I'm looking for some feedback on a couple of things: fuel brands and octane ratings. Here's why I ask:

I have the SCT Xcalibrator with the JLT CAI, and Bassani Exhaust (x-pipe and axle-backs). I have both the JLT tunes - one for premium and one for regular fuel.

In California, we only have 87, 89, and 91 octane (no 93). I have typically gone to the local Mobil station, and have filled up there for years. However, when I run the Premium tune with 91 octane Mobil, the car pings like crazy...and I have to take 3 degrees off the timing for it to go away.

I ran that tank out, and filled with Shell 91 Octane, and went back to the default Premium tune, and determined I can get away with taking only 1 degree out of the timing...and it feels like there is a lot more power.

I would think that the brand wouldn't matter that much...but apparently it does...or maybe Mobil is "watering down" there 91 with a little lower octane fuel??

For now, I'm sticking with Shell, and will try some others like 76.

Not sure what to think....anyone else have any ideas or comments on this?

 

All gasoline is blended to a specification and is rigorously tested to make sure it meets that spec. Also, just because you buy gas at a Mobil station doesn't mean it was manufactured at an ExxonMobil refinery. It could come from Chevron or BP or even Shell. The only differences between the Mobil 91 and Shell 91 are additives. The base gasoline should be identical. If there's any 'watering down" being done, it's being done by the station itself.

 

Very interesting, since many say gas is all the same. Obviously, since you noticed the difference in knock resistance, the 2 stations might be purchasing their gas from different refineries. It might also be related to a different blend for the warming weather. I honestly don't know how a station could dilute the blend, unless they were pumping some regular fuel into the premium tank.

 

Even if it was purchase from different refineries, the base gasoline is supposed to be equivalent. You could be right about the different blend for warm weather. One other possibility exists as well: if the Mobil station is privately owned, then they could be buying off-spec gas and passing it off as 91 (big no-no!!!).

 

Thanks for the feedback, folks...this helps in my analysis! I'll let you know if the other brands make a difference.

 

Latest update: Ran my tank full of Mobil 91 octane out last night, filled up with 76 91 Octane, then reflashed to the JLT tune minus 1 degree of timing (been running at minus 3 degrees of timing to get rid of the ping with the Mobil 91).

No ping, massive 2nd gear chirps, and even 3rd gear chirps - all day long! I've never been able to break the tires loose shifting to 3rd...but now it wants to!

More powerful than it ever has been!

The only other station near me is Chevron; I might give them a run for the next tank.

 

Interesting article on gas brands in Aug issue of Car and Driver, titled ""Your car is a temple, so put in the good stuff". Not about octane but additives the different brands have. BMW, GM, Honda, and Toyota got together to specify what makes a good fuel. These fuels are called "Top Tier" because they meet standards agreed to by eight companies that sell the fuel. Alot of good info in the article, but you can go to the link below to find out who the eight companies are that meet top tier fuel standards.


http://www.toptiergas.com/

 

I used to buy exxon gas but after the merger with mobile, the gas around here stinks. Both Exxon and Mobile gas give me worse mpg and performance. I now use Citco. BP also gave me less performance and hp, but not as bad as Exxon/Mobile. I wonder about all of the eco-friendly stuff in the gas too. Every summer, the new blend seems to reduce mpg by 10 percent or so. :notnice: Way to save fuel government nannystate! :bang:

 

Figure this was a good spot for a noob question. When you guys talk of hearing pinging . . . what causes it and what does it sound like? Are there dangers associated with the sound or does it only contribute to a loss of hp? Thanks!

 

It's caused by premature detonation of the fuel in the chamber. It sounds like a ping or a knock. Severe knock can mess up your engine.

As for the OP question - I believe your gas in Cali is all oxygenated, correct? Assuming they're making 10% ethanol blends out there, the problem might be the ethanol. Because of pipeline issues, ethanol has to be blended into the gasoline at the terminal. If the one station received gas blended at a terminal that had contaminated ethanol, the final octane of the blend will not be as high as advertised.

 

Thanks for the answer. I have heard what sounds like crackling when I am WOT. I only seem to hear it when I am next to a wall or barrier. Hopefully, this is something different as I have heard it called lifter noise? My ole 4-banger used to do it to.

 

The injectors make a ticking type of noise, you might hear near a wall at a drive through for example. You shouldn't hear any pinging unless the engine was under heavy load. It is definately a metallic rattling sound that goes away when you let off the gas. If you experience it, I'd step to a higher octane gas if possible. If not, some timing might need to be removed from your tune or fuel added. Hot weather only aggrevates the problem.

 

I've never heard my 05 GT ping.

It's my undestanding that the 05 GT has knock sensors that will automatically reduce timing to prevent pinging.

You might get a CEL though or may be not. I'm not sure what happens but I'm confident the event will be logged by the computer which can be extracted by a data logger (SCT2, Predator, etc ..)

 

Ever wonder about the pumps that have only one hose for all three octane ratings? What happens when the guy before you buys the 87? If the hose isn't evacuated, I bet it's at least a gallon or so worth of low grade fuel. Anybody know about this for sure?