Originally Posted by TriPinTaZ

I never understood people spending $75K + on a sports car where premium fuel is recommended and then try to run 87 octane to save pennies. We are all free to do as we please but I especially loath these kinds of people.

Originally Posted by TriPinTaZ

I never understood people spending $75K + on a sports car where premium fuel is recommended and then try to run 87 octane to save pennies. We are all free to do as we please but I especially loath these kinds of people.

Originally Posted by V Vette

Good in fo but now its is so confusing.. Can the real answer step forward? Thanks

Originally Posted by PatternDayTrader

The answer is this, run the highest octane you can buy at a typical fuel station. In the event you are stuck with a low octane fuel, then go easy on the throttle until you can fill up again with the proper fuel.

The claims about high altitude not requiring as high octane, seem to be centered around lower barometric pressure, which ultimately should result in lower cylinder pressure, and therefore would be less likely to detonate. This is why in certain Mercedes cars the supercharger wont engage, as described in post #33 by JoeC5, instead of just retarding spark timing. The intention is to keep cylinder pressure down so as to not cause knock. The problem with the low octane at altitude claim, is it only applies at wide open throttle, because in every other circumstance, you just push the gas pedal down further to accelerate at the same rate, to or maintain the same speed, and therefore end up with the same cylinder pressures anyway. So to me, the lower octane at altitude claim doesn't hold water.

Originally Posted by PatternDayTrader

The claims about high altitude not requiring as high octane, seem to be centered around lower barometric pressure, which ultimately should result in lower cylinder pressure, and therefore would be less likely to detonate. This is why in certain Mercedes cars the supercharger wont engage, instead of just retarding spark timing (described in post #33 by JoeC5). The intention is to keep cylinder pressure down so as to not cause knock. The problem with the low octane at altitude claim, is it only applies at wide open throttle, because in every other circumstance, you just push the gas pedal down further to accelerate at the same rate, to or maintain the same speed, and therefore end up with the same cylinder pressures anyway. So to me, the lower octane at altitude claim doesn't hold water.

Originally Posted by JoesC5

I like to watch the instantaneous gas mileage reading on my DIC as I'm cruising along at 75-80 MPH using the cruise control. Every time I drive I-70 across Kansas to Denver, I notice that my gas mileage increases(per the DIC) as I gain altitude. And it drops when I'm retuning home.

Originally Posted by LDB

The thing that you are missing is that you only get into detonation risk at high power settings or with fuel way below design octane. At moderate power settings, the worst you are risking is a bit of light knock or pinging, and unless the knock sensors are really in bad shape, they will handle that just fine. Or said another way, as long as a sea level car is only at part throttle, it doesn’t need 93 octane fuel to avoid detonation risk either. Comparing specific situations, a car at sea level at 80% throttle is developing about the same power as a car at 6000’ at 100% throttle. Neither car needs 93 octane fuel to prevent detonation at those conditions because both cars, as you mention above, have lower cylinder pressure than they would if at sea level at full throttle. The only point where highest octane is needed to prevent detonation is when the car is at sea level near full throttle and cylinder pressures are very high. Since the 6000’ car never gets to that point, it doesn’t ever need 93 octane to prevent detonation.

Originally Posted by PatternDayTrader

Any detonation will eventually hurt the engine. This can happen at any throttle opening or rpm. Obviously under a high power demand situation, the damage will occur faster, but why risk damaging the engine slowly ? Why risk it at all ?

Plus, I'm not so sure I buy into the lower octane at altitude, at any point, including wide open throttle. I understand the premise for the claim, but that doesn't mean its right. I would like to read the study that Amoco did because I suspect they used carbureted vehicles, without sophisticated timing controls. The study was done on 84-86 model year vehicles. GM didn't use fuel injection on their trucks until 87 for sure, and I don't think they had any fi cars in 84 except the Corvette. I'm just not going to spend the 30 bucks on the paper, because one need look no further that the owners manual to see these cars need premium fuel under all circumstances. Plus, the entire lower octane at altitude idea, doesn't apply to forced induction cars, including the zr1 and z06. So the claim is fatally flawed right there.

Originally Posted by LDB

This may be semantics, but I don’t think it’s a question of damage happening faster or slower. It’s a question of severity, meaning the spectrum from high to low of detonation to knocking to pinging. The higher the power being developed by the engine, the higher in that list you are likely to be, and thus the more potential for damage there is. Since the 6000’ engine can’t develop as much power as the sea level engine, its risk and need for octane is less.

Forced induction as installed in cars normally doesn’t change things much. While aircraft forced induction systems are designed with extra fat to compensate for increasing altitude, car forced induction systems normally are not. Once the internal bypass is fully closed (for superchargers) or the waste gate is fully closed (for turbochargers) the MAP at full throttle for forced induction systems will decrease with increasing altitude just like a normally aspirated engine. It will still have some boost, but MAP won’t be as high as it was at sea level. Yes, a car’s forced induction system could be designed with extra fat like an airplane’s, but it normally is not because that would cost mileage at sea level, a no-no in today’s world of economy standards. I admit in advance that I do not know the exact altitude at which the internal bypass in Vette supercharged engines is fully closed at full throttle, and thus the point at which a further altitude increase causes full throttle MAP to fall, but I’d be astonished if it were anywhere near 6000’.

Originally Posted by COCorGS

So the question is who else has experimented with different octanes and come up with their own results? And who out there has done some mods and seen some differences on the horsepower gauge, not just the dyno results? It'd be cool to hear about someone adding long tubes, an exhaust maybe with cat delete, a CAI and a tune and getting the gauge to the 460 mark really easily, like it wanted to keep right on going. I think it would be cool if you could use that gauge instead of having to dyno the car to find out if the mods effected the HP. Hopefully this topic hasn't already been exhausted, I tried to search and didn't find anything.

Originally Posted by PatternDayTrader

The forced induction removes the low barometric pressure argument from the discussion. No one is going to argue that.

Originally Posted by LDB

What physically happens in your engine when you use 87 and floor it is that it starts to knock, the knock sensors sense that, and the ECM almost instantaneously retards the spark to stop the knock. Some power and mileage is lost, but the Vette does so well in those regards that it probably doesn’t matter much to power or mileage unless you are a racer or are competing in an economy run. In my mind the real risk is that the knock sensors, while good, aren’t necessarily perfect. If they don’t retard the spark in near-instantaneous fashion, you are in big trouble. And even if they do retard as designed, you have still suffered mild pinging for an instant, and that can’t possibly do any good to the engine. Still worse, you keep going through that cycle over and over and over and over and over again, because the ECM tries to get back to design spark advance, so it keeps on going through those sensing cycles. Pings detected, retard, pings stop, timing creeps back toward design, pings again, retard, over and over and over again. It’s utterly inconceivable that does your engine any good. The only question is how much harm it does. If the system runs flawlessly, there probably isn‘t much harm. But how many things run flawlessly for how long? In my view, the system is there for a backstop in case you can’t get premium fuel now and then. But I don’t think it’s intended as a system to allow continuous running on 87. If that was the intent, it would not have the logic built into it to keep trying to get back to design spark advance, and you would not have to keep going through the cycles described above over and over and over again.

As to the altitude impact, yup, at that altitude, you need a couple of octane less. But the grades at the gas stations are also usually a couple of octane less at stations in high altitude areas. So while regular/mid/premium at low altitudes may be 87/89/93, at altitude, they are probably something like 85/87/91. So to avoid the cycles described above, you still need to be using at least 91 at altitude, which may well be premium as sold in your area.