Skip to main content

Additive ABCs

Fuel additives are basically fuel-soluble chemicals that can be added to gas or diesel to enhance certain performance characteristics.

Fuel additives are basically fuel-soluble chemicals that can be added to gas or diesel to enhance certain performance characteristics. They are typically divided into two categories: those that treat a problem and those that prevent a problem.

Gasoline additives

Gasoline additives are commonly used to increase the fuel’s octane rating or to act as corrosion inhibitors and lubricators, allowing the use of higher compression ratios that can lead to increased efficiency and power. The octane rating is the measurement of the fuel’s resistance to auto-ignite or detonate. Typical gas pump octane values range from 87 to 93, with the higher number being more resistant to detonation.

Deposit-control additives are designed to help eliminate carbon buildup on intake systems and within the combustion chamber. Deposits on intake valves can absorb fuel during engine warm-up, leaning out the air/fuel ratio. Within the combustion chamber, they can increase the compression temperature, which can cause detonation of the air/fuel mixture, and promote engine run-on, poor acceleration and increased emissions. Deposits on fuel injector tips affect fuel flow and, thus, drivability.

If you are certain that you regularly use high-quality gasoline, you probably don’t need additives. However, today’s gasoline is very different than just a few years ago. The burning quality and additional chemicals have made it prone to leaving significant carbon deposits in hotter-burning marine engines. In addition, engines that are sensitive (high-performance inboards and most outboards) or are run very hard — commercial towing, etc. — could benefit from additives.

I have found no consistency in the pump gasoline I feed my outboard engines, but I’ve been aware of performance consistencies when using my preferred additives. I also have noted visual improvement in the combustion chamber, as viewed through a bore scope following several tanks of additive-enhanced fuel.

Diesel additives

The diesel fuel injection system relies on its fuel for lubrication and, as such, requires the best you can buy. Today’s ULSD fuel (ultra low sulfur diesel) is required to meet the ASTM D975 standard for lubricity, which isn’t necessarily the same as the fuel system manufacturers’ recommendations. Unfortunately, as with gasoline, there are no guarantees regarding the quality of the diesel from the pump. The only consistency I am comfortable with is in maintaining my own standard of additive improvement.

In addition to increasing lubricity, quality diesel additives can increase the fuel’s cetane level, which aids in starting and reduces exhaust smoke along with increasing power and fuel economy. Avoid additives that emulsify water; the water will come out of suspension when the engine cools, causing component corrosion. You should look for a product that will leave the water in suspension and allow the water separator/ filter to do its job. Quality diesel additives also can clean injector deposits, help prevent corrosion, and add storage stability.

I don’t look to fuel additives as a cure for serious problems, but rather as a part of a maintenance program to enhance performance and reliability. They are not a substitute for fixing a problem, but they can certainly prevent or slow the return of one. I also don’t buy everything on the market that sounds like the ultimate answer. I purchase my additive packages from the same manufacturer, which ensures compatibility among the products and provides the most consistent tech info if needed.

The two companies I have dealt with over the years are Stanadyne Corp. ( ) in Windsor, Conn., and ValvTect Petroleum Products ( ) of Northbrook, Ill. Both have informative Web sites and telephone tech lines, and provide good, consistent information.