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I have a 2017 scat pack which recommends using 91 octane just wondering if there's really any difference between using regular and premium?
 

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Low octane can cause preignition and damage your engine. Works fine for lower compression engines but not for higher compression (assuming you have a 392).
 

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If you use 87 octane, your car's PCM will retard the timing to prevent pre-ignition and possible engine damage. You will lose power and your car will run like crap. If gas prices are an issue, you should have bought an SXT.
 

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I have a 2017 scat pack which recommends using 91 octane just wondering if there's really any difference between using regular and premium?
With 91 octane gasoline the engine controller can run more ignition advance which starts the combustion process at the more optimum time and thus obtain more mechanical energy from the chemical energy of the burning gasoline/air mixture.

This is important. Emissions are lower and fuel economy is better. If you switch to 87 while the initial cost is lower you will find the engine burns more 87 than it would under the same operating conditions as with 91. The difference will have you buying more 87 and often enough to wipe out the initial lower price of the 87 octane gasoline.

There's more bad news. With a lower grade of octane the engine controller will have to dial back the timing to avoid detonation. This lowers the pressure in the combustion chamber which eliminates detonation but raises exhaust gas temperature. The mixture starts burning later in the power stroke and stops burning later. Thus subjects all components exposed to the exhaust gas to higher operating temperatures. This includes the exhaust valves, O2 sensors, converters. All very expensive items to replace if they go bad.

You think because if you drive the car "easy" you can avoid detonation and the ignition timing retard? Nope.

Detonation is not a WOT only problem entirely. At part throttle at relatively low RPMs and in a higher gear -- which is with the automatic transmission equipped cars is how the transmission shift map works -- cylinder filling is quite good. As a result combustion pressure can be quite high. So driving around town easy with 87 octane gasoline and thinking one is doing no harm is just the opposite of what is going on.

High compression is a very good thing. The power it takes to compress the air/fuel mixture is almost all regained by the initial push back of this against the piston. If the spark can be triggered at the right time the mixture burns and develops pressure earlier in the power stroke which pushes down on the piston at the most optimum time. Running the correct octane grade allows the engine controller derive maximum benefit from the high compression.

Best I can in some way detail how this works is if you ride a bicycle you just know when is the best time to apply pressure on the pedal to get the most out of your leg muscles. If you delay applying pressure -- which is what happens with low octane gas and the engine controller retarding ignition -- you get a very good idea of how inefficient this delay in applying pressure really is.
 

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With 91 octane gasoline the engine controller can run more ignition advance which starts the combustion process at the more optimum time and thus obtain more mechanical energy from the chemical energy of the burning gasoline/air mixture.

This is important. Emissions are lower and fuel economy is better. If you switch to 87 while the initial cost is lower you will find the engine burns more 87 than it would under the same operating conditions as with 91. The difference will have you buying more 87 and often enough to wipe out the initial lower price of the 87 octane gasoline.

There's more bad news. With a lower grade of octane the engine controller will have to dial back the timing to avoid detonation. This lowers the pressure in the combustion chamber which eliminates detonation but raises exhaust gas temperature. The mixture starts burning later in the power stroke and stops burning later. Thus subjects all components exposed to the exhaust gas to higher operating temperatures. This includes the exhaust valves, O2 sensors, converters. All very expensive items to replace if they go bad.

You think because if you drive the car "easy" you can avoid detonation and the ignition timing retard? Nope.

Detonation is not a WOT only problem entirely. At part throttle at relatively low RPMs and in a higher gear -- which is with the automatic transmission equipped cars is how the transmission shift map works -- cylinder filling is quite good. As a result combustion pressure can be quite high. So driving around town easy with 87 octane gasoline and thinking one is doing no harm is just the opposite of what is going on.

High compression is a very good thing. The power it takes to compress the air/fuel mixture is almost all regained by the initial push back of this against the piston. If the spark can be triggered at the right time the mixture burns and develops pressure earlier in the power stroke which pushes down on the piston at the most optimum time. Running the correct octane grade allows the engine controller derive maximum benefit from the high compression.

Best I can in some way detail how this works is if you ride a bicycle you just know when is the best time to apply pressure on the pedal to get the most out of your leg muscles. If you delay applying pressure -- which is what happens with low octane gas and the engine controller retarding ignition -- you get a very good idea of how inefficient this delay in applying pressure really is.
Very thorough explanation.
 

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Discussion Starter #7
With 91 octane gasoline the engine controller can run more ignition advance which starts the combustion process at the more optimum time and thus obtain more mechanical energy from the chemical energy of the burning gasoline/air mixture.

This is important. Emissions are lower and fuel economy is better. If you switch to 87 while the initial cost is lower you will find the engine burns more 87 than it would under the same operating conditions as with 91. The difference will have you buying more 87 and often enough to wipe out the initial lower price of the 87 octane gasoline.

There's more bad news. With a lower grade of octane the engine controller will have to dial back the timing to avoid detonation. This lowers the pressure in the combustion chamber which eliminates detonation but raises exhaust gas temperature. The mixture starts burning later in the power stroke and stops burning later. Thus subjects all components exposed to the exhaust gas to higher operating temperatures. This includes the exhaust valves, O2 sensors, converters. All very expensive items to replace if they go bad.

You think because if you drive the car "easy" you can avoid detonation and the ignition timing retard? Nope.

Detonation is not a WOT only problem entirely. At part throttle at relatively low RPMs and in a higher gear -- which is with the automatic transmission equipped cars is how the transmission shift map works -- cylinder filling is quite good. As a result combustion pressure can be quite high. So driving around town easy with 87 octane gasoline and thinking one is doing no harm is just the opposite of what is going on.

High compression is a very good thing. The power it takes to compress the air/fuel mixture is almost all regained by the initial push back of this against the piston. If the spark can be triggered at the right time the mixture burns and develops pressure earlier in the power stroke which pushes down on the piston at the most optimum time. Running the correct octane grade allows the engine controller derive maximum benefit from the high compression.

Best I can in some way detail how this works is if you ride a bicycle you just know when is the best time to apply pressure on the pedal to get the most out of your leg muscles. If you delay applying pressure -- which is what happens with low octane gas and the engine controller retarding ignition -- you get a very good idea of how inefficient this delay in applying pressure really is.
Very thorough explanation.
Thanks for the advice I'm sticking to premium.
 

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Discussion Starter #8
Low octane can cause preignition and damage your engine. Works fine for lower compression engines but not for higher compression (assuming you have a 392).
If you use 87 octane, your car's PCM will retard the timing to prevent pre-ignition and possible engine damage. You will lose power and your car will run like crap. If gas prices are an issue, you should have bought an SXT.
Thanks for the advice I am going to continue using premium.
 

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My Canam 1000 takes 93 and the single hose gas pumps ripping me off with the 1st gallon or so of 93 being mixed with 87, it pisses me off. My tank holds 10 gallons but I seldom put more than 6 to 7 gallons in it so I'm being ripped off with a mixture of 87 when I'm paying for 93 octane.
Just something to keep in mind so don't just top off your tank with 5 gallons of fuel.
My Canam is an off road vehicle so i have to fill it before heading into the woods so i have no choice of running my tank empty.
 

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My Canam 1000 takes 93 and the single hose gas pumps ripping me off with the 1st gallon or so of 93 being mixed with 87, it pisses me off. My tank holds 10 gallons but I seldom put more than 6 to 7 gallons in it so I'm being ripped off with a mixture of 87 when I'm paying for 93 octane.
Just something to keep in mind so don't just top off your tank with 5 gallons of fuel.
My Canam is an off road vehicle so i have to fill it before heading into the woods so i have no choice of running my tank empty.

Many stations have a nozzle for each grade of gasoline. Maybe you need to find one and buy your gas from one?
 

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Thanks for the advice I'm sticking to premium.

Forgot that while I wanted to make it clear why premium is important to use the flip side is it is only important, can only provide the intended benefit, provided the engine is designed for premium.


I didn't stress this in my previous post. Let me do so now.



There is little benefit in running 91 in an engine designed to burn 87 or 89.



With a couple of possible exceptions.


One exception could be if you track the car, a higher octane provides a bit extra margin against detonation so the engine controller won't have to retard ignition. Engines as they are used can have their octane requirement increase a point or two and this and what the engine experiences on the track could require a higher octane.



Another exception might be if you were heavily loading the car with passengers and their luggage or towing a trailer. I have no observations though to back this up but just knowing the engine is going to be working harder and could encounter operating conditions in which the engine cylinder filling exceeds what it has ever before the next higher grade of octane could be called for. In this case the higher octane gasoline provides a bit extra margin.


But except for the above exceptions if the engine is designed to run with 87 or 89 then using a premium grade of gasoline is ill advised. It wastes money mainly.


Be sure you buy gasoline from a busy station. Premium gasoline, especially, goes stale and quicker than the lower octanes of gasoline and if you fill up you want the freshest gasoline for your money.
 

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Best way to see if going to higher octane benefits your vehicle, get a diablo sport trinity and log knock. If you see almost no improvement in ST knock from 91 to 93 then just run 91. On a 6.4 or a (5.7 with a manual trans) stay at or above 91.
 

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I have a 2017 scat pack which recommends using 91 octane just wondering if there's really any difference between using regular and premium?

Your 6.4 Hemi has a 10.9 : 1 compression ratio. Stay with the highest octane you can find. 91 minimum. Maybe your ECM will compensate for 89 but I would not count on that to protect a $10,000 engine.
 

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Best way to see if going to higher octane benefits your vehicle, get a diablo sport trinity and log knock. If you see almost no improvement in ST knock from 91 to 93 then just run 91. On a 6.4 or a (5.7 with a manual trans) stay at or above 91.
I found a local source of 90 octane non ethanol gas. Since it was not a top tier supplier I was skeptical but wanted to experiment with it. I was hoping to use it for drag racing by adding TORCO to bring it to the octane needed by my tune. I set up a gauge in the trinity to display knock retard. While it's impossible to read the values in the gauge it does flash green when timing is retarded because of knock. Well, the thing lit up like a christmas tree around 3.5K RPM. The logs showed considerable knock retard. Even with the gas bumped with TORCO to 1 octane point above my target octane for racing it still produced much more KR than top tier 93 octane ethanol gasolines bumped similarly with TORCO.

My advice is get 91 octane (93 if its available) from a top tier supplier for your Scat Pack.
 

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My Canam 1000 takes 93 and the single hose gas pumps ripping me off with the 1st gallon or so of 93 being mixed with 87, it pisses me off. My tank holds 10 gallons but I seldom put more than 6 to 7 gallons in it so I'm being ripped off with a mixture of 87 when I'm paying for 93 octane.
Just something to keep in mind so don't just top off your tank with 5 gallons of fuel.
My Canam is an off road vehicle so i have to fill it before heading into the woods so i have no choice of running my tank empty.

Many stations have a nozzle for each grade of gasoline. Maybe you need to find one and buy your gas from one?
In Texas I have yet to come across a top tier gas station with seperate octane fuel hoses. Diesel is the only designated hose. What state are you in where there are still seperate octane fuel hoses? Also most stations here the super unleaded is 93 a few have 91 but most are 93 here. 87, 89, and 93.
 

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In Texas I have yet to come across a top tier gas station with seperate octane fuel hoses. Diesel is the only designated hose. What state are you in where there are still seperate octane fuel hoses? Also most stations here the super unleaded is 93 a few have 91 but most are 93 here. 87, 89, and 93.
Marathon is the only brand I have seen that has separate hoses for each grade.
 

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Many stations have a nozzle for each grade of gasoline. Maybe you need to find one and buy your gas from one?
the majority of stations in NC / SC (I'm near the state line) have one nozzle for the three grades and separate ones for diesel.

The stations that sell Non-Ethanol gasoline will have a separate nozzle, but those are ~ 90 octane for those.

I tend to buy 15 - 16 gallons at a fill up, so that 93 octane fill up only has ~ 1 - 1.5 quarts of whatever the previous buyer diluting what I'm buying.
 

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Here in PA, I don't think I've ever seen a pump that only has one hose. They all have separate hoses for each "level" of gas.

EDIT: The more that I think about that, I may be wrong. Maybe I have seen some with only one hose... I'll have to pay more attention going forward! :)

EDIT2: Now that I *really* think about, I think that *most* DO only have one hose nowadays (one for gas and one for diesel)! Man, it sucks getting old...
 

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Here in PA, I don't think I've ever seen a pump that only has one hose. They all have separate hoses for each "level" of gas.

EDIT: The more that I think about that, I may be wrong. Maybe I have seen some with only one hose... I'll have to pay more attention going forward! :)

EDIT2: Now that I *really* think about, I think that *most* DO only have one hose nowadays (one for gas and one for diesel)! Man, it sucks getting old...
Around here it is common practice to have one hose for regular/mid/premium and one for diesel. Athought there is now a separate hose for elevated levels of ethanol fuel.

 

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the majority of stations in NC / SC (I'm near the state line) have one nozzle for the three grades and separate ones for diesel.

The stations that sell Non-Ethanol gasoline will have a separate nozzle, but those are ~ 90 octane for those.

I tend to buy 15 - 16 gallons at a fill up, so that 93 octane fill up only has ~ 1 - 1.5 quarts of whatever the previous buyer diluting what I'm buying.

Well, if there is just one nozzle for all 3 grades of gasoline not much you can do.


But I believe like you mention the amount of gasoline left in the hose that can dilute the high octane gasoline with the left over low octane gasoline from a previous pump use is small. Even if it was one gallon -- which I'm sure it is not that much -- 10 gallons of gasoline, 9 gallons of 91, and 1 gallon of 87, works out to 90.6 octane. Sure not "91" but hardly the end of the world.


Years ago when I rode a motorcycle I picked up the "trick" from others of draining the gas pump hose after the pump shut off. The amount of gas I got out of the hose was very small, not even close to a gallon -- I wished it had been a gallon for this would have represented a large fraction of the motorcycle gas tank's total capacity (less then 3 gallons IIRC).


It was such a small amount I soon stopped draining the hose of its gasoline. Just wasn't worth it.
 
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