Tuning Technical Point #1 - What is MBT timing?

[802SHO]

Okay, so if MBT is shifting with the change in drive pressure, if the drive pressure increases, what direction does MBT need to go?

As drive pressure increases, MBT generally shifts toward less spark advance (retards), because residual exhaust gases increase, burn rate slows, and peak pressure timing moves unfavorably earlier if spark is left unchanged.

Which is exactly why MBT appearing to plateau or retreat at high RPM doesn’t indicate an airflow limit — it indicates combustion phasing being constrained by exhaust backpressure.

 

[802SHO]

Develop a larger stock-location turbine and the entire system moves forward. Drive pressure drops, residuals fall, VE recovers at higher RPM, MBT advances again, and the same engine suddenly looks like it “flows more air” without touching cams, heads, or intakes. That’s not a new engine capability—it’s the exhaust side finally getting out of the way.

 

[mattr66usa]

Develop a larger stock-location turbine and the entire system moves forward. Drive pressure drops, residuals fall, VE recovers at higher RPM, MBT advances again, and the same engine suddenly looks like it “flows more air” without touching cams, heads, or intakes. That’s not a new engine capability—it’s the exhaust side finally getting out of the way.

You are trying to shift the scope of this thread.... You accused me of not tuning for meth properly on cars with stock location turbos. We are talking about MBT timing not turbine flow being a limiting factor of these engines. Try again.

 

[mattr66usa]

As drive pressure increases, MBT generally shifts toward less spark advance (retards), because residual exhaust gases increase, burn rate slows, and peak pressure timing moves unfavorably earlier if spark is left unchanged.

Which is exactly why MBT appearing to plateau or retreat at high RPM doesn’t indicate an airflow limit — it indicates combustion phasing being constrained by exhaust backpressure.

Again we are talking about MBT timing needs in this thread and the comparison of actual changes with methanol vs no methanol injection happening.

 

[802SHO]

You are trying to shift the scope of this thread.... You accused me of not tuning for meth properly on cars with stock location turbos. We are talking about MBT timing not turbine flow being a limiting factor of these engines. Try again.

I’m not shifting scope. I’m staying inside MBT.

MBT timing is determined by combustion phasing, and in a turbo, speed-density engine combustion phasing is affected by exhaust residuals and pumping losses, both of which increase with drive pressure.

So when meth changes knock margin and alters EGT, mass flow, and pressure ratio, MBT can move for more than one reason. Without accounting for exhaust pressure, you can’t attribute MBT movement solely to octane or charge cooling.

That’s not turbine flow theory — that’s MBT definition in a boosted engine.

 

[802SHO]

Staying strictly on tuning and MBT: MBT is a combustion-phasing outcome, not a fixed value. In a speed-density turbo engine, anything that alters residuals, burn rate, or cylinder pressure history will move MBT. Methanol is fuel and octane, not passive cooling, and any amount of meth flowing must be tuned for….that is non-negotiable.

Exhaust backpressure further alters residual dilution and pumping losses, which also shifts MBT. Without accounting for those effects, observed MBT limits are configuration-specific, not inherent engine limits.

 

[mattr66usa]

I’m not shifting scope. I’m staying inside MBT.

MBT timing is determined by combustion phasing, and in a turbo, speed-density engine combustion phasing is affected by exhaust residuals and pumping losses, both of which increase with drive pressure.

So when meth changes knock margin and alters EGT, mass flow, and pressure ratio, MBT can move for more than one reason. Without accounting for exhaust pressure, you can’t attribute MBT movement solely to octane or charge cooling.

That’s not turbine flow theory — that’s MBT definition in a boosted engine.

What does speed density have to do with anything relating to engine dynamics. Speed density is the chosen airflow model for the computer, not the airflow dynamics of the engine.
Points not relevant! Take away what the computer is trying to do and tell me what needs to happen if you understand this so much.

MBT timing..... tell me when methanol is flowing vs not flowing, how does the relative MBT timing NUMBER change? Can you answer that? I still feel like you don't even understand what MBT timing is in the frame of reference we are talking about.

 

[802SHO]

What does speed density have to do with anything relating to engine dynamics. Speed density is the chosen airflow model for the computer, not the airflow dynamics of the engine.
Points not relevant! Take away what the computer is trying to do and tell me what needs to happen if you understand this so much.

MBT timing..... tell me when methanol is flowing vs not flowing, how does the relative MBT timing NUMBER change? Can you answer that? I still feel like you don't even understand what MBT timing is in the frame of reference we are talking about.

When methanol is flowing, relative MBT timing generally moves forward (more advance) compared to meth off, because meth increases knock margin and can improve combustion stability. The exact number of degrees depends on mixture strength, distribution, and operating point, but the direction is forward, not backward.

However, MBT is not determined by knock margin alone. MBT is defined by combustion phasing and torque response, not by the ECU model. If exhaust backpressure is high, residual dilution and pumping losses increase, burn rate slows, and the torque response to spark diminishes. In that case, MBT may advance less than expected or even flatten despite improved octane.

Speed density doesn’t change engine physics, but it does matter for interpretation. In a speed-density system, VE is inferred from pressure, temperature, and modeled efficiency. When exhaust backpressure alters effective cylinder filling and residual fraction, the calculated load, VE, and spark sensitivity change together. That’s why MBT observations can’t be interpreted in isolation from exhaust conditions.

So yes, meth flowing typically allows more spark at MBT. The point I’m making is that without accounting for exhaust backpressure, you can’t know whether MBT movement—or lack of it—is being limited by combustion chemistry or by exhaust-driven dilution and pumping losses. That’s an engine-dynamics issue, not an ECU strategy issue.

 

[mattr66usa]

When methanol is flowing, relative MBT timing generally moves forward (more advance) compared to meth off, because meth increases knock margin and can improve combustion stability. The exact number of degrees depends on mixture strength, distribution, and operating point, but the direction is forward, not backward.

However, MBT is not determined by knock margin alone. MBT is defined by combustion phasing and torque response, not by the ECU model. If exhaust backpressure is high, residual dilution and pumping losses increase, burn rate slows, and the torque response to spark diminishes. In that case, MBT may advance less than expected or even flatten despite improved octane.

Speed density doesn’t change engine physics, but it does matter for interpretation. In a speed-density system, VE is inferred from pressure, temperature, and modeled efficiency. When exhaust backpressure alters effective cylinder filling and residual fraction, the calculated load, VE, and spark sensitivity change together. That’s why MBT observations can’t be interpreted in isolation from exhaust conditions.

So yes, meth flowing typically allows more spark at MBT. The point I’m making is that without accounting for exhaust backpressure, you can’t know whether MBT movement—or lack of it—is being limited by combustion chemistry or by exhaust-driven dilution and pumping losses. That’s an engine-dynamics issue, not an ECU strategy issue.

NOPE! Wrong! You are backwards again. I told you you didn't understand it so why don't you just listen for once. MBT is not the timing for keeping knock at bay. So tell me what is MBT timing.....

 

[mattr66usa]

surprised you didn't consult your trusty AI on this one...

 

[802SHO]

NOPE! Wrong! You are backwards again. I told you you didn't understand it so why don't you just listen for once. MBT is not the timing for keeping knock at bay. So tell me what is MBT timing.....

MBT timing is the spark advance that produces Maximum Brake Torque at a given RPM/load… meaning the point where adding more timing no longer increases torque, even with knock completely removed from the equation. It’s about combustion phasing for best net work, not “the most timing before knock”.

On your meth question, the direction of MBT shift is driven by burn duration and pressure-rise timing. If meth injection (as an oxygenated fuel with strong charge cooling) makes the combination burn “effectively faster” at that operating point, then MBT moves to less spark advance (retards) because you don’t need to light it as early to hit optimal peak pressure after TDC. If it slows the burn (distribution/dilution effects), MBT can move the other way, which is exactly why it must be determined by torque response, not assumed.

So yes, MBT is not a knock target. It’s the torque-optimal spark for that condition, and meth can shift it, often toward less advance when it improves burn effectiveness at that point.

Anyone claiming MBT always moves one direction with meth is no longer talking about MBT, they’re talking about a shortcut.

 

[802SHO]

surprised you didn't consult your trusty AI on this one...

MBT is defined by combustion phasing, not fuel identity. Methanol can shift MBT in either direction depending on whether flame speed, charge cooling, dilution, or residuals dominate, which is exactly why methanol must be tuned for rather than assumed.

 

[mattr66usa]

MBT timing is the spark advance that produces Maximum Brake Torque at a given RPM/load… meaning the point where adding more timing no longer increases torque, even with knock completely removed from the equation. It’s about combustion phasing for best net work, not “the most timing before knock”.

On your meth question, the direction of MBT shift is driven by burn duration and pressure-rise timing. If meth injection (as an oxygenated fuel with strong charge cooling) makes the combination burn “effectively faster” at that operating point, then MBT moves to less spark advance (retards) because you don’t need to light it as early to hit optimal peak pressure after TDC. If it slows the burn (distribution/dilution effects), MBT can move the other way, which is exactly why it must be determined by torque response, not assumed.

So yes, MBT is not a knock target. It’s the torque-optimal spark for that condition, and meth can shift it, often toward less advance when it improves burn effectiveness at that point.

Anyone claiming MBT always moves one direction with meth is no longer talking about MBT, they’re talking about a shortcut.

So you agree that the trend for MBT timing is less advance needed with meth vs no meth all else equal then?

 

[mattr66usa]

So you agree that the trend for MBT timing (and only mbt timing) is less advance needed with meth vs no meth all else equal then?

Had to stipulate only MBT timing so you can stick to the singular point I am trying to illustrate in this thread.

 

[802SHO]

The simplified trend you’re describing only holds if exhaust backpressure, residuals, and effective VE are unchanged — which is not true on stock-location, drive-pressure-limited EcoBoost systems. That’s why methanol tuning is non-negotiable.

 

[mattr66usa]

The simplified trend you’re describing only holds if exhaust backpressure, residuals, and effective VE are unchanged — which is not true on stock-location, drive-pressure-limited EcoBoost systems. That’s why methanol tuning is non-negotiable.

Let's not change the subject before moving on to the next point. Does the MBT timing need to be more or less advanced when running methanol vs not running methanol?

 

[802SHO]

Let's not change the subject before moving on to the next point. Does the MBT timing need to be more or less advanced when running methanol vs not running methanol?

Yes.. in the simplified case where all else is equal, methanol’s faster effective burn rate tends to move MBT toward less advance (closer to TDC).

However, that trend only holds if exhaust backpressure, residual fraction, and effective VE are unchanged — which is not the case on stock-location, drive-pressure-limited turbo systems.

Given that methanol can simultaneously improve burn rate and alter exhaust energy, residuals, and drive pressure, how are you separating MBT changes caused by combustion chemistry from MBT changes caused by increased exhaust backpressure if turbine inlet pressure is not measured?

 

[mattr66usa]

Okay

Yes.. in the simplified case where all else is equal, methanol’s faster effective burn rate tends to move MBT toward less advance (closer to TDC).

However, that trend only holds if exhaust backpressure, residual fraction, and effective VE are unchanged — which is not the case on stock-location, drive-pressure-limited turbo systems.

Given that methanol can simultaneously improve burn rate and alter exhaust energy, residuals, and drive pressure, how are you separating MBT changes caused by combustion chemistry from MBT changes caused by increased exhaust backpressure if turbine inlet pressure is not measured?

Finally we are getting somewhere..... Now that we have that out of the way.... go here: https://shoforum.com/threads/tuning-technical-point-2-borderline-timing-vs-mbt-timing.147788/

 

[802SHO]

Okay

Finally we are getting somewhere..... Now that we have that out of the way.... go here: https://shoforum.com/threads/tuning-technical-point-2-borderline-timing-vs-mbt-timing.147788/

No. I’m not jumping threads or changing topics like Mario from Mario Bros.

We already agreed on the simplified MBT trend only when all else is equal. My original and still-unanswered point is this: on a drive-pressure-limited, speed-density turbo system, methanol changes combustion energy, residuals, and effective VE, which means MBT and borderline timing no longer map 1:1.

Because meth is fuel mass and combustion modifier, any amount of meth requires explicit calibration. Relying on knock control or “letting the car sort it out” is neither safe nor technically correct.

If you disagree, the burden isn’t to redirect me to another thread — it’s to explain how introducing additional fuel energy without recalibration is not reckless on this platform.

 

[802SHO]

There’s also an important ethical distinction here.

It’s one thing for an individual to experiment on their own car and accept the consequences while the system adapts imperfectly. It’s another thing entirely to sell that approach to paying customers.

Customers are paying for expertise and intent, not for “let the car sort it out” behavior. Forwarding an untuned fuel and combustion modifier to customers is not a difference in philosophy — it’s a failure to calibrate responsibly.