Tuning for Dyno Numbers.
The first thing that comes to mind when comparing two similar engines is power or torque output. Since these are measured on a dynamometer, the ultimate goal when building or tuning an engine is commonly expressed by vehicle owners as some specific desired power level. It’s very easy to get lulled into this thought process as an engine calibrator as well, especially if you have worked on similar engines before. I can’t count how many times I’ve heard “This should make XXX horsepower” during bench racing sessions. The careful distinction comes, however, when it’s time to calibrate and measure. When all is said and done, the engine flows some fixed amount of air at maximum effort. Once the throttle is fully opened or the wastegate is working, there is no magical ECU table that forces more air into the cylinders. All the calibrator can do at this point is quantify how much airmass is trapped on each cylinder stroke and attempt to deliver the corresponding fuel mass and ignition angle for the present conditions. Sure, the calibrator can choose a less than-optimal fuel mass or spark angle, but once the optimum is found there’s not really much else he can do. If the final result is an engine that only delivers 495 wheels horse power on a chassis dyno safely, that’s it. Any attempt to show a bigger dyno number is usually the result of an eroded safety margin, unethical manipulation of the numbers, or some other error. Correction factors are imperfect at best, so don’t get wrapped up in the need to deliver beyond some magical threshold for power. The only exception here is that if power is way off from the expected, it may be time to look for mechanical issues. It may be belt slip on a blower pulley failing to deliver full boost, or it may be three cylinders down on compression. Either way, it would be prudent to step back for a minute and take a sanity check. Pull one or all of the spark plugs and investigate. If everything checks out mechanically, chasing a dyno sheet in the name of impressing everyone is seldom a rewarding venture in the long run.
Tuning Only on the Street
There are a lot of skeptics out there who don’t feel that a dynamometer accurately reflects the way we drive a vehicle. They claim that an actual vehicle on an actual road gives them the exact conditions they need to tune to. They’re right. However, loaded dynamometers do accurately reflect how the fundamentals of the ECU’s control system works. The real objective is to calibrate a control system to a mechanical device. Doing this means playing by the rules set forth in the controller. If the controller uses a series of steady state reference tables to determine engine conditions, it’s best to populate these tables in steady state. The load-bearing dynamometer allows the calibrator to dial in the exact conditions he is looking for and tune these steady state tables in fairly short order. At the OEM level, all ECU calibrations start out on the dynamometer before ever seeing a vehicle. The engine’s efforts on a load bearing dynamometer can be both above and below those seen when driving down the road. What’s important is that each breakpoint of the reference tables be individually optimized so that any time the engine is between two of them, an accurate interpolation can be made. Driving on the road can make it very difficult to hold the engine precisely at each cell in the table if the available load breakpoints don’t line up with easily achievable loads seen in the various gears. Even more importantly, it’s just about impossible for anyone in an urban area to find a road long enough with zero traffic to safely perform the necessary steady state measurements. Skipping the steady state measurements only opens the door for more confusion from transient fuel delivery as previously mentioned. The simple fact here is that a skilled calibrator can get more work done on a proper load bearing dyno than most “tuners” can do in weeks on the road. Getting the fundamental steady state tables correctly calibrated on a dynamometer can save endless hours and gallons of fuel during road testing. When done correctly, dynamometer calibration makes the overall tuning process faster, more accurate, and more enjoyable. It’s just as important to recognize that tuning usually doesn’t end with the last dyno pull.
Tuning Only on the Dyno
Just as there are those who swear by “real world” tuning only on road surfaces, there are those who insist that their dynamometer is the only method by which calibration can and should be done. Certainly, dyno tuning is an excellent start to any complete calibration procedure. The trick is to recognize that the engine does not live in a steady state world. Even steps on the dynamometer do not precisely replicate the transients seen as a vehicle shifts through the gears on the street or track. Worse yet, many paid tuning professionals swear by testing on an unloaded dynamometer as the only necessary approach.
At WOT, the sweep rate of an unloaded or inertial dynamometer can be much faster than the vehicle sees in the outside world. This allows less time for combustion temperatures to stabilize, perhaps giving a false sense of security when adding that last couple degrees of spark advance during the power pulls. Even though the engine may pull clean and strong on the dynamometer with lower load, it’s not uncommon to see the same engine self destruct at the 1,0W-foot mark of the drag strip where aerodynamic and thermal forces have aligned against an overly aggressive tune.
That extra 20 wheels horse power from the hot tune-up are of little condolence when staring at an expensive rebuild. At lower loads, an inertial dynamometer is still a handicap. Since the inertial dynamometer works fundamentally upon Newton’s second law (F=ma), any increase in the force provided by the engine results in some acceleration of the dynamometer. This makes it impossible to hold a constant load at anything other than light loads equal to the friction of the dynamometer and driveline. At best, the result is a series of sweep tests dynamically through the speed range that still does not yield valuable steady state calibration data for the base maps.
In short, an inertial dynamometer is the wrong tool for tuning speed density systems where it is critical to get a wide range of stable readings at specific speed-load points. While a loaded dynamometer certainly makes for a more flexible and precise tuning experience, it still doesn’t define the entire range of tests necessary for truly optimized engine calibration. The load-bearing dyno should be used as a tool to develop the steady state maps and controlled sweep rate tests at WOT in a safe environment. After these objectives have been met, it’s preferable to actually drive the vehicle in a free environment to experience as many possible operational conditions as possible.
A quick test drive after an initial dynamometer tune may help the calibrator zero in on a specific speed-load point in a particular gear where something does not feel right. This allows the calibrator to return to the dynamometer with the more specific task of fixing either fuel or spark trims in a narrower range of cells that may not have been completely optimized the first time around. Driving on the street also exercises the transmission, driveline, and brakes differently than on a dyno. The addition of new forces may open up the calibrator’s eyes to some other area of the software that may need additional attention such as dashpot or coast down airflow control. It is strongly encouraged to test drive the vehicle after tuning is completed whenever possible.