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.

Sinful2

Thanks LDB. Very interesting and informative post.

CROWVETTE

Good Post!!!!

dekaran

great post and thanks for all the info. I was always told that midgrade is fine for most of the time and mix it up to the high grade once in awhile to "clean" out the engine. Your post was well worth the read.

mako5

Excellent Info

sxeC7

You make some good and valid points. I'd love to see your thoughts on Mobil 1 and regular petroleum based oil. Obviously, you don't work for Exxon-Mobil.

ASZ2222

Great post. Thanks

Grumpy

Info - Top Tier Detergent Gasoline (Deposits, Fuel Economy, No Start, Power Performance, Stall Concerns) #04-06-04-047D - (March 17, 2005)

Top Tier Detergent Gasoline (Deposits, Fuel Economy, No Start, Power, Performance, Stall Concerns)
2005 and Prior All General Motors Passenger Cars and Trucks (U.S. Only)

2005 and All Prior Saturn Vehicles (U.S. Only)

2003-2005 HUMMER H2 (U.S. Only)

This bulletin is being revised to include additional gasoline brands that currently meet the Top Tier Detergent Gasoline Standards. Please discard Corporate Bulletin Number 04-06-04-047C (Section 06 - Engine/Propulsion System).

A new class of gasoline, called Top Tier Detergent Gasoline, is appearing at retail stations of some fuel marketers. This gasoline meets detergency standards developed by four automotive companies. A description of the concept and benefits of Top Tier is provided in the following question and answer section.


Gasoline Brands That Currently Meet Top Tier Detergent Gasoline Standards
As of February 23, 2005, the following gasoline brands meet the Top Tier Detergent Gasoline Standards:

• Chevron has markets in 29 states in the West, Southwest and South, as well as in Alaska and Hawaii. ALL grades of Chevron with Techron gasoline meet Top Tier Detergent Gasoline Standards.

• QuikTrip (not to be confused with Kwik Trip) QuikTrip operates convenience stores and travel centers in a number of metropolitan areas:

- Tulsa, OK

- Springfield, MO

- Kansas City, MO and Kansas City, KS

- Wichita, KS

- Des Moines, IA

- Omaha, NE

- St. Louis, MO and St. Louis, IL

- Atlanta, GA

- Dallas-Ft. Worth, TX

- Phoenix, AZ

- Bartlesville, OK

- Miami, OK

- Vinita, OK

- Columbia, MO

• Conoco has over 4,000 stations located in 33 states.

• Phillips 66 has locations in 32 states across the nation.

• 76 has over 2,700 stations located throughout the country.

• Shell Oil Products has a network of nearly 7,000 branded gasoline stations in the Western United States. Shell Oil Company markets branded products through more than 10,000 stations in the Eastern and Southern United States.

• Entec Stations offer gasoline through stations located in the greater Montgomery, Alabama area.

• MFA Oil Company offers gasoline at "Break Time" convenience stores located throughout Missouri.


What is Top Tier Detergent Gasoline?
Top Tier Detergent Gasoline is a new class of gasoline with enhanced detergency. It meets new, voluntary deposit control standards developed by four automotive companies that exceed the detergent requirements imposed by the EPA.


Where Can Top Tier Detergent Gasoline Be Purchased?
The Top Tier program began on May 3, 2004. Some fuel marketers have already joined and have introduced Top Tier Detergent Gasoline. This is a voluntary program and not all fuel marketers will offer this product. Once fuel marketers make public announcements, they will appear on a list of brands that meet the Top Tier standards.


Who developed Top Tier Detergent Gasoline standards?
Top Tier Detergent Gasoline standards were developed by four automotive companies: BMW, General Motors, Honda and Toyota.


Why was Top Tier Detergent Gasoline developed?
Top Tier Detergent Gasoline was developed to increase the level of detergent additive in gasoline. The EPA requires that all gasoline sold in the U.S. contain a detergent additive. However, the requirement is minimal and in many cases, is not sufficient to keep engines clean. In order to meet Top Tier Detergent Gasoline standards, a higher level of detergent is needed than what is required by the EPA. Also, Top Tier was developed to give fuel marketers the opportunity to differentiate their product.


Why did the four automotive companies join together to develop Top Tier?
All four corporations recognized the benefits to both the vehicle and the consumer. Also, joining together emphasized that low detergency is an issue of concern to several automotive companies.


What are the benefits of Top Tier Detergent Gasoline?
Top Tier Detergent Gasoline will help keep engines cleaner than gasoline containing the "Lowest Additive Concentration" set by the EPA. Clean engines help provide optimal fuel economy and performance and reduced emissions. Also, use of Top Tier Detergent Gasoline will help reduce deposit related concerns.


Who should use Top Tier Detergent Gasoline?
All vehicles will benefit from using Top Tier Detergent Gasoline over gasoline containing the "Lowest Additive Concentration" set by the EPA. Those vehicles that have experienced deposit related concerns may especially benefit from use of Top Tier Detergent Gasoline.

MitchAlsup

High altitude decreases the need for octane by decreasing the effective pressures within the cylinders (that lead to knock, ping, and detonation.)

p-md

Thanks for taking time to post this information

Firedog

He's right...Higher altitudes require LESS octane...

Bville-Bud

Thanks for the info, saved to the hard drive.

LDB

A few have made the comment that higher altitude requires less octane. Depends on how you look at it, and also where your engine is most sensitive to knock. If you consider constant throttle opening, octane need is indeed less at high altitude, for the reasons cited in those responses -- less cylinder pressure due to thinner air. But at constant throttle opening, there's also less power at high altitude, so I tend to think in terms of constant power output. To get to constant power, throttle must be open wider at high altitude, and the engine is working harder to suck in the thinner air, so octane need is higher at high altitude for equal power.

Now you must ask, where is your engine most sentitive to octane and knocking. If at wide open throttle, then true, octane need is less at high altitude, just as maximum power output is less. But if the spark curve in the engine is set such that it's most sensitive to knock at part throttle, and if you also are considering points of equal power output, then octane need is greater at high altitude because throttle must be open wider, and engine is working harder to suck in the air.

As this issue illustrates, it's tough to generalize in complicated situations.

TedG

Well lets drill down a bit, in regards to high altitude the comment about working harder to suck in the air is arguable, thinner air requires less energy to move. True, you need more of it to make equal power, but atmospheric pressure is going to dictate how much natural boost you are going to get. There is just so much pressure drop when the piston goes down. In reality the computer knows how much you opened the throttle, and slightly later it will know the manifold pressure. It will try to advance the timing as much as possible given the circumstances, but there is a limit for every RPM and it will not go past that. The knock sensors are used to detect and then back off the timing, but if you don't have enough compression (with a given octane) to cause knock it will not retard the timing. Excess octane can and or will slow down the flame front to the point of a performance loss because the computer will not allow enough timing advance. Timing is not dictated by the knock sensors, it is a safety valve, each time it pre-detonates bad things happen, so they do their best to never let it ping. IMHO.

TRJ

Some Midwest Tuners & Dyno operators have said that: Fuels that have a larger amount of "additives" in the fuel, reduce the amount of power producing fuel available, and thus the charge of fuel with all these additives will show up on the dyno with lower RWHP numbers, compared to a fuel that has less of these additives in it. They have demonstrated this using the same car on the dyno, but with different brands of fuel. Also, two Corvette Specialists have told me that the Shell gasoline has been found to be the culprit in the C5 gas gauge sending units problem, in their experience.

BigLabe

Terrific post LDB! Answers a lot of questions about gasoline.

DebtCraft

Great post, LDB!! Thanks for taking the time to type it.

crazyeddie

Good stuff LDB, thanks for the information--e

SimpsonH

Interesting discussion. Additive packages are not a significant factor in fuel costs. More importantly, the EPA is very explicit that oil companies cannot control engine cleaning, air quality or mileage through the cost of their products to consumers. When using the correct octane for your engine, higher-octane gasolines do not (i) outperform the intended octane, (ii) provide better protection from engine deposits, (iii) clean a dirty engine more effectively or (iv) yield better mileage. Bottom line, the federal government (EPA) requires all fuel manufacturers to provide similar cleaning and mileage additives in their fuels regardless of octane rating. A few companies do offer additional additives in their costlier gasolines for advertising purposes, but the additional additives have not been shown to provide statistically better cleaning or mileage in federal testing.

need-for-speed

Great post LDB. I must agree with the others that octane requirements are reduced with altitude. Why is it that as you get into the higher altitudes, the octane rating of the "locally sold" fuel is lower?