WB O2. Coolant temp doesn't even bat an eyelash, so I'd imagine the combustion temps are doing ok.

With the Gear Vendors behind the TH700R4, and 3.42 axle, I'm only doing about 2000 rpm at 85 mph in "8th" gear with the converter locked. I've calculated 28-29 mpg with highway cruising.

~420 hp Gen 1 383 in a 55 year old car with the aerodynamics of a brick getting just shy of 30 mpg... LMAO. My wife's RX350 only gets about 30-32.

However, I didn't add any timing highway mode spark timing (I already run pretty aggressive timing as it is).

 

Your car is not a brick, lol.

I can also attest to the MPG of lean cruise over the years. I run it in my P59 managed Express to this day. My 383 in the 97 Express conversion van (3500 swapped) made a 2,200+ mile road trip a while back and I mostly cruised 75-85 mph. My average for the whole trip was 19.7 and the best tank was 20.6 mpg. I am at 2,500 rpm @ 80 mph with the 4L85E, 3.73s and LT245/75R16s on it.

 

Considering the RPM and the aerodynamics of a conversion van that is pretty impressive Fast. All about finding that sweet spot in efficiency and torque.

 

Yes it is. Load vs RPM is more important than outright low rpm for fuel mileage. I found out years ago swapping a Hemi Ram from 3.55s to 4.56s and getting better fuel mileage everywhere that RPM alone does not dictate fuel mileage. That was a stock height 2wd with the factory 20s on it even. I also swapped cams since I built this engine and intake manifolds. The first cam was 218/228 @ 0.050 on a 108 LSA. The new cam is 219/219 @ 0.050 on a 112 LSA. The 2nd cam gets 3-4 mpg better overall. With cam 2 it has more low-midrange torque even with shorter runner intake manifold that has ports that are a larger CSA. I did a 24x coil near plug ignition conversion and drive by wire conversion when I did the cam and manifold swap.

For 7,000+ lbs, a 4L85E and 3.73s it gets out of its own way fairly well too. That is even locking the TCC right at the 1-2 shift to reduce slippage and heat. The way I have it tuned is not full send until ~30 mph. I have the 1st gear torque limit at ~470 ft/lbs, PE is delayed until 3,000 rpm or 5 seconds. Before I killed off some torque, it was flashing the converter hard enough it was spinning through a lot of 1st gear.

https://youtu.be/CyzgFIOwdQA?si=1QRS-mVBSQzI_6g4

This was in and out of lean cruise a couple of times due to load on the uphill sections but you would never notice it unless you looked at the wideband or datalog. I also really like how the DBW can be dialed in to tame the pedal a bit in your off-idle/cruising throttle range. Very smooth and controlled response to small changes in the pedal at cruise throttle. The only thing I have yet to figure out is why the P59 PCM is locking out the DBW cruise. I think it has to do with the 4 missing wires to the PRNDL I never wired in. Will be adding those soon as I want my cruise control functioning.

https://youtu.be/R5iUgeWADuA?si=SQz5vKpgdClfBknd