87 vs 91 octane, HP loss, MPG loss, knocking???
 

Okay so I only have about 2400 miles on my '19 GS that I just got at the end of August. No performance mods yet, bone stock, except for a Range AFM in the ODB II port to keep it in 8 Cyl. mode. For the first time I tried 87 octane to see what effect, if any, it had on the car. The salesperson at the dealer told me premium fuel was required, but I later read in the owner's manual that 87 will work. It said there may be some knocking and to use only premium if that occurs. It didn't. I figured it wouldn’t because it was the same with my '15 Camaro. There is a sensor in both models that tells the ECM to change the timing based on what fuel you put in it. The Camaro had an issue switching back and forth from 87 to 91 and losing performance and the forums talked about a fuse pull to reset the tables to 91. Hopefully the Vette doesn’t have the same problem. I'm aware that it may effect both mileage and horsepower if I put in the lower octane fuel, at least that's what the owner's manual says. And who wants lower HP? Certainly not me but I wanted to see if the butt dyno could feel it and if the mpg’s would go down.

Well so far I've only driven it about 50 miles, this morning, under both WOT and cruising at 65 using the instant mileage calculator gauge, and I'd have to say it's not that much of a difference. Said the guy who measures his car in tenths, LOL. I'm sure lap times would have shown a difference, but I experienced zero knocking or any kind of technical problems. It didn't really matter in daily driving. I was using the horsepower gauge in WOT though and it never went above about 380. That was up to 4th gear at close to redline. Unfortunately I'm not sure if I ever got it to 460 HP with premium fuel in it because I don’t remember and I hadn't looked at it in a while. I will say that where I usually drive is back country roads in the foothills of the Rockies and it's around 6600 feet high in elevation so that has to be effecting the HP. I read somewhere that your car loses about 3% of HP per 1000 ft above sea level, which means I would have already lost 91 hp where I drive. Here's the formula I found if anyone is interested: HP loss = Elevation x .03 x HP @ Sea Level / 1,000.

My instant fuel gauge was showing about 35 mpg cruising 65 mph in 8th gear on relatively flat roads at about 1500 rpm. I tried this several times for varying distances and this was the average number I kept getting. I know it's not a full tank over a long span but that's the scope of my research so far.

So the question is who else has experimented with different octanes and come up with their own results? And who out there has done some mods and seen some differences on the horsepower gauge, not just the dyno results? It'd be cool to hear about someone adding long tubes, an exhaust maybe with cat delete, a CAI and a tune and getting the gauge to the 460 mark really easily, like it wanted to keep right on going. I think it would be cool if you could use that gauge instead of having to dyno the car to find out if the mods effected the HP. Hopefully this topic hasn't already been exhausted, I tried to search and didn't find anything.

You are out of your mind running an 11.5 to 1 compression ratio engine at wide open throttle on 87 octane fuel.

Well I guess you can call me crazy for following the owner's manual then. Just went by what the manufacturer guidelines are.

"Recommended Fuel
(LT1 6.2L V8 Engine)
Do not use any fuel labeled E85 or
FlexFuel. Do not use gasoline with
ethanol levels greater than 15% by
volume.
Premium unleaded gasoline
meeting ASTM specification D4814
with a posted octane rating of 93 is
highly recommended for best
performance and fuel economy.
Unleaded gasoline with an octane
rated as low as 87 can be used.
Using unleaded gasoline rated
below 93 octane, however, will lead
to reduced acceleration and fuel
economy. If knocking occurs, use a
gasoline rated at 93 octane as soon
as possible, otherwise, the engine
could be damaged. If heavy
knocking is heard when using
gasoline with a 93 octane rating, the
engine needs service."

You wont hear it at wide open throttle.

Why would you use 87 when 93 is recommended for peak perfornance. Certainly the few pennies you would save makes no sense.

Why oh why??? That hurts just to consider doing that.

I think posting that portion of the owner's manual didn't help your argument. It seems to suggest all sorts of bad stuff could happen using 87 and states that fuel economy might suffer. I don't see any point to this "experiment" Prepare for an onslaught of criticism from folks a lot harsher than me.

Well my thought was if you daily drive your Vette, and you can’t really use anywhere near its full potential when driving in traffic around town, then why pay for premium fuel when you aren’t using that fuel’s potential either, as long as the manufacturer is saying it’s okay to use 87 octane?
If you reduce some of it’s potential by using 87, but you aren’t going to use it anyway and Chevrolet says it’s okay… I guess I don’t see what the issue is. Of course I’m not a mechanic and am just going based on the owner’s manual, I really don’t know the importance of octane to compression but I would hope Chevrolet Engineers would. Sure when I go to track it and when and if I do some performance mods, and possibly tune it, the tune will be for 91 octane and I wouldn't put 87 in it. Just seems kinda weird to me that it's not acceptable to do what the manual says is okay to do.

I'd stick to 91. Haven't seen 93 close...some in Denver I think. I wouldn't risk the chance by saving a few pennies.

Don't use 87 unless nothing else is available, and if you find yourself in this unfortunate circumstance, then keep the load on the engine as light as possible. This is how you need to treat an engine with such a high compression ratio.

Yeah it was more of a try it once with one tank and see if I could feel a difference in HP or see a difference in MPG. And both were very minimal so far. And with gas prices so low right now it doesn't really save much but I guess my curiosity just had to know what kind of a difference there was. If I thought there was a risk to do the engine damage and the manual hadn't said it was okay, I wouldn't have done it. I'm still curious to see what the HP gauge shows when I go back to premium, 91 around here.

Two big things need to be said here. Number one, you are at a higher elevation, so you can get away with lower octane up there because of the thinner air. So roughly speaking, that 87 octane at your elevation could be similar in performance to 89 or 90 octane at sea level. Another thing too, it's winter time so I'm sure it's a lot colder in your area too, and that's another thing that lowers your engine's need for octane.

So basically, running 87 octane at high elevation in the winter is probably roughly equal to running 91-93 at sea level in the summer.

Wow that's some good information Patman. Yeah it was 30 degrees at 6600 feet this morning. The winter tires worked great and the engine seemed good with the 87.

What physically happens in your engine when you use 87 and floor it is that it starts to knock, the knock sensors sense that, and the ECM almost instantaneously retards the spark to stop the knock. Some power and mileage is lost, but the Vette does so well in those regards that it probably doesn’t matter much to power or mileage unless you are a racer or are competing in an economy run. In my mind the real risk is that the knock sensors, while good, aren’t necessarily perfect. If they don’t retard the spark in near-instantaneous fashion, you are in big trouble. And even if they do retard as designed, you have still suffered mild pinging for an instant, and that can’t possibly do any good to the engine. Still worse, you keep going through that cycle over and over and over and over and over again, because the ECM tries to get back to design spark advance, so it keeps on going through those sensing cycles. Pings detected, retard, pings stop, timing creeps back toward design, pings again, retard, over and over and over again. It’s utterly inconceivable that does your engine any good. The only question is how much harm it does. If the system runs flawlessly, there probably isn‘t much harm. But how many things run flawlessly for how long? In my view, the system is there for a backstop in case you can’t get premium fuel now and then. But I don’t think it’s intended as a system to allow continuous running on 87. If that was the intent, it would not have the logic built into it to keep trying to get back to design spark advance, and you would not have to keep going through the cycles described above over and over and over again.

As to the altitude impact, yup, at that altitude, you need a couple of octane less. But the grades at the gas stations are also usually a couple of octane less at stations in high altitude areas. So while regular/mid/premium at low altitudes may be 87/89/93, at altitude, they are probably something like 85/87/91. So to avoid the cycles described above, you still need to be using at least 91 at altitude, which may well be premium as sold in your area.

Lol … No.
Thinner air just means you have to press gas pedal down further than you would otherwise need to, to make the same amount of power.
Show me some contrary data that considers altitude (and winter blend fuel), otherwise this one falls under the category of high ignore.

Sorry, but you are the one who is wrong. Why is regular only 85 octane at the gas station pumps in Denver, etc?

As for the OP. he shouldn't use less than 89 octane at 5,000' in a car that requires a minimum of 91 at low altitudes.

When I'm in Colorado in my C6 Z06, I do use 91 as I'm not trying to save a few pennies and 93 here at home(1320' altitude).

Google it, or simply read LDB's post above.

Here ya go. Read it carefully. Pay attention to the dates.

https://www.sae.org/publications/tec...ontent/872160/

Here's a nifty quote in case you don't want to read it

"We expect octane demand on gasoline suppliers in high-altitude areas to increase as these new cars make up a larger part of the vehicle population, and this could raise the cost of gasoline"