Lorenfb

Based on comments posted on the internet about the USA '84-'89 Porsche 911 Carrera,
there appears to be some confusion as to Porsche's specified minimum octane requirements.

Data from Porsche reference manuals:

I. Porsche Workshop Manual - WKD 482 020 & Porsche Technical Spec Book WKD 423 020

930/21 - 911 '84-'86 RON/MON 91/82 USA pump rating = (RON+MON)/2 = 87
("recommended 91" per manual WKD 482 020)

II. Porsche Service Manual '89 911 - WKD 495 421

930/25 - 911 '87-'89 RON/MON 95/85 USA pump raing = (RON+MON)/2 = 91

GothingNC

I've always used 93 in all my air-cooled vehicles.

$4 more a tank (20 gallon) vs RU is not going to break the bank.

John

84_Carrera

Thanks for posting Loren. At the risk of opening up a Cr@psandwich, I also use 93, as I have a Weltmeister chip.

Amber Gramps

"My point is that the Porsche engineers ALWAYS do there homework and in my PERSONAL oppinion I think they can RECOMEND a certain grade of fuel but the customer could use something lower in grade which is offten the case."

Oh O.k. So when Porsche defines a specific specification, just do what you think best, right:

1. When Porsche specifies the oil type as 20/50 for year round use, it's O.K. to use a 10 weight
in SoCal during the summer at 100+ degrees, right? Porsche must have designed their engines to use a 10 weight anyway knowing someone would being using a different weight because it's on sale.
2. When Porsche specifies the valve clearance at .1mm, it's O.K. to set them
at 0 clearance, right? They designed their valve systems knowing someone would
probably not have the correct feeler gauge or decide on another setting.
3. When Porsche specified the octane at 96, it's O.K. to use 87, right?
Porsche knew that some owners may have had a limited budget and could only
afford a 87 octane.

Bottom line: OEM specifications are just kinda rough guidelines to follow if one agrees with them, right?

Note: Using the octane calculation method used by fuel retailers, e.g. Shell & Chevron,
of pump octane = ( RON + MON ) / 2 and the Porsche spec for a minimum octane (where
RON = 96 & MON = 86), results in a minimum pump USA octane rating of 91 for a 911
Carrera. Porsche also indicated "Premium" in the owner's manual which infers the highest
grade available where a minimum grade would be 91.
__________________
Loren
Systems Consulting
Automotive Electronics


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Last edited by Lorenfb : 03-18-2008 at 08:46 PM.



THE ABOVE POST IS A QUOTE...from Lorenfb

I think we are seeing a rehashing of the old Steve Wong Thread about Chips and an effort to resurect the topic without continuing that thread. As many of you know I mixed 100 octane and 91 octane for the last year to have a 95 octane blend. Since I did my valve adjustment, and at Pete's request, I filled up with Chevron 91 and have found that my car still rocks my world.

dshepp806

Rehashing?

Peter Zimmermann

Douglas: Do realize that in Bakersfield your interior would get just as dusty with your makeshift cover as without ?


Chevron. Yes, Martha, their is a difference from one gasoline to another. I have used only Chevron Premium in my 911 and other cars that require 91, and nothing but Chevron Regular, along with a bottle of ProGard every couple thousand miles, in my F 150 (which is designed to run on Regular).

Amber Gramps

Pete, that isn't a dust cover, it's a cape!

Brads911sc

imho... if you are worried about running 92 /93 octane gas in your 911... how can you possibly afford the maintenance? i understand not wanting to throw good money away... but an extra couple dollars seems like nothing compared to the 7k rebuild on an engine not run with good fluids. even an oil change when using a good oil and mahle filter runs $60... if you want a car that loves cheap gas and oil... seems a civic or corolla is more what you want....

imcarthur

In reality, I am sure most 911 owners use top grade fuel. From what I have read, the bigger concern is the growing ethanol content. Sunoco 93 is 10% ethanol.

Loren & Pete: Is this a problem?

Ian

Amber Gramps

We don't get to really use what we put in on the street anyway. To let the car run up like it wants to is "exibition of speed". Just driving slow and our cars look fast. I was sitting at a light today and a motor cop pulled up next to me and said "the speed limit is 35MPH. You were past that in 2 seconds at the last light." I didn't get it out of first between lights and I'm exibiting speed! What do I do with the other 200HP? He went on to tell me he should write me up. I assured him I already had that ticket. The light turned green and he launched out so fast I could have never cought up.

Hester

Fuel with a high octane rating is more resistant to auto-igniting under pressure than fuel with a low octane rating. "Knock" and "ping" happen when, in addition to the combustion of the air/fuel mixture ignited by the spark plug, some of the air/fuel begins to auto-ignite and the resulting rapid increase of pressure inside the cylinder creates a shock wave that slams against the cylinder wall, piston, and cylinder head. Potentially, the rapid inrease of pressure inside a cylinder caused by fuel that auto ignites under pressure and is lit by the spark plug at the same time can result in catastrophic engine failure (e.g., burnt pistons).

Here in California, the best we can buy at the pump is 91 octane ((R+M)/2 method). That is pretty limiting.

I built up my Harley "Twin Cam 88" engine by boring the cylinders out and increasing the displacement to 95 cubic inches (1550 cubic centimeters) and installing high compression pistons (rated 10.5:1). I also installed a programable ignition so I could make my own timing advance adjustments.

By setting the timing so that the spark plug ignites the air/fuel mixture before the piston travels far enough toward the top of the compression stroke to compress the air/fuel mixture so much that it will auto-ignite, I can prevent the "knocking" and "pinging." Unfortunately, I also don't get much out of the power stroke that way. If I had higher octane fuel, I would be less worried about auto-ignition and I could advance the timing so that the spark fires later in the compression stroke and the expansion that is created by the burning air/fuel charge translate to more power.

Modern engines utilize a "knock sensor" and the ignition automatically retards the timing if the sensor detects knock. In a perfect world, with that system, you would not damage the car by running the "cheap stuff." In an older high-compression engine (Hester is a 1970 911S with a stated compression ratio of 9.8:1), the cheap stuff can cause "knocking" and/or "pinging" which will tend to shorten the life of the engine components. Most regular passenger cars can run on low-grade fuel with no problem because their designers could lower the compression ratios to accomodate low octane fuel without worrying about how much power they produce. My little Honda got 40.5 MPG this past weekend on regular unleaded.

Brads911sc

My question gets to a more basic level... simply, knowing what we know about our cars... why would anyone even ask what is the lowest octane they can run? who cares? you wouldnt run that in your car anyway... would you? shouldnt they be asking whats the highest octane I can buy and maybe what brand is best? never understood people that buy high end things and then maintain them on the cheap to save a few cents...

Dave Thomas

"Modern engines utilize a "knock sensor" and the ignition automatically retards the timing if the sensor detects knock."

There's currently a good thread on this issue on Pelican:

http://66.236.61.177/showthread.php?...78#post3851027

Especially see Steve Wong's post in general, and the section on advance - vs - intake temp.

I guess Loren won't be able to comment there since he's been banned...

Lorenfb

"There's currently a good thread on this issue on Pelican:"

Really! All/most is what's called hypebole:

"Motronic has a little known function that pulls back overall ignition timing as the intake air temps rise. This is the system's knock protection function."

That's totally misleading and has very little effect in reducing detonation,
as the key cause relates to head temp and engine LOAD. That's why
Porsche used/uses knock sensors on all later engines. As an example
the air temp could be moderate, but because of engine conditions (LOAD)
the head temp would very high resulting in a high probability of detonation.
This is VERY problematic in an air cooled vs a water cooled engine where
the head temps are more controlled.

"What this all means is that in general is the 87-89 Carreras can run on 87 octane fine."

A very ridiculous statement, which totally ignores Porsche's requirements, to promote
the sale of "performance" chips and only the very naive or those who rationalize their
purchase of such would believe.

Lorenfb

"If you've ever seen a piston with a hole burned through the top of it from predetonation, you would understand why $.20 more a gallon for the best gas available is cheap "insurance."

- Hester -

One of the few/only intelligent statements found on the Pelican thread mentioned.
But I guess that's normal for what gets posted over there!