Addendum to the thread on the term "MBT"

[TrackRat]

thread71-251159

I came across this older thread while searching for some other info. and thought I would comment based on firsthand experience.

While the acronym "MBT" can certainly have several meanings, in regards to engine mapping it is in fact an SAE term for Minimum ignition timing for Best Torque as several folks pointed out. There is in fact a minimum point that is different than the maximum point or points in between, as also discussed. You can have test points where the torque stays the same with several degrees additional advance, so MBT is what you calibrate for.

"MBT" is usually referenced with the SAE acronym "LBT" which is Leanest (AFR) for Best Torque. These mapping calibrations are used at WOT and standard procedure for car makers.

 

[turbocohen]

Hmm Thought it meant Mean Best Torque?

 

[Lou Scannon]

TrackRat, I daresay the torque stays the same over several degree, within experimental accuracy, but if you did a design of experiments to generate statistically valid data, I suspect there would be a smooth curve throught the data of torque vs spark timing, with a single maximum point.

I forgot what I was going to say

 

[TrackRat]

turbocohen-

For the purposes of engine calibration on spark ignition engines, MBT definitely means: Minimum ignition timing for Best Torque. As I said the acronym "MBT" can mean other things for other applications. MBT and LBT go together to produce the optimal WOT power mapping for an engine.

hemi-

The minimum timing and maximum timing at some tests points - but not all, can be the same. In other words there are test points where advancing the timing a half of degree more lowers torque so for all intent and purposes minimum and maximum are the same. At other points however the minimum and maximum point can be several degrees apart yet there is no change in torque until you add even more timing. So for calibration purposes, you calibrate to MBT so that everyone is on the same page.

 

[Lou Scannon]

I understand you are talking about observed data, with all the measurement limitation and experimental error. And the method you describe for tuning an individual engine makes sense. But from a physical/chemical analysis standpoint, with everything else held truly constant except spark timing, and assuming no appreciable knock, the torque will exhibit a smoothly curved response in the peak region (however flat it may be), with a single maximum point. Now for actual tuning, it may be wise to subtract a few degrees for knock margin, unless the engine management is intended to detect unexpected knock and adjust accordingly.

I forgot what I was going to say

 

[vagman2]

Hello all,
In my understanding LBT means Lean Best Torque and is what one should tune for usually for cruise spead/loads. There is also Rich Best Torque when WOT tune is on the table.
Here's a good video from one of the best dyno manufacturer:

http://www.youtube.com/watch?v=sRIbOCT1pO4&feature=player_embedded

 

[vagman2]

Also this screenshot:

 

[TrackRat]

vagman2-

Leanest for Best Torque (LBT), is the SAE AFR term for WOT calibration. Part throttle is usually calibrated to run at stoichiometric but can be run leaner depending on the application and engine design.

Hemi-

The true MBT/LBT engine calibration is determined with indolene certified octane fuel then a couple degree safety margin is mapped in. Final validation is with various street fuels. Knock sensing is normally used as a last resort as it is a reactive approach that in forced induction engines can be too little, too late.