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How It Works By Mark A. Desorbo, American Iron Magazine, May 2006

Dyno Dos And Don’ts, I

The original article with revisions

Editor’s note: First published in the April 2003 issue, this article has become required reading for anyone interested in how to correctly dyno tune a bike. However, there has been significant improvement in the dyno tuning process over the intervening three years (like the air/fuel ratio module Dr. Dyno added to his Dynojet dynamometer), so it’s time for a reprint and an update. To that end we asked the good doctor to revise the original piece and bring it up to date on what’s happening in the dyno-tuning universe. The result is the following, which is a condensed version of the original article written by Mark A. DeSorbo, as well as clarifications in the areas that raised some questions with our readers. However, one How It Works is not going to be enough to cover all that we wanted to tell you, so this issue is the first of a three-parter. Some of the subjects covered will be dyno tuning, how a dyno can be doctored to give higher-than-true readings, the difference in fourth- and fifth-gear runs, and how to dynotune a fuel-injected bike. We hope this series helps you get more from what you’ve paid to own — in both power and pleasure. — Chris

Dr. Dyno at work dialing in a Super Glide Sport after it received some performance modifications.


Benjamin Franklin once wrote, “A man in passion rides a mad horse.” The 18th-century sage indeed was clairvoyant about 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. That’s true of many American V-twin disciples. We’re eager and steadfast in modifying our respective iron horses, yet many do not realize the potential of the very detuned engine that lies beneath them.

And that boils down to some ending up riding a pig instead of a bonafide hog. “They got oink instead of thunder,” quips Fred Eno, aka 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, Fred’s helped enthusiasts like Glenn, 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, 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 insulted, in addition to what he perceived as a rather expensive injury. “I dyno’ed 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 ft-lbs. of torque!”

All Fred did to fix it was tune it; he did not 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 midrange torque — and slow bikes. It’s not the peak horsepower that the bike comes onto the dyno with that counts, it’s the useable midrange torque it goes off with.”

Dynamometers take measurements at various engine speeds and convert those numbers to horsepower and torque values using the formula horsepower equals torque, multiplied by rpm and divided by 5,252. This means the horsepower and torque values are always equal at 5252 rpm, if they are scaled equally. As you can see, this one is not, so the curves cross at about 4600 rpm.


Perhaps the greatest misconception about the dyno is 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.

Dynamometers take measurements at various engine speeds and convert those numbers to horsepower and torque values using the formula that horsepower equals torque in ft-lbs., multiplied by rpm and divided by 5,252. By definition, this means the horsepower and torque values are always equal at 5252 rpm. When graphed, the recorded measurements create horsepower and torque curves which will cross at 5252 rpm if they are scaled equally. These curves indicate 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 is 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 hp 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 hp.” 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(s), were also of the absorption type. Many are still in use in automotive performance shops and are common in the diesel truck testing industry.

If a freer-flowing intake or exhaust has been added, the carb’s jetting almost always has to be made a littler richer to avoid a too-lean condition.

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, Fred 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, which is 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 rearwheel, real-world power,” he adds.


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 says.

Burnout pits are another “smart” way to have fun. They provide a wall or restraint to put the front wheel against while the rear tire burns rubber on a steel plate. Fred tells about an official pit setup next to him (the keep-all-the-noise-in-one-place mentality) at the Harley Rendezvous last summer. “The first guy’s Harley revved way up like he missed the first-to-second shift,” Fred observed. “Nope, he’d snapped the drive belt, which became obvious as he and his friends pushed the bike back to the campground.” Fred smiled as the next guy, on another Harley, high-fived his buddies as he ran through all five gears. There’s only one problem: no smoke. The rear wheel didn’t ever spin! Talk about a cooked clutch! “Meanwhile,” Fred says with some pride, “I ran a whole season of dyno testing and tuning without a single broken belt or damaged clutch. A little later the same night,” Fred continues, “a guy takes a sportbike to the rev limiter in all six gears accompanied by copious billows of tire smoke. Then, to the cheers of the gathering crowd, he does it again!” Asked why he would beat up on his bike that way, the guy explains “Oh, it’s not my bike, I’m just getting back at my buddy for something he did to me earlier!” What’s the slogan? “Smart, very smart!”

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. (More on this and other tricks in the July issue.)

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, 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’re 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.”


For $80 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 useable power, not just measure a peak, so I want to see what it does as it warms up,” he says. This has caused some confusion, as some customers interpret “not warmed up” as it needs to sit for a couple of hours. With the dyno upgraded with the addition of the air/fuel ratio exhaust gas analyzer (more on this next month), watching as the bike warms up has become less important. Other owners think the bike has to be fully warmed up before going on the dyno, either for the safety of the engine or because the true peak horsepower won’t be read otherwise. “I run each bike on the dyno consistent with how hot or cool it arrives and peak power is always read,” he comments.

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 to check when evaluating Harleys. They are air, fuel, ignition, and exhaust. “These items, especially the jetting, timing, and backpressure, each make a variation of 2 to 3 hp,” 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 1 hp 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.

Getting the ignition timing dialed in is critical. If an adjustment is needed on a Twin Cam, change out the stock electronic module for one that allows you to adjust the engine’s static timing.

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, and he dials in 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.”


So with all that experience tuning bikes under his belt, we asked the good doctor if he had a 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 or 200 main is usually the best choice for a Twin Cam. “Those are optimum jets for those kind of 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. The Twin Cam, also has basically the same carb, but it comes from the factory with a 190 or 195 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 latemodel 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 it’s 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 says. “On the old Ironheads and Shovelheads with S&S carbs, you’d see a real lean jet like a 64 to 66, but once I tune the exhaust, I’ll end up running them with a 70 to 72 main jet.”


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 slashcut exhaust,” says Rich. “I was blowing everyone 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 [now 95 ft-lbs. and 140 hp].” 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 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 he 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 useable 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.’” AIM