Car Craft
Click here to find out more!

High Octane Mixing & Water Injection

By , Photography by

Pump Gas Blues - Water Injection

Gary Palmer; Byron, IL: With today's turbo and blower production cars requiring 91-octane or better, I need some guidance on gasoline. My past experience buying premium fuel has not been good. Several years ago, I was told that regular automotive gasoline has about a 30- to 60-day life expectancy before its properties deteriorate to the point of being noticeable. How true is this? I know the adage of buying your gas at the busiest gas station, but how many people are buying premium? Do current cars adjust boost and timing to correct for the fuel in the tank? What about water/ethanol injection? How do you set up a water/ethanol system and what are the limitations?

Jeff Smith: I spoke with my pal Tim Wusz at Rockett Brand Racing Fuel, and he confirmed that pump gasoline is designed to be stable for about that 30- to 60-day time period. The deterioration of the fuel begins with a gradual evaporation of what are called the aromatics or the light ends of the fuel. Gasoline is a complex blend of several different hydrocarbon chains mixed with a multitude of additives; among them are aromatics that are often associated with higher-octane fuels. These light ends also make up the portion of the fuel that evaporates before the heavier-end components. Losing the lighter ends reduces a fuel's ability to light easily. Keep in mind that most fuels across the country now use 10 percent ethanol as an octane booster instead of ethers like MTBE, which have been removed from gasoline because they have been determined to be carcinogenic.

Unfortunately, it's difficult to generalize because there are dozens of different gasoline formulations used throughout this country. Nevertheless, alcohol-based aromatics are generally considered less stable under long-term storage situations, so it would be best to try and use the fuel as quickly as possible, because allowing the fuel to remain in the tank over the winter, for example, could cause problems. Alcohol has an affinity for water, which means that alcohol attracts water out of the air and pulls it into suspension. The water then mixes with the alcohol and causes the mixture to create what is called phase separation, which separates the water/alcohol from the gasoline. None of this is good, and this process occurs when gasoline is allowed to sit in a vented tank. In this situation, a fuel-stabilizing additive like Sta-bil is a good idea because it prevents phase separation. It's also a good idea to keep the tank either as full or as low as possible if you plan to store the car for a long period of time, as this will reduce the formation of water.

According to Wusz, olefins in gasoline are the real troublemakers when it comes to long-term storage. Olefins represent a larger percentage of pump gasoline and serve an important function, but they are also the least stable additives and tend to oxidize, eventually converting to gum and varnish. Wusz says the amount of olefins is one of the main differences between high-quality race gas and pump gas, with the better race fuels having a lower percentage of olefins. This is one reason race gas can be stored in a sealed container in a cool, indoor location for as long as two years with no fear of degradation. Wusz also mentioned that even when pump gas degrades and loses some of its aromatics, that generally has a very minor effect on its octane rating. It's also important to note that octane rating on gas pumps is a combination of research and motor-octane ratings, and Wusz emphasizes that of the two, the motor-octane rating is the more important to a street engine under load. To further emphasize this point, most octane boosters you buy in a can add only minor numbers to the research octane number. When octane boosters claim an increase of four points, that means a 91-octane fuel improves to 91.4, not 95.

As for your question about whether late-model cars correct air/fuel ratio and timing to compensate for fuel quality, the answer is yes. All late-model engines use feedback air/fuel ratio sensors, and many now use wide-band sensors that are what most performance enthusiasts use to monitor engine performance. This feedback maintains the engine's optimal part- and sometimes full-throttle air/fuel ratio for best fuel mileage and driveability. Detonation sensors are used to help protect engines from harmful knocking if low-quality fuel is used. These sensors detect knock and automatically reduce ignition timing until the knock stops. This allows the engine designer to push the limits of timing, knowing that he has the ability to pull timing back should knock occur. There are also aftermarket knock sensors. We've used a J&S sensor on some dyno testing, which worked very well. The company even has a unit designed for late-model engines with distributorless ignition systems that uses smart coils and includes a boost and nitrous-retard feature.

Water Injection Water injection might be a reasonable solution for mild detonation problems for users currently mixing race and pump gas to raise the octane. Mixing fuel works directly along octane lines (half 91 plus half 100 will produce 95.5- octane fuel), but this tends to be expensive. Race gasoline on the West Coast is around $8 per gallon, and even mixing it at 25 percent with pump gas will increase the cost of fuel to roughly $6.50 per gallon. Water injection is most commonly associated with supercharged and turbocharged engines as a way to combat detonation, but water can also be employed on high- compression, normally aspirated engines as well. Many enthusiasts believe water (and water/alcohol) injection is equivalent to pouring water on a flame. While in one sense this is true, the more accurate evaluation is that small quantities of water work to reduce the extreme peak cylinder pressures associated with detonation. So in a general sense, the water does not put the fire out but instead reduces those harmful peak cylinder pressures that contribute to the "rattle."

Wheels Up

Jeff Paulin; Laguna Niguel, CA: Thanks for the info on our new drag car in the Ask Anything series in the Oct. '11 issue. I have just installed the factory front sway bar with Genuine Suspension bushings and added a BMR Pro-Series rear drag sway bar. Our best run last time out at the May PSCA meet was 10.20 at 131 mph with a 60-foot time of 1.47. That's from a 3,900-pound car with driver. With the changes, the car ran a best of a 1.37 60-foot time and 10.18 the first time out.

Jeff Smith: Jeff sent us the above photo of his new launch technique with the bars installed, and it's clear that the Chevelle launches much better now with the front end even and hooked! When you can run low 10s with a normally aspirated big-block at 3,900 pounds and pull the front tires like that, the suspension is definitely working.

More Info

BMR Suspension; Thonotosassa, FL; 813/986-9302;

Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!
David J Miller
David J Miller

thought the idea was to keep the wheels on the ground moving the car forward down the track pass!

Timothy Bremm
Timothy Bremm

Leaf spring car a torsion bar in the rear...depends on how much horsepower to the rear wheels.


I would like to know if the stock fuel line in a 1966 Mustang, will support a near 500 horsepower, Ford Racing, 363 cubic inch, crate motor. Any information or suggestions would be appreciated. Thanks, Joe

Car Craft