Octane requirement?

[Schurkey]

Using an 85 in computer controlled engine, would conflict with operating parameters of powertrain. Regular octane fuel 87. Manifold pressure, temp, timing, all factors and conditions based on first element of equation 87 octane.

But the purpose of "octane rating" is resistance to detonation. "Octane rating" has no other purpose.

Given that cylinder pressure will be reduced at altitude, detonation is also less likely, and even if the knock sensor detects it, it'll pull timing.

I'd rather take a chance on "low" octane on an engine WITH a knock sensor, than one without.

It's not like gasoline has a set chemical formula. It's a mix of hydrocarbons that varies from one refinery to another, maybe from batch-to-batch depending on what happens to be in the crude oil at the time. And in North America, that rating system ("Pump Octane" or "Anti-Knock Index") is the average of two testing methods--Research and Motor. A fuel that scores well on one, but poorly on the other can have the same number posted on the pump as one that scores mediocre on both tests. One or the other may run better in a given engine, though.

The only thing we've got going for us is that they make a lot of it, and they wouldn't want to reinvent the recipe every time.

 

[Frank Enstein]

Octane rating by itself has no effect on volatility, rate of burn, or power.

Higher octane simply resists knock.

Decent rule of thumb for octane requirements vs. cranking compression;

120 to 150 psi 87 octane

150 to 180 psi 89 octane

180 to 200 psi 91 octane

Of course there are many variables that effect the octane requirements such as good quench, efficient combustion chamber, aluminum heads, & etc.

Checking the Knock Counts will confirm your fuel choice.

 

[LC2NLS6]

I'd be surprised a stock 93 TBI truck can have enough power to do the speed limit up I-70 from Denver by Genesee...

 

[GoToGuy]

What is an octane rating? Is it not an indicator of resistance to self ignition. Higher compression ratio requires a higher octane rated fuel or preignition ( knocking) occurs.
In a non computer controlled engine, anyone who drove from 63 ft above sea level to Sierra cabin at 7800 above sea level learned using low octane regular was preferred. Seems backwards huh?
There are two conditions here. One being higher octane rating for higher compression gasoline engines.
The other condition carburetor, fixed timing pre computer engines, not able to adjust for reduced atmospheric pressure, reduced oxygen level.
I don't believe GM designed with the logic all vehicles are operated at sea level conditions. Sure as you drive higher elevations there will be a percentage of reduced power available, not the loss of earlier carbed vehicles. Don't you think the computer controlled system is operating more effective than any beforehand?

 

[AuroraGirl]

I'd be surprised a stock 93 TBI truck can have enough power to do the speed limit up I-70 from Denver by Genesee...

You live in michigan, we may be flat-earthers by these damn lakes but what do you mean "enough power" is a 350 tbi magically weak now.. or.. ? most modern cars can do speed limits as they exist, its a matter of in what gears or how labored or when it comes to eleevation, can they safely and efficiently engine brake on the downhills. The TBI is going to take elevation way better than a 350 from 86. If that truck was a sea level truck. High Altitude trucks were a factory "option" in that if they were to be operated at those conditions it was tuned that way. The owners manual and service data has all the information on how to adjust for those things.

Although, a 86 may have had a computer controlled quadrajet, im personally not aware of if they could adjust for elevation so lets say 1980 or 1981 whenever they still had manual qjets.

 

[AuroraGirl]

I read back further. my car says to use 91 and if in emergency, try to use as little 87 as possible and to avoid getting on the throttle since low manifold vacuum and in conditions not PCM commanded bypasssing, the supercharger is going to increase the compression but more importantly the temperature of the intake charge and the **** its cramming in the cylinders. Top end heat and temperatures is the problem being avoided when using low octane. its a low compression motor but supercharge makes heat and compression and then The late cooling fan strategy to keep the car at effieicent temp suddenly means spark knock possible in that way. also, Iridium and copper plugs of the same design and spec for heat range etc are going to move temperature out of the combustion chamber better than platinum. GM had tsb for cars with platinum plugs to go to iridum because people were chipping pistons with hot ambient temps, being on the throttle, and then underhood heat soak, the plugsbeing next to manifolds, no heat shields on half the plugs, etc andthen suddenyly you were having knock early because of what a diesel does.
The warnign is there because yes you can drive nicely and keep out of high load scenarios and high advancing scenarios to prevent preignition but if it does, it still had 2 knock sensors and a generous spark knock reduction programming(rip) but thats not because of your engine thats because of predetnoations effect on cat converters.
OBD2 misfire flashing is also only for a certain threshold of misfire/knock too, its not for stuff that can hurt your engine but fall low enough to pass the determined intensity for it.
HOWEVER even having 2 knock sensors, the idea to be nice until you have your fuel proper is goood because yes they are good at what they do but for your cars sake you wouldnt want to find out what happens when they dont prevent or fail to prevent oopsie. Basically dont be a dumbass and try to WOT if you have bad gas or see a large incline lol.

I would trust a TBI with knock sensors far more than old 80s ESC and janky components but i Would trust a 350 with 2 knock sensors more than 1

Also reducing gap on plugs would make for "Colder"(i know what it means but im not gonna break it down) spark that GM recommended .045 in small blocks that were equipped with .060 gap plugs as a means to reduce issues of load/misfire/chuggle(In transmission situations)

Gm also changed the LS iridium gaps to .040 or .045 on a TSB in the 2000s

 

[LC2NLS6]

You live in michigan, we may be flat-earthers by these damn lakes but what do you mean "enough power" is a 350 tbi magically weak now.. or.. ? most modern cars can do speed limits as they exist, its a matter of in what gears or how labored or when it comes to eleevation, can they safely and efficiently engine brake on the downhills.

Lolz, I lived there at 7500 ft and drove up to Mt Evans and all around the front range having fun. I had an '88 TBI Sierra 5 speed with 3.73's that barely could make it up that grade from Denver to Evergreen on a hot summer day. Sold the 88 (wish I kept it now) I bought a new 2004 WRX wagon (2.0L turbo 5 speed) as my beater, put my C5 Zo6 and 10 sec 87 Turbo Regal in the garage until winter was over. When the density altitude is over 10,000-12,000 ft, it sucks hp big time. My Buick trapped 125-127 mph at Bandimere and 20 years ago that would destroy 95% of the cars and bikes at altitude, in fact I never lost to a bike there (they got so mad too). The back road from Evergreen to Bandimere was always fun, as were a lot of other back roads in the mtns.

 

[thegawd]

Fauk I've never seen a mountain! can you believe that? yeah its true. I live in the middle of the great lakes here in flat ass Southern Ontario Canada.

with the dollar not going as far as it used to, I wonder if I will ever get to see one! Some Beach.

that sounds like a lot of fun!

 

[Frank Enstein]

What is an octane rating? Is it not an indicator of resistance to self ignition. Higher compression ratio requires a higher octane rated fuel or preignition ( knocking) occurs.
In a non computer controlled engine, anyone who drove from 63 ft above sea level to Sierra cabin at 7800 above sea level learned using low octane regular was preferred. Seems backwards huh?
There are two conditions here. One being higher octane rating for higher compression gasoline engines.
The other condition carburetor, fixed timing pre computer engines, not able to adjust for reduced atmospheric pressure, reduced oxygen level.
I don't believe GM designed with the logic all vehicles are operated at sea level conditions. Sure as you drive higher elevations there will be a percentage of reduced power available, not the loss of earlier carbed vehicles. Don't you think the computer controlled system is operating more effective than any beforehand?

The octane rating is ONLY an AKI Anti Knock Index. Or the fuel companies are all lying the same.

 

[LC2NLS6]

FWIW, I ran 91/92 or whatever I could get in my boosted cars at altitude that all wanted premium fuel, they all ran fine. I did run down to Denver and get GT100 from a sunoco station from time to time, I think I was in Lakewood, it was only $4 a gallon back then...