Riddle me this...Why does my SXT like 91 Octane?

[Fiveoh]

Last month I had my TCM and PCM flashed on my AWD SXT. However, I never did find out the new PCM number and the dealer didn't put the number on the sticker. However, they assured me they did flash the PCM. So here is the issue.

From the day I received my flashes I've been using the required 89 octane fuel. However, after a few days the engine was pinging loud and often. At first I thought I had bad quality fuel and, when empty, used fuel (89 octane) from another company. I pulled fuse 11, and even unplugged the negative battery cable. The pinging still continued. I ran to empty again and refueled with another brand (still 89 octane) pulled the fuse and battery cable. Yet the pinging continued. After a total of three refills from three different companies I decided to try 91 Octane. Well...lo and behold...pinging stopped and the engine actually performs better.

This confused me so I decided to use 91 octane on my trip to Daytona Beach (about 3 hour drive) to see if the long drive would produce pings. Performance was great at high speeds and still no pings. On my return trip I filled up with 91 Octane from another company. Still no pings. Today I'm at half a tank with 91 octane drove to and from work (approximately 60 miles total). Pulled up to my driveway and listened to the engine long and hard...no pings.

This is odd...considering the require fuel for my vehicle is 89 octane, but the engine seems to prefer 91 octane. The only thing that was done to my Mag was the two flashes.

Question is...did the dealership flash change any timing parameters and/or caused my engine to now prefer 91 octane?

BTW, I don't have or use the Jet Chip.

[MooNDoGGie]

The flash must have advanced the timing. I have a Jet StageII in my sxt and it also will ping unless I run 91 octane. I get better MPG and performance from 91 octane also. Go figure

[ZMagnum]

I don't have a flash or Jet Chip. On 87 and 89 octane it runs great. On 91 octane it gets sluggish. I've never heard a ping. AFAIK the knock sensors are supposed to change the timing to prevent pings and knocks.

When I look at Timing Advance on the computer (OBDII CAN bus) the numbers are all over the place. I might be able to graph Timing Advance. That might help to decode the mess I see.

[FloridaRT]

How many miles on the car?

Maybe carbon build up in the heads?

Causing a higher compression ratio?

Or you just buy cheap gas................LOL

Bob

[FOHN_JARGO]

I've never noticed any pinging with mine but it certainly likes the 91 way better than the 89.

Could it be DCX recommends 89 only for EPA reasons?

[thelastgoodbrother]

I have a 2.7 w/ the stage II and I made the big mistake of putting 89 in...
My car did not like it at all...it sounded like the episode of the Little Rascals when they made the birthday cake w/ the birthday prizes in it.

[Fiveoh]

How many miles on the car?

Maybe carbon build up in the heads?

Causing a higher compression ratio?

Or you just buy cheap gas................LOL

Bob

Normally that would be my first thought, but I had used the Chrysler combustion chamber cleaner two months ago. My spark plugs are fairly new...4 or 5 months old so that isn't the cause. I have a free flow exhaust and don't think there is any exhaust leakage in the valves. The Mag doesn't show any codes, so I can only assume it is the octane level.

[FloridaRT]

Maybe the headgasket is protruding into the combustion chamber, causing a hot spot?

Have you rechecked the spark plugs?

Did you install stock spark plugs?

Bob

[juvezak]

I really dunno whats the point of recommending 89 Im using the sunoco's Ultra 94 since I got my car and I never had any issues.. except more gas consumption when my foot is heavy on the gas