What Octane

[Hib Halverson]

Pre-ignition and detonation are, indeed, different anomalies of combustion in a spark-ignition IC engine but they can happen alone or together.

For example, pre-ignition is caused by ignition sources other than spark plugs such as glowing carbon deposits, glowing spark plug side electrodes, glowing shards or splinters of failing head gaskets, even glowing parts of combustion chamber walls is the surface is rough or has casting "flash".

One reason its advisable to put colder heat range plugs in engines which see severe duty cycles is avoidance of pre-ignition caused by a spark plug which cannot shed heat to the surrounding head material fast enough and runs hot enough to ignite the air-fuel mix without a spark.

This pre-igntion, depending on when it happens can also cause the engine to detonate if the pressure waves generated by the two flame fronts occur close enough together in time that they collide.

Detonation is caused by the flame front expanding away from the spark plug raising the pressure of end gases at remote areas of the chamber to auto ignite. The pressure waves emitted by both the normal flame front and the autoignition flame front run into each other. Detonation, when it occurs, causes combustion temperatures to skyrocket and, if heavy detonation continues unabated, it can raise the temperature of carbon deposits or spark plug electrodes such that they cause pre-ignition.

Needless to say an engine which is both detonating and pre-igniting will have very limited reliability/durability.
The characteristic sound of detonation--that 5000-6000-Hz "knocking" sound:bang-- is caused by pressure waves emitted by the colliding flame fronts causing the walls of the combustion chamber to vibrate.

 

[Vettehead Mikey]

detonation is the act of burning.

Not to be pedantic, but detonation is 'exploding' vs. the desired act of 'burning'.

 

[LLC5]

Not to be pedantic, but detonation is 'exploding' vs. the desired act of 'burning'.

Agreed. Word issue.

 

[kpic]

As there is always an explosion in the combustion chamber; normal combustion is detonation.



What we call detonation is the fuel-air mixture ignites before it should or the "explosion" occurs in a shrinking space which isn't good. The explosion is supposed to push the piston down; not be compressed further.

Without getting too technical, this is one of the best reads I've found on the subject. A few of the causes can be disregarded as their focus is forced induction and the OP's question was for naturally aspirated.

Detonation

Under normal conditions, the combusting air and fuel mixture inside the combustion chamber ignites in a controlled manner. The mixture is ignited by the spark, normally in the centre of the cylinder, and a flame front moves from the spark towards the outside of the cylinder in a controlled burn. Detonation, or engine knock, occurs simply when fuel pre-ignites before the piston reaches scheduled spark ignition. This means that a powerful explosion is trying to expand a cylinder chamber that is shrinking in size, attempting to reverse the direction of the piston and the engine. Causing sudden pressure changes in the cylinder (Up to 10x that normally experienced), and extreme temperature spikes that can be very damaging on engine pistons, rings, rods, gaskets, bearings, and even the cylinder heads.

Even the best engine components cannot withstand severe detonation for more than a few seconds at a time. More severe detonation obviously leads to more severe forms of engine damage. If there is enough heat and pressure in the combustion chamber, detonation can begin to occur before the spark plug even fires, which would normally initiate the combustion. Under these circumstances, known as "pre-ignition", the piston may be travelling up towards a wave of compressed, exploding gas. These are the worst kinds of detonation conditions, and can bend con-rods and destroy pistons.
What is detonation

The article explains the primary causes of what we call detonation.

BTW, knock is the proper term.

 

[Vettehead Mikey]

That explanation is so far wrong that words fail me.

The mixture is always ignited while the piston is still rising. The what ignition advance measured in degrees BDTC is.......

Sheesh. :ugh

As for 'As there is always an explosion in the combustion chamber; normal combustion is detonation' Uhhhh, no. :eyerole

 

[kpic]

As they said almost the exact same thing and you're missing it. Read the first three sentences slowly instead of trying so hard to correct them.

"Under normal conditions, the combusting air and fuel mixture inside the combustion chamber ignites in a controlled manner. The mixture is ignited by the spark, normally in the centre of the cylinder, and a flame front moves from the spark towards the outside of the cylinder in a controlled burn.

Detonation, or engine knock, occurs simply when fuel pre-ignites before the piston reaches scheduled spark ignition."

Point is, the ignition of the fuel-air mixture is happening at the wrong time and it is just that simple.

 

[Vettehead Mikey]

[UDetonation, or engine knock, occurs simply when fuel pre-ignites before the piston reaches scheduled spark ignition

Absolutely false and wrong. The above is a description of 'pre-ignition', not 'detonation'.

[/U]Point is, the ignition of the fuel-air mixture is happening at the wrong time and it is just that simple.

No, it's not that simple and has NOTHING in common with even the most simplistic understanding of detonation. If you don't understand the principles and the differences, please don't try to correct those that do. :eyerole

 

[Heyblue]

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!

Yes I got the answer, I will be running 87. The sort of referenced that today's "fuel injection" is an injector per piston. The 1984 Xfire just "injects" fuel in the intake manifold. A bit more than carburetor, but short of full fuel injection.

 

[Hot Rod Roy]

The 1984 Xfire just "injects" fuel in the intake manifold.

The TPI fuel injection system also injects fuel into the intake manifold. When an injector fires in the TPI system, some of the time the intake valve is open, and some of the time the intake valve is closed. The CFI spray pattern from the injectors looks like a continuous spray, but if you look at the spray pattern with a strobe, you'll see that it is also being pulsed. Not that much different that the TPI system, except that you can't see the injector spray pattern in the TPI system.

The CFI system has no comparison to a carburetor. The CFI (and the TPI) is completely controlled by the engine Electronic Control Module.

 

[KANE]

My personal thoughts having gone through a lot of learning about tuning over the last year and some change... is that GM sets their OEM spark maps for iron heads fairly conservative. I believe- but have not confirmed 100% by seeing it with my own eyes- the '84 Vette spark map would look almost identical to the '86 iron headed Vette spark map. I could be wrong, but odds of finding a BIN for an 84 to verify is probably quite low. That said, heads & flow characteristics, material, etc play a big role in the spark map from what I have learned.

When it comes to fuel... more octane will allow an 84 CFI engine to run a little better. How noticeable? Likely not much- but depending on engine / mechanical / ignition system wear, condition, fuel quality, etc... results could vary. Might run a little smoother- as in to run at bit more advance than it would with lower octane if it detects knock, etc.

I would think 87-89ish octane from a reputable supplier/gas station would be fine for a bone stock Vette in good mechanical shape- especially for daily driving and non-race usage. The ECM should be able to control and adjust the spark accordingly if it detects knock. Also... octane levels vary based upon altitude and then many fuels are blended with ethanol. Again, the ECM should be able to adjust accordingly with a properly operating knock sensor and good electrical & mechanical maintenance.

Just my $.02... though.

 

[Vettehead Mikey]

When it comes to fuel... more octane will allow an 84 CFI engine to run a little better. How noticeable? Likely not much- but depending on engine / mechanical / ignition system wear, condition, fuel quality, etc... results could vary. Might run a little smoother- as in to run at bit more advance than it would with lower octane if it detects knock, etc.

And that's the key- a low compression engine like the OP's with a conservative OEM timing curve does not knock on 87. It would be foolish for an OEM to release a car to the field and have it operate in a continuous state of knock due to low octane fuel.

Since there's no knock to cure, higher octane gas will achieve nothing. The engine has no idea what level of fuel is being used. It senses only two conditions- knock or no knock. If there's no knock, then the full OEM timing curve is followed. Feeding it fuel with higher octane does not move the timing curve beyond that.

Also... octane levels vary based upon altitude and then many fuels are blended with ethanol. Again, the ECM should be able to adjust accordingly with a properly operating knock sensor and good electrical & mechanical maintenance.

Again the engine has no idea what fuel it's being fed. An engine that is knock free on 87 at sea level will remain knock free at high altitude when fed 85 octane simply because the cylinder pressures are lower due to the thin ambient air. The knock sensors have no more to do with it than at sea level.

Dragging ethanol into the discussion is also a red herring. If gas is rated at 87 octane, it's 87 irrespective of ethanol content.

 

[KANE]

Mikey... Both of us are saying the same thing- 87 is fine.


Sent from my iPhone using Tapatalk

 

[Vettehead Mikey]

Mikey... Both of us are saying the same thing- 87 is fine.

Agreed- but I'm reiterating the point that nothing is to be gained by using higher octane in this particular case.

The myths and misconceptions surrounding the octane subject are enormous. I bought a used SUV a year or two ago that clearly states in several locations that 'premium fuel only' must be used. In going through the paperwork in the glove compartment, I found dozens of credit card slips indicating that 87 octane (regular) had consistently been used by the previous owner. In asking him about this he insisted he had complied with the requirement and had always filled up at Top Tier stations. To him, 'premium' meant Top Tier. :ugh

Turns out he sold the vehicle as it didn't seem to be as powerful as he had expected and figured the engine was on it's last legs. I gave him a ride after filling it with premium (91) and he couldn't get over how much more powerful it was. His conclusion was that it was the extra cleaning additives at work and that Top Tier was a scam. The concept of 'detonation' 'knock sensor' and 'octane' drew a blank stare.

If you've got time, watch this:

https://youtu.be/FPPkPAbzwbU

and you'll see that the investigative journalists and the 'instructor' at the college are at some point just as confused as the general public they're trying to educate.

Mac is probably familiar with this TV series.

 

[Mac]

If you've got time, watch this:

https://youtu.be/FPPkPAbzwbU

and you'll see that the investigative journalists and the 'instructor' at the college are at some point just as confused as the general public they're trying to educate.

Mac is probably familiar with this TV series.

Yup. This is what Canada's state broadcaster sells to the public as being "investigative journalism" but, as you can tell, the level of investigation & understanding is skin deep.

Regular or Premium? - Feature - Car and Driver

Mac

 

[hcbph]

If you're up for a little reading, here's an interesting article from some Racing Gas manufacturers: Everything You Need To Know About Octane

Hopefully it will dispel some of the myths.