The Cdswdrth Xd

and servicing costs. Perhaps inevitably, the decision became a blend of objective analysis and subjective opinion, coupled with the aforementioned desire not to follow the crowd. Menzies: 'Some of the big V8s seem to have almost unusable levels of torque, whereas the Cosworth XD seemed to be more like a big motorbike engine, with lower torque and lots of revs,' to which he might have added, 'like I had become used to with the supercharged Hayabusa'. And sequential gearboxes with flat-shift systems mean that the fewer shifts required with a large capacity, high-torque engine are less of a benefit.

February 2011 ยป www.racecar-engineering.com

David Gudd, head of development at Cosworth, offered seme recollections on the XD's genesis: 'The XD was designed in 1995 and introduced in 1996. It replaced theXB, the primary weakness of which was the timing drive. The XD's timing drive was re-located to the rear of the engine where, thanks to the inertia offered by the flywheel and clutch, the torsional amplitudes were considerably smaller. The timing chains on the XD were also significantly shorter, and the combination of these two changes allowed race engine speeds to be raised from 13,800rpm on the XB to 14,800rpm on the XD at the end of its development.

'The XD carried over many features of the XB. It retained the same bore size of 92mm, as extensive testing on the XB yielded no incentive to change. Port and valve sizes were also very similar, although the valve angles were reduced to make the combustion chamber more compact. And the inlet port was straighter, while each exhaust valve had its own port, rather than the siamese arrangement used on the XB. This was no advantage in terms of flow, but it allowed for more even load distribution on the head joint.

'Thp XD also had a significantly st.iffer block and head structure, with larger diameter head studs. The extra material required for this meant the engine was about 5kg heavier than the XB. The XD's reduced v-angle of 75 degrees [80 degrees on the XB] also made it easier to keep the exhausts clear of the car's underbody.

'The DJ Engineering Services project has been very much led by them, and isn't an official Cosworth project by any means. But the quality of the work D J has done is exceptionally high, and we are following the project with interest.'

consultant and 'wise head' for a few days. One of Ogilvie's ideas was the indent on the lower, rear ward sides of the tub, to allow for improved cooling packaging, but also because the engine of choice featured wide front upper mounts but narrow lower ones, so c square rear bulkhead shape like the Firehawk's was not optimal. The tub was also subjected to lots of ergonomic work too, incorporating a HANS device, as well as provision for a foam cockpit surround (still to come at the time of writing).

As with the Firehawk, the main chassis pattern was CNC machined (from CAD generated by Smith) from epcxy tooling block, prior to wet lay up of the carbon-epoxy tooling. The tub itself comprises 13mm aluminium honeycomb encapsulated in 1.5mm pre-preg carbon skins, with local thickening in areas of high loads. The bare chassis weighed in at around 35kg.

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