Effects of switching to a lower gas octane
 

I'm moving from a state where I feed 93 to my cars to one that tops out at 91 octane.
I was wondering about the effect on my cars, if any ?

On the Audi S4 (supercharged engine, contrary to the 3.0T badge), I imagine that the ECU will detect pinging - if any - and reduce power a bit ? Not much I can do anyway, but do you think that engine will lose a bit of power due to lower octane?

I also have an MFi 911 and a carburated 912. Should those be retuned (ignition timing I guess, more than anything) for lower octane or are those engines not prone to knocking because the compression is not that high to begin with ?

Or have i been fooling myself that my 93 was actually 93 to begin with? I know most gas comes from the same pipe and is mixed on site... Just wondering...I figure ethanol is probably a bigger issue than octane but not much I can do about that either...

don't forget that as altitude increases, octane requirements decrease. Here in Colorado (at 5280 feet above sea level) 91 octane gas provides anti knock properties that are very similar to those provided by 93 octane gas at sea level.

There is no benefit for a stock engine to using a higher octane than specified by the manufacturer.

On the S4 it may be any issue... otherwise I would just avoid the ethanol.

With the compression of your motor it won't suffer by going to lower octane fuel,it's the late model cars that really struggle with there high compression numbers,it won't damage then but you will have a peformance decrease as a result.

Porschetub, I've been running my 1982SC engine on that Mobil 97ron stuff. But it has 10% ethanol, do you think the car will be ok on ordinary 95ron petrol? The engine is the ROW 9.8:1 compression ratio model.

My manual states that my car requires a minimum of 96 RON octane. RON is the european rating, the US & Canada use RON+MON/2. The US rating system is "4 to 6 octane numbers lower" than the european rating values. 89 RON+MON/2 is too low based on that calculation, 91 should be fine. 87 is definitely too low according to the manual.

If you have a manual, check it. If not, post the year and someone can probably tell you. If your motor is not stock, then you may need to go trial and error.

PS - you may want to add the year to your signature.

About all we have in Oklahoma in 91. We can get 100% gasoline. I have run it for 19 years in 85 911and never had a problem.

Engine knock is usually due to a secondary point of fuel ignition in the cylinder (with the primary being from the spark plug). The secondary point of ignition creates a secondary flame front in the cylinder. This secondary flame front hitting the cylinder wall is the pinging sound that you hear. This is also called detonation.

A higher octane fuel burns slower due to more bonds needing to be broken, thus helping with prevention of detonation.

If the ECU in your Audi detects knock due to the lower octane fuel, it will automatically retard the timing. Retarding the timing lowers the peak cylinder pressure, which is another method to help with detonation prevention.

Yeah, go whit Shell V-Power, not 93 but clean 91 Nitrogen Enriched gas without ethanol (well, that is what is posted on the pump).

Sorry but you are incorrect. The above explanation is detonation and not preignition.

Preignition is exactly as it states. Ignition takes place before the actual called for ignition process by the ECU or the distributor. This is caused by extreme hot spots within the combustion chamber which ignite the intake charge prematurely.

Detonation is caused by your explanation but not preignition. I think that if you read the owners manuals on most of the North American made vehicles that you will see that they were spec'd to run on 87 North American octane. Feeding your car with higher octane may or may not help with detonation. Timing is much more critical than the octane level.

Now with that being said if you fill up with inferior 87 you may experience detonation. to be on the safe side you can utilize 89 octane and if inferior should not be any less than 87 unless their tanks are contaminated with water. A highly unlikely situation but not impossible.

Anybody with a stock engine filling up with 91 or 93 octane are not getting what they think they are getting, other than a less load on their wallet. I Can tell you that dyno testing has shown that there is little gain if any and a possible loss of power if you utilize a higher octane level than the motor was originally designed for and do not re tune the ignition and fuel distribution to go along with the higher octane.

As stated higher octane levels burn at a slower rate therefore the mandate for increased ignition timing to obtain maximum power. JMHO.

You are forgetting that the older 911S ('71 and prior) had a 9.8:1 CR.

I don't think I have not forgotten anything. As I and others have stated, if your owners manual specs out the octane level for whatever year then that is what you can utilize. It would be interesting to see what the octane spec was for the '71 and prior. If you are uncomfortable with the spec then utilize whatever makes you happy.

Bah! You are absolutely correct. Good catch! I'll correct the above post to prevent future confusion.

No problem. A lot of people are confused by pre-ignition and detonation.

Before the drop in compression ratios with the 1972 models, premium was suggested. In Europe, that meant 98 RON, around here 93-94 R+M/2 would probably have been what you used. There was a push to reduce premium to 91 in 1972, led by the fools in California.

The composition of the gas available today is so far removed from what it was available back then, it's hard to make any sort of comparisons. If I had a high-compression MFI 911, I'd probably try to run 93, if I could get it. Alcohol-free would be a bonus for me. If I had a '72, or '73, I'd run 87.

JR

My '78, US Targa loves 87 as it's a stock 8.5:1

Not sure what the quandary with octane ratings are always about.

Your S4, if mine, would be 91 minimum, always 93 where available.

It's a simple function of your Compression Ratio, which you left out in the OP.

Java...we were answering in the same spirit at the same time. Ha!:D

Keep in mind that the octane requirement for an engine can increase slightly with time, due to carbon buildup and the fact that it might not be running as well as it did when it was new. And, you can be experiencing problems a little before you can hear them, so I'd err on the side of caution. There are also sometimes better/more additives in premium fuel than the cheap stuff.

Then there's the issue of where the guy lives... temperature, humidity and altitude all play a part.

JR