87 octane vs 93 octane - ECOBOOST

[TheGoatman]

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.



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.



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.

Rest assured in hpt, 0 is tdc. Minus is after. Positive is before.

even on other platforms, people put on throttle body’s with crap port jobs that leak too much air during idle and the torque model goes hay wire. It goes into negative timing (usually see about -4 or -5) during idle to try to reduce the rpm back to target. You can feel the heat coming off the cats sitting there idling.

the idle gets a boomy drone sound at the tail pipe. It’s crazy but it’s true it’s atdc

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[Alfecupe]

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. I studied chemistry well at school and understand what I'm talking about. And my friend used https://essays.edubirdie.com/chemistry-help for his assignments. He got his good grades, but in general he was not very interested. And I spent many hours on textbooks. 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.

But octane still depends on gasoline's chemical composition, right? And probably Ford, with all their tuning, allows for extracting maximum power from it anyway. I don't think it's just about explosion protection.

 

[Gros Ventre]

But octane still depends on gasoline's chemical composition, right? And probably Ford, with all their tuning, allows for extracting maximum power from it anyway. I don't think it's just about explosion protection.

No, the energy content of a gallon of gasoline does not change with octane. The way octane works is it allows higher compression in the engine, as a matter of design. Octane only is a measure of resistance to auto-ignition, the rest is how the engine is able to use that. Peak compression is a design feature. So if you want to figure out how to make the engine use the energy more efficiently, which the computer control of things does, that works. But the energy content is a constant (there is a small variation depending on the reservoir the oil came out of).

 

[Snakebitten]

So 87 octane inhibits the 3.5 Ecoboost engine management to extract as much energy/power from the gallon of gas. (?)

Actually makes sense to me.

93 doesn't increase the amount of energy in the gallon, but does make more of it obtainable in the combination chamber. So more timing/boost, therefore more energy converted to torque. ?

 

[Cobraman428]

93 octane in my PB is like

of power when I'm towing...

 

[HammaMan]

Simple version is the computer is always trying to make more power. Timing is upped until knock sensors detect knock and it backs off. It's actually taking all variables into account including various temps, rpm, speed, etc... and uses it as a baseline map of sorts that automatically adjusts with temp. If you're out hot rodding on a cold night you're making more power than during the daytime when it's 35 degrees hotter by default. Going from a near empty tank of 93 to a full tank of 87 will impact 'the map' quite quickly as it's going to pull hard on excess knock. Going from 87 to 93 will take longer for the OAR to adjust as it incrementally brings the timing back in so long as knock isn't occurring.

I haven't paid enough attention to see if a new tank or substantial increase in the amount of fuel in the truck 'wakes up' the algo to try and be a little more aggressive as I don't continually monitor the variables. I do have a dedicated phone for it but just haven't found a mount that I'm happy with for this task plus I need to spend more time playing with the gauge layout, auto phone sleeping, etc. I've got a 93 performance tune and have blended to e25. It's cost me about 1.5 MPG going from 93e0 to 93e25 putting me about 96+ octane on my 'mountain trip' return (21.3 to 19.8, which ). I left the truck to its own devices and not locking out gears during the continual up and down valley run. I did notice it stayed in higher gears longer going into higher boost with lower RPM so I'm curious if gear selection itself might include watching knock as a variable to downshift and not just throttle position / RPM.

Given there was a PCM update and tune change since the last trip that could have been a variable (too many variables changed to really point at any of them specifically). At the same time it was also 40 degrees warmer so to see it holding off on downshifting was surprising. When I added the E85 into the tank upon pulling out the nozzle the spring loaded door was kind enough to throw a drop onto my cheek (the pump handle was continuously weeping fuel out of the nozzle after stopping as well as already being full when removed from the handle stowage on the pump itself). What was surprising was that I didn't smell even a hint of gasoline -- but it was textbook alcohol-evap cold. Not sure how this station mixes its e85 - ig: if it used the 87 + e98 to blend whatever "e85" fuel they're looking to serve based on demand. e85 is realistically supposed to be anywhere from 50% ethanol to 83%, but could present a situation where it could be even higher if they're using e98 and blending -- pure speculation on my part as if they are using e98 they should be advertising it / selling it separately. Given the price difference of the 2, maximizing the ethanol content would increase their profit by not having to use as much 87 octane. Given this station offers several grades and ethanol content in 1 pump housing with 2 handles, they're blending the "e85" to create e15.

Need to fill up a few tanks and play with it to see what's going on precisely. Didn't expect to wipe a drop of "e85" off only to not smell any gasoline at all. Typically even a tiny spec of gasoline can be smelled an hour later.
/tanrant

 

[Snakebitten]

I usually pump about 4 gallons of E85 to the 26 gallons of 93 e10.

Since I often refill with maybe 5 gallons of the previous blend still in the tank, I realize I'm increasing the e-content a little more than the actual recipe.
I did the math once when I went through the trouble of testing the E85 at the station I use. I think it was something like 67%? Or was it 65%. Don't remember exactly but I think I remember coming up with something just shy of E20 overall. Or maybe comfortably shy of E20.

E20 is my own line that I have drawn for the factory calibration being well able to manage safely and leverage the additional potential in the combustion chamber.

OAR always looks good.
The truck runs so well and knock sensors are happy.
I'm sure I lose some fuel economy, but it's insane to run around in this huge brick enjoying amazing torque at a whim, all while averaging better than 21mpg for the 25,000 mile Trip 2 record onboard. And I don't care if it's 1 mpg optimistic or pessimistic. It's outstanding either way.

Still waiting.

For any manufacturer to answer the Powerboost. Including Ford. It's crazy that there still isn't a direct alternative.

 

[JExpedition07]

Two words. Otto. Cycle. *Mic Drop*

 

[Gros Ventre]

Manual says for all motors 91 octane for best performance.

True, but remember, those sneaky Ford Lawyers edit the books and the books say 87 is just fine for all regions of performance. They're just trying to save your dealer from snake oil salesmen.

 

[Old Hat]

I like to stick with 93 in my Ecoboost, mostly for the higher mileage. Yes, I understand that if you do the math the higher MPG doesn't make up for the extra cost of premium. I'm ok with that. I figure if I'm using less fuel per XXX miles that equals less combustion byproducts, carbon, oil dilution, etc. being produced and building up in my engine. This along with the extra power makes it worth it to me.

 

[dochawk]

Remember the old days when we would rotate our distributor caps to advance/retard the timing?

bah.

kids.

In the old days, there was a spark advance lever to the left of the steering wheel hub!

And not setting it to fully retarded (at 12 O'Clock) could be fatal.

In fact, once such failure when Byron Carter stopped to help a stranded lady (who incorrectly thought that she had retarded the spark) led directly to the self-starter! When the engine backfires, it kicks the handle the other direction. Struck in the jaw, he died a couple of days later in the hospital. His friend Henry Leland, Cadillac's Founder and president, charged Charles Kettering and Cadillac's engineering department to create the self starter.

 

[HammaMan]

If you're not watching this guy, you're missing out on great info

 

[Samson16]

Full synthetic Motorcraft and Shell V power premium (because it’s convenient) . Thanks Lake and @HammaMan !

 

[FirstOnRaceDay]

Can’t afford Premium in Kalifornicstan so I run my jb4 with 87 and it’s still way better than stock other than the damn speed governor is still 106 MPH wish the folks that build the jb4 could eliminate that damn thing … When passing if your doing 75 and stab it its over 100 mph so quick yet it cuts out at 106 …try passing 3 big rigs in a row because they’re doing 45mph in a 55 for them .. it’s 65 for cars and no passing lanes .. happens everyday .. don’t care what cowards say .. Speed limits are meant to be broken .. they’re dumb and I don’t need a baby sitter .. BTW I set the cruse at 73 mph when the idiots are not in my way ..which is rare in this state full of the dumb .. I only need the speed gov gone for passing not to run over 100mph for fun .. I do that on the race track ..

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