Gasoline Thread from C6 section
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Gasoline Thread from C6 section
Got this from the C6 section, thought it was pretty intersting...
Originally Posted by LDB
Lots of gasoline comments lately. I’m an engineer for a major oil company, familiar with engine testing for performance, economy, and emissions. I think my company’s gasoline is slightly better than other majors, but I’m not going to use the forum for advertising. Instead, I’ll make a few points about what you should look for in a gasoline. If you do that, my company will get more than an average share of your business.
Octane is generally misunderstood. High octane gasoline isn’t more powerful, it simply offers better resistance to engine knock. Lower than needed octane is a big penalty from knocking and/or spark retard, but higher than needed octane gives no benefits. Sounds odd, but it’s true as described below. Premium gasoline does have some benefits even to a car designed for regular, but those benefits aren’t from octane. They are from some other characteristics which are described at the very end, DI and additives. Let’s talk octane first.
Two main factors set octane requirement: compression ratio and spark advance. Compression ratio is a design feature of the engine, with higher ratios giving better performance, but also requiring higher octane rating. Corvettes need premium gasoline because they are high compression ratio, which requires high octane. Best performance in any given engine is at a particular spark advance, and going in either direction makes poorer performance. But while it costs performance, less than optimum spark advance does have the advantage of lowering octane requirement. So your knock sensor can compensate for low octane fuel by retarding the spark, at a cost in performance. But the reverse is not true. Once octane is high enough to allow optimum spark advance, more octane doesn’t help, because greater than optimum spark advance does not increase performance.
The Corvette gasoline spec is 91. There is some variability in engine response depending on a myriad of other characteristics including engine age, but it’s safe to say that benefits of 93 versus 91 are small, and 94 versus 93 are nil. You can make similar comments about the benefits of greater than 87 octane in a normal car. Its compression ratio is lower, so it can get to optimum spark advance on 87 octane. Running 93 octane will not help turn it into a Corvette, because it doesn’t have the compression ratio to utilize the higher octane.
Bottom line on octane is that benefit of being over manufacturers recommendation is slim to none. The only three things that can cause you to get significant benefit from going higher are: 1) a 100,000 mile engine probably needs a couple of numbers more than new due to various irregularities in an old engine, 2) if you live at high altitude, a couple of numbers over normal spec are usually needed, or 3) if you do major engine modifications to raise compression ratio.
Other issues are a detergent additive package for valve and injector cleanliness, a reliable quality control system, and good DI, drivability index, a measure of how well the fuel evaporates. All majors are roughly equivalent in those areas, and most premium gasolines have more detergent and better DI than most regulars. There are shades of gray differences in additives, but once you are with a major oil company, you need careful tests to see them. All majors (including my own) have had quality control lapses, usually with sulfur (causes odor, emissions, and gas gauge problems), particulates (plugs filters prematurely), and/or water (the worst of all, and can devastate an engine, but this one is almost unheard of with the majors). But when the rare lapse does occur, we quickly correct the problem, including any damage it may have caused. When you buy from the grocery store, or Wal-Mart, or a convenience store, you are much more exposed. Some of the time, those places buy from the majors, and their gas is perfectly OK. But all too often, they buy on the spot market, the cheapest stuff that happens to be floating by on a barge. This gives you higher probability of a serious quality control incident, and also exposes you to poor additives and DI. In the short term, you’d never notice lack of detergent additives, but over time, you engine will foul. DI symptoms are subtle, mostly a rough warm up period, but poor DI also tends to foul the engine, especially if it is short on detergent. So you have to ask yourself, do you want to fuel a $50,000 car from the lowest bidder.
Anyway, at the very end of this tome, I’d advise you to set you minimum standard as being at least recommended octane, from one of the majors. Once you are at that point, differences become small. Does 93 versus 91 or 94 versus 93 make much difference? Probably not, unless you live in the mountains or have an aging engine. Is premium worth it in your regular car for the slightly higher additive concentration and slightly better DI if you don’t need the octane? You have to make that decision. There are benefits, but you are into small stuff. It’s kind of like Mobil 1 versus normal oil. Sure, Mobil 1 is very slightly better. It’s also $4 versus $1 per quart. But that’s another long post, and I’m out of energy for now.
Octane is generally misunderstood. High octane gasoline isn’t more powerful, it simply offers better resistance to engine knock. Lower than needed octane is a big penalty from knocking and/or spark retard, but higher than needed octane gives no benefits. Sounds odd, but it’s true as described below. Premium gasoline does have some benefits even to a car designed for regular, but those benefits aren’t from octane. They are from some other characteristics which are described at the very end, DI and additives. Let’s talk octane first.
Two main factors set octane requirement: compression ratio and spark advance. Compression ratio is a design feature of the engine, with higher ratios giving better performance, but also requiring higher octane rating. Corvettes need premium gasoline because they are high compression ratio, which requires high octane. Best performance in any given engine is at a particular spark advance, and going in either direction makes poorer performance. But while it costs performance, less than optimum spark advance does have the advantage of lowering octane requirement. So your knock sensor can compensate for low octane fuel by retarding the spark, at a cost in performance. But the reverse is not true. Once octane is high enough to allow optimum spark advance, more octane doesn’t help, because greater than optimum spark advance does not increase performance.
The Corvette gasoline spec is 91. There is some variability in engine response depending on a myriad of other characteristics including engine age, but it’s safe to say that benefits of 93 versus 91 are small, and 94 versus 93 are nil. You can make similar comments about the benefits of greater than 87 octane in a normal car. Its compression ratio is lower, so it can get to optimum spark advance on 87 octane. Running 93 octane will not help turn it into a Corvette, because it doesn’t have the compression ratio to utilize the higher octane.
Bottom line on octane is that benefit of being over manufacturers recommendation is slim to none. The only three things that can cause you to get significant benefit from going higher are: 1) a 100,000 mile engine probably needs a couple of numbers more than new due to various irregularities in an old engine, 2) if you live at high altitude, a couple of numbers over normal spec are usually needed, or 3) if you do major engine modifications to raise compression ratio.
Other issues are a detergent additive package for valve and injector cleanliness, a reliable quality control system, and good DI, drivability index, a measure of how well the fuel evaporates. All majors are roughly equivalent in those areas, and most premium gasolines have more detergent and better DI than most regulars. There are shades of gray differences in additives, but once you are with a major oil company, you need careful tests to see them. All majors (including my own) have had quality control lapses, usually with sulfur (causes odor, emissions, and gas gauge problems), particulates (plugs filters prematurely), and/or water (the worst of all, and can devastate an engine, but this one is almost unheard of with the majors). But when the rare lapse does occur, we quickly correct the problem, including any damage it may have caused. When you buy from the grocery store, or Wal-Mart, or a convenience store, you are much more exposed. Some of the time, those places buy from the majors, and their gas is perfectly OK. But all too often, they buy on the spot market, the cheapest stuff that happens to be floating by on a barge. This gives you higher probability of a serious quality control incident, and also exposes you to poor additives and DI. In the short term, you’d never notice lack of detergent additives, but over time, you engine will foul. DI symptoms are subtle, mostly a rough warm up period, but poor DI also tends to foul the engine, especially if it is short on detergent. So you have to ask yourself, do you want to fuel a $50,000 car from the lowest bidder.
Anyway, at the very end of this tome, I’d advise you to set you minimum standard as being at least recommended octane, from one of the majors. Once you are at that point, differences become small. Does 93 versus 91 or 94 versus 93 make much difference? Probably not, unless you live in the mountains or have an aging engine. Is premium worth it in your regular car for the slightly higher additive concentration and slightly better DI if you don’t need the octane? You have to make that decision. There are benefits, but you are into small stuff. It’s kind of like Mobil 1 versus normal oil. Sure, Mobil 1 is very slightly better. It’s also $4 versus $1 per quart. But that’s another long post, and I’m out of energy for now.
Drifting
Joined: Jul 2000
Posts: 1,289
Likes: 1
From: the West is the best
once you know that any one distributor trucks gas to a number of DIFFERENT gas stations out of the same truck, then all those eloquently established differentiators meld together to once again searching for the lowest cost point as the only thing that truly matters
Drifting
Joined: Dec 2004
Posts: 1,820
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From: San Diego
Originally Posted by BNLspeedaddict
Got this from the C6 section, thought it was pretty intersting...
.... In the short term, you’d never notice lack of detergent additives, but over time, your engine will foul. DI symptoms are subtle, mostly a rough warm up period, but poor DI also tends to foul the engine, especially if it is short on detergent....
... I’m out of energy for now.
.... In the short term, you’d never notice lack of detergent additives, but over time, your engine will foul. DI symptoms are subtle, mostly a rough warm up period, but poor DI also tends to foul the engine, especially if it is short on detergent....
... I’m out of energy for now.
On the "over time" remark. If you only keep your cars on the average of 2-3 years then you probably won't notice the "over time" effects. That doesn't mean I recommend burning lowest octane or cheapest gas all the time but, one NEVER knows when buying a car used, where the previous owner(s) cut corners.
..rickko..
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Joined: Jan 2005
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Originally Posted by rickko
I'm curious about two things, "over time" comment in first para above and how can he be "out of energy" if he works for a big energy company?
On the "over time" remark. If you only keep your cars on the average of 2-3 years then you probably won't notice the "over time" effects. That doesn't mean I recommend burning lowest octane or cheapest gas all the time but, one NEVER knows when buying a car used, where the previous owner(s) cut corners.
..rickko..
On the "over time" remark. If you only keep your cars on the average of 2-3 years then you probably won't notice the "over time" effects. That doesn't mean I recommend burning lowest octane or cheapest gas all the time but, one NEVER knows when buying a car used, where the previous owner(s) cut corners.
..rickko..
good point
