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The Dynapack Dyno

Looking at a hub dyno.

by Julian Edgar

Click on pics to view larger images


This article was first published in 2001.

In Australia, the Dyno Dynamics roller dyno is king. It is used more widely than any other single type, and over the last decade its measurement has become the standard against which 'wheel horsepower' is measured. However, just because it's ubiquitous, it doesn't mean that it is the only way. One of its more recent competitors is the New Zealand-sourced Dynapack hub dyno, a vehicle dynamometer that bolts the dyno heads directly to the wheel hubs, rather than using a roller against which the car's drive tyres bear.

So what's a Dynapack like? What are its advantages (and disadvantages - all products have both!) over a more conventional roller dyno? Recently ChipTorque on the Gold Coast negotiated the loan of a Dynapack four-wheel dyno for a few weeks; Dynapack was pleased to demonstrate the equipment to a potential purchaser, while Lachlan Riddel of ChipTorque is a technology junky, always keen to experience something new. ChipTorque's current dyno is a two-wheel-drive Dyno Dynamics roller.

The Dyno

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Each dyno head on the machine that we examined is rated to a maximum of 200hp (150kW), giving a total power absorption capacity per axle of 400hp (300kW). Thus on the four-wheel drive dyno shown here, the max power capability is 800hp (600kW). (Different models with a higher power rating are also available.) The dyno heads are housed within self-contained modules, each of which weighs about 260kg. These are equipped with small wheels, allowing the heads to be moved around with relative ease. While we weren't allowed a photo inside the boxes, we did get to perform an inspection. Each dyno module consists of a very hefty cast pump, which is connected via large diameter hoses to an internal oil tank holding about 20 litres. The oil is pumped in a circular path by the dyno head, with a variable restriction in flow apparently providing the load. The oil is cooled by an external supply of water, normally provided by a garden hose that links each dyno head to a tap.

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To set the car up for dyno'ing, each end of the car must be firstly jacked-up and the wheels then removed. The Dynapack hub adaptor is then bolted to the vehicle hub, using the original wheel nuts. Adaptors for both four and five-stud wheels are available, with the different PCD (pitch circle diameters) provided for by the slotted approach used by the very strong adaptor. PCD's from 95 - 155mm are catered for.

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Self-centreing of the adaptor on the hub is ensured by the tapered washers that sit within the adaptor. The washers have a shoulder machined into the other side, allowing each to slide up and down the slot in the adaptor.

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For cars with very long studs, a spacer plate is used.

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At the other end of the adaptor are a large roller bearing and an internally splined shaft. With the adaptor bolted to the car's hub, the dyno head is rolled into position, the splined adaptor sliding into the dyno head to make a positive drive connection. A lever then notches into place so that the adaptor cannot come out during the action that will follow. The angle that the dyno adaptor forms with the horizontal can be varied a little, allowing for different wheel cambers.

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A temperature probe is placed near the engine's intake....

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...and a third-party air/fuel ratio measuring probe placed in the exhaust. In addition, a manifold pressure sensing input is used.

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The car is then set up and ready for dyno'ing - once the correct data is entered into the controlling PC, anyway. The mechanical set up process takes about 30-45 minutes, Lachlan Riddel suggests. And while Dynapack representative Chris Spencer says that setting up any dyno takes about this time, that's not quite the case. While it is true that the dyno'ing of a high powered car on a roller dyno can take a while to organise (what with the tie-downs, wheel chocking and the like), a quick run on a roller chassis dyno can involve a set-up time of less than 5 minutes. There is no way in the world that a car could be placed on the Dynapack in five minutes!

The Software

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The set-up software requires some unusual inputs, most of which are present to allow the calculation of engine rpm from wheel speed and to predict actual on-road acceleration. Data required to be input includes wheel diameter, vehicle mass, gearbox type and so on. From this data the program calculates the final drive ratio. With this data punched in, the operator can select an AutoPlot set-up, which quickly and easily sets the parameters for a power pull - Min RPM, Max RPM, Settling Time and RPM Step Size. Alternatively, the dyno can be configured to allow a certain engine rpm speed to be held, or a certain load to be held.

One of the problems in calculating engine rpm from wheel (hub) speed is that if the car is an auto, below the lock-up torque converter's lock-point, the relationship between rpm and hub speed can vary. It would seem more logical to directly measure the engine parameter - eg by using an inductive pick-up or similar direct sensor on the engine.

Once the set-up software menu has been completed, power runs are quickly and easily made. The dyno operator drives the car with a whole PC keyboard sitting on his lap; something that could appear to be a bit awkward if they are also simultaneously steering a laptop PC for management mapping. However, with the problematic tyre/roller interface non-existent, the power runs are both very repeatable and the dyno extraordinarily sensitive. For example, the dyno was capable of measuring the tiny (~1kW) cyclic power variations of an Impreza WRX as the mixtures went rich/lean, rich/lean in closed loop cruise! Dynapack's literature suggests that the power difference with the headlights on or off can be measured - and I'd believe it.

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As with all PC-based dynos, the Dynapack is capable of recording and displaying multiple power runs on the one graph. The dyno scales the graphs automatically, using a slightly odd approach where, for example, the vertical power scale might read 29.4, 40, 60, 80, 100, 120, 140, 158.8kW. I guess it allows the immediate recognition of both the max and min power. The power at the hubs is displayed, but tractive effort is not. Instead, the software calculates (using the gear ratios data) the torque at the flywheel.

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Various recorded data can be logged and then displayed - here's the boost curve from '2002' rpm to '6496' rpm.

One very interesting function of the dyno software is to calculate and display predicted acceleration. This is primarily a function of the power, mass and gearing of the vehicle - all data which has been either entered by the operator or measured by the dyno. Here it can be seen that this WRX has a predicted peak acceleration of 0.343g at 4384 rpm in the tested gear. If this function proves accurate (and obviously aerodynamics, rolling resistance and other factors are also relevant!) it has massive potential, especially if the software is further refined (and another version of the software is due out shortly) to allow averaging of the data across certain rpm ranges.

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In operation, the dyno both looks and sounds unconventional. It is much quieter than a roller dyno, as there is a complete absence of tyre/roller interface noise. The dyno also has very little inertia. This allows the sensing of such tiny power variations as the closed loop mixture cycling, but it also means that the deceleration behaviour of the car is not as it is on the road. If the operator was fine-tuning the over-run injector cut off, or trying to trace an over-run backfire or exhaust popping, it would be difficult to do that on this dyno. Lachlan Riddel also commented that when he hit a boost cut, the car felt quite odd - not at all like the same scenario on a roller dyno or on the road.

Dyno power runs are far less dramatic than on a roller dyno, where it's not uncommon to see the car straining at its retaining tie-downs, trying very hard to leap off the rollers and straight through the door of the workshop! Instead, on the Dynapack the car sits unmoving, the torque reaction of the power being fed through the hubs causing nothing to move or apparently strain. It is obviously far safer, and the torque steering problems facing the operator of a roller dyno testing either a front-wheel drive car or a very powerful four-wheel drive car are non-existent.

Cheaper than some of its dyno opposition, the Dynapack is a viable option for workshops considering installing a chassis dynamometer. Whether it meets their requirements better than, say, a Dyno Dynamics, will depend a lot on the type of work that they are doing. From an enthusiast's point of view, the better choice will depend on the type of car that you have. For example, if you've never been able to bear the thought of your turbo FWD fighting its wheelspinning torque steer against lateral retaining straps, this is the dyno for you!

http://www.dynapack.com

Footnote: In this story we have spelt the name of the product as 'Dynapack'. However, throughout the company's literature, the product is also alternatively spelt as 'Dynopack'. Take your pick....

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