For an engine builder going to the dyno for the first time, the process can be intimidating, and the information gathered can be confusing. However, the rewards are well worth the time and money. In fact, dyno time is such a valuable asset to your engine program that it should be regarded with the same enthusiasm and preparation as a trip to the racetrack.
“Go to the dyno with specific goals in mind,” suggests Mike Giles, marketing manager for SuperFlow Dynamometers and Flowbenches.
A dynamometer is a measuring tool to help optimize engine combinations and find small problems before they turn into catastrophic failures. And they’re not just for racers. Everyday enthusiasts can tune their engine for better drivability, fuel economy and power.
While horsepower numbers dominate bench racing in the garage, the first-time dyno customer should have clearer understanding of the primary objectives for an initial dyno test.
“It’s important for an engine builder to use dyno testing to get a baseline and measure accurate improvements from there,” stresses Giles. “He should not to be concerned with an absolute dead-nuts correct horsepower number.”
In other words, you don’t want to get into a situation where the dynos get raced by the operators for bragging rights. Experienced and quality dyno shops provide consistent and repeatable tests so the builder can find and quantify improvements.
It’s a shame to waste your dyno reservation over a $5 part failure or a bad water pump. — Mike Giles
Finding a reliable dyno shop takes research, patience and sometimes a little luck. Ask around. Good shops have excellent reputations throughout a large region. Check local racing publications and racer forums on the Internet. Don’t be afraid to drive a couple extra hours to use a good shop. With dyno time costing anywhere from $500 to $2,000 a day, extra travel time will be worth the effort.
Once you’ve located a dyno facility, ask the operator what to bring–such as headers, tools, etc? Dyno operators have seen it all, and they will be happy to assist, so talk at length with them. The operator also needs to understand your intentions and the level of involvement you expect from them, especially if you’re going to be tuning the engine between runs.
Other tips when talking with the dyno shop: Make sure to clarify the appointment time and any cancellation policy. Bring fuel, nitrous, lubricants and an extra set of spark plugs. Double check all engine fasteners. Bring the laptop and extra battery or charger if specific software is needed to tune your engine.
“Bring everything that goes with the engine in its application: the same air cleaner, same carb or fuel system, spare parts,” says Giles. “Again, it’s a shame to waste your dyno reservation over a $5 part failure or a bad water pump. Take as many extra parts as you have.”
Headers can present a problem, however. Dyno shops usually store headers for popular applications but they don’t always cover every tube size and length.
“Bring the same headers you are going to be using in the intended application,” says Giles. “This is a huge deal. We’ve seen time and time again where an engine was tuned with dyno headers and then ran like crap in the vehicle.”
Of course, the reason dyno shops have their own headers is that they’re are modified to support EGT probes. Some shops have special mounting plates that go between the cylinder head and the customer’s headers but also have connections for the EGT probes. Again, those are the types of shops that pay attention to detail and should be considered for patronage.
Tools needed to change injectors or other special equipment are your responsibility. Don’t assume that the shop will have special tools.
Know the specs of your engine – inside and out. You may know what parts you used when your engine was assembled, but it’s hard for the dyno operator to diagnose problems or assist in tuning the engine if they do not know your engine combination.
Prepare your engine for transport by covering all openings and passageways into the engine. Remove the carburetor from the intake manifold and replace it with a lift plate that covers the intake manifold plenum at the carburetor mount pad. Hoisting the engine with the lift plate is much easier than using chains that are bolted to the cylinder heads.
Be punctual and respectful of the facility operators. Have your goals written out on paper. They can be as simple as measure the standard horsepower and torque numbers or as intricate as find maximum power or eliminate detonation. It’s important to review these goals with the dyno operator working with you.
“If you’re trying to find the best carburetor, take as many carburetors as you can and test them all,” adds Giles. “We’ve seen people do this with all aspects of an engine from water and oil pumps to the obvious ones like valve springs and cams. If you’re after a good tune for all your components make sure you take all of them. Air cleaner, headers, carburetor, fuel system, and the like. All the things that the engine will use in its application. All of these components are a major part of the tune.”
Testing on the dyno
The dyno operator will mount the engine and complete all the fuel, water and oil hookups for safe operation. If you have a fresh engine that needs broken-in, the dyno operator will run the engine gently at first to ensure that everything is in order. If no problems are indicated, he will proceed to set the timing and air/fuel ratio to safe values for break-in.
Exhaust gas temperature is important during engine break-in to ensure that the fuel/air ratio and ignition timing is kept in a safe zone. Too much fuel will wash down the cylinder walls with raw fuel and dilute the oil. Generally, engine timing for break-in will be slightly retarded to avoid any chance of engine knock. Once the engine has been broken-in and any adjustments like valve lash have been checked and adjusted, you can begin tuning the engine for wide-open-throttle (WOT) runs.
The dyno operator will likely begin with a fairly low rpm then gradually increase the load while listening for spark knock and checking the air/fuel ratio, stopping to correct any problems that he may see or hear. Repeating this procedure until he is able to make a complete sweep from 10% to full wide open throttle under load while maintaining a steady rpm.
Many operators will repeat that procedure at higher rpm levels (usually bumping up the rpm range from 500 to 800 rpm in subsequent runs) until the engine can be operated at 4,000 rpm steadily with an increasing load at WOT. Only then can part throttle or full throttle runs be attempted.
You dyno operator should begin by making some short WOT runs, verifying the air/fuel ratio and timing at regular rpm points to establish a matrix of proper values within the test range. Once a good safe fuel and ignition calibration has been mapped in the ECU, part throttle and full throttle testing can begin.
Circle track engines and those that are destined for street only operation should be tuned at part throttle as well as full throttle. Many dyno operators are only concerned with tuning at full throttle because that is where the max horsepower numbers are at. Circle track and street engines operate at lower rpm on a regular basis, therefore optimization of power and torque at part throttle makes the most sense.
A few words about correction factors
You may hear the term “correction factor” or “corrected horsepower.” Correction factors are formulas that standardize the dyno testing of engines under different atmospheric or weather conditions. It’s a necessity when comparing dyno results. Air enters the engine by atmospheric pressure. It’s not actually pulled into the engine by the downward stroke of the piston – the piston moving down in the cylinder only creates a void for liquid or air – the combustible mixture is actually pushed into the engine by the external atmospheric pressure.
Atmospheric pressure is affected by altitude and air temperature. For example; hot air expands and fills the combustion chamber sooner than denser air, but expanded air has less oxygen. Moisture in the air can displace oxygen molecules, so humidity also plays a role in the air/fuel ratio. Overall, any change in atmospheric conditions will affect engine performance.
SuperFlow engine dynamometers can change between two correction factors along with the raw engine data to calculate the final dyno results. Both correction factors take into account barometric pressure, vapor pressure and air temperature but different values area assigned to each factor based on formulas set down by the Society of Automotive Engineers (SAE).
Values assigned to STP J-607 correction factor:
• Barometric Pressure = 29.92 inHg
• Vapor Pressure = 0.0 inHg
• Air Temperature = 60 degrees F
Values assigned to the SAE J1349 correction factor:
• Barometric Pressure = 29.23 inHg (approx 800 feet asl)
• Vapor Pressure = 0.0 inHg
• Air Temperature = 77 degrees F
With SAE J1349 being the current accepted standard, if you tested on a day with respective readings of 29.23 inHg, 0.0 inHg and 77 degrees F, this would constitute a multiplier of 1. If these same atmospheric conditions were applied with STP J-607 correction factor, the multiplier would be 1.04. For simple comparison, an engine measured at 100 horsepower with SAE J1349 correction factor would be rated at 100 horsepower (100 X 1.0 = 100). The same engine raw data applied to the STP J-607 standard would end up with 104 as the corrected horsepower rating (100 X 1.04 = 104).
It’s easy to see why a dyno operator would use the STP J-607 correction factor for engine dyno runs – it records a higher horsepower number. This is not really an issue, you just have to be aware of what correction factor was used when reading engine dyno numbers and compare apples to apples.
Reading a dyno sheet
Most dyno operators will print a form with columns of numbers for the customer or tuner to reference. Reading the dyno sheet is fairly simple if you understand the categories. Dyno printouts come in various different formats but inputs or categories are usually consistent:
Engine RPM refers to the engine revolutions per minute (RPM) and serves as the reference to which all readings in the same row were taken.
Torque (either corrected or uncorrected) should be represented in a subsequent column. We prefer to see uncorrected torque and corrected torque in separate columns. This gives a visual indication that everything is recording correctly. If you happen to see a difference between the corrected and uncorrected torque values of more than 50-ft/lbs, there may be a problem that needs to be checked out.
Horsepower isn’t measured by the dyno, but rather the torque and rpm readings are run through a formula to calculate the horsepower. This number reveals much about your engine. Pay attention to where in the RPM range your engine hits peak horsepower and torque. Making component changes like fuel systems or intake manifolds can move your power band up or down in the RPM range. There is a large difference between horsepower numbers from an engine dynamometer and a chassis dynamometer because of the numerous variables that exist in the vehicle’s drivetrain. Chassis dyno horsepower numbers should never be used to verify engine dyno horsepower numbers.
Fuel Flow, displayed in pounds per hour (lbs/hr) can be a very useful means of balancing multiple carburetors on a single engine, or the data can be used to refine electronic or manual fuel injection systems.
Knowing the fuel’s specific gravity is important because it affects the accuracy of the BSFC readings and can mess with the optimum carburetor calibration. — Mike Giles
Mechanical Efficiency (ME%). This reference number is simply brake horsepower divided by indicated horsepower. Used by many engine tuners working on fuel use in their engine programs. Typical stock street engines register 70% to 80% mechanical efficiency. Above 80% of mechanical efficiency indicates an efficient engine.
Barometric Pressure. This column represents the local barometric pressure at the time of the test. Many engine dynamometers have a weather station that provides the barometric pressure for the location of the dynamometer on that date and time of the test. It is an uncorrected number and displayed as raw data.
Vapor Pressure. Vapor Pressure refers to the amount of water in the ambient air. When the vapor pressure is high, it indicates that the amount of oxygen in the atmosphere is being displaced by the atmospheric water.
Tuning on the dyno is simply optimizing the components of an engine within the framework of the engine build. The most successful engines are well designed with carefully selected components to create peak efficiency. Many times in an engine build, trade-offs are made, based on part availability or budget. Hopefully, engine builders make these compromises keeping the engine’s application in mind.
The common misbelief in internal combustion engines is that the air/fuel mixture explodes and forces the piston down to create power. In a properly tuned engine, the air/fuel mixture burns rapidly and does not explode. This rapid burn creates a controlled and predictable pressure rise from the expansion of gasses caused by the combustion.
The spark needs to be adjusted according to the engine rpm, design, equipment and intended use. Some engines need more advance as rpm increases while others may require that an ignition retard at due to boost, nitrous or cylinder-head design. Flame propagation (the way fire spreads in a cylinder) has been the focus of combustion engineers for the last decade. Your dyno operator should be able to help you reach peak cylinder pressures and optimal ignition timing.
Air/fuel ratio is approached conservatively. The goal is to have a safe mixture that allows maximum power without causing the engine damage. Heat damage can happen in an engine that is too lean, so starting with a rich mixture and adjusting from there is the best practice.
Using several sensors, like the exhaust gas temperature sensor and lambda sensors, your dyno operator should be able to guide you toward taking fuel away until the power starts to fall off, then richen the mixture back up until maximum torque is reached.
Again, keep in mind the application of your engine. If the engine is destined for a circle track racecar that competes in longer events where pit stops are made, fuel economy is a bigger issue. Races could be won or lost on an extra pit stop. Work with your dyno operator to figure out the best air/fuel ratio for your engine’s application.
A quick word on dynamometer accuracy
A test is only as good as the tool that is doing the measuring, with accuracy being the key buzzword. In order for a dynamometer to be accurate and able to repeat the tests as close to exact as possible, they must be calibrated. Try to verify the accuracy of the dynamometer calibration, if possible. Ask the operator about the last calibration and how they do the calibration. We’ve heard of tuners who carried their own calibration weights with them when they take a trip to the dyno facility. It’s all about your faith and trust in the facility to produce accurate test results.
In a short article such as this, it is difficult to produce an all encompassing “go-by” on engine dynamometer testing. Hopefully, we have given you enough information to take any fear away from taking your engine build to a dyno facility and what to do with the results. We recommend checking dyno facilities in your area that have a good reputation and are proficient in the application for which your engine is designed. Using some basic approaches and a little common sense, engine dyno testing can greatly enhance your entire engine program.