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Dyno Dos

One of the boons to performance enthusiasts in the last decade has been the development of quality chassis dynos—ones that don’t carry six-digit prices. Finally the grassroots gearhead can operate just like the pro teams, as more shops have been purchasing and installing these dynos and offering us average folks a chance to test and tune like the big teams.

If you’ve never had your car on one of these machines, you’re really missing out. If you’re prepared, you’d be amazed at what you can do and learn in 30 to 60 minutes. If you’re not prepared, however, you may have a mess on your hands—along with a huge bill for wasting the dyno shop’s time.

A dyno isn’t some sort of magical device; on the contrary, it’s actually quite easy to use and understand. Having a good game plan and properly using the data can often be the difference between success and failure.

Have a Plan

We never go to the dyno without a plan. Usually, we prefer to keep the plan simple.

While we’ll occasionally spend the whole day on the dyno making a lot of tests and pulls, we usually find it better to spend less than an hour looking at just a few variables rather than a whole day looking at a lot of them. The more variables you have in your testing session, the more chance you have to make mistakes or misinterpret your data.

As an example, we may decide to work on dialing in our car’s ignition. Our plan would be to first baseline the car with the stock ignition. Before starting, however, we’d check the timing and record it. Then we would install any parts we want to test, usually one at a time (more on this later), and see how they do with the timing set the same as stock. Once we find the best combination of parts, we’d adjust the timing a bit at a time to find the best setting for power. In the above example, we’ve outlined about an hour’s work and could probably make 15 to 30 pulls on the dyno. You’ll notice we’re doing nothing with the induction side of the engine—in an ideal world, we’d come back and do that at a different session.

If we did do it at the same session, we’d first do all of the ignition testing and then move on to the induction work (for which we’d have a separate plan). We would not intermix any of these tests as they could confuse our data and thinking.

Tune Your Car First

Contrary to what some people may think, a dyno session shouldn’t be used for much “tuning.” About 90 percent of your tuning should be done before you get to the shop, with just the remainder dialed in on the dyno.

We’ve been to a lot of dyno sessions and dyno days where cars go on the rollers and can’t even make a full pull because they’re running so poorly. While we’ve been able to get them running right on the dyno, we could have done the same thing without this piece of equipment and would have saved the owners a bunch of money.

If your car has a drivability problem, a rough spot in its acceleration, or parts that you know are weak, fix them before you go to the dyno. Some people will expect the dyno to magically identify these problems—and occasionally it will, but so could a good conventional tuning session.

Rehearse Your Plan

After we’ve made our plan but before we go to the dyno shop, we always rehearse the plan at home first. We install all the parts in the order we plan to install them at the dyno. Again, we usually install one part at a time and ensure the car still runs fine after each installation.

In addition to making sure all the parts fit and work before going to the dyno shop, this rehearsal lets us make sure we’re bringing all the correct tools with us to the shop. It’s pretty tacky to borrow tools at the shop, plus it’s just going to waste expensive time while you’re doing it.

Pack Your Parts and Your Tools

At the dyno shop, time is money and good preparation will save you both. Pack your parts and your tools in an organized way so that you can easily find them when you get to the shop.

If possible, lay the tools and parts out on a bench or even on the floor near the car so that you can grab each one when you need it. Make sure you keep them all together as much as possible so you can bring back everything you brought. No need to make the session more expensive by accidentally donating tools or parts to the shop.

Write Things Down

We keep a notebook that we take to every dyno session. We have our plan written out in the book.

Following the plan, we write down the baseline details of the car, including things like the parts used and present tune-up settings. We also write down the weather conditions.

Then, as we make each pull, we note the pull number, the changes we made to deviate from the baseline, and the results—for example, “down 2 peak horsepower,” “too lean above 4000 rpm,” etc.

While the dyno operator is hopefully putting similar notes into the computer, it’s pretty smart for you to have a separate set to corroborate and compare later on. You may find the operator misinterpreted something or used abbreviations that make no sense to you.

Communicate With Your Operator

Either before or upon your arrival at the dyno, go over your plan with the operator. The operator may offer some suggestions: If they’re helpful, take them; if they’re not, politely explain that you’d like to stick to your plan.

Put together a mutual understanding of approximately how many pulls you’ll be making and how long you’ll be on the dyno. That way you won’t find yourself rushing because of a miscommunication.

Then, after each change you make, discuss the results with the operator. Again, he may have some suggestions for you based upon his experience. Also, look at the data on the computer and make sure it makes sense and the notes are correct.

Make Your Baseline

The first pull you make for the session is your baseline, and you want it to be accurate. We usually make three or four baseline pulls and make sure that they all graph out the same. This will be important later in the session as well as for future sessions, since you always need a good baseline for reference.

If you really want to feel good about your session, you may even want to return the car to its baseline set of parts and tune, then make a couple of last pulls before leaving for the day to be sure it’s still the same as the beginning. That way you can be sure the weather conditions or something else didn’t skew your data.

Make One Change at a Time

There are occasional exceptions to this rule, but if you’re truly trying to test results of tuning or parts changes, you need to make one change at a time. If, for example, you make three changes and lose a couple of horsepower, you can’t really determine what cost you the power.

Make Your Pulls

Consistency is a key part of dyno tuning, so make sure you drive your car the same way for every pull. If you’re making your pulls in fourth gear, make sure you accelerate up to speed in the same manner each time. If you skip second gear the first time, skip it every time. If you drop down to 2000 rpm before your acceleration starts the first time, do that every time.

We usually make two pulls, back to back, for each test. If they don’t repeat exactly, we don’t move on until we figure out why and correct the problem. Then we repeat the pulls to make sure we’ve got good numbers.

Expect Bad Pulls

It’s very likely that you’ll have a few malfunctions during your session that make a few of your pulls bad. Be a skeptic when you see extremely good numbers and be an optimist when you see bad numbers. It could be that something threw off the data and it’s just wrong. Repeat the test until you’re confident things are working properly, then throw away the data from the bad pulls.

Get All of Your Data

Don’t just settle for a few graphs of your best pulls. Ask the operator to give you all of your data. We usually get graphs that compare some pulls against each other (for example, baseline, best and worst, all on one sheet).

On the graphs, we’ll ask to have horsepower and torque vs. rpm—preferably with horsepower and torque on the same scale—and we’ll also ask for graphs with air/fuel ratio vs. rpm. Sometimes, we’ll ask for similar graphs over time instead of rpm.

Once we’re done with the graphs, we’ll ask for a set of printouts of the numbers for each pull. We’ll get horsepower, torque, and air/fuel for every 100 rpm. Finally, we’ll get all of the data on disk so we can plug them into a spreadsheet later at home if necessary. Your operator may not be enthusiastic about spending the time to do this with you and you may not feel like it’s worth the time or extra expense, but it is. Your memory will fade over time, so the ability to go back to the recorded data is very helpful.

Study Your Data

You may have learned some things at your dyno session, but you won’t really know until you’ve looked at your data more closely. Read through the graphs and numbers carefully and think about what information is there.

Think about whether you can trust your testing: Does the data seem to support it, or are there numbers that just don’t make sense? If you’ve built a data library, you can compare your numbers to previous data (or look on the Internet at other people’s data if you can) and see what else you can glean.

Keep a Data Library

We’ve got data from every dyno pull we’ve made. (Well, except for some of the obviously bad pulls we’ve thrown away.) We’ve made copies of the originals and we keep it all organized in three-ring binders.

On the pages in the binders, we’ve included copies of our session plans and notes, and we’ve written comments on the graphs and printouts. As our library has grown, we’ve been amazed at how often we can use the data from past session to make predictions and plans about future sessions. It’s worth the effort.

That’s a Lot of Work!

If all you want to do is brag about how much horsepower your car makes at the wheels, you don’t really need to do all of this stuff—just go to the dyno and make a couple of pulls.

But if you’re serious about performance tuning, you’ll find that these practices are helpful, and you’ll find yourself going back to the dyno time and time again.

A Step-By-Step Guide to Your Dyno Session

  1. To get the car on the dyno, it is first driven onto the hoist and raised to position. The car is then parked so the drive wheels are on top of the rollers, and the car is securely strapped in place.
  2. Ideally, a wide-band oxygen sensor should be installed in the tailpipe if there is no catalytic converter, or directly into a bung in the exhaust system.
  3. After the car is warmed up, a pull can take place. Typically the operator will first gently accelerate up until a predetermined speed before flooring the throttle for the actual pull. Always using the same procedure will ensure repeatable, reliable numbers.
  4. While the dyno pull is taking place, a computer is used to record and interpolate the data.
  5. During a dyno pull, the car is going to be run at full throttle for several seconds. Since a few fabric straps are really all that hold the car in place, make sure the shop you choose uses quality equipment. The last thing anyone wants to see is their car launching off a dyno at maximum acceleration. Speaking of safety, the car's tire pressures, oil level and other parameters should all be within the recommended zones. While it’s tempting to stand next to a car that’s making a dyno run, remember that the drive wheels can be spinning in excess of 100 mph. If something goes wrong or comes loose, odds are that you don't want front-row seats.
  6. We always lay out our parts and tools right by the car so that we can quickly find them as we make changes and adjustments.
  7. Before our first pull, we record all of our baseline conditions and double-check settings.
  8. During the pull, the data acquisition computer is recording everything we need for good analysis later. Make sure to take all of your data home with you. Once back home, you should file your data for good analysis later. We keep a spiral binder of handwritten notes that we take at the dyno shop. We make copies of our dyno sheets and put them in three-ring binders with handwritten notes on the sheets and graphs. We also keep electronic versions on the computer and use software tools like spreadsheets to delve deeper into our data.
  9. The typical dyno chart shows horsepower, torque and the air/fuel ratio. While testing in a controlled environment can cost a few bucks, the dividends of a better running car are generally worth it.

Weather and Correction Values

A dynamometer can be very consistent from day-to-day, with little in the way of variance in horsepower or torque numbers. The engine being tested, on the other hand, is sensitive to atmospheric conditions, or more specifically, to the amount of oxygen that is sucked into the combustion chamber. Keeping things consistent allows more accurate tuning.

Air temperature, humidity and atmospheric pressure will all affect horsepower numbers by changing the amount of oxygen contained in the incoming air. The higher the temperature, the more dispersed the oxygen molecules become, leading to less power.

Humidity is a lot like temperature in this regard. As the water vapor content in the air increases, horsepower values tend to drop because the water vapor takes the place of oxygen molecules in the combustion chamber. Barometric pressure can also affect dyno numbers: Increases will actually force more oxygen molecules into the same physical space, leading to higher horsepower numbers.

Since maintaining constant outside air conditions can be tough to accomplish, correction factors are used to provide stable numbers. A correction factor is simply a multiplier applied to the raw horsepower numbers, which produces a corrected horsepower value. It is used to predict what horsepower numbers an engine would generate if it were tested at sea level under standard temperature and humidity conditions.

Say a car is being tested on a hot and humid day, yielding a correction factor of 1.03. This means the final, corrected horsepower and torque values will be 3 percent higher than the raw data produced by the dyno. At the other end of the spectrum, cold and dry days could have a correction factor of less than 1.00, meaning the corrected values would be slightly lower than the measured ones.

Each particular dyno manufacturer may have its own correction factor. The one used by Dynojet employs the following formula:

 1.18 x (29.22/ABP) x ((Square Root (T+460)/537)) – 0.18), 
 where T = Intake air temperature in degrees Fahrenheit and 
 ABP = dry ambient absolute barometric pressure.

ABP can be calculated using the following formula:

ABP = RelHg - EIev/1000, 
 where RelHg = Relative barometric pressure and 
 Elev = test location elevation in feet above sea level.

The Society of Automotive Engineers originally published this formula in 1990 in their scientific journal. If the math has you baffled, don't worry, the dyno's computer handles it.

Engine Dyno vs.Chassis Dyno

When the time comes to measure performance output on a dynamometer, one of two styles can be used: an engine dyno or a chassis dyno. As the names suggest, one measures the output of an engine while it is placed on a stand; the other measures output while the engine is still in the car's chassis.

The engine dyno can be used to measure the horsepower and torque at the engine itself, without taking into account the amount lost through friction in the rest of the drivetrain—the transmission, drive-shaft couplings, wheel bearings, CV joints, differential and so on.

Testing an engine on an engine dyno can require some extra work, as the engine must be removed from the vehicle, mounted on an appropriate stand and hooked up to cooling, fuel and exhaust systems. However, most professional race shops test this way in an effort to eliminate outside variables.

Of course, it's not always practical to remove an engine from a car just for testing purposes. This is where the chassis dyno steps in.

The chassis dynamometer uses eddy-current absorbers or inertia systems to measure the engine's output at the drive wheels. Instead of measuring the power at the engine's crankshaft, the chassis dyno's large rollers measure the horsepower of the engine as it is delivered to the road. Some tuners prefer to work with these numbers anyway, as they can look at the entire driveline picture while testing.

Since the chassis dyno doesn't require the engine to be removed, nearly any motorized vehicle—car, truck, motorcycle, golf cart or whatever—can be quickly and easily tested on one. Do note, however, that testing a vehicle with a full-time all-wheel-drive setup will often require a chassis dyno designed to accommodate the two pairs of drive wheels.

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Comments

View comments on the GRM forums
Jeff Blaine
Jeff Blaine New Reader
8/15/08 8:28 a.m.

Step 1: Never "tune" your car by just doing full-pull runs on a dyno.

chknhwk
chknhwk HalfDork
5/18/10 5:13 p.m.

I miss my '98 Cobra... :(

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