GAMIjectors-18 Years of Precision Fuel Injection

Once an exclusive property of big radial engines, GAMI brought lean-of-peak operations to general aviation by developing precision fuel injectors.


Last week I was on the telephone with Tim Roehl, one of the founders of General Aviation Modifications, Inc., the folks who created the tuned fuel injectors called GAMIjectors. We were talking about a number of things, including Lycomings fairly new IO-390 engine that puts out 210 HP and is getting attenion in the aviation community. I’d flown behind it recently and liked it. We then got to talking about what’s going on in the world of GAMIjectors.

I asked Tim how many sets of GAMIjectors had been sold. He told me that set number 22,000 went out the door recently. That got my attention. I understand that there are something over 200,000 general aviation airplanes in the world. To develop and market something that gets bought and installed on 10 percent of all of the airplanes in the world is no small matter. When you consider that probably fewer than half of the airplanes in the world are fuel-injected, the market penetration is impressive.

I also got to thinking about fuel savings. Trying to be conservative and only assuming 10,000 sets of GAMIjectors are being used on airplanes that are being run lean of peak, that the average fuel savings is two GPH, and that the airplanes are being only flown 100 hours per yearthats saving two million gallons of avgas a year. What is the effect of two million gallons less demand for avgas? Not gigantic in the overall scheme of things, but certainly not just a minor bump in the road. Even with an Econ degree, the aviation world is strange enough that Im not willing to assume that demand for avgas drives up its cost, yet I wonder if GAMIjectors are saving every pilot a fraction of a cent per gallon of avgas. Nevertheless, with five dollar avgas, not burning two million gallons worth is saving GAMIjector owners $10 million a year. Not bad.

Back in 1996, when GAMI of Ada, Oklahoma, announced that it had developed tuned fuel injectors called GAMIjectors, I was skeptical of claims that they made the engine smoother and more economical through lean of peak EGT (LOP) operation. Id seen too many bolt-on engine devices that promised to enhance engine performance, longevity and cut fuel use. None worked.

But GAMI published detailed engine operational data which helped sell the product to a pilot community accustomed to trust-us, it-works marketing. I had one of those, now I understand moments sitting with George Braly at Oshkosh one evening as he patiently went over masses of LOP test data with me. It didnt hurt that he also had material from Curtiss Wright and Pratt and Whitney from their LOP testing and operation of the big radials in the 1930s and 1940s. GAMI wasnt inventing something brand new in engine operation-it had developed tuned fuel injectors that allowed bringing a proven concept to horizontally-opposed aircraft engines. A day or so later, we went out and flew his GAMIjector-equipped T-34 and I got to see how lean of peak operation worked in real life.

With an open, heres the data approach to marketing, GAMI sold more than 1000 GAMIjectors in the first year. Users praised them to, insisting that they were burning less fuel and their engines were running cooler. They shared their experiences, especially engine monitor data, with anyone who cared to look.

Of course, a devoted core of skeptics argued that owners were going to burn up their engines, cylinder life would be measured in double digits and the engine manufacturers would deny warranty claims on modified engines. After 18 years, there are one or two still around. My understanding is that neither Continental nor Lycoming ever denied a warranty claim simply because GAMIjectors were installed.

Although most pilots seem to have heard of GAMIjectors, a lot of pilots I speak with dont understand how they work and still think that you can hurt an engine by leaning too aggressively. This article is to review how GAMIjectors work, what LOP is and point out that the way to hurt an engine is by wrong leaning, not leaning too much.

A Little History

The idea of tuned fuel injectors came about when GAMI founder George Braly began doing research into why Continental engines tended to run rough, especially as he tried to lean to near peak EGT. What he found was that the fuel injection nozzles, straight from the factory, varied significantly in the amount of fuel each delivered.

Braly also observed that the pipe-and-riser induction system was effective at consistently distributing air to the combustion chambers, however, it was not so good at delivering the same amount of fuel to each cylinder. Further, the continuous flow nature of the fuel injection system meant that some of the fuel injected against the closed intake valve of the rear and middle cylinders got carried forward, to the middle and front cylinders, causing the rear cylinders to run leaner and the front richer.

The combination meant significant variations in power output among the cylinders, which meant a lack of smoothness in operation. Further, it meant that when the pilot leaned to find peak EGT, the first cylinder to reach peak usually did so at a much lower fuel flow than the last, sometime by as much as three GPH. (This difference became known as the GAMI spread.)

George Braly brought in engineer Tim Roehl and together, using some of the most sophisticated test equipment in general aviation, they explored fuel distribution in Continental engines. They found that by assuring there were no induction leaks-which negatively affect air distribution to cylinders-and using fuel injectors engineered to provide precise fuel flows and installing the correct injector in each cylinder, fuel and air flow imbalances among the cylinders could be corrected. This allows each cylinder to produce the same power at the same mixture setting.

Benefits of Balance

These tuned fuel injectors, GAMIjectors, each installed in its correct cylinder, yielded overall smoother engine operation, lower fuel burns when ROP (none of the cylinders are running over rich) and meant the engine could be run LOP, opening up the full spectrum of engine operation, not just the ROP side of the engine leaning curve.

For the owner interested in installing GAMIjectors, the Website provides guidance, initially directing the owner to run a GAMI lean test, recording the fuel flow at which each individual cylinder peaks. That information is provided to GAMI and which then selects fuel injectors that have been bored and flow tested to assure specific fuel flows and packages each injector with a small plaque to attach to each cylinder so that the numbered injector is always matched with its respective cylinder.

Installation, under an STC, is carried out by the owners mechanic and usually takes less than half an hour, with paperwork. The owner then flies the airplane and conducts another GAMI lean test looking for the fuel flow difference between where the first and last cylinders reach peak EGT. The actual temperature of peak EGT for each cylinder is absolutely irrelevant. The important bit of information is the fuel flow at which each cylinder peaks.

Whats Acceptable

The GAMI spread should be 0.5 GPH or less if the injectors were selected correctly and there are no induction system leaks.

Once installed with a satisfactory GAMI spread, the initial benefit for owners who run ROP is usually a slight fuel savings, as much as one GPH, because there are no cylinders running way too rich.

What has turned out to be the most valuable application of GAMIjectors is the ability to take advantage of the so-called right half of the engine power curve, and run LOP. Because each cylinder is putting out nearly the same amount of power, the engine will run smoothly and predictably lean of peak EGT, typically saving at least two GPH, according to GAMI customers.

As the mixture is leaned, CHT peaks just before EGT peaks-both continue to cool off as the mixture is leaned further. As a result, the engine runs significantly cooler LOP and, for a given power setting, the internal cylinder pressure is lower because the power pulse is spread out over a longer period and peaks lower than it would ROP.

GAMI did its initial research on Continental engines, but as the success of GAMIjectors grew, owners asked about installing them on Lycoming engines. Because of the nature of the Lycoming induction system, the fuel/air mixture in each cylinder is more evenly distributed than in Continentals.

GAMI found that some Lycoming engines would run LOP straight out of the factory. Nevertheless, on many, the GAMI spread was significant enough that tuned injectors could bring it down to where the engines could be run LOP. Accordingly, in the last several years, GAMIjectors have become available for virtually all Lycoming injected engines.

Power Curve

To go into a little more detail on the engine power curve-lets begin with the engine running at cruise power and the mixture full rich. As we lean the mixture, power output increases to its maximum about 35 degrees rich of peak EGT. It then begins to drop off. Power output at 50 degrees LOP is about 10 percent less than at 35 degrees ROP.

EGT increases to peak and drops off at about the same rate as it increased. Internal cylinder pressures, a critical factor in detonation, increase slowly, reaching their highest point about 40 degrees rich of peak and then drop off rapidly. CHT increases and reaches a peak about the same time as internal cylinder pressures and power output are at their peak, about 40 degrees rich of peak.

The greatest risk of detonation and damage to the engine occurs in a range centered on about 35 to 40 degrees rich of peak EGT. However, that is also the area in which maximum power is generated. The fact that the very high internal cylinder pressures and head temperatures that accompany this mixture setting is of concern for engine longevity. During the last 18 years, owners who have installed GAMIjectors have consistently reported smoother operation and fuel savings generally in the range of two to three GPH when operating LOP, lower cylinder head temperatures, greatly reduced lead deposits, fewer fouled plugs and longer cylinder and engine life. Naturally, GAMI has taken to calling this operating green of peak.

For a detailed discussion of whats going on inside an aircraft engine as well as LOP operation, AVweb columnist John Deakin wrote a series of columns that have become the gold standard reference for general aviation engine operation around the world. They are here.

It Works

I am aware of a number of engines that are running well beyond TBO with LOP operations. Cirrus and Beech call for LOP as standard operating procedures for their piston aircraft. Continental now offers position-tuned fuel injectors.

Lycoming started advocating LOP operation at a well-attended seminar at Oshkosh in 2012. Reports we get from engine overhaulers tend to be consistent with what we heard from Powermaster Engines in Tulsa, Oklahoma. Owner Bill Cunningham advises that the engines he receives that have been run LOP display far fewer signs of heat distress and lead buildup. He noted that it is not unusual for him to get an engine that did not need to be overhauled.

Another benefit reported by users who run LOP with GAMIjectors is that saving some three GPH means that on a four-hour trip, they have 12 more gallons of fuel available as reserve or that they can safely avoid the hassle of a fuel stop.

With all the above, I am firmly of the opinion that an all-cylinder engine monitor is essential for safe operation lean of peak.

Show Me the Money

A set of GAMIjectors for a normally-aspirated Continental engine is $799; for a turbocharged engine its $949. For Lycomings the price varies between the four-, six- and eight-cylinder models. See GAMIs Websitefor pricing. GAMI holds an STC and PMA for virtually all fuel-injected general aviation engines, and its working on the ones it doesnt have, such as in the Beech Duke.

Using GAMIjectors and running LOP, an owner can safely operate the engine at some 85 percent power because of increased detonation margins due to lower temperatures and internal cylinder pressures. On a 300-HP engine, that works out to be a power output increase of 30 HP or 10 percent, according to GAMI claims.

It also means that GAMIjectors create an effective increase in engine power-to-weight ratio. From what I can tell, the last time general aviation had an increase in power to weight ratio was when Cessna developed the turbocharged Models 320 and T210 in the early 1960s. Getting a 10 percent power increase for the price of a set of installed GAMIjectors is not to be dismissed lightly.

GAMI offers a 30-day, no-questions-asked, full-price refund policy. Assuming that a user saves just two GPH and flies 100 hours a year, that means, at $5 a gallon fuel, the user has saved $1000, more than the price of purchase and installation of the GAMIjectors.

Rick Durden is the Features/News Editor of AVweb and the author of The Thinking Pilot’s Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing It, Vol. I.