Developing an inconel exhaust system for a special Subaru
GoodFabs worked with Roger Clark Motorsports' Gobstopper Time Attack cars and developed a custom-made and heat-shielded twin scroll inconel manifold finished with white ceramic coating to suit a very tight engine bay.
The name Roger Clark is well known by many motorsport fans and has its roots in the racing and development of rally cars from the 1960s to the present day.
Founded by the late Roger Clark himself, the company is now run by sons Matt and Olly and is based in Leicestershire in the United Kingdom.
Their latest Time Attack development is Gobstopper II, their 780bhp Subaru Impreza STI that won the Goodwood Festival of Speed Hill Climb this year.
At GoodFabs, we were privileged to help make this car what it is today.
Gobstopper I
Back in 2010, Roger Clark Motorsports (RCM) approached GoodFabs to help develop an exhaust system to suit their Gobstopper I Time Attack car.
After some discussions with Phil Levett and Geoff Rayner at GoodFabs to work out the objectives of this development, the 1999 Impreza STI was brought to our workshop in Long Crendon, near Oxford for a couple of weeks.
When the car was finished, RCM tested the new exhaust and clocked a staggering 780bhp on the engine.
The Gobstopper I car was too much for the competition and quickly became one of the most popular racing Subarus.
Gobstopper II
In 2014, the development of the new Gobstopper II began and once the development of the chassis was finished, it was time to create a new exhaust system to suit the new 2008 Subaru Impreza STI based Time Attack car, using the same base engine as the Gobstopper I.
Since the development of the exhaust system for the original Gobstopper was a success, RCM quickly knocked on the door of GoodFabs again. Sadly, this was one of our busiest times of the year with Formula 1 and we unfortunately had to decline the project.
As no time could be wasted, RCM had to turn to another race exhaust supplier, who developed a small and tight exhaust system to suit available space to fit the Garett hybrid 4094 twin scroll turbo.
When the system was finished and tested, various runs on the dyno proved that the car had lost about 70bhp.
In March 2015, RCM contacted GoodFabs to ask if we could develop a system which delivers the full 780bhp again. At this time we were able to take on this challenge.
When the car was brought to the workshop, it quickly became clear that this would not be easy. The new Time Attack car still had the EJ20 - Closed Deck, 2000cc boxer engine, but also had a very close underfloor with very tight constraints.
Since the exhaust system also needed to be heat shielded, the constraint package became even tighter.
When we had an initial look at the system developed by the other company, some fundamental problems were discovered.
The diameters of the exhaust system were too large which reduced the exhaust gas velocity and had a negative effect on the turbo.
A second apparent problem was the centrelines used on this system; they appeared to be very tight, which is very detrimental to exhaust gas flow.
Since GoodFabs owns a massive range of bend tooling, we were able to use the largest possible centrelines to keep the flow inside the system optimal.
Also, the experience we already had with Gobstopper I and the entire Prodrive Subaru Rally programme quickly enabled us to create a system with the correct diameters and an optimised flow geometry.
Heat shielding Gobstopper II
When the system was fully finished within the constraints, GoodFabs' in-house heat shielding department tailored an inconel 625 custom-made heat shield in order to retain all the exhaust energy inside the system so it could be fully utilised in the turbo.
When the full exhaust was finished, this was again tested on the engine dyno to prove out the system and remap the engine.
The results showed that the lost 70bhp had been completely restored and RCM could live up to its stated aim of "Always Pushing, Always improving, Making the Best Better, the Fastest Faster, the Impossible Possible, Perfecting the Perfect".
This was impressively demonstrated at Goodwood in front of an international audience made up of motorsport's elite.