Thrust SSC's driver/pilot was Wing Commander Andy Green, and in August this year he will be attempting a new land-speed record in something much closer to the everyday reality.

This time it's the diesel land-speed record. He will be using a car driven along by its wheels and not by jet thrust.

The present record of 235.756mph was set in August 1973 by Virgil W Snyder in his Thermo King Streamliner, but nowadays Audi's diesel Le Mans race cars aren't far behind that speed. In August, if all goes to plan, the record will hurtle to more than 300mph. And the vehicle to do it will be a JCB.

The JCB Dieselmax, a 9m-long streamliner powered by two highly tuned diesel engines, has been used in all kinds of JCB machinery since 2004. It is, however, different from your usual digger.

"For 25 years it was the dream of the company founder, Joe Bamford, to build our own engine, " says JCB engineering director Dr Tim Leverton. "We started production in 2004, and we've made 10,000 of them so far. Now, to prove we have entered engine manufacturing, we want to use the engine in a recordbreaking car. Extreme performance is part of our brand values."

The JCB444 engine . . . four cylinders, 4.4 litres . . . is designed by the British engineering consultancy company, Ricardo. It comes in many forms to suit its different applications. Normal outputs of the 16-valve turbo-diesel are up to 125bhp and 457lb ft of torque with a speed limit of 2,800rpm, but the Dieselmax clearly needs much more than that to break the 300mph barrier. So the capacity is increased to five litres, and it gains modified pistons, camshafts and fuel injection to increase the flow of fuel and air.

Both an intercooler and aftercooler help to make the intake air denser, and water injection and extra oil jets keep things cool when the two-stage turbocharger is blowing at its maximum 5.2 bar boost (more than turbocharged Formula 1 cars used to have).

The result is an engine with 750bhp, 1,105lb ft of torque, a rev limit of 3,800rpm and an appetite for only the very best high-cetane diesel fuel . . . and the JCB Dieselmax has two of them. One engine drives the front wheels, the other the rear wheels, and the driver sits between them. There are two gearboxes as well, linked electronically so there's no danger of one changing gear when the other does not.

The Dieselmax's shape was designed by Ron Ayers, who also shaped the Bloodhound missile and Thrust SSC.

Unlike these two, the Dieselmax won't be reaching supersonic speeds, but even at 300mph there are enough airflow quirks to preclude normal wind-tunnel tests. So Ayers used computational fluid dynamics; this means that the first time he'll know if the Dieselmax really works will be during the first test runs at RAF Wittering.

"The underfloor shape is critical, " explains Leverton, "because we want neutral lift and no downforce."

JCB's Dieselmax will reach about 200mph on the 2.5-mile runway. The engineers will be looking very hard at the airflow characteristics around the front wheels, because the record attempt is to take place on the Bonneville salt flats in Utah . . . the world's largest dead-flat space.

The powdery surface causes salt spray, and the idea is to divert that away from the rear wheels so it doesn't spoil traction.

Richard Noble, the driving force behind the Thrust SSC record-breakers, is advising Leverton's team. Leverton himself has the unusual distinction of having helped to engineer the remakes of three iconic British designs: the Mini, the Range Rover and the Rolls-Royce Phantom.

If all goes to plan, the diesel land speed record will hurtle to more than 300mph.