mattr66usa
Active Member
I will ask the mods to remove any response in this thread that isn't strictly technical in nature. I'm going to keep this semi-relative so I will over-generalize a few things so if this confuses anyone, or you want better elaboration, let me know.
MBT Timing:
MBT Timing is the timing at which the power to cylinder pressure gain starts going lopsided and you should stop advancing timing with unlimited octane (lets say 120 octane in this case). So let's just say you have good 93 octane fuel. At 6000 rpms with an IAT2 of 100 degrees you can run 16 degrees of timing at a VE (load) of 1.65 (165% Volumetric Efficiency) before you experience the knock sensors saying "nope" and pulling timing. Is this MBT timing? Absolutely not!
Now let's say you are running 100 octane unleaded race fuel. In this scenario, for every 1 point of octane gained, you get about 2 degrees of spark you can run extra before you experience knock. Let's say the MBT spark at this Load and RPM point is 23 degrees (timing that makes the max safe power). That means we would need about 3.5 octane points to allow for another 7 degrees of timing to get to that spot. But let's just call it 4 octane points to allow for a little safety margin. This means we need 97 octane fuel to get to the full MBT timing in that load/rpm point of the engine's operation.
But wait.... We have 3 extra octane points we aren't using, let's keep advancing the timing right? No, this isn't advisable. The further you are from the MBT (optimum) spark point, the more power is gained for every degree of spark you add. As you get close to the MBT timing, the returns keep get smaller and smaller. From 16-17 degrees you might see an extra 10 HP, 17-18 you may see 7, 18-19 you might see 5. Once you approach MBT, you will get smaller and smaller gains for every degree of spark you add. Here's why this is happening:
Engine power is generated through pressure produced by the combustion process. Timing is when the spark fires (usually before top dead center (BTDC)) to light the air/fuel mixture. I'm oversimplifying a bit here, but what you want is the cylinder pressure to occur after TDC when the engine is actually in the power stroke and can harness that miniature explosion to transmit power to the crankshaft. When you "advance" the timing, you are moving this pressure curve closer to TDC (earlier start point). You get to a point where you advance too far and you increase the cylinder pressure stress seen by the components, but don't gain any power because the engine is literally fighting against itself and wasting power to make the power (extra pressure before the crankshaft breaks over into the power stroke that literally does no good). This is past the point of MBT timing and very bad things start happening here.
Now Ford calls the timing that can be run on current fuel that is octane-limited BORDERLINE timing. The way most Ford logic works is that you have a borderline table that is trimmed by the knock sensors that allows addition of timing up to the MBT timing tables or the often artificially low "maximum spark for combustion pressure limit" table that only allows timing up to a point even if you haven't reached MBT for that load/rpm point of the system. We can touch on that aspect in another topic, but for now let's talk about MBT timing only because it will lead into the next topic.
Let's keep this technical so we can talk facts and not treat engine tuning like magic anymore. Once we put this topic to bed, we can move on so save other technical aspects of tuning for the appropriate thread.
Matt@Gearhead
MBT Timing:
MBT Timing is the timing at which the power to cylinder pressure gain starts going lopsided and you should stop advancing timing with unlimited octane (lets say 120 octane in this case). So let's just say you have good 93 octane fuel. At 6000 rpms with an IAT2 of 100 degrees you can run 16 degrees of timing at a VE (load) of 1.65 (165% Volumetric Efficiency) before you experience the knock sensors saying "nope" and pulling timing. Is this MBT timing? Absolutely not!
Now let's say you are running 100 octane unleaded race fuel. In this scenario, for every 1 point of octane gained, you get about 2 degrees of spark you can run extra before you experience knock. Let's say the MBT spark at this Load and RPM point is 23 degrees (timing that makes the max safe power). That means we would need about 3.5 octane points to allow for another 7 degrees of timing to get to that spot. But let's just call it 4 octane points to allow for a little safety margin. This means we need 97 octane fuel to get to the full MBT timing in that load/rpm point of the engine's operation.
But wait.... We have 3 extra octane points we aren't using, let's keep advancing the timing right? No, this isn't advisable. The further you are from the MBT (optimum) spark point, the more power is gained for every degree of spark you add. As you get close to the MBT timing, the returns keep get smaller and smaller. From 16-17 degrees you might see an extra 10 HP, 17-18 you may see 7, 18-19 you might see 5. Once you approach MBT, you will get smaller and smaller gains for every degree of spark you add. Here's why this is happening:
Engine power is generated through pressure produced by the combustion process. Timing is when the spark fires (usually before top dead center (BTDC)) to light the air/fuel mixture. I'm oversimplifying a bit here, but what you want is the cylinder pressure to occur after TDC when the engine is actually in the power stroke and can harness that miniature explosion to transmit power to the crankshaft. When you "advance" the timing, you are moving this pressure curve closer to TDC (earlier start point). You get to a point where you advance too far and you increase the cylinder pressure stress seen by the components, but don't gain any power because the engine is literally fighting against itself and wasting power to make the power (extra pressure before the crankshaft breaks over into the power stroke that literally does no good). This is past the point of MBT timing and very bad things start happening here.
Now Ford calls the timing that can be run on current fuel that is octane-limited BORDERLINE timing. The way most Ford logic works is that you have a borderline table that is trimmed by the knock sensors that allows addition of timing up to the MBT timing tables or the often artificially low "maximum spark for combustion pressure limit" table that only allows timing up to a point even if you haven't reached MBT for that load/rpm point of the system. We can touch on that aspect in another topic, but for now let's talk about MBT timing only because it will lead into the next topic.
Let's keep this technical so we can talk facts and not treat engine tuning like magic anymore. Once we put this topic to bed, we can move on so save other technical aspects of tuning for the appropriate thread.
Matt@Gearhead
