matthewb

Hi,

I'm a fairly new owner of a 1991 C2. The previous owner installed an Autothority Mass Flow kit. I contacted Autothority, and they recommended using an octane above 91. In my area (Logan, UT), this isn't so easy to come by. I've been buying Sunoco 260 GT (100 octane) in a 5 gallon can and mixing it with 91 octane, which is the highest octane I can buy at a pump. By my calculations this brings the octane up to (15*91 + 5*100)/20=93.25 since I have a 20 gallon tank. This is pretty expensive since the 5 gallon can runs around $50 (last year's price). It is also inconvenient if I want to go on a trip (Should I carry a spare 5 gallon can of 100 octane in my car?). I've had other thoughts:

Someone told me that since I live in the mountains (altitude about 4500 feet), 91 octane should be just fine. He said that the octane requirement is decreased by about 2 at this altitude, so 91 at 4500 feet gives the same anti-knock performance as 93 at sea level. If this is true, then I probably don't need to raise the octane level

Then, I also wonder about octane boosters that are available in many stores. I've never looked into them, but I'm assuming they would be a much cheaper way to go than spending $50 for 5 gallons. But then I worry about whether these boosters will harm my nice engine in some way. Does anyone use an octane booster in their Porsche?

Can anyone clear all this up for me? Thank you!

Matt

ilko

91 will be fine. I'm hesitant about using octane boosters.

kos11-12

I think octane is related to timing and compresion ratio, altitude will only alter richness (less air) so it will run richer, but maybe the ECU or air mass metring will correct,

Indycam

Do you have a code reader ?
Can you get someone to read the codes on your car ?
If yes , run pump gas and have the reader look for knock count .
Then run the mixed gas and the reread the knock count .
If there is no improvement in the knock count , the mixed gas is not doing anything for you .
On the other hand , if the knock count goes down , then the mixed gas might be a good thing .

People who drive lightly don't need as much octane as those who drive hard .
People who don't clime hills don't need as high a octane as those who drive hard / fast up big hills .

When a manufacturer recommends one octane level , its a general recommendation that does not cover all situations . Your driving style , location , etc etc etc

matthewb

Thanks for the responses! I do not have a code reader. Which ones are recommended? My understanding is that the engine computer prevents knocking by retarding(?) ignition timing when pre-detonation (knocking) is sensed. In spite of this, reading the codes should still yield numbers to make a determination about octane? Sounds like a good approach if the engine computer doesn't spoil the results by automatically compensating for insufficient octane (and therefore reducing power).

bgiere

Tolulene

http://www.elektro.com/~audi/audi/toluene.html

Fred's 964

MatthewB, the computer doesn't eliminate knocks until it detects them. It adjusts the timing after a knock is sensed (hence the "knock sensor" label). The knocks will be recorded and immediately compensated for with timing.