Chassis Dyno questions -

Firstly, chassis correction factors are very difficult to get right and are more fudge than factor. Many different issues are affecting the amount of power that finally makes it to the driving wheels. Soem very nice research from Rototest in Sweden even seems to suggest that it is not a fixed percentage loss but a much more variable loss linked to engine torque output.

Front engine/rear drive (FR) does chew up more power than a engine/transaxle assembly (either FF or RR). The extra rotating mass and extra joints are the culprit.

Four wheel drive dynos are in existence. I know of two within a stones throw of my front door. For those who have never seen one, check out thepics attached.

What gear do you run it in. As low as you can without sacrificing traction and as high as you need to. Remember that most good chassis dynos are speed limited so it's no use using a gear that overspeeds the dyno half way through a run. 3rd or 4th is sufficient most fo the time.

The fourth question is the magic one. Comparign between different dyno brands canbe challenging. For example, their is no use in comparing the results from a Dynojet to a Mainline. A Dynojet is an inertial dyno reknowned for giving "optimistic" numbers whereas a Mainline is an eddy current brake dyno which usually gives depressingly accurate figures! However, comparing results from the same dyno type is usually quite valid. The environment the dyno is operated in starts to take a toll though. A couple of things to remember. The atmospheric correction factor should be adhered to correctly and is you can smell exhaust gas inthe dyno room, then it's already effecting your engines output. Dyno rooms should be well ventilated but not so mush that it effects the ambient pressure inside.

I hope this helps.
Image Unavailable, Please Login
Image Unavailable, Please Login

 

I pretty much agree with Patrick here, but one thing that should be considered are the different types of dyno. The most commonly used dynamometer is an enertia type. It operates by placing the wheels on drums that are of considerable weight. I believe one has 800 pound drums. They are free wheeling, and they work by calculating the time needed to accelerate them to a certain speed. The problem with them is that except in the case of making comparisons after adding parts or modifying your car, they are almost useless at measuring real HP. The reason is because of how they work. If you run a lighter flywheel, lighter wheels, actually anything in the driveline that weighs more or less, it will allow the engine, and conversely the dyno, to accelerate differently which will skew the result.

The real dynamometers are brake type. The drums or rollers are hollow, much easier to spin up. The ends of one roller are connected to a generator or water brake, and torque can be applied at any RPM to "hold" the engine back at wide open throttle. Because torque is easily converted to HP, that alone adds remarkable accuracy. But also, because the power can be held at a static RPM, no enertia is present to skew the results. If the dyno says your developing 300 HP at the rollers at 4700 RPM, its really 300 "BRAKE" HP at the wheels RPM calculated directly by torque, not some mathmatically calculated number based on weight and acceleration via enertia.

Then you have losses to figure in. On 308's there seems to be this 17% gearbox loss that gets tossed around. But there is also a lot of debate that a gearbox does not make a flat percentage of HP across the board. There is also some argument that gearbox power losses flatten out once a certain level is reached. An article I was reading regarding gearox efficiency on Allison tubine engines shows this very clearly. So for example, a 308 probably has a more or less flat 35-40 HP power loss once the engine level of about 180 HP is reached. Because the 308 gearbox does not have any changes in driveline direction, no right angle turns, it makes for one of the more efficient gearboxes, simular to a modern FWD car. What throws off the front engine rear drive cars, or any car that changes drive angle 90 degrees, are that there are basically two gearboxes. And dont forget gear oil, lighter oils would show greater efficiencies.

Of great interest is the 308 GT4 service manual. In the section on running in the engine it gives specific torque to apply to the engine while maintaining a particular RPM, and gives the equivalent HP that results. But because they have you taking the power directly at the flywheel, it gives no idea as to gearbox loss. Measuring power of an engine this way would give true flywheel HP. If you could then install the engine in the car and run the car on a brake type chassis dyno, you could get actual losses through the driveline. Remember that everything in its path effects HP. Drive axle joints, especially if angled at all, wheel bearings, brake pads, alternator, air pumps, AC compressor, etc..

 

Dyno pulls are made in whatever gear is one to one ratio. 4th is usually the case but on a ferrari like a 308 or boxer, no gear is one to one. For the dyno to calculate correctly your car either has to have a one to one ratio or the dyno has to be calibrated accordingly otherwise I cant see how it would get it right. Ive heard of ferrari owners getting numbers that are way out to lunch so Im assuming this is the reason. The 17% loss is also in the favour of the car owner leaving them satisfied that the car makes what ferrrari says it does. A tweeked out euro GT4 made 235HP at the flywheel, should be 255HP wet or drysump doenst matter when its sitting still, too bad in chassis numbers werent checked just to verify the % loss through the drivetrain. I use 15% myself, its more honest I think. Ferrari says 340HP for my boxer, I would love to know.