Who runs 87 octane on their daily driver?

[lucas287]

I don't really have an interest in running 93 due to cost (~.60 cents/gallon more). I drive about 1,500 miles/month so it adds up. Being E85 converted I can run it whenever I want the power gains.

There's a lot of rhetoric out there about 87 octane. Some fact some fiction. Yes these Gen 5 LT engines have high compression but they are also direct injected which I believe helps a lot with keeping combustion chamber temps down. I've done quite a bit of data logging on my truck (L86/8L90) and I don't see dangerous amounts of KR. In fact, I don't see much at all. It makes me think I'm missing something,...

So how many of you run 87 on your Gen 5? In the KR trouble areas is there anything that can be done besides reducing timing? SOI? Cam retard - for EGR effect?

[TriPinTaZ]

When you run 87 the knock learn will gradually shift the timing table towards the Low Spark Table. Nothing wrong with this if you have no mods. You're not going to get away with trying to fix anything though. Low octane means it can only handle so much timing even if it is DI. If you're flex fuel, why not run 5 gallons of E with your 87?

[lucas287]

When you run 87 the knock learn will gradually shift the timing table towards the Low Spark Table

It only does this IF you're experiencing KR though, correct? Also, each time I write a new calibration to the ECM, does that reset Knock Learning back to the High Octane tables?

[lucas287]

If you're flex fuel, why not run 5 gallons of E with your 87?

I feel like something's off in the OE flex tables at E20-E40 because it doesn't feel as good as one or the other (E10 or E60+). Maybe that's the path, though - getting those low blends dialed in and it would be a great compromise between fuel efficiency and knock resistance.

[TriPinTaZ]

It only does this IF you're experiencing KR though, correct? Also, each time I write a new calibration to the ECM, does that reset Knock Learning back to the High Octane tables?

yes and yes

[lucas287]

yes and yes

Well then I don't know what's different with my truck, but I don't get much KR at all on 87. Not even in the summer. Mid-high load - yes - a few degrees. But just casually driving and highway - not much to speak of.

[Higgs Boson]

Well then I don't know what's different with my truck, but I don't get much KR at all on 87. Not even in the summer. Mid-high load - yes - a few degrees. But just casually driving and highway - not much to speak of.

you wouldn't get much KR if it has already learned down from running 87.

every GenV I have seen knocks on 93 and learns down some on that. you probably are getting very little timing compared to what you could be. it makes a difference, but not if you don't care.

[lucas287]

you wouldn't get much KR if it has already learned down from running 87.

every GenV I have seen knocks on 93 and learns down some on that. you probably are getting very little timing compared to what you could be. it makes a difference, but not if you don't care.

That's why I asked about the knock learn factor being reset when I write a new calibration. Since I'm in the early phase of tuning I'm making lots of little changes, almost daily, and following what TriPinTaz says - the knock learn is being zero'd/reset so defaulting back to the High Octane table. If that's the case, it doesn't seem right that I'm not getting much KR on 87. Follow me?

[TriPinTaZ]

That's why I asked about the knock learn factor being reset when I write a new calibration. Since I'm in the early phase of tuning I'm making lots of little changes, almost daily, and following what TriPinTaz says - the knock learn is being zero'd/reset so defaulting back to the High Octane table. If that's the case, it doesn't seem right that I'm not getting much KR on 87. Follow me?

Post your tune and a log. Could be your knock sensor settings. Could also be the truck engines have a different cam, cam timing and compression ratio which would reduce the overall cylinder pressure. Lower cylinder pressure would be less prone to knock on any fuel, 87 or 93.

[MikeOD]

I have a stock 2018 5.3 Silverado, it's never seen anything but 87 octane, a little over 40k miles on the odometer now.

[10_SS]

They have them setup to they knock most of the time with 87 in normal summer weather, just cruising down the highway with some load. Premo, makes it go away. This is with my 2017 5.3, 2019 5.3, and 2020 5.3. Cooler weather, not so much knock with 87.

And yes, it will continue adding the timing back as soon as it starts to see less knock, literally, as soon as the knock goes away, it starts putting full timing right back and decreasing the Knock Learn back to 0.00. Learning doesn't mean it's permanently adjusting anything, it's just like a LTFT but for retard (using more of the Low Octane Table), instead. And it happens pretty quick, starts putting timing back in seconds, not days, just to see more knock, then retarding timing and incrementing KNock Learn, then knock stops, and Knock Learn starts moving back to 0.00 pretty quick, this cycle repeats constantly until the cows come home, or knock stops.

So when you want no more knock and the most responsiveness from full timing, add 92 and that will occur in a matter of a few miles of driving, with no knock.

This log is with 87 octane, 2017 5.3, high 80's for air temps.

The log in this pic is about 10mins long, see how fast the knock Lean tries to come back to 0.00, within a few minutes if not having knock.

Knock Learn.JPG

[NotSure]

Who runs 87 octane on their daily driver

Not anymore since I am now down to 2 vehicles, and both are supercharged
But the stock L67 gets 30 mpg so.....there?s that.

[NotSure]

And yes, it will continue adding the timing back as soon as it starts to see less knock, literally, as soon as the knock goes away, it starts putting full timing right back and decreasing the Knock Learn back to 0.00. Learning doesn't mean it's permanently adjusting anything, it's just like a LTFT but for retard (using more of the Low Octane Table), instead. And it happens pretty quick, starts putting timing back in seconds, not days.....

This is very true. Ive even noticed on a short drive where it felt like one wot pull was stronger or weaker than another. Go back and look at the log and see that some random knock kicked up the Learn, lowering timing, but may be learned back down to zero during the next, stronger pull. I have since adjusted settings so occasional random knock won’t trigger knock learn, like anything less than 2 degrees.

[lucas287]

Haven't checked the forum in a while. Good stuff here!

I made some VVT changes at part-throttle to get rid of the EGR effect and that really shows the instability of 87 octane. I had knock EVERYWHERE. It learned it down pretty quickly to where I was actually getting single digit/negative values at high-load/low-rpm. 4 gallons of e85 (18.8% alcohol effectively) pretty much completely eliminated it.

[NotSure]

If you changed your VVT, you need to also make changes to the VCT spark adder table, under Spark/Advance/Base Corrections/VCT.

[lucas287]

If you changed your VVT, you need to also make changes to the VCT spark adder table, under Spark/Advance/Base Corrections/VCT.

Yep - did that. Zero'd it out under 2800 RPM also. That increase in cylinder pressure quickly showed the weakness of 87.