Premium Fuel in 5.3L. More power or placebo effect?

MikeNH · September 23, 2015

Blackbear proved even on port injection engines that a lot is left on the table using 87 vs 91/93. So the feeling of additional power is quite possible.

gone_fishing · September 23, 2015

I have a ScanGauge in my Sierra 5.3 that can monitor knock retard (KR) and timing advance. I have been monitoring these two parameters for over a year now. From what I can tell from watching KR/timing, there does appear to be some sort of computer logic that is always trying to maximize timing advance. Almost like it's getting a "feel" for how far it can take it. Doing a WOT run with either 87/89 I may get a "blip" of KR but for the most part not much. It seems the WOT timing tables are set about right for 87 octane.

It's at partial throttle and/or cruising where KR likes to show it's ugly face. This is especially true when in V4 mode. Even with 91 octane, there is almost some KR when cruising in V4 but it's less then 89 and even less then 87. It comes and goes mainly with engine load (aka going up a hill/wind gust). I did some testing with 87 and V4 cruising at highway speeds I had to slow down quite a bit to keep in in V4. Basically the more load on the engine, the more difficult it is to stay in V4 mode because KR will pull back timing till its down to maybe 3-4 degrees advanced. Engine will then switch back to V8 mode. This seems to be the computer's way of knowing when to go back to V8 mode. If it can't keep at least 6-8 degrees of timing advance while in V4, back to V8 it goes. That appear to be the tipping point of when the computer decides V8 mode is more efficient. I think if you run 91 long enough, the computer seems to get used to it. Not saying KR is gone but its reduced, Switch back to 87 and it comes back. I think the computer might get used to having 87 again but I haven't ran 87 long enough to see. I usually switch back to 89 as it's kind of a happy medium. Not that much more expensive then 87 but usually a lot cheaper then 91. 89 doesn't eliminate KR but does seem to reduce it some.

What I find fascinating is as soon as I put E85 in the tank, KR is all but gone. Doesn't matter if it's WOT, partial throttle or V4 cruising I will pick up VERY little KR. I have seen upwards of 22-24 degrees of timing advance while cruising in V4 with E85 (and that's not going downhill or with a stiff wind....just normal engine load). This is why I think E85 fuel economy isn't as bad with this truck compared to other vehicles. The high octane of E85 allows for more use of V4 mode. I think this helps offset the usual decrease in fuel economy associated with ethanol. I can average 16-17 MPG tanks with E85 compared my 19-20 MPG tanks with gas. I don't think 3 MPG's is that bad of a reduction. If E85 is at least 20% less then gas it's about the same cost per mile for me. At 30% is actually cheaper to run E85. I can also tell there is more power during partial throttle or WOT.

Long story short it appears the 5.3 can take advantage of higher octane fuels. Whether the advantages are worth the price difference is up to the driver.

Northside · September 23, 2015

The 5.3 is just screaming "I need a tune!"

The 5.3 has 11:1 compression. That's pretty high. Spark has to be retarded a lot to run on 87 octane. That leaves a lot of power untapped. You'd need a tune to really get the benefit of the engines potential and the 93 octane fuel.

A flex fuel vehicle will tune itself for E-85. E-85 is over 100 octane in these parts. It's just what the doctor ordered for 11:1 compression and the power gains prove it! Believe it or not I got 20.8 m.p.g. last Saturday cruising the 2 lanes in the Colorado Rockies on E-85. Never saw 6th gear. A lot of M4 going up and down the mountain roads. I normally get 14 in town 18 on the highway.

jamin87ss · September 23, 2015

Only difference I've noticed is 91 non e, about 2.5 miles to the gallon better, stupid govt

Northside · September 23, 2015

Only difference I've noticed is 91 non e, about 2.5 miles to the gallon better, stupid govt

Two sides to that "Stupid Gov't" stuff. Nobody likes being told what to do, myself as much as anyone else but... The government forced the automaker to get better m.p.g. I remember 160 h.p 350s getting 10 m.p.g. Now we have 400 h.p. engines getting 20 m.p.g. The air is cleaner now and the new engines are amazing. I doubt we'd have this without the gov't sticking it's nose in where it wasn't wanted. Necessity it the mother of invention, and the automakers responded to the challenge.

robvas · September 23, 2015

Better computer controls really helped cars get better mpg and less emissions

TxTruckMan · September 23, 2015

Better computer controls really helped cars get better mpg and less emissions

Especially in small diesel engines...

Jackseric · September 23, 2015

Especially in small diesel engines...

I see what you did there...

mike17em · September 23, 2015

damn every one has different experiences for me I own a 2014 Silverado double cab v6 2wd and ran only 89 (chevron exclusively) for almost three months I filled up 5 or 6 times a month. Now I'm filling up 4 or 5 times (rarely) a month running only 91 from Sams club or Costco.... I commute 80 to 83 miles if I drive around for lunch that's 5 days a week that's not including my little detours around the city on the weekends. when I used 89 I would fill up on the morning of my 5th day but now with 91 I'm filling up 6th day at night and on rare occasions the 7th day... I think I'm getting 70 to 80 miles more since running 91 but I didn't notice it till probably my 4th or 5th fill up I've been using 91 now for the last 7 months now and yes my car did feel a little bit more responsive I just cruise and bump my music I'm not a heavy foot driver

TxTruckMan · September 23, 2015

Switch back to regular gas and see if it changes again.

Do you have the 6.2L?

tnchevy · September 24, 2015

I ran two tanks of 89 octane (I only fill up once a month) and saw no measurable difference in mpg than I do with 87 octane so I'm back to the cheaper stuff. One of my motorcycles in an air/oil cooled engine that requires 91 octane or higher. The experts on my bike say to run as low as an octane as you can that don't ping because it gives more power. I can also run lower octane in my bike in the winter (usually 87) and it does fine. I know it is not the same as a liquid cooled DI engine but something to think about.

Aubie_01 · February 11, 2016

Two sides to that "Stupid Gov't" stuff. Nobody likes being told what to do, myself as much as anyone else but... The government forced the automaker to get better m.p.g. I remember 160 h.p 350s getting 10 m.p.g. Now we have 400 h.p. engines getting 20 m.p.g. The air is cleaner now and the new engines are amazing. I doubt we'd have this without the gov't sticking it's nose in where it wasn't wanted. Necessity it the mother of invention, and the automakers responded to the challenge.

Any passed the costs along to you, the buyer.

HondaHawkGT · February 11, 2016

Even when stock, my truck ran like shit on 87. Stumbling idle, subpar fuel economy, pinging in the summer when I used my AC. My lawnmower can barely run on 87, why would I fill my truck up with the stuff?

Most modern engines are designed and tuned to maximize performance based on the fuel it's running on. It's just that on 87 that means mediocre performance.

Edited February 11, 2016 by HondaHawkGT

airman734 · February 11, 2016

I also used to run the "cheap" stuff in my truck and my truck hated it! It would stutter and "felt" like it was struggling to go when I needed it to go. Switching to 91 (the highest in Idaho without a specialty store) fixed the horrible stutter and go issues. I have a log of hand calculated mileage since mile 4 (when I bought my truck) and it did slightly improve when I switched. Based on how much better my truck felt like it was running I have never switched back.

I have now recently added a BB Tune and that solved all the other issues. Made my tuck awesome, and I even found out that Justin can make my truck stay in V8 below 50mph and in gears 1-4 so that if I am on a flat back road doing 55mph it can go into V4 mode and I can enjoy a little better MPG. I had no clue that had that kind of control in the tune. Saves me from having to deal with the constant switching around town and I still get the benefit on back roads!

I am with the OP in this one - may not have added "power" but definitely made my truck run better.

fondupot · February 11, 2016

Only difference I've noticed is 91 non e, about 2.5 miles to the gallon better, stupid govt

I tried some 91 non-ethanol for a few tanks and I didn't notice any difference in fuel mileage compared to 93 with 10% ethanol. The 93 with 10% ethanol is a lot cheaper too.