Don't care if it reduces peformance, I only care if it will damage my car.

Can lower octane fuel like Exxons lowest octane fuel rather than highest octane fuel actually damage my engine?

I looked through the manual, but it doesn't say anything, it just says Porsche 'recommends 91' but my understanding is if you use less that it just retards the timing to compensate and everything is fine.

Can anyone confirm?

It just cost me $75 to fill up the Cayman with 91 octane. wouldn't mind saving a few bucks per fill up. It's getting really pricy up here in Canada.

while i can't tell you the pronouce effects but if its money you are trying to save than perhaps the recommanded grade is still your best option, this is because the engine management will make best use of the recommanded fuel type (thus achieveing best efficency), and the rest is up to you driving economically.

I don't know the answer to your question but from the picture, it looks like there is a tanker truck that may have just filled up the tanks. I recommend not getting gas at a station where a truck is filling the tanks or just has filled the tanks as it tends to stir up the settled sediment at the bottom of the gas station's tanks, which then finds its way into your car and is not good. Now I am not an expert on this and I don't know how true it is, but it makes sense to me so i stay away when i see a tanker truck at the gas station.

You won't get knock, the ECU will compensate and power will be reduced accordingly. The service manager at the Mercedes dealership I worked for put in regular on his new 2007 ML, he said he couldn't notice a difference and it produced no ill effects, he said he wouldn't hesitate to do it in any Mercedes except maybe the supercharged ones.

Personally, I think the thought of it is crazy. I mean you just bought a $50,000+ car, 20c. per gallon is not the place to cheap out and treat the car badly. The only way I'd ever consider it is if the car was a leased car and I just didn't care at all because I was getting rid of it in two years, but even then, I'm not one of those who would compromise the performance. Also, even though the ECU will compensate by dropping power, I really doubt the engine will run as smooth as it currently does. The more octane in gas, the longer+smoother it burns.

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- Suneet

Well my car is leased, but I still want to look after it. Anyway, if it's not damaging it then I'm not hurting it right? It's just a few less horsepower. The cost savings will be about $28 per month. Not much I know. Anyway, I'm mostly riding my 848 these days and the Cayman is just kind of a rainy day car so I'm not all that worried about squeezing every last bit of performance out of it. Maybe it was just the psychological effect of seeing over $75 for the first time when I'm use to being in the sixties.

I agree with Suneet. Penny-wise, pound-foolish.

I don't know exactly how the Porsche engine management system works, but most go to a separate low-octane timing and mixture map when they count a certain number of pings of a certain magnitude.

If you look at the tops of pistons that have suffered from pinging, the surface looks very slightly scarred or marred. Is that damage? Well, it's not serious damage. Pinging is the heat-induced pre-ignition of the mixture, that causes shock waves that literally cause a "ping" as the wave hits the combustion chamber wall. Knocking, on the other hand, is very serious, and is an explosion that can literally blow a hole in the top of the piston. Every make & model engine is different by how much the low octane map retards timing and richens the mixture, and the necessary amount to protect the engine varies by temperature and air density, so it's really difficult to say if gas of a certain octane will fall within the safety range of a given engine under particular conditions. I think the evidence is that the auto makers are very conservative, and for most engines even 87 octane rated gas will not cause knocking. Some manufacturers allow some level of pinging in normal operation, some are more aggressive and retard or richen enough that the only differences are power and fuel economy. In my experience, the US and Asian makers let the cars ping, the Euro guys are more aggressive.

So, if they adjust the timing and mixture on detecting pinging what's the big deal? Well, for starters, retarding timing can increase the degree of carbon build-up in the combustion chamber, and the carbon raises the octane requirement of the engine needed to avoid pinging, so over time your performance can degrade and pinging can get worse, and even knocking can become a possibility.

Retarded timing also decreases fuel economy, as does richening the mixture. How much will fuel economy degrade running 87 octane gas in your Cayman? I can't predict; maybe not at all. Try a tank or two and measure it. There will be less of a difference in the winter than the summer, because the temperature of the intake air affects the engine's octane requirement. Hotter air means you need a higher octane rating.

So if you're so concerned about saving 20 cents per gallon, I say try it and do some measurements. If you hear no pinging, feel no roughness, and your fuel economy doesn't plummet, go ahead. Do with your car as you please. It's only a mechanism.

My best guess - in the summer months you'll see a fuel economy decline of about 5% on 87 octane gas. Assuming $4.50/gal, 5% is about 22 cents per gallon... a wash in economic terms! Any more than 5% and it's better to run premium at these gas prices.

Even assuming the same fuel economy the cost savings on 87 octane rated gas is only about 5%. Is it really worth a 5% cost savings to have the possibility of your Porsche operating with retarded timing more often?

I only fill up a half tank at a time (~35) for me, it keeps the sticker shock away for a full tank but mostly it's because I have a stigma about the weight of a full tank affecting my 0-60 (not that realistic I know...) Also there are some weeks that gas prices are lower here than others, so it's a gamble.

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- Suneet

Maybe your comment is why I would never, and would never recommend, buying a car that has been previously leased. I think I would give up eating out 1 X per month or Starbucks and not compromise the car.

thats very noble of you sir, if everyone is so nice to lease car then the world would be a better place.

If i were u i'd use the lower grade then, your're not going to floor it anyway if you want to save on fuel so i don't see too much problem for the engine.

What kind of world have we come to when a gallon of gas costs as much as a Venti Caramel Machiato... not one I want to live in!

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- Suneet

The difference is $28/month? You're buying 140 gallons of gas per month for your Cayman?

I heard that too never fill up when a tanker is refilling the main tanks---way too much crude gets stirred up and clogs your fuel filters