Octane


#1

Isn’t it true that with it he invent of computor controls in new autos that the engine will adjust for a lower octane gas even though a high octane was recommended(by the manufacturer)


#2

The computer will compensate within limits. It will mask the noise but the internal damage will continue unabated. Using lower-than-octane in a high-compression engine is one of the best ways I can think of to shorten the engine’s life.

If you buy a vehicle that needs premium gas, you’d be foolish, in my opinion, to use anything else.


#3

thanks for the advice


#4

The ECU will adjust only to a point. You have a “knock sensor” in the head of the engine that senses shock waves from preignition. If the lower octane results in mild preignition, basically a secondary detonation resultant from the gasses already being hot that crashes into the spark induced detonation, in essence the gasses combust too quickly such that the flame front bangs into the piston while it’s still rising rather than pushing it down the cylinder at a controlled rate, (these conditions are called “pinging”) then the knock sensor will sense the shock wave and tell the ECU to back off the ignition timing to prevent the pinging. If the gasses self-detonate without the spark having been fired from compression generated preheating, the system will be unable to compensate and the shock waves will cause internal damage.

Note that even if the knock sensor compensates for pinging as described above, the retarded spark timing can cause reduction in power and efficiency.


#5

It depends if the mfr. says recommended or must regarding premium usage. Most vehicles since the 1990s have knock sensors and can adjust for 87 octane fuel. For more reading on this Google Jakob Neusser Porsche premium, Gottfried Schiller Car and Driver and also Loren Beard Volvo Edmunds. If your ears are normal and if you don’t hear sustained knock/detonation, then your engine is not being damaged.


#6

Using lower octane than recommended: You will lose some horsepower; you will lose some mpg. The engine may run slightly hotter because the spark will be retarded up to 5?. Chance of damage to the engine: NIL.

What to do? Compute mpg with regular gasoline, then with premium. Select the cheaper alternative, or whatever your wallet and/or aggressive foot can live with.

Knocking? An engine can have a damaging knock and you will never hear it. If you can hear the knock, you have stepped way over the line. Have a little faith in your knock sensors; they can detect a knock long before you can hear it.


#7

It has always been my experience that any pre-ignition condition that was causing damage is ALWAYS audible. If it’s there you WILL hear it…

Look at the listed compression ratio for your engine. THIS is the factor that determines octane requirement. The magic number is AROUND 9.6 to 1. Below that, 87 octane usually does fine and is the recommended fuel to use. Above that, OR turbo or supercharging, usually demands 92 octane. FORGET “mid-grade”. The 2 octane points will not make ANY difference one way or the other. Besides, how do you know what you are REALLY getting? Can YOU test octane? Do you think ANYBODY does?? High altitude? 85 octane does just fine.


#8

True of some cars but not all. Always check the owner’s manual. It it says use high octane and nothing more use the high octane or risk damage. If it says use regular, use regular, using high test will not give you more power and could cause damage. If it says you can use either, then using regular will result in a little less power and mileage. You may or may not save any moeny.

In an older engine that calls for regular, you may need to use premium due to carbon build up, especially if you have been using premium. The carbon buil up increases compression and the need for permium.

Using premium in an engine designed for regular WILL NOT give more power or mileage. Premium is not more powerful fuel it just does not burn as easily.


#9

Yes it handles it nicely…It handles it by retarding the timing. This will result in a decrease in performance. As long as the engine isn’t knocking then no damage will occur. Back in the 70’s when I had my Firebird with the 11:1 compression ratio pistons I needed 102 octane to keep it from pinging. Most of the time I just set the timing about 2 degrees retarded from OEM. Ran fine that way for over 100k miles.


#10

quote:

It has always been my experience that any pre-ignition condition that was causing damage is ALWAYS audible. If it’s there you WILL hear it…
The knock sensors will detect knock and retard spark before the spark is audible.


#11

quote:

Premium is not more powerful fuel it just does not burn as easily.
Correction: Higher octane is not a more powerful fuel, it just does not burn as easily. Premium may or may not contain more calories per gallon. Aviation gasoline is a premium fuel; it definitely has a higher energy content per gallon than anything one can buy at a service station…


#12

quote:

The carbon build up increases compression and the need for permium.
I doubt if the “carbon build-up” on an older engine is thick enough to be measured, let alone change the compression ratio. It could glow and cause hot spots to produce pre-ignition, but the mass of carbon is nil. Nothing like what can occur on the backsides of intake valves.


#13

It will adjust the engine trying to compensate for the lessor fuel and likely knocking and pinging. The problem is it only adjusts if knocking/pinging occurs so basically your motor is running between a state pinging/knocking then not by adjustment if your vehicle requires premium. If you car recommends premium the algorithm (programming) is better and will keep your engine running at the proper state to prevent the flip/flop.

My ex-college roomate is a engineer for automotive computers.

No blanket statement can apply except to every vehicle.