Drivetrain Losses
There are two ways to measure horsepower with dynos: at the flywheel or at the rear wheels. Unless you're an automobile manufacturer or have a very big dyno cell, most people don't test engines in the same configuration they will run in the car. For example, you probably won't have the exact same exhaust system with all the right bends and routing or the complete accessory system with all the belts and pulleys or fan, all of which rob some horsepower. Also, the engine dyno doesn't account for the most important source of horsepower-robbing friction: the rest of the drivetrain.
Drivetrain loss generally ranges from about 15-17 percent for a manual transmission to 20-25 percent for an automatic. In other words, an engine producing 400 hp at the crankshaft may transmit as little as 300 hp to the rear wheels. Automatics eat more horsepower than manuals primarily because they have heavier rotating assemblies and torque converters, which cause some horsepower to be lost through slippage (lockup-style torque converters are more efficient, but most are not recommended for high-horsepower applications). The type of rearend installed in the car can also affect rear-wheel output. For example, a GM 12-bolt or Ford 8.8-inch eats less power than a Ford 9-inch due to the way the ring-and-pinion gears intermesh. The bottom line is that it's how much power you ultimately put to the pavement that counts, and a chassis dyno is the best way to measure that.
Another issue to consider with an automatic is the torque converter. When chassis dyno'ing any engine with a torque converter, the dyno won't start taking accurate torque and horsepower readings until the converter has reached its true stall speed and is transmitting 100 percent of the power to the rear wheels (in reality, most converters never transmit 100 percent, because there is always some loss to hydraulic slippage unless it's a lockup-style converter). The chassis dyno thus becomes a very effective device to determine your converter's specific characteristics as installed in your car, as we learned with our SuperNova.
Be Prepared
Most dyno shops we've worked with are set up to handle just about any car that comes through the door, and usually part of what you pay for is their tuning and diagnostic expertise. Generally all you have to do is show up with your car in decent running condition and the dyno operators will take it from there. One critical item that's often overlooked until it's too late are tires. In top gear at WOT, it's not uncommon for tire speed to reach levels your car could never actually attain on the road because of aerodynamic drag. But strapped to a dyno, the actual tire speed is limited only by the rpm the engine is capable of turning and the rearend gearing. This means it's easy to far exceed the tire's speed rating. For example, with 3.08:1 gears, a 28-inch-tall tire will spin to about 135 mph at 5,000 rpm. So what you say? You won't be laughing if the tread decides to peel off at a buck-thirty. We've seen it happen, so make sure you've got good tires on your car. You can check the speed rating on the sidewall and ask any tire store what the top recommended speed is for those tires. Borrowing a set of slicks if you don't own your own might also be a good idea, because the sticky compound will adhere better to the dyno rollers, giving the most accurate readings, and they're designed to handle brief high-speed bursts. Here are some other things to consider before your first dyno session.
* Do a thorough tune-up before the session and check or replace the air and fuel filters, spark plugs, and wires
* Not all timing lights read the same, so check and set timing with your own timing light and bring it with you for consistent readings
* Lash the valves if you have a solid cam
* Make sure your engine has a legible timing mark on the balancer
* Torque all the lug nuts to spec
* Air up the tires to recommended pressure and remove trim rings or hubcaps if you have them
* Bring any tune-up parts you may need, such as carb jets, power valves, and gaskets
* Fix leaks and drips so your car doesn't make a mess of the dyno cell CC