Thursday, December 22, 2016

SCCA Trans Am Boss 302 mods by Kar Kraft

The first step in Kar Kraft’s chassis preparation was minimizing weight. Even though the SCCA rule book for the ’69 Trans-Am series specified a minimum weight of 2900 lbs for Group II 5.0 litre cars, the aim was to build the cars as light as possible and then bring them up to meet the minimum weight limit.

This was done by securing lead or steel ballast down low at various key points in the chassis, to move the car’s standard 55.9% front v 44.1% rear weight distribution nearer to the ideal 50/50 split.

This process of moving weight rearward was further enhanced by relocating the battery from the engine bay to the boot and discreetly lowering the engine by around 50mm and moving it back as far as the firewall would allow.

 Mounting these heavy items as low as possible also had the effect of lowering the car’s centre of gravity (CoG) for greatly improved handling and cornering capabilities. The same thinking applied to construction of the fuel tank.

This was made out of two flanged halves (ie upper and lower shells). However, a Boss 302 racer featured a much deeper bottom section than standard to drop the fuel load as close as possible to the road. This idea was duplicated in the Bathurst-style “drop tanks” seen on Torana L34 and A9X racers in the mid 1970s.

The Mustang bodyshells had already begun a weight loss program on the production line, as they were built without any weather sealing or sound deadening compounds. Kar Kraft then removed any brackets not required for competition and either drilled a zillion holes in any component that had to remain, or re-made it in aluminium. Not a single nut or bolt was overlooked in this weight loss process, right down to the internal window winding mechanisms which even had shorter crank handles to save weight.

Although acid dipping was strictly outlawed under SCCA rules on safety grounds, the practice was in fact widespread in Trans-Am to trim fat from a race car.

the weight loss program for the Boss 302 racers was very effective, which included significantly thinner window glass and bolt-on panels (bonnets, boot lids, door skins, guards etc) stamped from thin gauge sheet metal.

Lessons learned from running stock-bodied sedans at 200mph on the NASCAR super speedways and Kar Kraft’s own GT 40 Le Mans program had exposed the considerable performance gains to be made from cars with good air penetration.

Kar Kraft began by trimming 25mm from the height of the radiator support panel; the engine bay inner guards were then tapered down from the firewall on each side to match.

 This substantially lowered the front aerodynamic profile of a Boss 302 race car. The inner halves of the rear wheel housings were also discreetly moved in-board by as much as 75mm on each side to provide adequate clearance for the 12-inch wide rear racing tires, as only minimal flaring of the external wheel arch lips was permitted.

To maximise torsional rigidity, the shell was fully seam-welded and two sturdy braces were connected to the front suspension towers; one spanned directly across the engine bay between the two towers and another braced the towers rigidly to the firewall. The base of the towers were also treated to some substantial reinforcing plates as fitted to the road going Boss 302.

The front suspension subframe was notched about 20mm on either side where it bolted to the chassis, which had the effect of raising the sub-frame further into the car and permitting a lower static front ride height. This left only 25mm of belly clearance above the road surface and was another important gain in lowering the centre of gravity for optimum handling and cornering power. It also explains why (in combination with the tapered front sheetmetal) a standard 1969 Mustang looks so high at the front compared to Moffat’s Trans-Am version!

A special track test conducted by Road and Track magazine had one of the factory cars driven by George Follmer at the quarter mile in 12.9 seconds, it generated just over one g of cornering force and almost halved the Boss 302 road car’s 130km/h braking distance from 90 metres to just 54. A lot of that was due to Lincoln Continental brakes.

The combination of Lincoln front brakes and Ford rears resulted in different wheel stud patterns front and rear, but under Australian racing rules all four wheels had to be interchangeable. Moffat had to re-drill his wheel centres so that they could be bolted to either end.

the race car had a factory race spec engine, with a pair of 1080 cfm Holleys, and a street version for sale to customers had only a 780 cfm Holley. The race engine had a huge oil pump with 3 pick ups, and a baffled sump, and 12 to 1 compression for 470 hp. Street cars were 10.5 to 1 and only had about 350 hp.


  1. Great read thanks for posting it. I loved Alan's Mustang that thing had balls.

    1. welcome! I enjoy learning the little details in how anyone made a race car a bit better, usually by cheating in a way that wasn't obvious. 55mm here, 70 mm there, etc

  2. Well Smokey did it for years...hahaha