MBT Timing changes effecting Idle

[joepeek]

2005 Mustang GT, 5 speed, Airaid CAI, Flowmaster 40's......I was reading somewhere that the FORD 4.6 3v calculates two possible timing values. One from the Borderline table and one from the MBT table. The lower number of these two after all kinds of adders are thrown in is what the engine uses. In order to know what timing I am commanding, I copied the Borderline table to the MBT table and then added 4? to all the copied values. Therefore, the Borderline number will always be the "lower" number and what the engine gets. The problem I have is after copying the Borderline and adding 4? to the MBT, the next day when I started my car the engine screamed to 3000 RPM, then took about 25 seconds to calm down to around 1000 RPM......but never really settled down to a steady idle. I drove it to work that day and thought it was going to die at several lights. Got home that night, put the previous tune back in, and all is back to OK. Anyone know why my engine searches for idle and screams to 3000 when I mess with the MBT timing table?

[Bugasu]

2005 Mustang GT, 5 speed, Airaid CAI, Flowmaster 40's......I was reading somewhere that the FORD 4.6 3v calculates two possible timing values. One from the Borderline table and one from the MBT table. The lower number of these two after all kinds of adders are thrown in is what the engine uses. In order to know what timing I am commanding, I copied the Borderline table to the MBT table and then added 4? to all the copied values. Therefore, the Borderline number will always be the "lower" number and what the engine gets. The problem I have is after copying the Borderline and adding 4? to the MBT, the next day when I started my car the engine screamed to 3000 RPM, then took about 25 seconds to calm down to around 1000 RPM......but never really settled down to a steady idle. I drove it to work that day and thought it was going to die at several lights. Got home that night, put the previous tune back in, and all is back to OK. Anyone know why my engine searches for idle and screams to 3000 when I mess with the MBT timing table?

The MBT table serves more than just being one of the spark limits. MBT table also indicates to the ECU which timing for a given load/RPM produces an indicated torque. As such your spark delta from MBT is utilized extensively in the torque control features of the ECU (one of which is idle speed control), and when you mess with MBT, you essentially break those calculations. The various tables, such as Engine Torque in torque management, are based on that torque being the torque at MBT. So MBT is a tricky table to dial in. It should coincide with your actual MBT timing, and your torque tables should be dialed in with MBT torque values as well to keep this relationship in-tact.

There are more sources than just BL and MBT as well. The aforementioned spark target is the "base" spark. There's quite a few sources at play. If you want to instead know where your current timing comes from, I'd log Spark Source.

[joepeek]

The MBT table serves more than just being one of the spark limits. MBT table also indicates to the ECU which timing for a given load/RPM produces an indicated torque. As such your spark delta from MBT is utilized extensively in the torque control features of the ECU (one of which is idle speed control), and when you mess with MBT, you essentially break those calculations. The various tables, such as Engine Torque in torque management, are based on that torque being the torque at MBT. So MBT is a tricky table to dial in. It should coincide with your actual MBT timing, and your torque tables should be dialed in with MBT torque values as well to keep this relationship in-tact.

There are more sources than just BL and MBT as well. The aforementioned spark target is the "base" spark. There's quite a few sources at play. If you want to instead know where your current timing comes from, I'd log Spark Source.

Thanks for the reply. So if the MBT table is used for other calculations, that are sensitive to messing things up, and in my stock tune the MBT values are lower than Borderline knock, (except at low RPM/high load), then in general, the MBT value will always be the lower number (except if I nail it in 5th gear going 10 mph), then what good is changing Borderline Knock values if they are always going to be greater than the MBT number and therefore not used? Since my engine is mostly stock, and I don't have a dyno to plot my own iterations of different timing values vs. conditions to determine actual MBT timing better than Ford did, I'm starting to feel a bit lost in just what I can do that will let me at least give a few more degrees and go datalog the change. I don't want to just get random, and really don't want to use the hack of bumping up the global adder.

[joepeek]

Thanks for the reply. So if the MBT table is used for other calculations, that are sensitive to messing things up, and in my stock tune the MBT values are lower than Borderline knock, (except at low RPM/high load), then in general, the MBT value will always be the lower number (except if I nail it in 5th gear going 10 mph), then what good is changing Borderline Knock values if they are always going to be greater than the MBT number and therefore not used? Since my engine is mostly stock, and I don't have a dyno to plot my own iterations of different timing values vs. conditions to determine actual MBT timing better than Ford did, I'm starting to feel a bit lost in just what I can do that will let me at least give a few more degrees and go datalog the change. I don't want to just get random, and really don't want to use the hack of bumping up the global adder.

Looking at other tunes in the repository, it seems the upper left cells on the MBT table are pretty much stock. Which makes perfect sense for not effecting the idle. Looks like the lower right where the load an RPM are high is where most tunes are modified. I'll have to try adding a few degrees to some lower right cells and leave the upper left alone. I'll have some time this weekend to see how it goes.

[Bugasu]

Looking at other tunes in the repository, it seems the upper left cells on the MBT table are pretty much stock. Which makes perfect sense for not effecting the idle. Looks like the lower right where the load an RPM are high is where most tunes are modified. I'll have to try adding a few degrees to some lower right cells and leave the upper left alone. I'll have some time this weekend to see how it goes.

MBT and Borderline model two concepts. Borderline indicates the knock borderline, i.e. where knock starts to occur on whatever fuel the BDL table was calibrated for (typically 91 octane). MBT on the other hand represents what timing you make best torque at. Two other common ones you will see are Cylinder Pressure timing, designed to set the timing to limit overall cylinder pressure, and Preignition Timing, designed to prevent preignition at high loads and RPMs.

If your BDL > MBT that simply means that you have enough leeway to hit maximum torque without knock in that region of engine operation. For most engines, this will be true at low loads regardless, where you can essentially run any timing you want without knock. The 3V is one where at High RPM, High Load, Ford has indicated that MBT is actually before the knock borderline when IMRC is closed. If true would mean that running more timing in this region would actually result in less power.

I'm not well versed in 3V tuning, so I don't know if Ford is actually off here, or if that's actually true for a stock motor. From most of my testing, Ford's MBT table is usually pretty spot-on. You'd have to do a spark hook test to verify if this is indeed the case, and whether more timing in that zone would be beneficial. Keep in mind the BDL and MBT tables are calibrated at stoich, so there is a lambda adjustment on both tables. As well, there is an adjustment to MBT for IMRC as well, so keep that in mind depending on your IMRC position. Open IMRC raises MBT almost 15* in the high load/high RPM region, which is above the BDL spark in that region.

[Eric@HPTuners]

Thanks for the reply. So if the MBT table is used for other calculations, that are sensitive to messing things up, and in my stock tune the MBT values are lower than Borderline knock, (except at low RPM/high load), then in general, the MBT value will always be the lower number (except if I nail it in 5th gear going 10 mph), then what good is changing Borderline Knock values if they are always going to be greater than the MBT number and therefore not used? Since my engine is mostly stock, and I don't have a dyno to plot my own iterations of different timing values vs. conditions to determine actual MBT timing better than Ford did, I'm starting to feel a bit lost in just what I can do that will let me at least give a few more degrees and go datalog the change. I don't want to just get random, and really don't want to use the hack of bumping up the global adder.

MBT on your stock tune is not lower. Look at the MBT Spark Adder for the IMRCs. That adds quite a bit of timing to the total MBT value.

[joepeek]

MBT on your stock tune is not lower. Look at the MBT Spark Adder for the IMRCs. That adds quite a bit of timing to the total MBT value.

Thanks Eric..............

[svoman8284]

Usually @ wot spark source typically comes from the borderline table. But yes i agree, log source