Flyin' Miata : Tech : The Rototest dyno

What do Volvo, Cosworth Engineering, VW, GM (and most of the rest of the major auto makers) and Flyin' Miata have in common? The Rototest dynamometer.

Our old Dynojet was simple, easy to use and consistent. It's also the standard in North America. However, it had certain limitations. It didn't allow us to do steady-state testing. Tire slip on the rollers gave a certain inaccuracy to the readouts. If the ignition pickup was lost, the run was useless. So it's for sale!

Our Rototest is one of only two in North America at this time. In order to learn how to take full advantage of it, Peter and John flew over from the factory in Sweden for a week to train us. We would have liked to keep them longer - they were full of all sorts of information learned from working with automakers. They learned a few things from us as well, as tuning shops have different needs than automakers.

So what makes the Rototest different? Well, the biggest change is how it hooks up to the car. The driving wheels are removed, adaptors bolted in place and then attached to a dynamometer unit on each side. These are basically big hydraulic pumps. An electronically controlled valve restricts the flow from the pumps, allowing us to set the load accurately. Strain gauges measure the torque being applied to the pump. Because the connection between the engine and the dynamometer units (ie, the transmission) is solid, the readout is extremely accurate. Since we know the gear ratios of the car, we don't even need an ignition pickup to determine engine rpm.

A neat trick - the pressure that the pumps generate is used to run two 40hp hydraulic motors with cooling fans. These both cool the fluid through four large radiators as well as the car. Oh, and if you have four dynamometer units it's a 4wd dyno. It'll also fit in the back of a pickup truck so we can move it around.

The Rototest can be run in three basic modes: fixed speed, fixed load or sweep. The sweep is the classic Dynojet run through the rpm range. We can vary the speed of the sweep in order to minimize inertial factors or to decrease the amount of heat soak. We can even do it backwards, starting at 7000 rpm and slowing down to idle to reverse the inertial loads.

Fixed speed holds the engine speed at a set rpm regardless of load. This is fantastic for tuning the ECU, as we can center the car in a zone and tune it while watching the real-time torque output. It's hard work for the car, though - even on Keith's naturally aspirated engine, the headers were glowing as the engine was held at 6500 rpm at full throttle. We can also work through a column in the tuning tables without changing row. For example, we can run through zones 320, 325, 330, 335, etc easily.

Fixed load means that the dynamometer units will provide a fixed amount of resistance. This will allow us to run through a particular row easily.

Eventually, we can even factor in the coefficient of drag and frontal area of the car and do a simulated top speed run or add aerodynamic load based on road speed.

The adaptor bolted to the hub, ready for the dynamometer unit.
Ken hooks up the dynamometer.
The valve that controls the restriction for the pumps, and therefore the load.
It looks like the mad scientists have been at work!
From left, Bill, Ken, and John listen to Peter.
John confers with whoever has taken the wheel.
Lots of air to cool the car.
The display screens. Obviously, there's not much going on at the moment.
The tower of power. Well, the tower of power measurement anyhow.