Switching from 93 to 89 octane......

[Sunroof]

My 1973.5T/CIS is not my daily driver nor a real gas gizzler, but through the years all my 911's were fed 93 octane or "high test". Mechanics I have met along the way always pushed the "good stuff", but now I question what's the negative for using a lower octane (89) on the 2.4 engine?

We have been through this before, but I was not paying attention then.....

Thanks

Bob

[Steve@Rennsport]

Bob,

Your 2.4T motor has prrety low compression and you should be OK with 89. That said, Altanta has hot summers so it cannot hurt to throw some 93 in when its more than 85 deg outside.

[onlycafe]

detonation.
if you are running a dead stock engine there is not much need for 93.

[scottcip1562]

Both my 85 944 and 79SC require 91RON. If you look in the owners manual for the 79SC is 91RON or 87 R+M/2. I have been using 87 for the past 5 years in both cars. Both are stock motors. This is from Wikipedia:

Measurement methods

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of iso-octane and n-heptane.

There is another type of octane rating, called Motor Octane Number (MON) or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.

In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States, Canada and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90-91 US (R+M)/2, and even deliver 98 (RON) or 100 (RON).

The octane rating may also be a "trade name", with the actual figure being higher than the nominal rating.[citation needed]

It is possible for a fuel to have a RON greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels, straight ethanol, AvGas and liquified petroleum gas (LPG) typically have octane ratings of 110 or significantly higher - ethanol's RON is 129 (MON 102, AKI 116) reference[1]. Typical "octane booster" additives include tetra-ethyl lead, MTBE and toluene. Tetra-ethyl lead is easily decomposed to its component radicals, which react with the radicals from the fuel and oxygen that would start the combustion, thereby delaying ignition. This is why leaded gasoline has a higher octane rating than unleaded.

[4flyboy]

How about a 3.2 I've been in situations where I can only get lower octane "regular" it's dead stock w/out a cat. Am I doing damage? Sorry for the hijack.

S.

[stlrj]

My 86 3.2 has been on a steady diet of 87 regular for the past 150,000 miles. Should I be worried too?

[Komenda Fan]

I find my car runs better on 87 then on the high octane stuff. I really can't figure out why, but it always seems to be the case...

[Sunroof]

Appreciate the responses.

Amazing to hear that 911's are runing on 87, but that does not appear to be a problem. I might try and alternate (89 - 93) this summer and see if I have any changes in performance.

Bob

[911s55]

Originally Posted by Komenda Fan

I find my car runs better on 87 then on the high octane stuff. I really can't figure out why, but it always seems to be the case...

Lower octane has fewer additives so it burns more efficiently and a little hotter.

[jhubs]

This is interesting because here in Colorado we do not have 93 Octane (I was told that it's an altitude/oxygen thing...we don't have oxygen here).

Our highest is 91, then 87 and 85. I have been alternating tank fill ups between 91 and 87 for two years. I don't know if this causes any damage to the engines (an 87, 911 and a 95, 993) but I would be curious to know from some "engine guys" if this may cause problems down the road...so to speak... Both cars have run great with no knocking, etc.

All I know is that once every month I have to take the car out and give it an "Italian Tune-up"...100 mph+...I just hate that!

Jerry

[Quicksilver]

Originally Posted by 911s55

Lower octane has fewer additives so it burns more efficiently and a little hotter.

The number of BTUs per gallon is identical between the lower octane and higher octane fuels of the same brand. The higher octane fuels sometimes have more cleaning additives but that doesn't effect the burning of the fuel.

The difference between lower octane and higher octane is how much of the other parts of the petroleum is in the mix. Lower octanes will have more of the heavier forms of petroleum in it such as asphalts. To get it to a higher octane they have to refine it to a higher degree but the increased exclusion of the non gasoline portions of the petroleum means they end up with less finished product. They need lots of gasoline so they don't want to leave anything out.

[greghershner]

In NC mountains our highest is 91 octane as well. When I had my engine rebuilt by John Forbes of Black Forest Racing, he said I should use 87 for daily driving and the 91 for DE/Auto-x only. Interestingly some of our local stations have three pump handles (87/89/91) but only 2 tanks in the ground. The 89 pulls from the 91 tank but costs a dime less per gallon!

[GH85Carrera]

The 89 is just a blend of the 87 and the 91.

[GH85Carrera]

The 89 is just a blend of the 87 and the 91.

[stlrj]

Originally Posted by greghershner

In NC mountains our highest is 91 octane as well. When I had my engine rebuilt by John Forbes of Black Forest Racing, he said I should use 87 for daily driving and the 91 for DE/Auto-x only. Interestingly some of our local stations have three pump handles (87/89/91) but only 2 tanks in the ground. The 89 pulls from the 91 tank but costs a dime less per gallon!

That's because the 89 is actually pulling from both tanks as a blend that results in 89. You might also notice that all gas deliveries, at least here in California, come from trucks with two tanks only and 89 is not one of them.

Cheers,

Joe

[3.2 CAB]

Most of the gasoline tankers that I have been around have more than just two compartments. Most have four or five, while others may have only one, or others still may have more than five. Av-gas is usually in a dedicated tanker, due to the quality and trying to avoid contamination. It is not good to have crappy gas in your plane. It was always inspected for octane, quality, and contaminants. I have been at the fill docks when the tankers were being loaded, they had a premium, mid-grade, and a regular(the lowest rated unleaded) there are dry-break fill heads and valves for each compartment. When one of the buyers was a place that had been using the alcohol blend for many years, you would have to figure out the amount of gasoline needed in that compartment, and the amount of alcohol was then added to obtain the 90/10 mix. The Circle K stores, Stop-N-Go, and the 7/11 stores have been using the 10% blend for 15 or more years in the Houston area, and other areas as well. It was not unusual for one truck to have the Premium, Mid-grade, reg. no-lead, and even diesel loaded on the same multiple compartmented tank trailer. That is why there are so many different Haz-Mat placards holders and placards on the front, sides and rear of the equipment. At some stations or stores, you will see the different color tank cover lids, most were red, white, blue, black, and even green or yellow, for diesel. Just some hands on observations.

[fredmeister]

I thought that there was more energy content in 87 versus 93. Octane actually slows down the burn rate and therefore prevents detonation, but you actually sacrifice a little energy content doing so....tell me if I am wrong.
The stock engine in my 79 911SC with 8.0 to 1 compression ran fine on 87 octane for 70k miles after I made the switch. No problems.
Hell my 79 turbo will run on 89 according to the owners manual!
If I tracked them then I used premium.

[Quicksilver]

Octane refers to how hard it is to get it to initially start to burn. It doesn't in any way refer to energy content.

Because it is harder to get the fire started you can run more ignition lead or compression because the higher pressures (and resulting temperatures) won't light off the air-fuel mix.

[rudderboost]

I had a buddy the other day grieving his car that required high octane. He was looking into buying the "cheaper" gas and then putting an octane booster in it. Good grief. If 98 is .20 higher and you put in 15 gal that's $3.

Reminds me of my dad who used to drive all over town to find the station that was .02 cheaper. Great way to save .30!

[Sunroof]

Not to get too far off the subject.................as a general rule.............

If you enter a gas station to fill up and a tanker is present dumping a load of gas, turn around and come back later! The dumping will disturb the sediment and crap at the bottom of the underground storage tank and make it to your tank when pumping from the dispenser. Although the fuel passes through a small fuel filter at the dispenser a slug of contaminants can make it through and into your tank.

Bob