What is the highest octane recommended for a supercharged engine

[ZXMustang]

I'm a little confused about what you're saying because when my car is reporting negative knock it is definitely adding timing on the other hand when it is reporting positive knock it is pulling timing so I'm not sure what you're talking about

I can show you. This is from my car in a log of a WOT pull. The KR is -3.0 meaning because the car is so happy and not sensing any knock, its adding 3* to the total timing. The car will do this all by itself within the guardrails of the tables in the tune.

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[engineermike]

Thanks for trying to clear that up but that still doesn't sound right to me

Which part?

knock retard just takes timing away from the current advance as a function of knock intensity. The result could be positive or negative depending on how much knock advance it had at the time and the intensity of the knock. You could have 10 deg of knock advance, experience mild knock, and still have 9 deg of knock advance even though knock happened.

 

[Grimreaper]

^that.
Gotta log and see the entire curve/ pull. Looking at one moment of a pull and saying knock sensors were happy is not enough.

It helps to know what's in the tune. Some are setup to add xx degrees until xxxxrpm and then less after that. The recovery rate after a detected event can cause some oddities in the way the curve looks too.

I prefer to use the mbt tables to set my end target. Logging mbt/ borderline shows very clearly if you hit it or not.

 

[engineermike]

@Grimreaper exactly. It depends so much on how the calibration is “designed”. For instance, I set mine up to be below knock at 2 deg knock advance and that’s where I limit it. Cyl pressure and preignition are moved up out of the way. If any cylinder is not -2 then I had a knock event, even if it’s -1.

 

[Markuaw1]

I can show you. This is from my car in a log of a WOT pull. The KR is -3.0 meaning because the car is so happy and not sensing any knock, its adding 3* to the total timing. The car will do this all by itself within the guardrails of the tables in the tune.

Thanks I got plenty of my own log files from the track I understand how it works it's not what he was saying though

I can show you. This is from my car in a log of a WOT pull. The KR is -3.0 meaning because the car is so happy and not sensing any knock, its adding 3* to the total timing. The car will do this all by itself within the guardrails of the tables in the tune.

I have got plenty in my own log files in the track and understand how it works that's just not what he was saying

 

[engineermike]

@Markuaw1 , zxmustang’s post isn’t exclusive of what I’m saying. However, if his -3 turns into -2 and it’s not hitting another limit (mbt, preignition, cyl pressure), then it experienced a knock event even though it’s still adding timing.

 

[Markuaw1]

@Markuaw1 , zxmustang’s post isn’t exclusive of what I’m saying. However, if his -3 turns into -2 and it’s not hitting another limit (mbt, preignition, cyl pressure), then it experienced a knock event even though it’s still adding timing.

every car does that, so 100% every car Knocks every time according to that

 

[Grimreaper]

How does every car do that?

Post a full wot log. Let's see

 

[engineermike]

every car does that, so 100% every car Knocks every time according to that

Most fords do, but not all cars. On most of my logs I have zero knock events. I think ford designs the engines strong enough to withstand an occasional knock event in stock form but probably not at the cylinder pressures boost on top of 12/1 gives you.

That said, I’m still not understanding what you took exception to in post 27 when now you say it’s normal.

All I was trying to say is that negative knock retard doesn’t mean it’s not knocking.

 

[ice445]

Most fords do, but not all cars. On most of my logs I have zero knock events. I think ford designs the engines strong enough to withstand an occasional knock event in stock form but probably not at the cylinder pressures boost on top of 12/1 gives you.

That said, I’m still not understanding what you took exception to in post 27 when now you say it’s normal.

All I was trying to say is that negative knock retard doesn’t mean it’s not knocking.

I think his issue might relate to total timing over a run. I'm actually curious too, since I don't entirely understand the "MBT" limit. Is that just running into a wall of base timing? I do know that the higher the RPM, the less timing you can run typically, but since I've never logged these values the right way, I'm not sure if knock adder will essentially taper off the closer to redline you get as a result of that. For example, if I'm tipping in at 3500RPM, and my knock adder goes to -4, and then blends out to -.75 by redline, have I experienced knock, or is that just normal base map limitations?

I'm just starting to learn some of these tuning related magic happenings, so apologies if this is a silly question.

 

[engineermike]

I think his issue might relate to total timing over a run. I'm actually curious too, since I don't entirely understand the "MBT" limit. Is that just running into a wall of base timing? I do know that the higher the RPM, the less timing you can run typically, but since I've never logged these values the right way, I'm not sure if knock adder will essentially taper off the closer to redline you get as a result of that. For example, if I'm tipping in at 3500RPM, and my knock adder goes to -4, and then blends out to -.75 by redline, have I experienced knock, or is that just normal base map limitations?

I'm just starting to learn some of these tuning related magic happenings, so apologies if this is a silly question.

No worries, the system is hard to understand at first but makes total sense once you get it.

The genius in the Ford logic is that it's always calculating spark timing 4-5 different ways and then does a "low-select" for the commanded.

MBT - Maximum Brake Torque, which gets you the highest efficiency and power output regardless of any other limit. This varies as a function of load, speed, cam timing, and lambda.

Borderline - Where it expects to find knock on low octane fuel. This is a function of load, speed, cam timing, lambda, coolant temp, and charge temp. If this is found to be the lowest spark timing calculated, then it will start adding timing per the knock advance limit. If knock is sensed, then it will take timing away. There is a lot to this subject. The thing that messes people up is that if the initial borderline calculation is the lowest then the "spark source" will show borderline even if it hits some other limit before it hits the max advance limit.

Pre-ignition, Cylinder Pressure - Other limits calculated as a function of load, speed, and other things. Cylinder pressure is likely how they limit mechanical stresses on the engine.

Torque Control - In certain situations, it will reduce timing in order to control torque. Most common is at idle where it uses timing to control idle speed, and on the shifts, where the torque cut is usually done through spark retard.

As for your knock advance tapering down close to redline, without looking at your tune file it is impossible to say. Some tuners reduce max knock advance at high rpm. A stock ecoboost F150 doesn't allow any knock advance above 6000 rpm, for instance. Some tuners use max cylinder pressure to apply an easy timing limit. And still, it is possible that you are hitting MBT. You can see this if you log MBT.

 

[ZXMustang]

No worries, the system is hard to understand at first but makes total sense once you get it.

The genius in the Ford logic is that it's always calculating spark timing 4-5 different ways and then does a "low-select" for the commanded.

MBT - Maximum Brake Torque, which gets you the highest efficiency and power output regardless of any other limit. This varies as a function of load, speed, cam timing, and lambda.

Borderline - Where it expects to find knock on low octane fuel. This is a function of load, speed, cam timing, lambda, coolant temp, and charge temp. If this is found to be the lowest spark timing calculated, then it will start adding timing per the knock advance limit. If knock is sensed, then it will take timing away. There is a lot to this subject. The thing that messes people up is that if the initial borderline calculation is the lowest then the "spark source" will show borderline even if it hits some other limit before it hits the max advance limit.

Pre-ignition, Cylinder Pressure - Other limits calculated as a function of load, speed, and other things. Cylinder pressure is likely how they limit mechanical stresses on the engine.

Torque Control - In certain situations, it will reduce timing in order to control torque. Most common is at idle where it uses timing to control idle speed, and on the shifts, where the torque cut is usually done through spark retard.

As for your knock advance tapering down close to redline, without looking at your tune file it is impossible to say. Some tuners reduce max knock advance at high rpm. A stock ecoboost F150 doesn't allow any knock advance above 6000 rpm, for instance. Some tuners use max cylinder pressure to apply an easy timing limit. And still, it is possible that you are hitting MBT. You can see this if you log MBT.

Mine uses the boarderline. It says that in the logs when Im at full swing.

 

[ice445]

No worries, the system is hard to understand at first but makes total sense once you get it.

The genius in the Ford logic is that it's always calculating spark timing 4-5 different ways and then does a "low-select" for the commanded.

MBT - Maximum Brake Torque, which gets you the highest efficiency and power output regardless of any other limit. This varies as a function of load, speed, cam timing, and lambda.

Borderline - Where it expects to find knock on low octane fuel. This is a function of load, speed, cam timing, lambda, coolant temp, and charge temp. If this is found to be the lowest spark timing calculated, then it will start adding timing per the knock advance limit. If knock is sensed, then it will take timing away. There is a lot to this subject. The thing that messes people up is that if the initial borderline calculation is the lowest then the "spark source" will show borderline even if it hits some other limit before it hits the max advance limit.

Pre-ignition, Cylinder Pressure - Other limits calculated as a function of load, speed, and other things. Cylinder pressure is likely how they limit mechanical stresses on the engine.

Torque Control - In certain situations, it will reduce timing in order to control torque. Most common is at idle where it uses timing to control idle speed, and on the shifts, where the torque cut is usually done through spark retard.

As for your knock advance tapering down close to redline, without looking at your tune file it is impossible to say. Some tuners reduce max knock advance at high rpm. A stock ecoboost F150 doesn't allow any knock advance above 6000 rpm, for instance. Some tuners use max cylinder pressure to apply an easy timing limit. And still, it is possible that you are hitting MBT. You can see this if you log MBT.

Oh okay, I see. Thanks for the clarification. So the "low select" is basically the car testing the most likely value to inhibit spark timing based on things it knows, like load and rpm and such? And it's always trying to achieve MBT assuming no other limits have been reached? Makes a lot of sense and is pretty smart if that's the case.

As far as the knock example, that was just something I noticed "logging" the stock tune a while ago. And I wasn't even logging, just watching for a few seconds as I floored it. Knock adder slowly reduced through the RPM range. Given that we have crappy 91 here, it very well could have been real knock. Now I have the Ford Performance tune, so I'm definitely curious to properly log it and see if anything has changed.

 

[engineermike]

...So the "low select" is basically the car testing the most likely value to inhibit spark timing based on things it knows, like load and rpm and such?

Right, the way I envision this is that each engineer or department at Ford has a specialty and they each submit their limits for spark timing. One guy might do all the modelling and testing to map out MBT at all the different loads, speeds, cam angles, lambdas, etc. He submits his data and they plug it into the calibration. The logic then looks at all the different spark timing limits and selects the lowest (most conservative) one. The GDi flow and desired torque are handled similarly.

And it's always trying to achieve MBT assuming no other limits have been reached? Makes a lot of sense and is pretty smart if that's the case.

I guess you could think of it that way. MBT makes the most power and is the most efficient, so it should be the goal. But in actuality the logic just sees MBT as a "do not exceed" value just like pre-ignition and cylinder pressure. But pre-ignition and cylinder pressure aren't a function of cam timing, so MBT gives it the flexibility of moving the value as a function of cam timing as well. I would say it's always calculating all of the spark limits and selecting the most conservative value.

 

[monte87]

I have a 2021 gt. I got a roush stage 2 supercharger, midpipe delete and everything else is stock still has stock roush tune. I can barely find 93 here but have access to race fuel. Usually not unleaded but i am looking for sunoco 260gtx and usually can find VP C12. Question: will mixing either with 91 and getting my octane to 98 give me any benefit. I know lead is bad for o2 and cats but idc thats guna be gone eventuallyand i prefer the sunoco but i hafta take what i can. And i can find this easier and mix rather than travel 20 miles for 93 every 5 days.

21 GT w Roush phase 2 blower here on stock Roush tune still under warranty, and i run 93. Im curious about this as well? I always wanted to run a real octane booster, but wasnt sure and never got a real answer myself.
Thanks Anthony

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