Good reason to run a 6.2 on premium fuel

[Snowbound]

Kind of a fun business, at least until you have broken your nth engine down.

Also an interesting thread. After reading it and thinking about it, I checked the compression ratio of my LMG 5.3L and in my model year it is pretty darn close to 10:1. Might be 10:1 with as many miles as it has on it, who knows? I always assumed it likely was more like 9:1 or less, given the regular gas endorsement by GM

I have always run 87 Octane (regular) in it, but looking at the close to 10:1 compression ratio it really gives me pause. I was out and about running errands this morning, so I put my cheapo ODB scanner in live mode and watched the timing advance to see what GM does with it with regular gas stated as GTG. For the most part, under load with the throttle on it hard, the timing never got much above 25 degrees advanced, which was comforting. It would pop way up when I got off the throttle to well into the 50+ degree range, but dropped right back down when the throttle was opened again.

It would be really interesting to know how intelligent the timing curve is managed, based on things like knock sensor input, etc. If it is anything like the fuel pressure management in this first model year 2007, I might be disappointed with the answer.

This sounds totally wrong. When getting off the throttle your injector pulse width should drop to .5 milliseconds and timing should drop to - advance. This is called DFCO (deceleration fuel cut off) and I can’t see why you would be seeing high advancements on decel. I wonder is someone got in there and played with the tune and didn’t know what they were doing with DFCO.

 

[mikeyss]

Isn't this what happened to @mikeyss? I know a valve seat dropped, but maybe it was because the head cracked between them.

Yeah, the valve seat was broken in half, and half of it went to the piston, the other half got stuck in the intake valve. Mine never got hot, but I wonder if the square port heads have hot spots since there is no rear steam ports. I installed the 4 corner steam kit on my new motor. I always use 91 octane or E85

 

[iamdub]

Yeah, the valve seat was broken in half, and half of it went to the piston, the other half got stuck in the intake valve. Mine never got hot, but I wonder if the square port heads have hot spots since there is no rear steam ports. I installed the 4 corner steam kit on my new motor. I always use 91 octane or E85

Was it one of the rear cylinders?

 

[wsteele]

This sounds totally wrong. When getting off the throttle your injector pulse width should drop to .5 milliseconds and timing should drop to - advance. This is called DFCO (deceleration fuel cut off) and I can’t see why you would be seeing high advancements on decel. I wonder is someone got in there and played with the tune and didn’t know what they were doing with DFCO.

No, stock tune. DFCO is not on (according to my Tech 2). I assumed it was just the anti unburned fuel committee at work, but that is how it is working. Maybe I can figure a way to record the advance reaction on my cheapo ODB scanner and post it.

 

[mikeyss]

Was it one of the rear cylinders?

Passenger side, 3rd one back

 

[wsteele]

This sounds totally wrong. When getting off the throttle your injector pulse width should drop to .5 milliseconds and timing should drop to - advance. This is called DFCO (deceleration fuel cut off) and I can’t see why you would be seeing high advancements on decel. I wonder is someone got in there and played with the tune and didn’t know what they were doing with DFCO.

All I could figure out how to do with my cheapo ODB app on my phone was do a screen capture of the traces. The TPS trace is green and the advance red. When I pull off the throttle, the advance jumps before settling back down. I have no idea why. I will try and get this stuff on the Tech 2 where I can capture DFCO status, etc. But driving and recording the Tech 2 screen gets kind of busy.

 

[wsteele]

I did get a chance to fiddle a little with the Tech 2 while driving and the advance response was predictably the same as I recorded with the ODB app. I forgot to call up the DFCO status, but when I looked at it in the past, I have never seen in enabled. Maybe I have to be coming off of higher RPMs to get it to enable?

One thing I found interesting was I selected some of the knock sensor items. Accelerating on flat ground, not even ******* it, but briskly, I saw as much as 20% knock retard kick in. I am going to call up the individual cylinder knock data next time out, but it looks like with 87 octane (regular), the engine is retarding timing to compensate for knock.

After I get some baseline knock information with 87 octane, I will switch to E85 and also try some 91 octane tanks and see what the knock retard numbers look like.

 

[Geotrash]

I did get a chance to fiddle a little with the Tech 2 while driving and the advance response was predictably the same as I recorded with the ODB app. I forgot to call up the DFCO status, but when I looked at it in the past, I have never seen in enabled. Maybe I have to be coming off of higher RPMs to get it to enable?

One thing I found interesting was I selected some of the knock sensor items. Accelerating on flat ground, not even ******* it, but briskly, I saw as much as 20% knock retard kick in. I am going to call up the individual cylinder knock data next time out, but it looks like with 87 octane (regular), the engine is retarding timing to compensate for knock.

After I get some baseline knock information with 87 octane, I will switch to E85 and also try some 91 octane tanks and see what the knock retard numbers look like.

Very cool. Thank you for the data points. Looking forward to hearing what you learn with premium.

 

[Snowbound]

Wow, that really has me scratching my head as to why timing would jump advance to 40° during decel. I can see why you’d be having KR on 87 octane though. Thanks for sharing, I’m gonna dig into a couple tunes tonight at work to see if mine does that too. Only thing that makes sense is you would increase timing to eliminate decel popping.

 

[wsteele]

Very cool. Thank you for the data points. Looking forward to hearing what you learn with premium.

James (@swathdiver) has been preaching the virtues of running higher octane fuels for a long time. I promise never to doubt him again.

I had to run down to the grocery store and the fish monger this afternoon (higher IATs, coolant temps and ambient than this morning), so setup the Tech 2 to monitor knock detection by cylinder. Even with very modest (in town city street driving), modest acceleration away from a traffic lights caused persistent knock in every cylinder. Only after leveling off the throttle and with the knock retard function operating, did the knock sensors settle down and the knock retard number follow slowly thereafter.

It is impossible for me to record with my phone the Tech 2 display while driving (not enough hands). I will try and do a few screen captures of my ODB app live data with the TPS, knock retard and timing reaction displayed.

The knocking seems so sensitive and persistent, I really wonder if 91 Octane or E85 will be able to eliminate it

I am kind of surprised these engines last like they do with the whole "ride the knock sensor timing curve management". Doesn't seem like much margin for error or component failure.