jalden

just bought a 2018 cayenne. it says only use 91 octane fuel. is this a must or has anybody used anything less. and will it hurt?

Avec

You just bought an $80,000+ car and you want to go cheap on fuel?

Petza914

If it says use 91 then use 91 minimum. It means the engine is tuned for that level of octane (timing advance, etc) and using a lower octane could result in predetonation and engine knocking, especially under load, which can damage the motor.

Avec

The knock sensors will retard timing to prevent pre-detonation, unless it is really, really low quality/octane fuel. The true detriment to running lower octane fuel is lower performance, and lower fuel economy - both because of the aforementioned timing. Higher octane fuel also tends to have more cleaning additives, so carbon deposits are kept to a minimum.

Overall, the car will run better, with better fuel economy, and stay that way longer with higher octane fuel.

nodoors

If you don't have an option because you are in a remote location, then it will probably be okay. It will just not run optimally. If you are doing this by choice to save money, then it is a poor decision and you bought the wrong car.

1. Your gas savings will be negated by higher fuel consumption in an engine designed for higher octane.
2. I wouldn't run it on really hot days or if I was loading the car down
3. Over time, you are more likely to have problems with fouled plugs, O2 sensors, catalytic converters, etc. due to cooler and incomplete burn of the mixture when the computer retards timing because it senses the preignition detonation from the knock sensor

oldskewel

All good advice above.

Fair question. Yes, you should avoid less than 91 octane whenever you can. You can safely ignore any recommendations regarding brand of motor oil though. Shell, Mobil, Porsche, etc. (just pointing out that some recommendations that come with your new car are pure marketing - gas octane is not, though).

Also, conversely, if you had a car that required 87 octane, you'd actually be worse off paying the extra $$ for 91. 87 will give better fuel economy if the engine is designed for that. Higher performance engines are typically designed with high compression ratios and the higher octane serves to suppress unwanted ignition. So again, if your different car asks for 87, give it 87.

deilenberger

Jaldin, your Cayenne is a 958 - and we have a forum just for it. I'm moving this post over. As others have said - why cheap out? It will likely cost you less in the long run to follow the manufacturers recommendation.

ScootCherHienie

Myth Number 1 - Lower Octane reduces fuel mileage. This is so untrue it's difficult to even know where to begin. A gallon of regular and a gallon of premium fuel both have PRECISELY the same number of BTUs. A gallon of each fuel produces the same amount of energy. The fuel itself does not change MPG as long as the car runs properly on the low octane fuel (most any modern car will). The only difference between high octane fuel and low octane fuel is that the high octane fuel burns SLOWER than low octane fuel and resists detonation. The engine computer will adjust ignition timing and fuel delivery to make the regular fuel. And if the vehicle has variable cam timing as many Cayenne engines do, the valve timing can even be adjusted to make regular fuel work perfectly well.

Also at high altitudes (like Colorado where the LOWEST altitude in the whole state is 3500 feet), all gas sold has lower octane because the altitude causes lower air pressure and hence, lower pressures in the combustion chamber because there is less air drawn in on each stroke. So "premium" in Colorado is 89 octane... NO PROBLEM and it is actually BETTER to use lower octane at higher altitudes. If you are at a low altitude and anticipate driving to a higher altitude and staying there while driving, fill with mid-grade or regular instead. And if you find yourself in a situation where you need fuel and regular is the only choice... go ahead. Fill up. Everything will be FINE.

That said... it is kind of silly to go for a lower grade of fuel on these cars unless premium fuel simply isn't available. It saves maybe $4 per fillup to use regular. 1 less cup of Starbucks coffee and you paid for the difference in cost. If you do find yourself in a situation where regular fuel is

nodoors

You have it backwards. Running high octane in an engine designed for low octane will just waste money. Running low octane in a high compression or turbocharged engine designed for high octane will result lower gas mileage and poor performance. It will absolutely affect the power and efficiency when knock is detected and timing is pulled. I have experienced this first hand most recently when making the same long drive with my wife's car many times and trying different octanes while recording the resultant mpg. Unloaded car, limited elevation change, slabbing it on the interstate with no hotdoggery or other factors that might induce extra knock (primarily heat and high cylinder pressures at low rpm) the cost savings of the cheaper fuel is almost perfectly cancelled by worse fuel economy.

Agreed.

ScootCherHienie

I didn't have anything backwards. What I wrote is accurate... but I didn't specify I was talking about a modern computer controlled engine the manufacturer specifies 91 Octane fuel for-like a Cayenne engine. If the engine is said to require 91 octane, and you put regular fuel in it, your peak HP will drop off slightly... 5 HP maybe in a V8. High compression in a normally aspirated engine is no impediment to running it on regular in a modern computer controlled vehicle.

Turbocharging and supercharging are HUGE variables and you can't make a blanket statement that 87 in one of those cars will do ANYTHING at all in regards to fuel economy. In engines designed for vehicles whose primary use is going to be as a daily driver, the will be accommodations in the turbo/super system to cope with a range of fuels. They do that for no other reason than giving the engine latitude to operate safely when there is no high-octane fuel available or if one of the car's systems begins causing problems like low fuel pressure, or loss of variable cam timing, or most any other condition you could imagine... high temperatures, high altitudes, below sea level, very cold environments. In Cayenne types of engines, regular could cost 100 HP maybe even more than that, but the engine will still run fine and still get fuel mileage that's not much different than premium. Turbo/Super engines are simply lower compression versions of high compression engines with a turbo/super that can potentially make the compression ratio higher than the compression in a naturally aspirated version of the same engine. As long as the vehicle can limit boost, the regular fuel won't hurt anything except HP and TQ.

NA 2005 BMW 3.0L 87 octane 29 mpg @ 70 mph, 91 octane 29 mpg @ 70 mph, over 2000 miles run on each fuel. BMW says it's a 91 octane vehicle. You MUST do things like this over many miles or the variables get out of control. Wind, temperature, tire pressures, more or less air conditioning, and throttle use will all affect MPG more than 87 vs 91 octane. I would estimate the horsepower loss with 87 octane to be pretty similar to the loss you get when the A/C compressor on vs. having it off all the time. With the BMW it felt like about 5 HP or so.

nodoors

I don't understand why you would try to confuse the issue. We are specifically speaking about Porsche Cayennes here.

RED HORSE

The bottom line is, use what your owner's manual specifies. They put the info in there for a reason, and I don't buy to assertion it is all a marketing decision.

ScootCherHienie

I didn't try to confuse the issue. The reply to my post implied I wasn't talking about modern high compression engines like those in Cayennes. I responded to the reply appropriately.

element

I run 91 in my 1978 VW bus! it makes the motor run cooler.. have you calculated how much the difference is? under $5 for a full tank - there are much better\easier ways to save $5!

ScootCherHienie

That is impossible, scientifically. There are the same number of BTUs in both regular and premium. Therefore, both fuels produce the same amount of heat and do the same amount of work when they are burned. The ONLY difference between regular and premium fuels is that premium fuel burns more slowly. In air cooled engines, in hot ambient temperaatures, premium fuel can stop knock/ping that can happen because of elevated engine temps. Premium will stop the knock/ping at high ambient temps, but it won't make the engine run cooler. Only cooler air or more air (or splashing it with water or something similarly extreme) can make an air cooled engine run cooler.