Originally Posted by JerryU

Respect your opinion. Sam's high test probably has some additive but since it is probably not consistent, understand why it's not labeled Top Tier. I used to fill my Street Rod (8.2 L BB) up in a small independent near where I lived. One day the owner showed me the gas deliveries he received and they were from all major brands! However that station was purchased and now also has crazy hi test pricing!
Was thinking I would use the GM additive twice a year in the C7 since I typically only put enough miles on for a once a year oil change. Would using Top Tier every 4th fill be worth the effort?

Originally Posted by LDB

That’s about $100 per year at the prices you are quoting, making it another non-obvious answer. One of my uncertainties is that I saw very little data on direct injection engines before retiring 5 years ago. Directionally, top tier makes less difference for those engines since the detergent is less able to get at the intake valve stems. I don’t buy the extreme positions of some like COSpeed who say additives are totally worthless on DI valve stems, as there’s too much talk from reputable sources about pulsed injection cycles (meaning that at low to medium power settings, a brief pulse of fuel gets injected before the intake valve closes, and with its high pressure, some of it can penetrate against the incoming air and get as far as the valve stem). But it’s also clear that additives aren’t the total cleanliness answer for DI engines like they were for PFI engines. So unfortunately, you’re left having to make a choice on the basis of what feels right to you rather than what you can prove or be at least reasonably certain about.

Originally Posted by LDB

There’s a bit more to octane than that. Octane refers to a gasoline’s anti-knock characteristics. A rating of 100 was assigned to the specific compound 2,2,4 tri-methyl pentane. It was chosen because it makes up a major part of a blend component called alkylate, developed during WW2, which was one of the earliest high octane blend components. It’s also where the test’s name “octane” came from, because that compound has 8 carbon atoms, and is hence one of many octane isomers. The specific compound n-heptane was assigned an octane rating of zero. To determine the octane rating of any other material, you run it in a test engine to see where it’s anti-knock characteristics fall in relation to those two specific compounds. So for example, 93 octane would behave the same in the octane test engine as a blend of 93% 2,2,4 tri-methyl pentane and 7% n-heptane.

As a minor side issue, as others in this thread have noted, there are two different octane test methods, R and M. The difference is that in the test engine, you vary either compression ratio or spark advance, with the former giving an R octane rating and the latter an M octane rating. R numbers average about 10 higher than M. There are also some special rating methods for aviation gas involving rich mixtures and supercharging, that give numbers about 30 higher than R.

Temperature does have a modest effect on how much octane an engine requires, but it’s hard to determine the quantitative effect because there are offsetting issues. Higher temperature by itself increases the octane need, but reduces air density which reduces octane need (unless the engine is supercharged). The exact way those offset depends on the engine’s cooling system and computer control algorithms. Most of the numbers I’ve seen suggest that it takes an ambient temperature drop of at least 25 degrees and probably more to reduce octane need by one number. So since I don’t think Florida temperatures are all that much cooler during winter, if you’re using lower octane gas in the winter, it may simply be a case that your engine’s knock sensors are just cutting more timing when you’re using lower octane during the winter.

And finally, while I suppose it’s a minor point, it’s not so much that higher octane gas is harder to ignite, it’s that it’s less prone to detonate. All gas is very easily ignited by the spark plug. But the gas is supposed to burn rapidly once ignited, not explode. Sort of like going “phhhht” in a very rapid burning front across the combustion chamber, not “bang” throughout the whole combustion chamber all at once. It’s kind of like the difference between dropping a 5 pound steel ball versus a 5 pound rubber ball on your foot. They both deliver a 5 pound impact, but the rubber ball doesn’t hurt your foot as much.

Originally Posted by tuned

I agree with u that it probably takes a -25 degree situation to take effect. My cooling fans did not come on til 225deg with stock. Now I run at 160 deg operating temp all the time. I don't agree that higher octane is not harder to ignite,cuz it is. It burns longer,and cooler during that burn too. Pre ignition is the cause of detonation. U could have the same engine with same octane, at 210deg it pre ignites. At 160deg it may not. Temperature is everything with how an engine runs,why it runs,and how it runs. Higher compression engines make more heat that is harder to dissipate. Anything above 11to1 probably is the cutoff. Go out to the yard with 2 gas cans premium/regular. Light a stick on fire and put it in both puddles for an experiment on the pavement. Depending on how hot or how cold it is outside will make the fuels burn the same,or burn different. I'm only referring to my tuned port injection 9.5to1 naturally aspirated motor. This is what I have seen driving this car and putting 225k on odometer. Not all vehicles will perform the same in different environments. I drive to the track,run consistent u.7 in 1/8mile. Been doing some rebuilding. I expect to be in lower 8's here in a month. Not bad for a poor man's toy who races 40,000$ cars and beats them by a fender

Originally Posted by Vetteman Jack

In a pinch it can run on regular, but I would put the recommended premium fuel in it.

Originally Posted by meyerweb

Thanks LDB. Your explanation is much better than mine would have been. I'll only add that pouring gas on the ground and lighting it has about as much relationship to what goes on inside the cylinder of our engines as does lighting a candle.

Originally Posted by rkhegler

You guys are lucky. In our state, Nebraska, there is about an $0.80-$0.90 difference between mid-grade and premium fuel.
http://www.gasbuddy.com/Station/17244

Makes one think hard about using mid-grade. In some cases that means $17-$18 per fill up. Twice a month that comes to $36.00 or $432.00 annually. That is almost my annual insurance payment.