When i finally started to cut through the BS fog of the gas marketing, i found out a few things.
first , regular has a higher energy potential than premium. both are the same quality, but one has more octane. (resistance to detonation, as its causes a flame front to propagate slower).
If you want max hp, you need to have timing and compression to match the octane rating of the fuel. If i could get more power, i would run 110octane at the races, but i cant take advantage of it. i cant adust timing and my compression is only moderate, being a moderate 10.3:1 stock S4. having compression and timing to take advantage of the octane, allows for more energy being released under combustion. even though the gas has less energy potential, the greater compression and timing allow for more power to be generated, without dentonation.

now, around town, i run 91 octane, because every time i get in the car, im WOT a few times to do 60-100tests, etc. With the gas crisis, im not doing it anymore (or near as much). I would have no problem putting 87 octane in it if it was my daily driver.

My BMW 328i specs the use of 91 octane. However, since i NEVER floor this car being my work car, i run regular and have never run 91 octane in it. It runs great, plugs are clean, gets great gas mileage and ive never heard a ping, even under WOT, which i have only done a few times in its life.
In theory, you might even get better gas mileage by using regular than premium due to its higher energy potetial. try tank and see. Use Cheveron as it has the techroline if you are worried about injectors keeping clean. premium vs regular has nothing to do with the quality of the gas, just the octane, and that might not be needed if you drive more mellow. If you hear a ping under the higher throttle settings, then you know, you cant use those throttle settings there. under cruise, it shouldnt be an issue and you can save $5 a tank!

mk

 

Mark how big is your fuel tank ?? 100 gallons worth mentioning that detonation at low levels may or may not be something you hear. In the end the biggest factor for fuel costs ends up being how you drive not what you put in the tank.

 

Now that my car is not supercharged for the time being (almost 2 years now), I've been running regular gas with zero issues. There is so much mythology around premium gas. It's just higher octane. Octane is a measure of resistance to detonation. I see no difference at all in mileage or performance and there shouldn't be any, providing there is no detonation. Even this stuff about Arco being somehow dirtier than Chevron is made up. I may not buy gas from Rotten Robbie, but my car has had zero problems with anything else, still running the original never-cleaned injectors approaching 191K miles. Because of the way the US calculates octane (RON+MON)/2, it can be confusing to determine the requirement, but I concluded that RON 91 or 92 is approximately US 87. Regular works very well in my S4. This probably wouldn't be the case for a GT or GTS. My car is very fast even though it is back to being completely stock - I beat the Holbert car in a street drag - sorry, Mark!

 

Ive been running 87 octane in the BMW for almost 10 years now. its 10.2:1 compression. I dont think there is any issue with pinging, especially at the lower power settings im operating at. If i tracked the car, sure, i would use premium.

mk

PS, fixed the $ savings. Still, 5 bucks a tank at 20 gallons!

 

I noticed from another post Paintball is 17 years old...owning a car, any car, is a new experience. Driving a car older than you are can get interesting especially for someone like me

 

Arco may not be be dirtier, but it doesn't have the detergents in it that Chevron does to keep your engine clean. My direct experience with this was several years ago in my '87 BMW 325. I had two gas stations on my block, Chevron and Arco. Being a poor recent-graduate at the time, I always used Arco. At some point performance tanked, particularly when accelerating onto freeways. Step on the gas, and little would happen for a few seconds and then it would go. I took it to my mechanic and he told me to put a bottle of Techron in it next fill up, and then at half tank top it off. So one bottle of techron in 1.5 tanks of gas. By the time that tank and a half was done, the problem was gone. From then on I filled up with Chevron at least 2/3rds of the time, and the problem never came back.

I started using the Premium grade of Chevron too, and by my estimate I got about 5% better gas mileage too, measured in identical situations (Between Santa Barbara and Fresno, a trip I make several times a year). This was the case in both the 325 and 525. I think the ignition senses lower octane and retards the timing, lowering efficiency(?). I never heard knocking in either car. Both cars had a little sticker on the fuel gauge that said Premium Only, and both owner's manuals said to use Chevron. I thought that was "marketing" too until I had the problem with the 325.

That said, I'm running regular grade in my 928. I heard pinging once under hard acceleration. The research I did last year indicated that back when my BMWs were built, Chevron was the only gas that had "good" detergent in it (techron) but that since then all the majors except Arco and Exxon have followed suit, so that's what I go with (usually Shell or 76/Phillips) now.

So, at least in some situations, the brand of gas does matter. But it matters less than it used to.

 

you didnt get better gas mileage with the premium, as it doesnt effect gas mileage. If 5% could be found, i think everyone would run premium.
plus, the 928 doesnt adapt to the gas, (either does the bmw) however, if you do ping, it does retard timing. (but that realy only happens higher throttle settings) keep in mind, that fuel efficiency is an entire new set of factors vs looking at just raw power. your gas mileage tests were not controlled, and certainly fall in the acceptable range of normal variance. if you were getting 21mph, and you then run premium, and get 22mph, based on all the variables, its pretty hard to determine that that was the reason. if it was on the freeway where a BMW 87 gets near 30mpg, that is still 1.5mpg more and could be due to headwinds, among the many factors.

I do think the techroline in cheveron is good for the injectors and fuel systems, and only use cheveron when possible with the 928. however i do often use 76, but thats because NASCAR uses it too

try that test with the premium and regular again. i bet there is little differnce, if any. theoretically, if you are not pinging, the regular should offer better mileage.

Ive only heard the pinging on a 928 once, and that was on a stroker. when it pinged, the pinging never went away for the given powersetting. dont know if the knock sensors do that much, but it didnt help this situation. a good question is what provides better gas mileage at cruise as far as timing goes. advanced just before ping, and high temps, or retarded. certainly, you could run REAL lean, just before the engine starts missign, and get lower egts and chts, but then the NOX would go out the roof! (airplanes do this to get better fuel mileage). Now, that would be something i would rig up if i was doing a lot of long driving runs. bypass the o2 sensor and pull back the fuel to 18:1 with some type of adjustable fuel regulator setting ****. put the car at 70mph cruise control and probaby get near 30mpg!

mk

 

I've used 87 octane in my '78, Euro '79, '82, '83S, and '84S. Never once had a problem. It's what they were designed to run. Running higher octane in these cars doesn't do anything more than cost you extra money. I ran 91 octane in my '83 Euro engine, because it has higher compression and the engine specified a higher octane.

I use 91 octane in my '88 and '89. I believe both cars specify 90 octane. Both run fine on 89 as well.

My GTS used to ping slightly on 91, but that crap is all I can get here in NorCal. The car specifies 93 octane, but for some stupid reason, we don't rate 93 octane here on the Left Coast, so I run "Premium" 91 octane.

Your owners manual and the inside of your gas flap should specify the octane rating required for your particular car.

 

My car is sensitive to brand/quality as well as octane rating. Poor quality premium, a 'regular' regular, no matter, it still pings under load in hot conditions in my normal driving. That timing retard from the knock detection does trash the mileage a bit, so in this case premium does in fact get better mileage than regular. Difference between reg and prem is still twenty cents here. With premium pushing close to $5/gal this morning, twenty cents is getting to be a smaller and smaller 'premium' for premium, as a percentage anyway. Only about 4% cheaper for regular, right?

 

I have always used premium in my 87. 93 oct when available. This is my fun car and is driven as such. Drive gently until warmed up, then into the throttle as much as possible.

My concern is what to do on the road in the rare cases when 91 is not available. Can octain booster be added to reliably increase octane. Or is this going to FUBAR my injectors?

 

Detonation is not pre-ignition. The issue with fuel vs the Otto cycle engine revolved around internal cylinder pressures. That is why many people run fine around town on 87 octane. The ICPs are moderately low, and there is little thermal stress in the combustion chamber. Detonation margins are high.

However, If you run hot and hard a lot, the detonation margins are reduced and it is a discrete problem, you are either going to detonate or not detonate. It is also a self-sustaining problem, in that detonation causes detonation.

Winter and summer blends also affect detonation margin. You should test your driving style with mid-grade, and verify non-detonation. If that is appropriate given the climate conditions, keep going. If you detonate, move up to premium. You can then try 87, and decide if you want to accept occasional detonation, which is not immediately fatal. Pre-ignition is often immediately fatal in the form of bent rods, or holed pistons.

Test and verify.

 

I of course can't confirm this, perhaps someone in the oil industry can clear it up, but I think the oil companies share resources whenever they want. Back in the '90's I used to live in Linden, NJ, where the old Exxon refinery was located. If you parked across the highway, over the course of a week you would see just about every brand of gas truck going in and out of the place, coming out sitting squat, fully loaded. We of course speculated that the oil companies buy from each other as necessary and convenient, and as long as the truck at the station has the right logo, no one ever seemed the wiser. So, it seems that you never really know who's making your gasoline. Last I heard, Tosco owned the refinery in Linden. Who owns Tosco, I don't know.

 

Don--

Drove by the Bayway refinery a few times in the last week. I was at the south end at Woods Ave-- the one you are talking about? Still has Exxon signage up down at that end. Drove past that to the PSE&G power plant out in the Grasselli marshes. Still plenty of tankers winding there way around that area, all with independednt branding on them. Where did you end up after leaving beautiful Linden?



We (L.A. area Kalyphornyens) endure a variety of seasonal blends of fuel, pretty much all spewing from one of three major refineries in the L.A. basin. Exxon and Mobil are the same, Texaco and Shell are the same, Valero is the 'new' Unocal I think, and Arco and BP are the same. The independents go wherever they can get the cheapest product. I can't remember ever seeing cross-supplying trucks from one of the major partnerships, but I also know that they all take turns pushing petrol through a pipe to Las Vegas, and another to Phoenix. So there must be ways to get the right valves open at the corect times to get blends. I know they don't pig between batches in the shared pipe. If vendor A is pumping into the pipe at this end, that must be vendor A's product dropping out at the other end.

Ive spent a few hours in refineries in my short life. Not a career I'd like.

 

Knock sensors don't affect timing? Timing doesn't affect gas mileage? I did notice less effect on my 525, more on my 325, maybe 10%. On premium I approached 30mpg in the 325. 27-28 on regular. No, it wasn't a controlled test but I did it often enough that I believe it. Maybe it was just the way that particular car was tuned.

That I never claimed, particularly on my '79. I don't even think it has knock sensors does it?

Both Bimmers are long gone...

Interesting idea in these days of $4.50 / gallon gas. When I first got it my original mechanic leaned mine way out to pass smog and didn't reset it, and the power decrease was noticeable. I do think it got better mileage though.

 

It's not unreasonable to think that a particular refinery - no matter who owns it - can refine in batches with each batch carrying a different formulation per the buyer's specs. I don't know that they do this, but I would be surprised if they couldn't do it if they wanted to.