87 octane vs 93 octane - ECOBOOST

[SALEEN961]

Attached is the pressure-volume diagram for the Otto Cycle that our engines employ. What is important is Point 2 and 3 on the diagram. Point 2 theoretically represents top dead center or the completion of the compression stroke. The line from 2 to 3 is the burning of fuel. Line 3 to 4 theoretically represents the downstroke or power stroke of the piston. The reality is line 2-3 is the burning of the fuel which takes a measurable amount of time. The purpose of ignition timing advance is to so time things that the combustion is just complete at TDC or a hair after TDC. The purpose of octane is to prevent the fuel from pre-igniting before the spark arrives. That theoretical point #3 is determined by engine design. It presumes certain things like intake charge, sea level pressure or boost pressure, spark advance, and so on. It cannot be changed. The rest of the things like variable valve timing or spark advance or other computer controls improve engine operation by approaching that perfect design point but the basic design determines how the engine operates in light of these variables. When you throw in wear and tear, hot summer days, carbon deposits (that may be glowing), less than advertised octane, a possibly overloaded trailer... using 93 octane is a good idea because it gives you some margin to bad things.

The issue is that the 3.5EB often fails to start the combustion process before top dead center, in fact it is very common to see a negative value for spark advance at WOT. Ford's calibration file lists the spark values at which the engine makes the most power in the MBT tables. We can easily confirm that we are octane limited and making less power than what the factory calibration will allow by looking at the spark advance values in MBT tables.

If you look at the actual values in the factory calibration file, you will see that Ford designed the calibration file to automatically adjust the ignition timing and boost pressure. The factory calibration is designed to limit boost pressure and spark advance based upon the inferred octane value that is calculated from knock sensor activity.

Power output will increase with higher octane fuel because that is how the factory calibration is designed to work. On the factory calibration, the upper limit is an inferred octane value of 98oct.

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[HammaMan]

Unless I am once again misunderstanding the point.

I too was looking for the substance on the latest part of this thread.
I'd begun to get some of these vibes

 

[Gros Ventre]

The issue is that the 3.5EB often fails to start the combustion process before top dead center, in fact it is very common to see a negative value for spark advance at WOT. Ford's calibration file lists the spark values at which the engine makes the most power in the MBT tables. We can easily confirm that we are octane limited and making less power than what the factory calibration will allow by looking at the spark advance values in MBT tables.

If you look at the actual values in the factory calibration file, you will see that Ford designed the calibration file to automatically adjust the ignition timing and boost pressure. The factory calibration is designed to limit boost pressure and spark advance based upon the inferred octane value that is calculated from knock sensor activity.

Power output will increase with higher octane fuel because that is how the factory calibration is designed to work. On the factory calibration, the upper limit is an inferred octane value of 98oct.

A spark timing After TDC represents a loss of available power. Not knowing where Ford set the zero reference means you don't know that a negative value means after TDC. As to adjusting spark and boost like you described, that means they are adjusting toward that max point in the diagram. As they approach that max point... of course more power is available. The theory says that points 2 & 3 in the diagram occur at TDC. However, since the Qin cannot be instantaneous the spark is introduced before TDC to allow the burning to get to completion at TDC. If the completion of burning of fuel were to happen after TDC that would be lost energy.

 

[gh8toes]

Here's an article that may interest some 3.5 owners.

Ford 3.5L V6 EcoBoost Engine Failure Baffles Tech: Video

 

[Snakebitten]

Here's an article that may interest some 3.5 owners.

Ford 3.5L V6 EcoBoost Engine Failure Baffles Tech: Video

The cheap pistons that Ford uses in the 3.5 Ecoboost are not news amongst the 3.5 performance engine builders. Nice crankshaft, according to them. (especially the new Gen3 version). Decent "enough" rods for OEM. But the pistons are very very HO-hum cost-point pistons for a 100HP per liter mass produced motor. It was pointed out to me personally by one such builder several years ago and his opinion is that if you bought 6 of them, statistically you would probably get 6 good ones. But if you bought 600 of them, he could almost guarantee that the skirt would fail on one at some point.

I will never forget him saying that if Ford spent $3.00 more per piston, it would make all the difference. And then he compared the Raptor piston. To me it was basically the same. But he described while it was nothing to write home about, it was a better piston, skirt-wise, and might be $3.00 more cost to Ford.

Your post above sure seems to confirm exactly what he was talking about.

 

[Gros Ventre]

The cheap pistons that Ford uses in the 3.5 Ecoboost are not news amongst the 3.5 performance engine builders. Nice crankshaft, according to them. (especially the new Gen3 version). Decent "enough" rods for OEM. But the pistons are very very HO-hum cost-point pistons for a 100HP per liter mass produced motor. It was pointed out to me personally by one such builder several years ago and his opinion is that if you bought 6 of them, statistically you would probably get 6 good ones. But if you bought 600 of them, he could almost guarantee that the skirt would fail on one at some point.

I will never forget him saying that if Ford spent $3.00 more per piston, it would make all the difference. And then he compared the Raptor piston. To me it was basically the same. But he described while it was nothing to write home about, it was a better piston, skirt-wise, and might be $3.00 more cost to Ford.

Your post above sure seems to confirm exactly what he was talking about.

Unlike the days when engineers used slide rules... and usually achieved +10% over minimum spec... in these computer days if the spec is X% that's what you get...

 

[JExpedition07]

The cheap pistons that Ford uses in the 3.5 Ecoboost are not news amongst the 3.5 performance engine builders. Nice crankshaft, according to them. (especially the new Gen3 version). Decent "enough" rods for OEM. But the pistons are very very HO-hum cost-point pistons for a 100HP per liter mass produced motor. It was pointed out to me personally by one such builder several years ago and his opinion is that if you bought 6 of them, statistically you would probably get 6 good ones. But if you bought 600 of them, he could almost guarantee that the skirt would fail on one at some point.

I will never forget him saying that if Ford spent $3.00 more per piston, it would make all the difference. And then he compared the Raptor piston. To me it was basically the same. But he described while it was nothing to write home about, it was a better piston, skirt-wise, and might be $3.00 more cost to Ford.

Your post above sure seems to confirm exactly what he was talking about.

Ford has a bad habit of doing that. The 5.8 Trinity rods were crap in the older GT-500’s. Due to the shitty rods Ford had an 8-second “over-rev” feature that would keep you from revving over 6,000 rpm too long because the rods had “durability concerns”. Yes, they used to do that for the GT-500 rather than sourcing better rods. The old Terminators used Manley H-Beams, much better. Ford improved the rods big time with the 5.0 Coyote utilizing a fatter I-Beam, but they are still relatively cheap compared to a A/M forged H-beam. The crankshaft on the 5.0 is also great and 100% forged. The crankshaft and block are absolutely STOUT, The rods are “good” up from “meh” in previous modular motors. The pistons are also “good” but not superb. I don’t think any Ford motor is using forged steel pistons barring the 6.7 HO Powerstroke, and don’t get me started on the 6.7. It is weak in other areas when compared to the L5P.

 

[Snakebitten]

Even if Ford isn't going to fork over for forged pistons, there's a ton of upside in cast piston design that they are sourcing below.

 

[Gros Ventre]

Let's talk "School of the Boat" Attached are two diagrams of what is going on inside cylinders. These are the Pressure-volume and temperature-entropy diagrams of the theoretical Otto Cycle. Isentropic means no heat transfer into or out of the cylinder. Point 1 on the diagram is when the intake valve closes and the piston begins upward travel. The purpose of octane is to ensure the fuel-air charge does not ignite before the spark arrives. Point 2, theoretically is TDC (it isn't really-I'll explain) and represents the maximum pressure and minimum volume of the fuel air charge. Points 2 and 3 show the burning of the fuel air mixture as if it was instantaneous. Since it isn't really, then there is spark advance so that the fuel air mixture just completes combustion as the piston reaches TDC. Points 3 and 4 show the expansion of the burned fuel air mixture from TDC to BDC. Points 4 to 1 show the heat lost with the exhaust as the exhaust valve opens and the piston moves up to push the burned gasses out of the cylinder. The exhaust valve shuts and the intake valve opens and during the intake stroke and the new fuel air charge is pulled in (with a turbo it is pushed in). The line between points 1 and 2 then repeats the compression of the new charge. The key point here is that the point represented by Point 3 is a design feature of the engine and cannot be changed. It is a function of the compression ratio coupled with the turbo pressure and the energy contained by the gasoline being used. Octane does not mean more energy. The work out between points 3 & 4 is the max that the engine can extract from that fuel. So what's all this other stuff, variable cam timing, spark advance control, detonation detection and spark retard, etc? they all serve to extract energy that is already in the fuel by enabling the engine to utilize the energy that is present. So why does Ford recommend higher octane when doing things like towing a heavy load in the hot summertime? Well consider an engine that is off peak. How could a manufacturer ensure you don't have trouble towing that heavy load up the West Virginia I-64 7 mile long hill, in August? If you up the octane then you give more margin for that off peak situation. So let's look at the T-S Diagram. In that diagram point 3 is the peak temperature. This is a function of the chemistry of the gasoline. Octane does not contribute to this. Entropy is a representation of U (internal energy) plus PV (pressure times volume). Once again these are design points of the engine and based on the petroleum companies advertised values of gasoline chemitry that Ford uses in designing the engine.

 

[SALEEN961]

A spark timing After TDC represents a loss of available power.

Agreed, this is why using higher octane fuel that allows for spark advance closer to MBT values increases engine power and lower octane fuel that requires increased spark retard to avoid detonation and pre-ignition decreases power.

Not knowing where Ford set the zero reference means you don't know that a negative value means after TDC.

Do you have any evidence that proves or suggests that Ford is referencing a random point or using an offset instead of ignition TDC for all of their timing advance values? All of the values shown perfectly align with what would be reasonably expected when using industry standard practices and referencing ignition TDC. I have not seen anything in the service manual, calibration file, or scan tool data that would suggest the use of anything but ignition TDC as the reference point.

As to adjusting spark and boost like you described, that means they are adjusting toward that max point in the diagram. As they approach that max point... of course more power is available. The theory says that points 2 & 3 in the diagram occur at TDC. However, since the Qin cannot be instantaneous the spark is introduced before TDC to allow the burning to get to completion at TDC. If the completion of burning of fuel were to happen after TDC that would be lost energy.

Agreed, the issue is that in most conditions the 3.5EB cannot hit MBT spark timing or peak cylinder pressures with 87oct or 93oct for that matter. Under high load conditions, it is normal to see spark advance that is 10-20° less than MBT.

This is also true for the 5.0V8 where boost pressure isn't a factor, higher octane fuels allow for significantly increased spark advance, and there is usually a delta of roughly 8° between what the engine will tolerate on 93oct and what it will tolerate on 100ct.

You have to remember that the 3.5EB engine is designed to adjust for inferred octane up to a set point of 98oct. Anything less than 98oct pulls boost or timing using the factory calibration because these engines were always octane limited and do not fully utilize the fuel to hit the maximum cylinder pressures that could theoretically be achieved.

Regardless of what reference point Ford used for their spark advance values, the large delta between observed ignition timing, borderline ignition timing, and MBT ignition timing shows that spark advance is severely limited and far from the theoretical ideal in which the maximum amount of energy is extracted from the fuel and the maximum amount of power is produced.

For the 2021+ 3.5 EB engines, Ford even added additional knock sensors (4 in total) and switched to using individual cylinder knock control. In prior years they used 2 knock sensors and global knock control. These actions speak to the fact that these engines are octane limited and are expected to have lots of knock sensor activity when using pump gas.

 

[Gord0]

Let's talk "School of the Boat" Attached are two diagrams of what is going on inside cylinders. These are the Pressure-volume and temperature-entropy diagrams of the theoretical Otto Cycle. Isentropic means no heat transfer into or out of the cylinder. Point 1 on the diagram is when the intake valve closes and the piston begins upward travel. The purpose of octane is to ensure the fuel-air charge does not ignite before the spark arrives. Point 2, theoretically is TDC (it isn't really-I'll explain) and represents the maximum pressure and minimum volume of the fuel air charge. Points 2 and 3 show the burning of the fuel air mixture as if it was instantaneous. Since it isn't really, then there is spark advance so that the fuel air mixture just completes combustion as the piston reaches TDC. Points 3 and 4 show the expansion of the burned fuel air mixture from TDC to BDC. Points 4 to 1 show the heat lost with the exhaust as the exhaust valve opens and the piston moves up to push the burned gasses out of the cylinder. The exhaust valve shuts and the intake valve opens and during the intake stroke and the new fuel air charge is pulled in (with a turbo it is pushed in). The line between points 1 and 2 then repeats the compression of the new charge. The key point here is that the point represented by Point 3 is a design feature of the engine and cannot be changed. It is a function of the compression ratio coupled with the turbo pressure and the energy contained by the gasoline being used. Octane does not mean more energy. The work out between points 3 & 4 is the max that the engine can extract from that fuel. So what's all this other stuff, variable cam timing, spark advance control, detonation detection and spark retard, etc? they all serve to extract energy that is already in the fuel by enabling the engine to utilize the energy that is present. So why does Ford recommend higher octane when doing things like towing a heavy load in the hot summertime? Well consider an engine that is off peak. How could a manufacturer ensure you don't have trouble towing that heavy load up the West Virginia I-64 7 mile long hill, in August? If you up the octane then you give more margin for that off peak situation. So let's look at the T-S Diagram. In that diagram point 3 is the peak temperature. This is a function of the chemistry of the gasoline. Octane does not contribute to this. Entropy is a representation of U (internal energy) plus PV (pressure times volume). Once again these are design points of the engine and based on the petroleum companies advertised values of gasoline chemitry that Ford uses in designing the engine.

You are correct in many of your posts, but what you are assuming is that Ford set the programming/tuning of the Ecoboost to achieve Xhp with 87 octane under absolute ideal conditions, and then to never go higher than that. And also that increasing octane only gives you a higher safety factor, since under your theory it will never achieve more than Xhp. Your theory would be correct, but we can see in real time the programming adding timing to the base/mean advance. With all other variables being ideal, an OAR of 0 will give you your Xhp number stated by Ford. Increasing octane moves that number to the negative, which advances ignition timing. That gives more HP than the stated number from Ford.

 

[Samson16]

Remember the old days when we would rotate our distributor caps to advance/retard the timing? Now they have a fancy computer controlled thingy that does it for you. Put 93 in your tank and the timing will advance/delay automatically to give you more power for towing! It’s in the manual even ?

 

[Chili]

Remember the old days when we would rotate our distributor caps to advance/retard the timing? Now they have a fancy computer controlled thingy that does it for you. Put 93 in your tank and the timing will advance/delay automatically to give you more power for towing! It’s in the manual even ?

The boost gauge is analog and has other variables but it certainly seems like it maxes out more often now that my truck has adjusted to the 92 octane. Better acceleration too, even though it's warmer outside.

 

[Calson]

This thread reminds me of how religious scholars would argue endlessly as to how many angels would fit on the head of a pin. No idea what they finally agreed upon.

 

[Samson16]

This thread reminds me of how religious scholars would argue endlessly as to how many angels would fit on the head of a pin. No idea what they finally agreed upon.

As many as it takes

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