9x8

I'm amazed by how often such discussions come up. Seriously, there are cayenne turbo owners discussing that 92RON is actually better than recommended fuel! I mean, come on, - manufacturer recommends some sort of fuel and you just use that, it's that simple.
Of cause sharktuned, custom EFI, supercharged and all other non-stock vehicles are whole another story, but for stock ones just stick with manufacturer recommendation.

If you can afford the vehicle, be ready to pay for the fuel.

Andre Hedrick

Good Answer !!!!

tilac999

I run premium but I've tried mid-grade and even regular, no ill effects and performance seems the same. I don't have a knock sensor.

Ispeed

Dumbest question and thread in 2011?

SeanR

Nope, go look at his anti-Jew comments in OT, it gets worse. This guy drinks more than anyone I've encountered. He's gotta be close to being banned.

aj986s

Not just bearings. When you hear a "knock", an explosion is happening where it shouldn't. Normally, the sparkplug fires, and the air/fuel closest begins to burn. This burn travels as a flame front across the piston. The heat causes air to expand which results in the forces pushing the piston down on its power stroke. Though it happens in a short amount of time, its supposed to be a smooth, gradual process. But when the flame front causes enough heat & pressure to build up at the other end of the piston, if conditions are right, the air/fuel that hasn't been burned by the flame front yet, can spontaneously combust, which is the knock/detonation.

Octane helps fuel by raising the level of heat/pressure needed for spontaneous combustion. Other factors can also affect, like timing, air temperature, air/fuel ratio, boost pressure, compression ratio, etc. Knock sensors usually retard the timing to delay when the sparkplug fires, which lessens the pressure buildup, but causing loss of power and usually more pollutants in the exhaust (unburned air/fuel).

In addition to jarring the bearings, it can also bore a hole through the piston over time.

Jfrahm

IMO the octane requirement is not a great concern for street driving in cool weather. My owners manual and gas flap do not suggest what you can run in January is any different than what you need in July but experience suggests otherwise.

Based on some very hands-on experience with turbo cars that are sensitive to intake and ambient temp, if the air intake temps are cool you can get away with murder on low octane gas. Likewise if you have not heat-soaked your heads good before you test for knock you will not see it so if you are knock counting take that into account.

If you compare premium to regular performance in a 928 when it's 40F out, You probably won't see a difference in street driving. If you do this test again in 90F, you probably will.

First - Fill up often so if you get bad gas you do not get a whole tank of it (probably unnecessary if you get name brand gas from a busy station but this can be critical on long trips.)

Second - Buy gas from busy stations, and if you are buying premium from a place that sells a decent amount of premium.

Third - Buying gas from name brand stations is IMO more likely to get you a quality product that is as advertised. Personally I also buy gas from our busy supermarket chains but my feeling there is that they have a lot to lose from selling bad gas so are not going to cheat much.

Never buy premium from some sketchy station that rarely sells premium, they chance of getting bad gas is much higher. If I desperately needed gas and stopped in Bugtussle I'd buy whatever they sell the most of and not risk the premium. If it's really hot out I'd take it easy until I could top off with quality stuff. Nowadays it's also wise to carry some fuel drier and not fill up to the top if you are away from home and need some gas and have to get it from a shady station. These are the guys who think they can sell water for $4 per gallon and add a fair bit to their tanks.

More than once around here stations have been busted for selling regular as premium or, more likely, midgrade. Having had cars in the past that ran perfectly on midgrade as long as it really was midgrade, I got burned a few times.

http://www.thedenverchannel.com/news...24/detail.html

Cheers,
-Joel.

Tom in Austin

Interesting discussion. Here in Austin, every drop of gas comes from a single Koch refinery in Corpus Christi. Trucks go to a terminal on the east side of town, load up, and then throw in a specific dose of additives that theoretically differentiate Shell from Exxon from whatever. So, for us, the only variable in 'quality' would be the additives and then, perhaps, how long the stuff sat in the tanks at the retailer before being dispensed.

It's kind of amazing to think of all the marketing dollars spent to differentiate brands when the raw product is exactly identical wherever you fill up.

hacker-pschorr

How do you know unless you keep pushing it until the engine goes...? The answer is, you don't

You have a 10.4:1 compression engine with a "wedge" style combustion chamber. If any engine requires premium it's yours.
Unless you never go past 3/4 throttle and shift at 3,500rpm.

I've said this in every octane discussion thread. I've been at the dyno, standing next to the engine with the hood open and nobody could hear it knock / ping, yet the shark tuner said otherwise.

I use to agree with that until one winter I put 87 in my Audi Coupe (20V 5-cylinder with 10.3:1 CR). First run at WOT I could hear it ping (yes I had the window slightly cracked open).

This brings up the discussion of what they add to "winter gas" in the snow belt though. I agree 100% if you live in an area that runs the same blend year round.


Back to Mike's original post. Any engine designed to run a higher octane gas will run less efficient with each drop in octane. So even if your engine never knocks or pings from the cheaper gas, you are most likely pissing away any savings by running your engine less efficiently than designed.

the flyin' scotsman

Mike.....this thread, as usual, is starting to drift so here's my last input:

internal combustion engines like cool dense air with cool fuel. Premium fuel gives slighly more performance but more importantly prevents engine damage especially in high compression high performance engines. You dont live at sea level so the air is compromised (thinner) and the engine management system adjusts accordingly and leans out the fuel input to keep the air/fuel mixture at optimum. In hot summer driving this could cause the engine to detonate and eventually wear with catostrophic result.

What do racers do to optimise performance at different race tracks? adjust the fueling to suit all conditions for best performance and ensure engine longevity. The variable conditions are weather and altitude that affect the air (temperature, humidity and density) and on different days the fueling is changed to compersate with old schools carb jetting or more modern fuel mapping in eproms.

As stated I use premium, mostly Shell F1 in the 928, VP race fuel in race applications.

Jfrahm

That's only true if you are not getting full advance. In my experience, in cool temps, I could get full advance on quality gas purported to be 85 octane and so any increase in octane would just cost money and reduce the energy content of the fuel (the additives that increase octane having less energy than the fuel they replace.) You do not get more power by putting race gas in your 928 so there is a point where you are wasting your money on octane, and that point is a moving target due to the environment and the quality of the product you get (not the quality of the product you buy, the quality of what actually ends up in your tank.)

I have also gotten knock on some ripoff gas in any weather so the quality of the product is also a big factor. Winter blend also is a question mark.

The safest course of action is to just pay for premium but IMO in cool weather you are wasting some money. I put premium in my 928 unless it's winter, but it does not get out much in the winter. My A8 gets regular except for the hottest months. I am not sure what I would buy if I was DD'ing the 928, maybe a Sharktuner would be the first purchase.

If your knock sensors are broken you might have a conservative timing map also and not, under normal driving, notice the improvement you might get from premium.

-Joel.

blown 87

I do not think they add anything, it is a change in the base stock to a different "Reed Vapor Pressure" stock.

That is why a airplane with a gasoline engine can be so friggen hard to start in cold weather, the RVP has to be high enough that it will not vapor lock at altitude and it just almost will not flash over to vapor when cold.

Markmandude

Speaking of altitude, we are at a mile high here in Colorado and very few gas stations sell above 91 octane. Anyone know how much lower octane is safe at a higher altitude? We loose about 1 second in the quarter mile up here for most cars.

SteveG

S4ordie

Knocks are often unheard as stated previously. When Rob Edwards and I were doing the initial SharkTune on my Zyclamrot car there were a lot, I mean a lot, of knocks that were recorded. Not one was heard. I learned a lot from that as I previously only thought engine knocks were an issue when you could hear them.

Jfrahm, instinctively I want to agree with you regarding cooler ambient temperatures, but still, without a some sort of monitoring device there is really no way to know for sure whether the engine is knocking or not. When it comes to my oil and my fuel I will err on the side of conservative; I've got too much money in my cars not to use that which represents the best insurance.