What Octane

[Vettehead Mikey]

When we have a runability (sp?) issue with a vehicle under warranty we are sometimes required to call our tech line. The first question they will ask if the vehicle is an 87 octane engine is: what brand of fuel is the customer using (preferably Top Tier and at a high volume dealer) AND what octane fuel. If the customer is using 92 octane in an 87 octane engine, their response is to get the customer off the higher octane fuel immediately because of carbon build up issues that affect valve coking and injector contamination. Apparently the chemicals used to get the octane rating higher promote faster carbon build up. 87 octane fueled vehicle still have carbon build up issues, but at a slower rate.

This flies in the face of the conventional wisdom that more additives is better and the assumption that high octane gas- the so-called 'premium' has more additives that 'regular'. In particular, the 'Top Tier' brands promote themselves as having more and better additives than non-member brands, so the recommendations you're getting from your tech line are kinda contradictory.

I'm half tempted to call BS on this one, but would be interested in hearing more.

 

[LLC5]

This flies in the face of the conventional wisdom that more additives is better and the assumption that high octane gas- the so-called 'premium' has more additives that 'regular'. In particular, the 'Top Tier' brands promote themselves as having more and better additives than non-member brands, so the recommendations you're getting from your tech line are kinda contradictory.

I'm half tempted to call BS on this one, but would be interested in hearing more.

They way it was described to me was that the additives that are used to reach a higher octane number (I'm no engineer so I don't know what those additives are) do promote faster, and more, carbon build up and the additives in Top Tier fuel do not promote carbon build up and are detergents to help get rid of carbon build up. It is known in the industry that "premium" fueled vehicles can require more decarbonization procedures than non "premium" fueled vehicles.

 

[kpic]

The additives are detergents; not octane boosters. Although some other additives are octane boosters or so-called lead replacements; the additives we are discussing are kinda "laundry soap" for engines.

"Mark Phelan from the Detroit Free Press recently raised that question. He asks "Should you care how much detergent is in the gasoline you buy? How can you even tell?"

He reports that leading automakers BMW, General Motors, Honda, Toyota and Volkswagen think you should care, and they want to help you find stations that sell the cleanest gas. They want some fuels certified as best for fuel economy and emissions.

The automakers formed a consortium, Top Tier Gasoline, that certifies retailers as meeting their standard for detergent. They say it's necessary because detergents prevent deposits of leftover material from building up in engines and exhaust systems. The deposits are like the ash that remains in a fireplace, and their presence in your car's engine can reduce fuel economy and performance.

Top Tier also certifies there are no additives that reduce catalytic converter effectiveness. Top Tier stations account for just more than 50% of U.S. gasoline sales.

“We strongly recommend Top Tier detergent gasoline to keep your engine clean,” GM fuel specialist Bill Studzinski said. “Fuel economy, emissions and acceleration all suffer when there are deposits in an engine."

Just like a fire leaves a pile of ash, burning gasoline leaves residue in an engine, said Matthew Mio, associate professor of chemistry and biochemistry at the University of Detroit Mercy. The residue is like the plaque that can clog your arteries. “Leaving untreated buildup reduces the efficiency of an engine over time. Detergents take the plaque away.”
https://blog.gasbuddy.com/posts/Does-your-gasoline-contain-enough-detergent/1715-543864-1864.aspx

Basically the "gas buddy" is quoting others.

Tier 1 gas, no matter if it is regular, mid-grade or premium has more detergent additives. Same as everything else some Tier 1 gas has more additives than other Tier 1 gas; however all Tier 1 gas has more additives than Tier 2 gas. Tier 2 gas meets the minimum requirements; Tier 1 exceeds the minimum.
There is a website somewhere which identifies which Tier 1 gas has more than others.

Where one has to be careful is some tunes disable the knock sensor.

References:
Top Tier Gasoline
https://fastlane.gm.com/2014/10/13/5-things-didnt-know-top-tier-gasoline/
Top Tier Gasoline

 

[1cooldaddyo]

Sure, why not. Might as well throw in my own opinions.
I'm no ASE certified nothing, but I've apparently been building engines longer than a lot of you have been alive.
My rule of thumb has always been 10 points of octane for every 1 point of compression. So a 9:1 engine would need 90 octane.
That said, there are some ways to cheat that. Aluminum heads will let you run more compression (about a full extra point), same with high swirl heads like the Vortecs.
Some say fuel injection will let you run higher, but I think that's more due to the very conservative advance curves that usually come with injection.

A 9:1 motor can certainly run on 87, particularly with injection so it won't diesel, but you're going to be giving up some power by not running as much advance as you could with the higher octane. That said, a lot places just add ethanol to the 87 to get 91 and that opens up a whole other can of worms. But if we're talking pure gas vs pure gas, 87 vs 91, I'd guess there's more more power to be had from the 91.

Carbon build up is a whole other animal that can be related, but might not be. I've never had a problem with carbon build up on any engines I built. Of course they've all had carbs, this is my first extensive foray into fuel injection. But I've had motors with everything from 7.5:1 to 11.5:1, always ran the highest octane available. Never had an issue with carbon. Let me qualify that- every new car I've had since about 2000 has carbon-ed up like crazy. I swear I could mine coal in the tailpipes. But my carb'd motors are running the same gas, so it ain't the gas.

 

[Vettehead Mikey]

My rule of thumb has always been 10 points of octane for every 1 point of compression. So a 9:1 engine would need 90 octane.

So 10:1 would need 100 octane and 11:1 would need 110?

 

[1cooldaddyo]

On a cast iron engine, open chamber heads, mild cam. Yes.
With a knock sensor, the computer can pull timing to allow it to run okay on lower octane but it really wants the good stuff.
Here's an article that maybe says it better than me:
How do cams affect compression?

 

[Vettehead Mikey]

What a track-only engine can be built to in terms of octane requirement is light years away from the OP's stock '84. Even the mightiest of GM's production offerings in the 60s and 70s did what they did on pump 93.

 

[kpic]

What a track-only engine can be built to in terms of octane requirement is light years away from the OP's stock '84. Even the mightiest of GM's production offerings in the 60s and 70s did what they did on pump 93.

Mikey,
Although it doesn't answer the OP's original question; a correction is in order.

It was 103 research or 95 motor octane. In the 1960s, Sunoco 260 was ~103 research octane. It was blue colored gasoline; I know because I bought it.

All L88 Corvettes came equipped with a special label that read:
"Warning: Vehicle must operate on fuel having a minimum of 103 research octane and 95 motor octane of engine damage may result" This emblem was mounted in the interior of the car behind the transmission stick shift"

See: 1967 - 1969 L88 Corvette Options & Production - Corvette Action Center

To see one: https://www.mecum.com/lot-detail/DA0912-135346/0/1969-Chevrolet-Corvette-L88-Coupe/4-Speed/

 

[DonB]

Mikey,
Although it doesn't answer the OP's original question; a correction is in order.

It was 103 research or 95 motor octane. In the 1960s, Sunoco 260 was ~103 research octane. It was blue colored gasoline; I know because I bought it.

All L88 Corvettes came equipped with a special label that read:
"Warning: Vehicle must operate on fuel having a minimum of 103 research octane and 95 motor octane of engine damage may result" This emblem was mounted in the interior of the car behind the transmission stick shift"

See: 1967 - 1969 L88 Corvette Options & Production - Corvette Action Center

To see one: https://www.mecum.com/lot-detail/DA0912-135346/0/1969-Chevrolet-Corvette-L88-Coupe/4-Speed/

That Sunoco 260 was some great gas and yes it was blue-ish almost a purple in color. Speaking of purple, does anyone remember "Purple Martin Ethyl?" It was also some potent stuff. Those were the days, cheap, powerful gasoline and Big Block Engines.

 

[Vettehead Mikey]

Mikey,
Although it doesn't answer the OP's original question; a correction is in order.

It was 103 research or 95 motor octane. In the 1960s, Sunoco 260 was ~103 research octane. It was blue colored gasoline; I know because I bought it.

All L88 Corvettes came equipped with a special label that read:
"Warning: Vehicle must operate on fuel having a minimum of 103 research octane and 95 motor octane of engine damage may result" This emblem was mounted in the interior of the car behind the transmission stick shift"

See: 1967 - 1969 L88 Corvette Options & Production - Corvette Action Center

To see one: https://www.mecum.com/lot-detail/DA0912-135346/0/1969-Chevrolet-Corvette-L88-Coupe/4-Speed/

This is true but the L88 (very low production numbers, not generally available to the public), was the only engine that actually required such high octane which would be roughly equivalent to today's 98 AKI.

The rest of the line up did just fine on 93 AKI, which was labelled as 98 RON back then. We all used Sunoco 260 back then in everything cause we didn't know any better. Even my Tecumseh powered lawnmower went like stink!

 

[gmhowe]

So after reading all these threads have one question, why if I run 91 octane in my 96 CE, jumping on the gas it still knocks even with new sensors ?
Thanx
Gordon

 

[LLC5]

So after reading all these threads have one question, why if I run 91 octane in my 96 CE, jumping on the gas it still knocks even with new sensors ?
Thanx
Gordon

If your '96 CE is recommened to run on 87 octane and it pings on a higher octane fuel, then one of two areas are most prevelant. The first is base timing and advance, and since yours is comptuer controlled this should not be an issue. The second is carbon build up, and the PCM is not able to back off enough timing to stop the pinging. Remember that these are just guesses, with out scanning the vehicle, but they are the two most common issues. Good luck with it.

 

[Vettehead Mikey]

So after reading all these threads have one question, why if I run 91 octane in my 96 CE, jumping on the gas it still knocks even with new sensors ?
Thanx
Gordon

How do you know for certain that it's detonation?

 

[kpic]

It is a sound about ~6400 Hz and you hear it. In new vehicles, knock sensors are sensors. They hear the noise and retard the timing.

Engine Basics: Detonation and Pre-Ignition by Allen W. Cline

What is a knock sensor:
Knock Sensor Function Symptoms and Problems

gmhowe,
Listen to LLC5.

 

[Hot Rod Roy]

1984 crossfire fuel injection (sort of) I assume the proper gas for this year is 87 Octane. Don't have the owners manual and the service manual does not seem to address this issue. Comments please

Lots of interesting debate here, but did the OP get his question answered? I'm not sure what Heyblue meant by "(sort of)", but to answer the question, yes, 87 octane is correct. That's what I run in my '84. My compression has been bumped up a bit, and I run 7 1/2 degrees static ignition advance. It runs just fine, thank you!

 

[LLC5]

For clarification: pinging (similar to my wife at times, and you know it is not going away until you do something about it) or knocking is what you generally hear inside the vehicle, and detonation is actually what is occurring inside the combustion chamber generally because of a pre-ignition event.

 

[Vettehead Mikey]

and detonation is actually what is occurring inside the combustion chamber generally because of a pre-ignition event.

Again, pre-ignition and detonation are NOT the same thing nor does one typically cause the other.

 

[LLC5]

Again, pre-ignition and detonation are NOT the same thing nor does one typically cause the other.

Yeah, I know. Pre-ignition happens before the act of detonation and yes pre-ignition of the fuel air mixture does need to occur before detonation can happen.

 

[Vettehead Mikey]

Yeah, I know. Pre-ignition happens before the act of detonation and yes pre-ignition of the fuel air mixture does need to occur before detonation can happen.

Sorry, wrong on both points.

'Pre-ignition' is the air/fuel igniting and burning normally, but as the name suggests occurs before the spark plug actually fires.

'Detonation' is the air/fuel igniting and burning normally and initiated by the spark plug, but part of the mixture elsewhere in the combustion chamber spontaneously explodes (detonates) rather than burning.

There is no requirement for pre-ignition to occur prior to a detonation event and in fact is extremely rare.

https://www.savvyanalysis.com/articles/detonation-and-pre-ignition

https://en.wikipedia.org/wiki/Engine_knocking

 

[LLC5]

Sorry, wrong on both points.

'Pre-ignition' is the air/fuel igniting and burning normally, but as the name suggests occurs before the spark plug actually fires.

'Detonation' is the air/fuel igniting and burning normally and initiated by the spark plug, but part of the mixture elsewhere in the combustion chamber spontaneously explodes (detonates) rather than burning.

There is no requirement for pre-ignition to occur prior to a detonation event and in fact is extremely rare.

https://www.savvyanalysis.com/articles/detonation-and-pre-ignition

https://en.wikipedia.org/wiki/Engine_knocking

I have always been taught that pre-ignition was the act of igniting the air fuel mixture before the normal ignition cycle timing (hence pre or before), and yes that detonation was the "part of the mixture elsewhere in the combustion chamber spontaneously exploding rather than burning" which is well worded. Taking the words literally, pre is technically before and detonation is the act of burning.