Old School Approach, tune for best (lowest) MAP reading?

[Mr. P.]

This weekend I drove hundreds of miles, took the opportunity to do some mileage tuning and had an unexpected observation; after dialing-in the VE table for freeway crusing, I ran two tests:

1) I entered 0.97 EQR in the commanded AFR table in my "cruising" cells, flashed the truck, and drove an hour on cruise control at 73-mph.

2) I entered 1.01 EQR in the commanded AFR table in my "cruising" cells, flashed the truck, and drove another hour on cruise control at 73-mph.

Both tests were driven in Kansas on I-35 - terrain wasn't much of a factor; head wind during that two hours was the same also, weather was consistent the whole afternoon. I was looking forward to the 'lean cruise' AFR table (test #1) to get better economy than the 'rich cruise' test, but the opposite happened - I got worse mileage when commanding 0.97 EQR than 1.01 EQR, almost 1/2-MPG worse in fact.

But I noticed something else as well, the MAP readings during cruise when on my 'rich' commanded AFR table were consistently lower than during the lean cruise run; the motor ran with noticeably more vacuum, and more 'pep' & was much more responsive. Which then made me remember tuning carburation back in the day, we would dial-in jetting by using a vacuum gauge and adjusting mixture with jets until we read the highest possible cruise vacuum, and that was usually the 'calibration' where the motor would have the crispest throttle response/transitions as well as pretty good economy too.

So the question I have is: with all the best-practice of using a wideband for tuning feedback, has anyone also tuned using MAP feedback, i.e. tuning for lowest/best MAP reading during cruise?

Mr. P. :)

Advanced question - I wonder if the motor will have lowest MAP when you have found MBT?

[macca_779]

Yep lower map means better efficiency. I've been tuning for economy like this for years. But your eq testing isn't complete. What you have to do is also work the timing to find the most vac. Leaner than stoich will return better economy when coupled with appropriate timing over running richer than stoich even though the vac might be lower as you are reducing torque by running leaner. But that's fine as you don't need max torque while cruising.

[WeathermanShawn]

Mr.P, I have noticed anecdotally some of what you are describing. I.E., finding the lowest MAP at cruise makes sense..less load>>less Airflow>>less fuel.

I have found relationships with anything engine-wise vs gas mileage hard to pin down. Even the ethanol content will alter it.

Interesting observation though. Sometimes just observing and Tuning for what is best for your vehicle is the best method there is.:)

[redhardsupra]

congratulations, you've discovered that in a airmass formula of GMVE*MAP/TEMP, if GMVE and TEMP don't change, MAP is the dominant factor in airmass (thus airflow, thus fuel consumption). go algebra! :)

[Mr. P.]

congratulations...Yeah I'm learning!! lol


...What you have to do is also work the timing to find the most vac. Leaner than stoich will return better economy when coupled with appropriate timing...
Gotcha. :thumb_yello:

Ben Schrader told us (albeit several years ago) the calibration for best efficiency is also always the calibration that yeilds best reliability, is also always the calibration that yeilds best performance. So, according to that statement one would expect when you find (for any given operating condition) the fueling & spark combo that delivers best/lowest MAP, it will consequently also be the calibration that gives best MBT, and best performance/efficiency, right??

It's times like this I wish I had my own dyno to experiment with. :wallbash:

Mr. P.