The turbochargers are supplied by Borg Warner--as announced last May at Indianapolis Motor Speedway--and have specific A/R compressor covers for road/street courses and for ovals.
The Borg Warner-supplied compressor cover for the single turbocharger that Honda intended to use has an A/R of 0.74; compressor cover A/R sizes are not mentioned in the published INDYCAR rules. The permitted and published turbine cover A/R sizes for the Borg Warner EFR9180 single turbocharger is either 1.04 or 1.45.For reference, twin turbocharger installations must use Borg Warner EFR6758 with A/R of 0.64 or or 0.85.
Engine rules do state that changing between the two permitted turbocharger A/R specifications "may only take place between events," which is what HPD attempted to do. The rules also say "should turbochargers fail to meet the durability targets set by Borg Warner, the manufacturer will take steps to remedy the situation." Since HPD isn't telling us how the swap occurred and why (because the hearing hasn't been set yet), we have to assume they were looking for better throttle response and posed the A/R of 0.74 to Borg Warner, who agreed that was appropriate for the application.
The hearing on Thursday will by held by INDYCAR, the sanctioning body for the IZOD IndyCar Series, in reaction to a protest filed by General Motors (GM) regarding an April 18 decision to approve use of the 0.74 A/R compressor cover on Honda's HPD-built engines. The hearing will take place in Indianapolis and both Honda and Lotus have been invited to participate in the hearing.
In his role as Race Director, INDYCAR president of competition Beaux Barfield has elected to use a three-person panel to resolve the protest. There will be one individual selected by Honda, which is the only engine manufacturer currently using a single turbo application; one individual selected by GM and a third individual mutually agreed upon by Honda and GM.
Pending the decision of this panel (which has not yet been named), the April 18 decision of INDYCAR isn't going to be stayed prior to the hearing. It could have been stayed if Barfield felt it appropriate.
Randy Bernard, the CEO of INDYCAR made remarks on the protest: "It is INDYCAR's job to review, enforce and uphold the current rules. It must be our position to make the best decision possible from the rules that we all established."