I just read an article saying that folks are hardly buying premium now because of the price even though they think it's better for their car & mileage. Some points I've learned:
1. Higher octane gas is not higher energy gas & won't usually give better gas mileage or power. Premium gas is regular gas with a "quencher" additive. Since heat expansion is what pushes the pistons if you use premium the pistons are getting pushed less with premium which runs cooler. The only motors that need premium are high compression motors that will pre-ignite or "detonate" without it. Same principle as a diesel motor without spark plugs-compress the fuel mixture enough & rather than burning with a smooth flamefront ignited by the sparkplug, the mixture will go all at once causing a diesel type rattle sound. This is the situation where higher octane is needed to prevent this damaging "pinging" that will end up damaging a a gas motor that's not built to take the pounding that a diesel gets.
2. Octane has nothing to do with power. Octane is the ability to resist pinging. Water has higher octane than premium gas. There are of course high energy racing fuels that have high octane but that's not what premium gas is. I have with several of my high-compression muscle cars used a water injector to be able to use low octane unleaded regular (cheaper) gas while cruising cross-country. This has avoided pinging by raising the octane of the cheaper gas-just like premium gas but without the cost. This is with 12:1 compression motors.
3. Find out what compression ratio your motor has. As an example, my Corvette that had a stock compression ratio of 9:1 (piston compresses fuel/air mixture from one volume down 1/9 volume as the piston moves) & had a owner's manual note to run premium fuel, ran just fine here in Utah at 4500 feet one unleaded regular low octane gas but when I drove it to California & was at sea level with more oxygen in the air I noticed my car was more responsive(more power) & it pinged when I got on the gas. It ran fine on mid-grade gas in California even floored all out with no pinging. Then I re-built that motor with 10:1 pistons & got pinging while floored unless I used premium gas. So now I use only premium gas for that car.
4. Try "cheaper" gasses in your vehicle-first mid-grade & then regular unleaded & check for pinging. Up to about 9:1 ratio cars should cruise fine on regular & under heavy loads the knock sensor can kick in & retard your timing while you climb hills to prevent pinging. Not all vehicles have knock sensors so pay attention.
5. If your vehicle runs without pinging on lower octaine fuels you should end up getting more fuel mileage-lower octaine gas burns hotter giving more power & mileage.
6. Premium gas is not "cleaner" gas. Gas companies add their same additives to prevent sludge buildup to all their fuels regardless of grade.
I've ended up learning these things thru years of playing with cars, building muscle cars, drag racing, running my little 17 vehicle fleet for my business & working for the oil companies for 25 years & talking with their people & many, many gas station owners-who, by the way tell me that all the gas brands tend to come from the same refineries & may or may not end up getting the special "additives" you see advertised on the pump added to what the refineries put in the tanker trucks. I have a sign company & we mostly install the signs at gas stations for our work in a three state area.
Last edited by Popgunner; 06-19-2008 at 10:50 AM..
Thanks Pop! I never knew exactly what "octane" was rating in the gasoline, but now I do!
I have an '01 Ford Ranger, and it seems to get the best mileage out of 89 or 90 octane gas (both higher and lower octanes drop it a bit). I don't really know why, but this is something I spent three months (and several thousands of miles) testing.
My last car, a '95 Chevy Lumina with 260k+ miles, always did best with regular (87 octane). It's about 270k now, and my little brother is still driving it daily.
I don't know the compression ratio on either of those vehicles, but I think maybe I ought to look that up.
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Re; Gas additives. I recently toured a gasoline refinery and I can confirm that many different brands come from the same facility. They blend whatever additives each brand specifies on the spot. The refinery claimed to not be making much money and I believed them based on some of the obsolete infrastructure being used.
Very interesting post, Pop. Thanks for sharing that information. I, for one, only recently realized I could run regular gasoline in my pickup and that it worked just as well as the higher grade I had been using.
One thing to keep in mind is milage. When you switch to a higher grade gasoline, you might see an increase in gas milage. However, take the time to calculate mile per penny, also. I have found on a couple of cars I used to drive that the regular grade gave me fewer miles per gallon, but gave me more miles per penny.
To add to your description Pops, Octane is proportional to the speed of a gasoline's burn. Higher octane gas burns more slowly, and less explosively than lower octane gas (thus the source of pinging). When you advance an engine's timing as an example, you start the spark at a point earlier in the piston's stroke. With 87 octane, the full energy of the burn may be released well before the piston's maximum compression point. This causes pinging and over time will cause engine damage. With a 93 octane or higher gasoline, your burn lasts longer and the gasoline's energy is expended over a longer duration.
So, if you have your timing advanced so that your spark occurs at a point well before the piston's maximum compression point it's OK to a point with 93 octane. What you're doing effectively is maximizing the amount of time fuel is burning in the cylinder to create an optimum level of power (more area under the curve for you geek types)... Higher compression engines want a fuel that burns more slowly as well. That more explosive ignition of an 87 octane fuel is going to burn too quickly for a high compression motor to optimally use the fuel...
Conversely, if you have a motor with stock ignition timing, no forced induction such as a turbocharger or supercharger and no higher compression than ~9.0:1 or so, you are only wasting gas by using 93 octane. The engine may appear to run more smoothly because you're having a less explosive burn in the cylinders but you may not be burning all of the gas in the cylinder at a point where the engine is making optimum power (therefore you'll lose efficiency).
I have to run 93 in the Mustang because I've got the timing set at 17 deg. initial (7 degrees more than stock). It's only able to do that because of the aluminum heads, which keep the hot spots down in the cylinders. I've contemplated moving the timing back down to 10 deg. to be able to run 87 again without pinging but with the baby the car hardly gets driven, therefore it costs very little...
If your vehicle manufacturer recommends 93 octane, by all means use it... Mitsubishi is one company that usually gives their cars high-compression engines, and they do require 93 octane on some models. Supercharged or turbocharged cars will ALWAYS require 93 octane from the factory. BMW is another one that asks that you use 93 octane because of their compression and optimized timing...
I'm sure a lot of you are going to already know this stuff. Pops, if you're into muscle cars, I know you do. I just wanted to elaborate a bit...
Ok, I'm done on my dork rant...
Last edited by ponycar17; 06-20-2008 at 05:57 AM..
Reason: Changed 'before' to 'because'.... Duh on me...
Ponycart, I'm not into muscle cars any more. I got my Internation Racing License in Belgium in '59 and raced in the United States on an informal basis. I was running a '59 Ford with the 292 Police Interceptor engine blown out to 305. We had dropped an experimental hydraulic shifted 6-speed transmission from GM into it and mounted Citron air-oil shocks with a compressor run off the fly wheel. The transmission pressures were boosted and the shocks had a dump/fill valve on each one.
New Mexico State Police clocked me at 148mph westbound and 152mph eastbound on the White Sands speed track. They used to set up weekend runs for us every once in awhile. They would gather all the road racers, check their cars for legal and safety, then start us out at 0'dark thirty, 20 seconds apart. The highway would be blocked off and swept for some miles and we would get an elapsed time ticket at the each check point. Then they would close and sweep another stretch and we'd take off again. All day Saturday going out, dinner at a chosen restaurant and sleep overnight. Next morning, they would head us back, often by a different route.
we're on the same page
With the older cars it was easy to get better mileage with lower octane gas-you just pushed down the pedal till you heard pinging, backed off & took on higher octane fuel of need be. We used to freeway cruise on lower octaine & enjoy the better mileage that way. Nowdays it's worth trying lower octane & checking mileage especialy if you drive at higher altitudes where oxgen content in the air is lower & pinging happens less. If you're at sea level & pulling a boat & climbing steep hills then you're surely going to need all the octane the owner's manual suggests. The knock sensors on newer vehicles are what can end up causing premium gas to end up giving better mileage when they kick in & retard timing when using regular unleaded, but that's usually only under heavy load.
I guess my point was if you're just easy cruising cross country it is possible to save gas by using a lower octane gas that actually has more BTUs in it. Just be sure to avoid pinging. As the piston compresses the air fuel mixture it creates a lot of heat-so much so that it a diesel that commonly runs about 22:1 compression-enough heat is created that the diesel which is harder to ignite than gas goes ahead & detonates. You may have a cylinder full of regular gas fuel & air mixture that has almost compressed all the way & gets the spark a little before top dead center & has a normal flame front started & then gets enough heat at TDC that the rest of the mix detonates like a diesel. Premium fuel avoids this because it's less volatile.
To explain another point, in talking with lots of gas station owners & de-branding them-some of them several times over the years-I've found that a lot of the gas we buy that's supposed to be Chevron with TECHRON or whatever is really just the cheapest Maverick gas the station owner can find. Most stations are not owned by the oil companies but by independant owners. When the independant owners go several months without ordering gas from the distributors that they're supposed to to oil companies step in & call me to pull all their signage. Later when these owners sign new contracts & promise to buy only company gas I go back & put the signs back up. We branded & de-branded one guy 5 times in as many years because Conoco-Phillips wanted their brand in a small tourist town where the guy owned the only station & constantly bought gas from whoever pulled in with a tanker & offered him cheaper fuel. Happens all the time. That's how I make my money
Popgunner; in the corvette with a 9;1 compression ratio, you could easily run regular, you're right so I wonder why they said you should use premium. Also, currently, a large amount of foreign cars, namely hondas/acuras use high compression, usually in the 10+:1 range, so a lot of them require premium gas or at least medium. My prelude requires premium(10.6:1 compression) and I can even feel a difference between 93 and 91, as some gas stations go with 91 instead. On my old mustang, I had to use race gas, but she was fully built. The octane boosters were really just gimmicks, or at least I really didn't think they worked. Some of them didn't keep the motor from pinging, so it was always race gas. That was over a year and a half ago that I sold it, thank God because of gas prices now. She got 5-7 mpg on race gas. Can't imagine how much I'd spend on that now. My prelude with 22mpg on premium is rough as it is.
Anywho, back on topic; it should be noted that there is a difference in quality of gas, depending on where you get it. Shell seems to keep very good quality gas, as does Exon for the most part, BP, QT, etc. However, I've seen some bad cases come from Sunoco and the Kangaroo gas stations. Usually stuff like Kangaroo and other hole in the wall places sell cheaper gas, and you get what you pay for most of the time. God knows what kind of gas they use.
Last edited by preludese111; 06-20-2008 at 08:24 PM..
Here in Northern Michigan, I used to switch over to high octane gas in the winter, for quicker starts when the temperature is below zero. But, I never got better gas mileage using the high end gas.
Most newer cars have sensors to detect preignition (pinging.)
These computer controlled cars will retard ignition timing automatically.
That does help, but especially if the car has higher compression, timing can only be backed off so far. The local BMW manufacturing facility offers lease cars to employees for a montly/per pay check fee. The factory has started making employees sign a contract saying that they will only use 93 octane gas in the lease cars. If employees are found to be using regular, from my understanding they can face disciplinary action. This comes from a coworker of mine, who's wife works there and takes advantage of the lease cars... It seems people found out that when no one's looking 87 is just fine until the car gets turned back in with problems...
Location: Depends on Uncle Sam's whim every 3 yrs.
Re: Premium gas not usually needed
There are actually people who think that adding "octane boost" additives from gas stations increase their horsepower. I guess it's how it's advertised maybe. Or could be not understanding how compression effects ignition etc etc....most drivers couldn't explain the difference between a rich mix or lean mix for that matter either so advertisers have much margine to play with.
I have 2 vehicles that require 93 octane to drive. 1 will go into limp mode if it detects a lower octane/engine knock....the other has a simple CPU and high-compression timing that will just let the engine rotating assembly thrash itself apart. I keep octane boost handy just in case an event where no 93 octane was available.
Since 93 is getting over $4 a gallon, I been leaving those parked anyways to be honest.
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