2018 GT MPG 87 vs. 93 octane

[Qcman17]

The unstated assumption in all of these is "in a clean engine".

Assuming the engine is clean and unmodified, there's no economic benefit to using higher octane levels (you'll never get the cost differential back in improved mileage) and likely little to no performance benefit on public roads at legal or moderately illegal speeds.

Add a bit of hot carbon to the mix and things can go a bit off. Carbon based detonation is harder to control with knock sensors and under some conditions (lugging the engine down comes to mind) may not be able to be avoided even with added knock sensors.

So, post-2018 with the dual-fuel path keeping the valves clean, you're likely fine with good quality 87 (major manufacturers with more-than-minimum detergents). Using the cheapest 87 you can find from a station you never heard of before? There could be some issues.

Same thing if you've got an engine that's burning some oil. You could get carbon buildup in a cylinder, and the detergent levels in any gasoline may not help. In that case, a bit more headroom before detonation with a higher octane level could be advantageous.

I notice on the Motorcraft oil container that the oil is blended to combat LSPI. I know that is generally a problem more for turbo engines but I wonder if the octane of the fuel could help that potential condition as well? I think that is what you are basically saying too.

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[OldPhart]

Not necessarily true. The benefit of higher octane in an engine like the gen 3 is to utilize the higher compression which is too much for 87 octane, even with the controlled snd cooler charge of DI. The VCT system will retard the cam timing to effectively reduce the compression ratio and allow 87 octane to be used. This won't have a noticeable impact on peak HP but it will impact your low end torque. This is where most daily drivers live and where most of the seat of your pants feeling comes from. This is one of the reasons why e85 people feel such a noticeable difference.

Hi - An educational question you can help me with. When I read about some people who drag race their cars, they mention using E85 gas and getting an E85 tune. Is that a different E85 than ordinary flex fuel cars use. I’m guessing it’s a higher octane rating. As I mentioned in another post, we don’t have traditional E85 gas readily available in this area. Thanks... Bruce

 

[Andy13186]

Hi - An educational question you can help me with. When I read about some people who drag race their cars, they mention using E85 gas and getting an E85 tune. Is that a different E85 than ordinary flex fuel cars use. I’m guessing it’s a higher octane rating. As I mentioned in another post, we don’t have traditional E85 gas readily available in this area. Thanks... Bruce

Pump e85 is the same e85 flex fuel cars use and most people who run e85 tunes run that. Pump e85 is actually e54 in some states though so you have to be a bit more conservative with the tune most likely in those areas. Some people test their e85 or run a flex fuel sensor in their fuel line that says the ethanol content, I just dont worry about it though since I live in Florida and I think its almost always above e75 here. Ive also read you can use basically the same amount of timing with e54 as e85 from TheTuningSchool (youtube video they have on testing different fuels) so I am not sure if its actually a problem to use e54 on an e85 tune. Some people with more serious setups use VP racing C85 which is basically just guaranteed to always be 85% ethanol but costs like 5 times as much as pump e85, so thats mainly for drag cars.

 

[OldPhart]

Pump e85 is the same e85 flex fuel cars use and most people who run e85 tunes run that. Pump e85 is actually e54 in some states though so you have to be a bit more conservative with the tune most likely in those areas. Some people test their e85 or run a flex fuel sensor in their fuel line that says the ethanol content, I just dont worry about it though since I live in Florida and I think its almost always above e75 here. Ive also read you can use basically the same amount of timing with e54 as e85 from TheTuningSchool (youtube video they have on testing different fuels) so I am not sure if its actually a problem to use e54 on an e85 tune. Some people with more serious setups use VP racing C85 which is basically just guaranteed to always be 85% ethanol but costs like 5 times as much as pump e85, so thats mainly for drag cars.

Thanks. I’m not interested in using it, just curious... Bruce

 

[Bluemustang]

The E85 around my area is almost always 85% or close to that. I wouldn't recommend running E50 or "flex fuel" on a dedicated E85 tune. Lund recommends E70 or above is fine. Any less and I'd err personally with a flex fuel tune that can adjust accordingly.

 

[millhouse]

Why buy a $50K plus car and even consider low octane fuel??

Some of us care where our money goes. It’s the same reason I chose a truck that takes 87 octane over one that requires 93.

 

[WildHorse]

Premium. #winning

All tests in 6th gear. So if your on a dead flat road, yeah.. not much difference between 87 & 91.
Add a little bit of load, premium shines.
red = economy blue = horsepower

blue = 87 red = 91

 

[Ebm]

Tell you what, Ebm, I've been running high-performance engines since the mid-1960's, quite successfully. I recommend that you, Ebm, run your car on 87 octane gasoline. Everyone else should use 91 octane or better.

Good for you I guess? If you feel the need to justify yourself go ahead. But this isn't the 60's, 70's, 80's, or even 90's anymore. High performance engines can run off of regular, mid-grade, and premium fuel just fine. Technology has made this happen. The fact that there is a computer or sensor for every component of a car these days is the reason why it is possible. Yes, the timing is retarded on regular gas compared to premium fuel, but the car runs just fine.

Before you start picking apart my statements, no, I wouldn't run regular fuel in a full time race car making 1000 hp or a supercharged GT making 700 or 800 hp. But for a stock or lightly modified Ecoboost Mustang or a GT Mustang, heck yeah, nothing wrong at all running regular or premium fuel depending on stock tune or aftermarket tune. The 2018+ GTs haul tail on regular fuel with that 10 speed just like premium fuel.

People think just because the compression ratio of the 2018+ Mustang GT's is 12.0:1 that it automatically needs premium fuel. That's complete bull. You can't judge fuel grade off of 1 component here. Yes, the compression ratio plays a part in it, but that's only 1 slice of the pie.

Premium. #winning

All tests in 6th gear. So if your on a dead flat road, yeah.. not much difference between 87 & 91.
Add a little bit of load, premium shines.
red = economy blue = horsepower

That test is pointless in 6th gear. On a steep grade, most automatic transmissions would shift into 5th or less. The rear end gears will cause the car to bog down. Maybe 6th gear in the 10 speed but certainly not 6th gear in the automatic in the 15-17 Mustangs or the MT-82.

 

[Bikeman315]

Good for you I guess? If you feel the need to justify yourself go ahead. But this isn't the 60's, 70's, 80's, or even 90's anymore. High performance engines can run off of regular, mid-grade, and premium fuel just fine. Technology has made this happen. The fact that there is a computer or sensor for every component of a car these days is the reason why it is possible. Yes, the timing is retarded on regular gas compared to premium fuel, but the car runs just fine.

Before you start picking apart my statements, no, I wouldn't run regular fuel in a full time race car making 1000 hp or a supercharged GT making 700 or 800 hp. But for a stock or lightly modified Ecoboost Mustang or a GT Mustang, heck yeah, nothing wrong at all running regular or premium fuel depending on stock tune or aftermarket tune. The 2018+ GTs haul tail on regular fuel with that 10 speed just like premium fuel.

People think just because the compression ratio of the 2018+ Mustang GT's is 12.0:1 that it automatically needs premium fuel. That's complete bull. You can't judge fuel grade off of 1 component here. Yes, the compression ratio plays a part in it, but that's only 1 slice of the pie.

That test is pointless in 6th gear. On a steep grade, most automatic transmissions would shift into 5th or less. The rear end gears will cause the car to bog down. Maybe 6th gear in the 10 speed but certainly not 6th gear in the automatic in the 15-17 Mustangs or the MT-82.

Come on Guy, admit it. You’re just a cheapskate.

 

[OldPhart]

Hi - On the lighter side - The dealer said “It’s the no-lead gas. It doesn’t have enough octane to do the job.” I said “please, I hear that from my wife.” Or how about “You need some more lead in your pencil instead of the gas”. There are also some good “passing gas” jokes. A related modern old folks joke is “ My wife and I don’t tweet, but we’re pretty good at tooting.” It was enjoyable and educational at times, but I’m bowing out of this conversation / debate and moving on. I think it’s starting to run out of gas. Happy travels... Bruce

 

[Ebm]

Come on Guy, admit it. You’re just a cheapskate.

I've been known to use coupons before lol.

 

[bootlegger]

That test is pointless in 6th gear. On a steep grade, most automatic transmissions would shift into 5th or less. The rear end gears will cause the car to bog down. Maybe 6th gear in the 10 speed but certainly not 6th gear in the automatic in the 15-17 Mustangs or the MT-82.

I have never had to downshift from 6th to 5th in my GT on any grade, and I have been up some steep grades. At 60-70mph, 6th is just fine.

 

[Zooks527]

I have never had to downshift from 6th to 5th in my GT on any grade, and I have been up some steep grades. At 60-70mph, 6th is just fine.

I'll agree it's fine at a steady speed on any grade. In traffic with speeds yoyo-ing up and down, I've needed to downshift to be able to keep the space between myself and the guy in front of me from opening up too much.

 

[Ebm]

I have never had to downshift from 6th to 5th in my GT on any grade, and I have been up some steep grades. At 60-70mph, 6th is just fine.

What rear end gears do you have? 3.73's? I personally don't have any problems going up a mountain but I also have 4.10's. Someone with 3.31's won't be so lucky without going really fast. Especially since 6th is an overdrive gear.

 

[Hack]

Fun thread. I have put the highest octane available in my Mustangs occasionally and I really could never tell the difference. When it's hot outside (very rare for it to be over 90 around here) or on track I'm more likely to buy the high octane just for the heck of it. But when it's under 60 F (most of the year) and traction is limited anyway, and I'm street driving I feel wasteful putting higher octane in the car. It absolutely will not do anything at all for me.

I think if you are keeping your Mustang virtually forever and putting several hundred thousand miles on it you might realize a difference due to a top tier fuel or something. If you are in a really hot climate with lots of hills you might notice a difference with higher octane. But I've never kept a car that long and it's not that hot here very often. If I'm fortunate I will continue to be able to swap cars every few years.

And I'm not going to worry excessively about fuel octane.

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