2018 GT MPG 87 vs. 93 octane

BlackandBlue

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Ding ding. Calling it "premium" is a silly marketing gimmick. Auto ignition is when the air/fuel mixture ignites without a spark, due to too much heat and compression. This is bad in ICE engines. If it occurs too early, it will try to push the piston down as it's traveling up, causing damage. Higher octane resists this. Higher compression means more power, so running higher octane enables the engine to have a higher compression ratio. However, the mixture needs to burn so that maximum cylinder pressure occurs at the optimum moment of the stroke. Higher octane fuel takes longer to burn, so it requires more advanced timing so the mixture ignites earlier. If it doesn't ignite early enough, then the mixture will burn after top dead center, wasting energy and losing power. So 93 isn't better than 87. Which octane is best is entirely dependent on the design of the engine because it's a balancing act. Although, anyone who says the engine was designed around 87 octane is kidding themselves. The engine isn't designed with a fuel's octane rating as its primary influence. It would be the compression ratio, with changes in timing made to accommodate different octane fuel. If the engine were designed around 87, then the jump to e85 wouldn't be so substantial. In fact, e85 providing such a nice power bump implies that 93 is as much of a compromise as 87 is. An example of an engine preferring 87 would be the pushrod 5.0l with its 9:1 CR. Running 93 would just result in wasted energy and unburnt fuel.

This leads me to the following. The 12:1 compression ratio is a theoretical number because the CR is dynamic. There are several factors that influence this but, for the sake of 87 vs 93 octane, I'll focus on the cam timing (hello VCT). The sooner the intake valve closes, the more cylinder pressure occurs and the more low end power is made. This hurts top end power because the engine also has to work harder to compress the mixture, and this is why vct offers so much flexibility (like running 87 octane). If the intake valve closes later then there's less cylinder pressure and reduced risk of 87 octane fuel auto igniting. Direct injection further helps because of the cooler charge and more controlled fuel delivery because it's independent of the valve timing. But you're giving up some good low to mid range power. Also, at high engine speeds, the amount of time the mixture is fully compressed is very short, so auto ignition is less likely to occur. All the above is why peak horsepower is not affected as much by running 87. Oh if only peak horsepower were everything...

I guess that's a bit off topic, but there was plenty of arguing about the power benefits so... ya. As for MPG, a richer mixture is cooler and resists auto ignition so if ecu richens the mixture for 87 then it would get worse gas mileage. Although, if I had to guess, it wouldn't be by very much when cruising around and would only show if driven hard on a regular basis. I, personally, wouldn't use mpg as my reason to pick 87 or 93.

That’s all great info.

The part about the piston being pressed down is slightly misleading though.

The part of higher compression detonation that causes damage is the combining flame fronts. The energy pushing piston to push down the other way is not what causes the problem.

This is why a lower compression engine can knock(ping) its whole life and a higher compression engine will start to eat itself up over a shorter amount of time.

A higher compression engine can have multiple flame fronts of detonation inside the cylinder. These flame fronts can combine into a point to create serious damage inside the the cylinder. It’s the multiplying effect that causes problem.





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jpogi

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The part of higher compression detonation that causes damage is the combining flame fronts. The energy pushing piston to push down the other way is not what causes the problem.

This is why a lower compression engine can knock(ping) its whole life and a higher compression engine will start to eat itself up over a shorter amount of time.

A higher compression engine can have multiple flame fronts of detonation inside the cylinder. These flame fronts can combine into a point to create serious damage inside the the cylinder. It’s the multiplying effect that causes problem.
I may have not have explained it like this, but this is the reason why I do 91-93 octane, Not because it’s “premium”. I hate it that it is expensive, partly due to the fact that people use it even if they really don’t need it.

I rather have those small pings to a minimum. At the same time, knock sensors won’t have to pull the timing and prevent the injectors from spraying more fuel than necessary, keeping the engine running in it’s most efficient combustion cycle operation.
 

Troutwrangler

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I never knew that there were so many intricities with the interaction of compression, timing, fuel, etc...
Thanks for all the info, I'm learning a lot.
Being a carpenter, I can't contribute much unless we start discussing how to build structures and shit.
 

BlackandBlue

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I may have not have explained it like this, but this is the reason why I do 91-93 octane, Not because it’s “premium”. I hate it that it is expensive, partly due to the fact that people use it even if they really don’t need it.

I rather have those small pings to a minimum. At the same time, knock sensors won’t have to pull the timing and prevent the injectors from spraying more fuel than necessary, keeping the engine running in it’s most efficient combustion cycle operation.
I wasn’t trying to nit pick just wanted to add in detonation is not going to hurt anything in lower compression engines.

I had a 1996 s10 2.8 V6 that pinged everyday all day. Pulled the motor apart at 260k from a bad thermostat overheat and the besides the warped head, everything was great. Higher compression is defiantly different though.

Read an interesting article years back when gas prices collapsed from $4 that found cheaper gas prices meant people buy more expensive fuel(plus/premium). Most people don’t understand the differences, but more expensive means better right?
 

13razorbackfan

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I never knew that there were so many intricities with the interaction of compression, timing, fuel, etc...
Thanks for all the info, I'm learning a lot.
Being a carpenter, I can't contribute much unless we start discussing how to build structures and shit.
I know the feeling! If you want to discuss how to grow and maintain a successful landscaping and grounds crew corporation then I'm your guys. This stuff? Way over my head which is why I ask so many questions
 

Dfeeds

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That’s all great info.

The part about the piston being pressed down is slightly misleading though.

The part of higher compression detonation that causes damage is the combining flame fronts. The energy pushing piston to push down the other way is not what causes the problem.

This is why a lower compression engine can knock(ping) its whole life and a higher compression engine will start to eat itself up over a shorter amount of time.

A higher compression engine can have multiple flame fronts of detonation inside the cylinder. These flame fronts can combine into a point to create serious damage inside the the cylinder. It’s the multiplying effect that causes problem.
You're not wrong, but I wouldn't say I was misleading. I wasn't specifically referencing detonation. What I'm talking about is when there's pre ignition. Detonation doesn't always follow (it can, and can also be the cause of pre ignition on the following compression stroke after detonation occured). The piston is now trying to compress this pre ignited force that wants to push it back down because it has no where else to go. It can't because there force of the other pistons on the crank will carry it along. This causes a lot of stress on the piston and connecting rod. However, like detonation, it depends on the severity.
 

thunderstrike

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My previous Ecoboost ran fine on 87, but with GT engine knocks when starting out. I have to use Premium. I buy Premium gas exclusively from Sam's Club or Costco as they are 25-30+ cents cheaper per gallon. Exception is when traveling, I have to pay higher on the road, but with 26+ MPG I'm good with the 5.0. To show my appreciation, I recently purchased a set of 5.0 valve stem caps.

I don't understand how other GTs are running fine on 87.
 

Troutwrangler

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My previous Ecoboost ran fine on 87, but with GT engine knocks when starting out. I have to use Premium. I buy Premium gas exclusively from Sam's Club or Costco as they are 25-30+ cents cheaper per gallon. Exception is when traveling, I have to pay higher on the road, but with 26+ MPG I'm good with the 5.0. To show my appreciation, I recently purchased a set of 5.0 valve stem caps.

I don't understand how other GTs are running fine on 87.
Mine ran terribly when I ran the tank of 87 through it. Even my wife riding as a passenger commented on how bad the Mustang was running.
 

The_Phantom

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From your Ford Mustang owner's manual, page 135:

"FUEL QUALITY: Your vehicle is designed to operate on regular unleaded gasoline with a minimum pump (R+M)/2 octane rating of 87.

For BEST overall vehicle and engine performance, fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel is most noticeable in hot weather as well as other conditions."

It's that simple. The brains who turned your car from a sketch to a final product say 87 runs just fine in it. If you want maximum performance, run 91 or higher.

There's no reason to even have threads on this as nothing anyone says about their personal experience or otherwise trumps the owner's manual. Should you have an engine failure and be running 87, it will be warrantied the same as if you ran 91 or higher.

Premium is not required in this car. I ran 87 in my '18 the whole time I've had it, and I've fed my '19 nothing but Shell 93+Nitro. No detectable difference in performance by seat of the pants, and anyone claiming otherwise is just suffering from the placebo affect.

I run 93 because I build cars for a living and I've seen the internals of engines run 100k+ on regular vs 100k+ on premium. There's no arguing the fact that premium gas and it's extra detergents cleans the engine internals much, much better than 87. But if you're running 87 Top Tier fuel, you are still coming out WAY ahead of those running the cheap stuff, believe me.

I don't race or track my car in any way. Like I said, I'm a bit different from most in that I pick my fuel grade based on how it cleans my engine internals. I'm a bit OCD, I admit. I just don't want anyone here to think that 87 isn't safe to run in these cars, because it is and Ford specifically designed the car to run on it. They believe it is safe, enough to put that recommendation into print which makes them legally liable. That's good enough for me.

Now, carry on. :beer:
 

Troutwrangler

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From your Ford Mustang owner's manual, page 135:

"FUEL QUALITY: Your vehicle is designed to operate on regular unleaded gasoline with a minimum pump (R+M)/2 octane rating of 87.

For BEST overall vehicle and engine performance, fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel is most noticeable in hot weather as well as other conditions."

It's that simple. The brains who turned your car from a sketch to a final product say 87 runs just fine in it. If you want maximum performance, run 91 or higher.

There's no reason to even have threads on this as nothing anyone says about their personal experience or otherwise trumps the owner's manual. Should you have an engine failure and be running 87, it will be warrantied the same as if you ran 91 or higher.

Premium is not required in this car. I ran 87 in my '18 the whole time I've had it, and I've fed my '19 nothing but Shell 93+Nitro. No detectable difference in performance by seat of the pants, and anyone claiming otherwise is just suffering from the placebo affect.

I run 93 because I build cars for a living and I've seen the internals of engines run 100k+ on regular vs 100k+ on premium. There's no arguing the fact that premium gas and it's extra detergents cleans the engine internals much, much better than 87. But if you're running 87 Top Tier fuel, you are still coming out WAY ahead of those running the cheap stuff, believe me.

I don't race or track my car in any way. Like I said, I'm a bit different from most in that I pick my fuel grade based on how it cleans my engine internals. I'm a bit OCD, I admit. I just don't want anyone here to think that 87 isn't safe to run in these cars, because it is and Ford specifically designed the car to run on it. They believe it is safe, enough to put that recommendation into print which makes them legally liable. That's good enough for me.

Now, carry on. :beer:
I appreciate the owners manual reference...
The poster that restarted this thread was curious about running 91 or 93 octane.
For those of us that don't build cars for a living, there is a lot of useful info on this thread that will never be printed in any owners manual.
 

13razorbackfan

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I appreciate the owners manual reference...
The poster that restarted this thread was curious about running 91 or 93 octane.
For those of us that don't build cars for a living, there is a lot of useful info on this thread that will never be printed in any owners manual.
I agree and it was the purpose I revived the thread. I simply wanted to know if there were any significant differences between the 91 and 93 and now I know there isn't I'm happy. I filled up 2 days ago with the 91 instead of 93 because of the answers given and I feel more confident now in my decision. Now I don't have to drive clear across town for the 93
 

Snake Plissken

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Ding ding. Calling it "premium" is a silly marketing gimmick. Auto ignition is when the air/fuel mixture ignites without a spark, due to too much heat and compression. This is bad in ICE engines. If it occurs too early, it will try to push the piston down as it's traveling up, causing damage. Higher octane resists this. Higher compression means more power, so running higher octane enables the engine to have a higher compression ratio. However, the mixture needs to burn so that maximum cylinder pressure occurs at the optimum moment of the stroke. Higher octane fuel takes longer to burn, so it requires more advanced timing so the mixture ignites earlier. If it doesn't ignite early enough, then the mixture will burn after top dead center, wasting energy and losing power. So 93 isn't better than 87. Which octane is best is entirely dependent on the design of the engine because it's a balancing act. Although, anyone who says the engine was designed around 87 octane is kidding themselves. The engine isn't designed with a fuel's octane rating as its primary influence. It would be the compression ratio, with changes in timing made to accommodate different octane fuel. If the engine were designed around 87, then the jump to e85 wouldn't be so substantial. In fact, e85 providing such a nice power bump implies that 93 is as much of a compromise as 87 is. An example of an engine preferring 87 would be the pushrod 5.0l with its 9:1 CR. Running 93 would just result in wasted energy and unburnt fuel.

No, no, no, no, and NO

This is wrong. Higher octane fuels do NOT take longer to burn. This is NOT why timing needs to be advanced for higher octane fuels. It's completely backward. Timing can be increased BECAUSE of higher octane fuel.

Why??

It's called mean piston speed. A gasoline/air fuel mixture can only burn so fast. Increasing gasoline's octane rating does not affect the burn rate one little bit. Higher octane fuels (as already discussed) resists pre-ignition/detonation, period. Here's the deal: As an engine is revved higher and higher in order to make more horsepower, piston speed increases. It increases on the up-stroke and the down-stroke. As piston speed increases, the initiation of spark in the combustion chamber needs to happen sooner so that the piston on its down-stroke does not outrun the flame front of the fuel/air mixture that has been ignited.

A higher octane fuel, especially in a high-compression engine, simply helps to prevent the fuel/air mixture from self-igniting prior to the optimum moment of spark that would otherwise be initiated by the ignition system as designated within the programming of the car's ECU.
 
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The_Phantom

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I appreciate the owners manual reference...
The poster that restarted this thread was curious about running 91 or 93 octane.
For those of us that don't build cars for a living, there is a lot of useful info on this thread that will never be printed in any owners manual.
There is. I just wanted to chime in with what Ford says is appropriate and also just express that there is more to consider when it comes to fuel than just octane rating (how it burns, cleans, etc.)

No disrespect to anyone here and if it came across that way, I apologize.
 

ALLSTOCK

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i'd never put 87 in my mustang.

keep that filthy swine juice for the lawn mowers.
 

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