Good reason to run a 6.2 on premium fuel
- Thread starter Geotrash
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iamdub
Full Access Member
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.
The LMG is advertised as having 9.9:1 SCR. But, GM's actual chamber volumes tend to be a few CCs larger than advertised. So, who knows?!
When my LMG and tune was completely stock, I'd get knock with hard throttle during hot weather on 87. It'd be noticeably less on 89 and non-existent on 91+ (93 is common here). There was a distinct improvement in power and MPG on 91+. I've only used top-tier fuels.
I thought that, back when a similar discussion arose, it was found in the user manual that 91 was recommended for the 6.2. Maybe I'm thinking of the GMT800 with the LQ9? It's "only" 10.1:1 CR, but a Gen3. Maybe it's because the timing control wasn't as strong and/or accurate for that generation. Or maybe I dreamed it all up.
I did a lot of towing in the first 60K miles of its life with an enclosed 5,000 lb trailer. It was all out west (lots of mountains) and while I have always been a "baby it" kind of guy with my Yukon, there were times when I was hauling up a steep grade where I was pretty close to or at WOT, passing a slow moving truck, etc. I never noticed any knocking and I think I am pretty sensitive to those kinds of things, but I can't say for sure. I think for the first 100K this truck only saw Chevron 87 (or whatever the "regular" grade was for that location), so obviously Top Tier, but nothing special I think.
As soon as I get the nerve up to do some runs with the analog fuel pressure gauge taped to my windscreen, I will see how it looks at WOT on a pretty steep grade. It has been hot as hell here lately and for the foreseeable future, so I likely will get a chance at an acid test.
swathdiver
Full Access Member
The 2007-2009 5.3 trio had flat top pistons and a 9.95:1 CR. The 2010-2014 5.3 duo had valve reliefs in their pistons and a 9.6:1 CR.
I used to be able to hear knocking and pinging in my Buicks but cannot hear a thing in the Yukon, can't hear much anyway anymore with tinnitus. So, when checking for such, I use my instruments, err Tech-2 and or Torque Pro app. Been drooling over Banks I-Dash with air density mouse...
I used to be able to hear knocking and pinging in my Buicks but cannot hear a thing in the Yukon, can't hear much anyway anymore with tinnitus. So, when checking for such, I use my instruments, err Tech-2 and or Torque Pro app. Been drooling over Banks I-Dash with air density mouse...
What's more common to break or go wrong with the L94 and LY6 is not the pistons (ring lands) but the cylinder heads.
The issue is how tight the intake and exhaust valves are on the rec port heads; they like to crack between the valve seats. It in turn make them junk or not cost effective to repair.
Bottomline though, if you can't afford to feed it the right fuel......you might want to rethink the purchase.
The issue is how tight the intake and exhaust valves are on the rec port heads; they like to crack between the valve seats. It in turn make them junk or not cost effective to repair.
Bottomline though, if you can't afford to feed it the right fuel......you might want to rethink the purchase.
swathdiver
Full Access Member
Didn't know this, thanks!
Just Fishing
Can't fix stupid
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.
Looking at a stock tune from my old 5.3, its not 100% clear since it's referencing voltages.
[ECM] 12970 - Knock Sensor Threshold Init: Initial learned threshold for knock detection. If knock sensor exceeds this learned value then the engine is considered knocking..
[ECM] 12971 - Knock Sensor Threshold Min: Minimum learned threshold for knock detection. If knock sensor exceeds this learned value then the engine is considered knocking..
[ECM] 2958 - Knock Sensor Transient Mode Airmass: Minimum airmass required for knock sensor transient operation.
Bunch of other tables that reference how sensitive the knock sensors are from there.
This is mostly gibberish to me.
FrankU
Full Access Member
Dropped valve seats are usually the result of overheating with square port heads.
Great thought, I will bring along the Tech 2. Lots to look at anyway.
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