What is MBT?
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What is MBT?
So I thought a discussion on what the actual proper meaning of this acronym is might be fun. Most people will tell you that MBT varies with fuel or boost (Strader says fuel I believe) or both. It is defined as Minimum Best timing for Torque or the point at which timing has achieved 99% of peak torque.
Practically, it means that our cars make 99% of torque at 18-19* out the top and this point where it stops making torque. We can generally only achieve this at higher rpms on racegas or E85 and cannot do it static as the octane requirements would be immense...or methanol.
It is generally accepted around here that this is the correct definition of the term. An ME site I still frequent had this to say-
http://www.eng-tips.com/viewthread.c...=251159&page=8
Specifically-
Discuss?
Practically, it means that our cars make 99% of torque at 18-19* out the top and this point where it stops making torque. We can generally only achieve this at higher rpms on racegas or E85 and cannot do it static as the octane requirements would be immense...or methanol.
It is generally accepted around here that this is the correct definition of the term. An ME site I still frequent had this to say-
http://www.eng-tips.com/viewthread.c...=251159&page=8
Specifically-
Originally Posted by PDMEAT
MBT timing = Maximum Brake Torque timing. Knock limit aside, at a given speed and flow rate, it is the spark timing that gives maximum torque, (and minimum bSFC). It doesn't mean maximum (or minimum) spark advance. The word "maximum" refers to torque - not spark.
Basically, if combustion starts too soon, the gas pushes back on the piston as it is coming up in the compression stroke, creating negative work which reduces torque. If the spark is too late, peak cylinder pressure is reduced, and hence, so is expansion work. So there is an optimum spark for maximum torque. If one were to draw out a curve of torque vs advance, (starting out below MBT), as spark is advanced torque will increase until MBT timing is reached, after which point further advance will cause torque to go back down. The top of the curve is somewhat flat, so the percentage increase in tq drops off as spark advance approaches MBT.
In the ideal Otto cycle, MBT timing would be at TDC, as mentioned above. But due to the non-zero burn duration of a real engine, MBT timing is not at TDC as mattsooty mentioned.
I have also heard it referred to as mean best timing, but since MBT depends on engine speed and load, that terminology just doesn't seem to fit, in my opinion.
Basically, if combustion starts too soon, the gas pushes back on the piston as it is coming up in the compression stroke, creating negative work which reduces torque. If the spark is too late, peak cylinder pressure is reduced, and hence, so is expansion work. So there is an optimum spark for maximum torque. If one were to draw out a curve of torque vs advance, (starting out below MBT), as spark is advanced torque will increase until MBT timing is reached, after which point further advance will cause torque to go back down. The top of the curve is somewhat flat, so the percentage increase in tq drops off as spark advance approaches MBT.
In the ideal Otto cycle, MBT timing would be at TDC, as mentioned above. But due to the non-zero burn duration of a real engine, MBT timing is not at TDC as mattsooty mentioned.
I have also heard it referred to as mean best timing, but since MBT depends on engine speed and load, that terminology just doesn't seem to fit, in my opinion.
Sorry Im so late to the party Aaron! LOL
Here is a great video to help "see" what is going on, and how "ADVANCED" really works...
http://www.youtube.com/watch?v=sEf8va1S7Sw
Hope this can kinda help people understand how timing can effect TQ ect.
Evan Smith
Here is a great video to help "see" what is going on, and how "ADVANCED" really works...
http://www.youtube.com/watch?v=sEf8va1S7Sw
Hope this can kinda help people understand how timing can effect TQ ect.
Evan Smith
Would the MBT on the lower rpm range be more advanced towards TDC as the burn rate is the same but the piston is rotating more slowly?
Does having the timing more advanced in the lower rpms help spool the turbo?
So, the timing target would be to set the timing to spool up the turbo then start heading towards making max torque? When does this transition in timing objectives occur (50% max boost, etc.)?
How does AFR affect MBT? Is the goal to get rich of stoich and then tune boost/MBT to knock?
Does having the timing more advanced in the lower rpms help spool the turbo?
So, the timing target would be to set the timing to spool up the turbo then start heading towards making max torque? When does this transition in timing objectives occur (50% max boost, etc.)?
How does AFR affect MBT? Is the goal to get rich of stoich and then tune boost/MBT to knock?
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