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The following is the How It Works section of the April 2003 American Iron Magazine, an article written by Mark A. DeSorbo.  The layout has been optimized for internet browsers.  Enjoy!


Benjamin Franklin once wrote, “A man in passion rides a mad horse.” The 18th century sage indeed had clairvoyance for the two-wheeled machine. He even spoke of a man who could name a horse in nine languages but still bought a cow to ride on. That’s true of many American V-twin disciples; eager and steadfast in modifying our respective iron horses, yet never realizing the potential of getting robbed of pleasurable horsepower by the very detuned heart that lies beneath.  

And what that boils down to is that some end up riding a pig instead of a bonafide hog. “They got oink instead of thunder,” quips Fred Eno, a.k.a. Dr. Dyno. Fred, of Stonington, Connecticut-based Eno Enterprises, LLC (www.drdyno.com), has seen this all too often. For he’s handed down numerous power prescriptions to inadvertently detuned Harley-Davidsons and the like after runs on his dynamometer, or dyno as it is most often called.

With it, he’s helped enthusiasts like Glenn Dillon, the proud owner of a record-holding 1997 Dyna Wide Glide that grunts out over 200 horses to the rear wheel. “Freddy knows what he’s doing,” says Peter Rich, a retired real estate broker from Storrs, Connecticut, who enlisted Dr. Dyno’s help. “I’ve stood there and seen guys with bikes that couldn’t get out of their own way, and he’s tuned them right.”  

Consider the $35,000 custom with a 96” S&S motor that produced less horsepower than a stock Sportster. Or the guy who spent more than $4,000 on a big bore kit, cam, heads, ignition, and carb for his FXDX. Although it most certainly had more peak horsepower, the Super Glide Sport didn’t feel any better or faster, leaving the owner feeling insulted, in addition to what he perceived as a rather expensive injury. “I dynoed his bike and it was making 104 (peak) horsepower, but no more torque in the mid-range (82) than most stock Twin Cams,” Fred says. “When we were done tuning, he still had 102 horsepower, but the bike now made 100 foot-pounds of torque!

All Fred did to fix it was tune it; he didn’t change any parts. “A lot of people have the right parts on their bikes; they’re just not tuned correctly,” Fred says. “I could care less about the peak horsepower of a guy’s bike, and usually the guys who go for that have no mid-range torque -- and slow bikes. It’s not the peak horsepower the bike comes on the dyno with that counts, it’s the useable mid-range torque it goes off with.”  

Deciphering The Dyno

Perhaps the greatest misconception of the dyno is the belief that somehow it can measure what Fred calls “dollar power input” instead of “engine power output.” It’s the exact opposite, and Fred, a retired electronics engineer, estimates that 75 percent of Harley-Davidsons on the road are running 10 to 50 percent below potential. That’s where the dynamometer comes in handy. It’s the polygraph machine of power.

According to www.howitworks.com, a dynamometer places a load on a running motor and measures the amount of torque and horsepower an engine can produce under that load. For example, a dynamometer can be used to apply enough of a load to keep an engine at 6000 rpm. Once that load is recorded at that rpm, the engine speed can be taken down to 5500 rpm and so on. Measurements can also be taken starting at fewer than 1000 rpm and up.

Dynamometers measure torque in foot-pounds and convert it to horsepower by simply multiplying torque by rpm and dividing by 5,252. The recorded measurements at different engine speeds create a horsepower curve, which indicates where the engine is strongest. This information can mean the difference between a horse and a cow when one is faced with a challenge at a traffic light.

Dynos, according to Fred, are classified as either absorption or inertia. One, or combinations of the two, technologies can be found in a hybrid called a chassis dyno, which are used widely by purveyors of power prescriptions, like Dr. Dyno. Absorption dynos, as described above, are also called brake or engine dynos; hence the term brake horsepower.

“With absorption dynos, which are generally used to test engines that are not installed in a chassis, you load the motor down to hold the engine at a certain speed,” Fred explains. “To put it another way, an absorption dyno simulates a hill that you can make steeper and steeper until the engine can no longer accelerate.” Engine manufacturers, rebuilders, and performance shops that work with engines before they are installed typically use absorption dynos for research and development, tuning, break-in, and to verify specifications.

The machines are often large and complex, with many parts affecting calibration. Fred says, “They produce a lot of heat. Ten horsepower generates enough power to light a bank of 75 100-watt light bulbs, and most people are trying to measure a lot more than 10 horsepower.” Computers have helped considerably with the heat buildup problem by allowing technicians to automate load adjustments and record readings more rapidly than a human ever could. Fred also warns that power measurements on engine dynos do not accurately reflect vehicle performance, because transmission losses and gear ratios cannot be taken into account.

The first chassis dynos, which measured power at the vehicle’s drive wheel or wheels, were also of the absorption type. Many are still in use in automotive performance shops. They are common in the diesel truck testing industry.

Computers also made possible a new type of chassis dyno, the inertia dyno, which measures an engine’s ability to accelerate a heavy mass such as a large roller. Unlike the “steep hill” load applied by an absorption dyno, the inertia dyno simulates a level road with no wind resistance. The engine simply works against the rotating mass instead of the vehicle’s weight as it accelerates. Horsepower on an inertia dyno is determined by the product of the mass, speed, and acceleration of the roller, while the torque curve is computed by multiplying the horsepower by 5,252 and dividing by the rpm.

The inertia dyno, he adds, would not have been feasible without a computer recording the data, because rpm and speed are constantly increasing during a test run. Now, instead of one point at a time, the operator gets the whole graph in mere seconds. Fred also points out that inertia dynos have several other benefits, noting the engine is only “loaded” for a few seconds, so less heat is generated. And with only one moving part, the calibration stays within spec.

On Dr. Dyno’s machine, an inertia chassis dyno, a motorcycle is ridden up and secured so that the rear wheel is centered on top of a single, large rolling drum -- a system that Fred says generates less resistance than a two-roller unit and provides more accurate readings with virtually no tire wear. “The rear wheel sits on top of the drum, which weighs 875 pounds, rotates on a shaft 2” in diameter, and can spin up to 200 miles per hour,” he says. And because the engine’s actual intake, fuel, ignition, and exhaust systems are all used, a chassis dyno makes an excellent whole-system tuning tool. “Chassis dynos, especially the inertia type, offer a quick way to check performance, or what I call rear-wheel, real-world power,” he adds.

Dyno Dos & Don’ts

Dynamometers can certainly be proving grounds, where braggarts go to die and bikes are born again. But some of the diagnostic information people walk away with can be misleading, while many potential customers may not fully understand the reason for loading their steeds onto a dyno in the first place. “‘What’s the highest horsepower you’ve seen today’ is not the right question for a Harley guy to ask,” Fred says. How often does this dyno have to be calibrated, do all dynos read the same, and will the dyno hurt my bike are just some of the questions that he fields regularly, and the answers reveal the beauty of the inertia dyno.

“You’re measuring the velocity and acceleration of the drum. There’s one pulse with every revolution, and the only thing that can go out of calibration on an inertia dyno is the operator,” says Fred. “As far as it hurting a bike, I tell them if something is going to break on their bike in the next 10 miles, I’ll find it for them on the dyno, and most people appreciate that.” Of course, most of the time he is asked these questions by the same people who ride their bikes up onto a dyno drag racing trailer after a few beers and then proceed to beat on their bikes, blowing belts, tires, and clutches. “Fun, but nothing more than a circus ride for bikers, where you bring your own ride,” Fred adds.

But those who seek the Dalai Lama of dyno doctors should keep their eyes open, for there is such a thing, he says, as a “happy dyno,” a machine that has been modified to read high. “Every 875-pound drum has its exact weight stamped on it, and that is the mass the computer works with. If the computer has been hacked into thinking the drum is heavier for higher readings, it isn’t a legitimate dyno business,” Fred says.

Those seeking to avail themselves of the services of dyno diagnostic testing and tuning should get a feel for the overall operation before jumping on, and beware if there are no tuning tools in the area, or if the dyno operator is revving bikes way beyond the powerband. Revving beyond the powerband could needlessly damage the engine.

Another potential problem is that the bike could overheat, too, if the dyno is not equipped with a cooling blower. “I’ve seen guys do three dyno runs, and the horsepower gets lower and lower each time,” says Fred. “They overheated the bike. With an adequate blower, you can do run after run without overheating the engine."

So, what do you look for? “You want someone to approach it from a cautious point of view; not someone who doesn’t care about your machine,” Fred adds. “It will be obvious, when they are overheating engines or revving them way beyond the powerband, that they do not have a feel for it or an appreciation for your bike. They just want to give you a printout and send you on your way.”     

The Dr. Dyno Difference

For $75, Fred will load your bike onto his dyno for a diagnosis. Any unlocked power that was tucked among out-of-sync performance parts is subject to additional charges. “When I test a bike, I don’t want it warmed up. My goal is to provide usable power, not just measure a peak, so I want to see what it does as it warms up,” he says.

On Harley-Davidsons, Fred will make a series of runs, the first up to 4500 rpm. Then, he’ll take the engine up to 5000 and then to 5500 rpm. “If I’ve gone past the peak horsepower, there’s no sense in running it to 6000 rpm, especially if it’s a Big Twin or Twin Cam. There’s no information there,” he says. “If I ever see a run go lower than the previous run, or if it goes down below what it should be, I stop testing immediately and shut the bike off.”

There are four areas that he checks when evaluating Harleys. They are air, fuel, ignition, and exhaust. “These items, especially the jetting, timing, and backpressure, each make a variation of two to three horsepower,” says Fred. “If you’re trying to tune by the seat of your pants, you might get one component that is minus two, one that’s letting you gain three, while another is adding zero to the engine’s output. So, that means you get a net gain of one horsepower and don’t notice any difference. On the dyno, we can see these variations separately, and make all of them add up for a valuable, noticeable increase.”

First and foremost, he checks for a high flow air cleaner. “What you need is a high flow filter and a high flow air box, so get rid of the black plastic box in the back,” he says. 

The next step is ensuring the right fuel-to-air mixture ratio through correct jetting or programming of the electronic fuel injection system. “The fuel does not read the name on the jet on the way through. It’s the right size that counts, not the brand or how many you have left over from your jet kit. With fuel injection, it’s a matter of remapping,” Fred says.

From there, Fred moves on to ignition timing. “Timing is everything,” he says. “With Evos, we set the static timing and then it’s done. We adjust it manually, and the dyno tells us if we went in the right direction. We massage it until we find the sweet spot.”

Timing Twin Cam engines is a tad more involved, because they do not have an adjustable ignition. “So, if adjustment is needed, we change out the electronic module for one that allows you to adjust the static timing,” Fred adds.

The last thing Fred checks is exhaust tuning, setting the appropriate backpressure with small adjustable baffles he calls TorqueTuners. “I have five different sizes,” he explains. “I insert them into pipes, and they work like wood stove dampers. I just change the angle to adjust the backpressure. I love doing Ironhead Sportsters and Shovelheads because 90 percent of it is in tuning the exhaust.”

Jetting Recommendations

So, with all that experience tuning bikes under his belt, we asked the good doctor if he had any few jetting tips for our readers. And while conditions like altitude, what pipes are on the bike, and a multitude of other factors can come into play, Fred did give us the following recommendations for those who have mostly stock components, but have added a freer-flowing air cleaner or exhaust system.

When it comes to jetting 100 percent stock carburetors on Evo Sportsters, Fred recommends using a 175 or 180 main jet. He also suggests a 180 main jet for Evo Big Twins, while a 195 main is usually the best choice for a Twin Cam. “Those are optimum jets for those kind bikes, and what’s interesting is these bikes all have the same carburetor,” says Fred, adding the difference is in the factory jetting. “There’s very little difference between the Sportster and the Big Twin carb. On the Twin Cam, it’s also basically same carb, but it comes from the factory with a 190 main jet in it.” However, if the carburetor has been modified, meaning if the needle and spring have been changed, it’s back to square one with the right main jet. “I’ll have to start all over. Those jet numbers won’t necessarily work. Other parts were changed, which affects the correct size of the main jet,” Fred states.

With ignition timing, Fred says late-model Evo engines, from 1994 to 1999, usually end up two degrees advanced. “Everything else is a special case,” he says. “If a guy brings me a 1992 or ’93, there’s not much point in touching the timing, because I know that’s it going to be right on. Sportsters are almost right on from the factory, too. Once in a while, you’ll find one that’s off.”

As far as Ironhead Sportsters and Shovelheads go, Fred says many no longer have stock carburetors, and therefore represent individual case scenarios. “The older the bike, the more likely it is that it’s been changed, so they are special cases,” he says, declining to speculate on timing averages. “A lot of them are running drag pipes and S&S carbs,” Fred explains. “When they run drag pipes, the lack of backpressure causes the midrange to get real rich, which causes a lack of power. Typically, they will jet their S&S carburetor leaner, essentially to try to get the engine to run smoothly on the highway.”

Running lean, he says, means it has a Grand Canyon in the middle of the torque range. By adding backpressure to the exhaust, an older Sporty or Shovel will produce more torque. “The other part of the equation is now I can jet the carb richer, so that it has more power at the high end,” Fred adds. “On the old Ironheads and Shovelheads with S&S carbs, you’d see a real lean jet like a 64-66, but once I tune the exhaust, I’ll end up running them with a 70 to 72 main jet.” 


Peter Rich felt his 1997 Dyna Wide Glide was running well, but he wanted to know if it was running its best. “I had all the right components -- a cam, ignition, and slash-cut exhaust,” says Rich. “I was blowing every one away beforehand, but after he tuned it, the bike was even faster.”

A rider and enthusiast of 46 years, Rich then rode his 2002 V-Rod onto Fred’s dyno. “It has a Screamin’ Eagle exhaust, and Fred messed with the intake and remapped the fuel injection,” Rich says. “We first got 111 out of it. Then we got 79 pounds of torque with 115 horses to the rear wheel.” While he is more than happy with the turnout, Rich says having power on tap is what it’s all about. “Horsepower isn’t what you want,” he adds. “You want a riding range, and that’s what he can do for you.”

Leon Jurasik agrees. When the self-employed ceramic tile installer from Dorothy, New Jersey, picked up a 1995 Sportster 883 for short money, he immediately set out to make it his own. First, he had the motor punched out to 1200cc and armed it with 10.5:1 compression pistons. Then, he topped it all off with a Mikuni HSR 42mm carb and Andrews N-4 cams. “I don’t think the local dealer set it up right. There was something missing. It ran well, but I think they may have rushed it,” Leon says.

Last summer, he rode the bike to Pattersonville, New York, the site of the Harley Rendezvous, where he was able to watch Dr. Dyno at work. “I was watching him put the power where people wanted it,” he said. “I wanted it off the line. I don’t want to go 120 to 130 mph. I wanted usable horsepower.” And that’s just what he got, he adds. “The bike ran pretty good on that six-hour ride to the Rendezvous, but after he got done with it, it ran a whole lot better. What a difference,” Leon adds. “People will still look and laugh because it’s a Sportster, but when they catch up at the next light, they’re saying, ‘Nice bike.’”