5.3 vs 6.2 octane

[intheburbs]

6.2L can & does run on 87. The motor dials things back to accommodate the loss of energy being supplied vs demand.

“IF” you’re running 87 towing, under load (hills), taxing the motor w/ in high temps or speed, your normal margin of safety gone.

My folks can’t fathom paying for premium fuel. It’s something that truly upsets them. They feel premium fuel is a rip off. Because of that, they only purchase vehicles made to run on 87 here in Cali.

I have another buddy that feels the same way but also felt that his wife’s Lexus RX300 was ok to run on 87 driving up to 6k mountain roads w/ heavy loads. When the alternator bearing went out the dealer offered them $10k+ toward a RX350 on trade. They laughed and said jay fix the alternator. The dealer let them know they prolly had a cracked piston. They did.

If you don’t want to pay for premium fuel, no biggie. Just don’t purchase a premium fuel (recommended) vehicle and run 87. The road you’ll go down may be expensive. It’s a roll of the dice.


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There's no "loss of energy" between 87 and 91/93 octane. Both grades of fuel have the same amount of energy per gallon. The only difference in how easily the fuel burns/detonates. 87 burns/detonates faster and at lower pressures. The 5.3 has a compression ratio of about 9.5:1. 87 octane runs fine in these engines. The L9H 6.2 in my Denali has a 10.5:1 compression ratio. Anything over 10:1 is generally considered to be high compression. This means the air/fuel mixture is subjected to higher pressures, and the ideal gas law states that as the volume goes down (it's compressed more), the pressure and temperature go up. A low-octane gas might get so hot under that higher compression that it ignites before the spark plug fires, and then the "wave front" of the explosion crashes into the piston as it approaches the top of its stroke. That's the knock/ping you hear, and as you described in the Lexus, that's what leads to cracked pistons.

There's no "safety margin" when you're talking about octane. If you're running low-octane gas in a high-compression engine, you're "hoping" the knock sensors and the computer can adjust the fuel mixture to eliminate/reduce the knock. But it's not instant, and the owner's manual says you may hear a "slight" knock. To me, that's unacceptable. I'm not going to trust those systems to prevent damage to the engine. Instead, I'll feed it the gas it was designed for. Running lower-than-recommended octane gas in a high-compression engine will cause damage. Period.

And up in the mountains, at 6,000 feet, the reduced oxygen levels in the air actually cause gas to behave like it has higher octane. That's why regular gas in Denver is 85 octane, and premium is 91. The thinner air effectively increases the octane of the gas by about two points, so 85 in Denver behaves like 87 at sea level.

 

[jeffbco]

This is straight out of the owner's manual for my 6.2 Sierra Denali (2009):



Yes, it says you can use 87. I don't. Why? Because the 6.2 is a "high-compression" engine that inherently needs higher-octane gas. And frankly, I don't trust the knock sensors and the computer to be able to adjust the timing well enough to fully control knock/ping. The manual even says that, you might hear a "slight" knock. Using 91+ octane gas is, IMHO, cheap insurance against possible engine/piston damage.

That, plus the fact that when I floor the throttle, I want all 400 ponies present and accounted for.

I'm actually tempted to take it to the other extreme and run E85 in it. If I'm willing to tolerate the abysmal mileage and limited range (10 MPG with a 26-gallon tank), E85 has an effective octane rating over 100.

I didn't realize the 6.2 was a flex-fuel engine.

 

[BG1988]

91 is best. It is due to the compression ration, cam timing, spark timing etc.
It will run fine on 87, but the knock sensor will be triggered quite a bit..
This then tells the computer to make reductions in timing and cam timing VVT.
So you will lose fuel MPG and HP for sure.
Def always run good premium if you are towing or have heavy loads

87 oct is fine if you in a pinch
enough to get you to the next station

this 6.0 is running 10.8 compression

compression can increase " if the cylinder is dirty"

 

[PG01]

87 oct is fine if you in a pinch
enough to get you to the next station

this 6.0 is running 10.8 compression

compression can increase " if the cylinder is dirty"

Shhhhhhhhhhh

 

[intheburbs]

I didn't realize the 6.2 was a flex-fuel engine.

I know the L9H is Flex Fuel. Not sure about other iterations.

 

[h2100]

There's no "loss of energy" between 87 and 91/93 octane. Both grades of fuel have the same amount of energy per gallon. The only difference in how easily the fuel burns/detonates. 87 burns/detonates faster and at lower pressures. The 5.3 has a compression ratio of about 9.5:1. 87 octane runs fine in these engines. The L9H 6.2 in my Denali has a 10.5:1 compression ratio. Anything over 10:1 is generally considered to be high compression. This means the air/fuel mixture is subjected to higher pressures, and the ideal gas law states that as the volume goes down (it's compressed more), the pressure and temperature go up. A low-octane gas might get so hot under that higher compression that it ignites before the spark plug fires, and then the "wave front" of the explosion crashes into the piston as it approaches the top of its stroke. That's the knock/ping you hear, and as you described in the Lexus, that's what leads to cracked pistons.

There's no "safety margin" when you're talking about octane. If you're running low-octane gas in a high-compression engine, you're "hoping" the knock sensors and the computer can adjust the fuel mixture to eliminate/reduce the knock. But it's not instant, and the owner's manual says you may hear a "slight" knock. To me, that's unacceptable. I'm not going to trust those systems to prevent damage to the engine. Instead, I'll feed it the gas it was designed for. Running lower-than-recommended octane gas in a high-compression engine will cause damage. Period.

And up in the mountains, at 6,000 feet, the reduced oxygen levels in the air actually cause gas to behave like it has higher octane. That's why regular gas in Denver is 85 octane, and premium is 91. The thinner air effectively increases the octane of the gas by about two points, so 85 in Denver behaves like 87 at sea level.

excellent info, thanks!

 

[Oldewing]

87 round town, 91 when I am dragging the wagon....

 

[mikeyss]

I'm sorry, but if you're going to buy a premium vehicle that has an engine that requires premium fuel to keep it from pinging or whatever... Than don't be a cheap ass and use the wrong fuel for it because you can't afford it. Don't buy a vehicle that you can't afford to operate properly. I LOVE escalades, but I didn't buy one because I know I can't afford some of the higher end stuff that would eventually break, like the MRC suspension. It really gets me that people buy a 6.2 that is meant to be a upgrade compared to a garden variety 5.3, and then treat it like it's a 5.3 and claim it's ok. High compression engines require better fuel, no matter if you're towing or not. Why buy a premium engine/vehicle if you're gonna just trash it by using inferior shit to keep running it? If you want to use 87 octane in a 6.2, knowing it's gonna make WAY less power because it's adjusting for the shit fuel.... Then why not just buy a 5.3?

 

[IROCQUE]

My 2018 Tahoe with the 5.3 runs good with mid-grade 91 octane, however I did run 93-94 Octane when I tow my travel trailer and it seems to run much better. Now that I have the 6.2 in my 2020 RST, I plan to stick with 92 octane as best as possible especially when I pull the travel trailer.

 

[h2100]

I'm sorry, but if you're going to buy a premium vehicle that has an engine that requires premium fuel to keep it from pinging or whatever... Than don't be a cheap ass and use the wrong fuel for it because you can't afford it. Don't buy a vehicle that you can't afford to operate properly. I LOVE escalades, but I didn't buy one because I know I can't afford some of the higher end stuff that would eventually break, like the MRC suspension. It really gets me that people buy a 6.2 that is meant to be a upgrade compared to a garden variety 5.3, and then treat it like it's a 5.3 and claim it's ok. High compression engines require better fuel, no matter if you're towing or not. Why buy a premium engine/vehicle if you're gonna just trash it by using inferior shit to keep running it? If you want to use 87 octane in a 6.2, knowing it's gonna make WAY less power because it's adjusting for the shit fuel.... Then why not just buy a 5.3?

thanks, but i'll stick to the data driven analysis

 

[wjburken]

I'm sorry, but if you're going to buy a premium vehicle that has an engine that requires premium fuel to keep it from pinging or whatever... Than don't be a cheap ass and use the wrong fuel for it because you can't afford it. Don't buy a vehicle that you can't afford to operate properly. I LOVE escalades, but I didn't buy one because I know I can't afford some of the higher end stuff that would eventually break, like the MRC suspension. It really gets me that people buy a 6.2 that is meant to be a upgrade compared to a garden variety 5.3, and then treat it like it's a 5.3 and claim it's ok. High compression engines require better fuel, no matter if you're towing or not. Why buy a premium engine/vehicle if you're gonna just trash it by using inferior shit to keep running it? If you want to use 87 octane in a 6.2, knowing it's gonna make WAY less power because it's adjusting for the shit fuel.... Then why not just buy a 5.3?

I agree 100%.
When I made the decision to buy a “premium” suv like my Denali’s, I accepted the fact that they may cost a little more to operate and maintain. That being said, to each their own.

 

[gooffeyguy]

Not all spark knock is audible, and usually already doing some kind of damage by the time it's heard.

Knock sensors are good to have, but they're just basically "listening" for knock then making the ecu retard the timing when it hears it, hopefully in time and not doing any major piston damage.

Best to use the fuel the manufacturer recommends.

Premium 93 octane is currently $1.88 around here. I always use either premium or E85 in my 6.2l

2001 Yukon SLT
2012 Yukon Denali XL
2011 Yukon Denali RIP 5/20/18

 

[donjetman]

I bought a 12 yr old Denali for less than $10k. I wish it had a 4.8L in it for better gas mileage. I have very little need for its 6.2L and hp. I bought it for the AWD, amenities, and its awesome quiet smooth ride. I run mid grade 89 octane in it and get 16.5 mpg. I did run 87 octane until I got a DIC knock sensor message once on one of our road trips. Maybe I'll run premium for the next yr and see what the data says.

 

[donjetman]

This 4.8 will do
https://www.enginelabs.com/news/a-look-at-the-worlds-quickest-stock-bottom-end-4-8-liter-ls-engine/

 

[gooffeyguy]

This 4.8 will do
https://www.enginelabs.com/news/a-look-at-the-worlds-quickest-stock-bottom-end-4-8-liter-ls-engine/

That reminds me of my buddy Jon Huber and his '79 mustang 2.9l (2.3 stroked to 178ci running a single PT-74 turbo) 4 cylinder turbo'd motor about 10-15 years ago running 9's (and eventually 8's) and kicking all the v8's asses!

 

[petethepug]

There's no "loss of energy" between 87 and 91/93 octane.

Wow I like this guy. I've just been acknowledged for using armchair talk and I like it. I did not know they had 85 in Colorado.

 

[donjetman]

It has a lot to do with atmospheric pressure. At sea level the pressure is about 30 inches of mercury or 15 psi. At 10,000 ft, Leadville, CO, its 20 inches of mercury or 10 psi. Lost about 1/3 of the pressure that's being forced into a normally asperated engine, so we lose about 1/3 of our horse power. So my 380hp 6.2L only makes about 250hp at Leadville. Cylinder pressure is reduced as altitude increases, so detonation, pre-ignition, knock is reduced, so the need for higher octane is reduced.

 

[avalonandl]

Seen lots of Denalis with broken pistons from running low octane gas in them. The 5.3 in the later generations make a lot more power than yours, test drive both engines. I run high octane fuel in everything, even my lawn mower and generator, more power more power!!!!

Swath: There is a Racetrac nearby selling E85 for 1.45 vs. 1.95 for premium at costco -( 2.25 at Racetrac)

Am I better off trying a tank of E85?

 

[swathdiver]

Swath: There is a Racetrac nearby selling E85 for 1.45 vs. 1.95 for premium at costco -( 2.25 at Racetrac)

Am I better off trying a tank of E85?

I would! We have E85 for $1.35, my wife apologized for letting the mileage drop and I said who cares, we can fill the whole truck up for less then forty bucks! That last tank delivered about 10.5 MPG and cost just thirteen cents a mile which is this truck's record low. E85 is good for the motor and injectors, cleans them up.

Didn't have enough fuel on board to run down south yesterday so we refilled a little early with E15 on top of the E85 and still averaged 16 MPG @ 75 MPH. That should bust the thirteen cent record nicely!

If you run a couple tanks of E85, it will get up to full strength and the computer will adapt to it, even the transmission.

 

[Leardriver]

I like Intheburbs answer.

I ran a dyno in Denver at high altitude. Several absolutes were clear.

In a normally aspirated engine, meaning no turbo or supercharging...
Your octane requirements go down about one point for every 1,000 feet above sea level. If you need 91 at SL, 85 runs the same in Denver.
Run the lowest octane fuel that your engine doesn't detonate on if energy and wallet efficiency matters to you. High octane fuel isn't better fuel, it could be argued just the opposite. It has more additives to resist detonation, and it burns slower. Lower octane burns faster and creates more energy IF the engine can peacefully exist, which at high altitude is virtually always. The argument that 85 octane makes more power than 93 is true-ish, but that's an oversimplification. The tradeoff for high octane burning slower than low octane is a tiny bit less explosive power, but you would never feel it.

We had a dirt bike dyno, and would measure bikes with great smelling 104 octane race gas in the CRF450R, and they would make 2 hp less than 91 octane pump gas. I raced MX for 25 years, and always said that if you want to go faster, twist the throttle. Those bikes make more power than mortals can use.

If you are towing a heavy trailer up the mountains, more octane doesn't hurt for that tank as an extra dissipator of heat. The higher altitude of the mountains, and warmer summer air actually reduces your octane requirements even more, but when you are on the gas non stop on long hills, more octane gives you a cheap source of additional margin.