If you’re an AlphaLuxe regular, you’ve likely read my earlier article describing my transcendental experience on both road and track with ‘Bonkers’ – a heavily modified 800 wheel horsepower Porsche 911 GT2 sporting bespoke, inboard pushrod suspension. If not, go check it out HERE! In that writeup, I promised a subsequent technical deep-dive. So, strap on your horn-rimmed glasses and pocket protectors – this is going to get nerdy.
Extracting 800hp from a 3.6L 6-cylinder motor is, by most accounts, no small feat. Akram, of aptly-named Akram Haus in Houston, would beg to differ. His stance? 800hp from this flat six is really no big deal, and perhaps even qualifies as a “mild” state of tune – not until cresting 1000hp does he start to consider upgrading to forged internals, or any other bottom-end work. Those rock-solid, reliable 800 horses come courtesy of a pair of tried-and-true Garrett GT3076 turbos, pumping out 27psi of boost on an E85 tune, and slightly less on a more conservative pump gas tune.
The legendary Mezger block, a race-derived design straight from GT1-class racing, was more than up to the task. But, cramming all that air through the factory 996 intake manifold just wasn’t going to happen without serious flow restriction, so Akram got to work adapting the 997 turbo unit to mount onto the 996 block, a project described succinctly as “painful”. The resulting flow increase through the revised 997 plenum justified the mounting difficulties, as Akram now had the freedom to push the boost sky-high without worry about starving the motor of air. With the manifold sorted, the next weak link in the chain became the supply of air to those hungry turbos. A full redesign of the air induction path was in order, as the single, top-mounted intake duct of the stock setup was woefully inadequate.
Repurposing the massive intake ducts on the rear fenders, Akram fashioned a dual-path setup, feeding both the turbo air induction, as well as a Protomotive intercooler on either side, with 5″ core and integrated blow-through intake plumbing. In addition to the improved flow, over 4 feet of induction piping were eliminated, resulting in expedited spooling of the sizable turbos, and a resultant improvement in throttle response. Protomotive‘s 3″ stainless steel X-pipe exhaust was also chosen for the build, though the original GT2 exhaust tips were retrofitted, to maintain that factory charm.
Of course, the stock Bosch Motronic ECU wasn’t capable of adapting to the copious boost on tap, so the unit was flashed with custom timing and air-fuel mapping likened to “black magic”, courtesy of Todd at Protomotive – masters of tuning turbocharged P-cars of every vintage. A GReddy digital boost controller was the only add-on beyond the Bosch DME, allowing precise control of boost application to smooth out the brutal power delivery, and ensure the car retained its impeccable road manners at sub-max throttle applications. To cope with the gargantuan fuel demands of an 800-hp motor, the injectors were up-rated to 160 pound units, and a relay-controlled dual fuel pump setup was implemented, with the second pump kicking in on-demand at heavy throttle.
Surprisingly, the factory transmission internals were more than capable of handling the additional thrust, though a short-throw shifter kit was fitted to ensure crisp, accurate gear swaps, and a hybrid GT3 cup-RSR clutch and lightweight flywheel kit replaced the factory bits. These driveline enhancements worked synergistically with the rest of the package, as the aggressive clutch showed zero signs of slippage at the brutal onset of full boost, and the lightened flywheel complemented the lightning-quick throttle response afforded by the efficient turbo intake plumbing.
It’s easy to take for granted the multitude of interactive systems working harmoniously behind the scenes, resulting in power delivery with OEM-like smoothness at reasonable speeds. That the car is capable of such docile behavior, while such a fire-breathing beast lurks beneath the rear deck lid, is a testament to Akram’s delicately balanced and masterfully-tuned powertrain design. Very few shops in the world could have pulled this build off, and with 20+ full-throttle laps under my belt, with speeds regularly exceeding 160mph, as well as several days on the road at Los Angeles speeds (<5mph), it’s safe to say Akram has managed to strike a perfect balance of “bonkers” power delivery and reliable daily-drivability.
But, ‘Bonkers’ is far from “all bark and no bite” – namely, the ability of its rear tires to bite hard into the pavement. While the front wheels roll on traditional, upright Ohlins coilovers, keeping those rear R888’s contact patches firmly planted on the ground is a bespoke, inboard, pushrod suspension setup, designed and fabricated by Joe Scarbo of Scarbo Performance in Lake Forest, CA.
It sports horizontally mounted double-adjustable Ohlins dampers attached to a frame milled from solid 6061 Aluminum billet, plus rockers of the same material adjustable to five discrete positions. The result is a linkage ranging from perfectly linear to extremely progressive, depending on the driver’s intended application and driving style. While the finished product is undoubtedly a work of mechanical art, the system is a fine balance of function and form.
Finding space for the sizable dampers in the space-constrained rear of the GT2 required some creative packaging – Scarbo played with several iterations before deciding on the horizontally-opposed setup of the finished product. Since the left and right pushrods were constrained to the same plane, a serious challenge imposed by the horizontal packaging was dealing with the unavoidable offset loading on one of the rockers.
With one damper mounted significantly fore of the other, the right-side load had to be transferred from the pushrod to the damper via an offset rocker with impressive torsional stiffness. Early iterations of the rocker weighed nearly two pounds more than the final design present in ‘Bonkers’ – with extensive 3D modeling and FEA (Finite Element Analysis) employed to ensure the rocker met the colossal stiffness requirements.
Another feat of Scarbo’s engineering might was the sway-bar torsion spring “blade” of the rear suspension – a manufacturing challenge so complex that it came back at double the manufacturer’s original quoted price. Heat treating to a hardness over Rockwell C 60 was necessary to achieve Scarbo’s desired specs for the blade. Designed with an OD spline, it could be removed and reinstalled in different orientations to affect the rear roll stiffness – another hallmark of the extreme adjustability built into every Scarbo design.
Finite Element Analysis
Joe meticulously walked us through the 3D CAD model of the whole design, with the motion of each component constrained to behave exactly as it would in the real world, allowing for accurate simulation of the dynamics of the entire system under realistic conditions. An FEA model of the afore-mentioned offset rocker showed how the load was distributed evenly across the full volume of the rocker, with no stress concentrations or large deflections present anywhere in the body, despite the large swaths of material milled away from its original shape.
On-the-fly, Joe set up a 2D representation of a pushrod suspension setup using a CAD sketch and Excel® spreadsheet, providing a simple demonstration of how the angle between the pushrod and rocker centerline could affect the ratio of the linkage – and shedding light on the rationale behind the five separate mounting points for each rocker.
One of the coolest moments of the visit, for me, came as we searched Google® for photos of comparable pushrod suspension designs – only glancing at the linkage design, Joe could distinguish the linearity of the motion ratio between the pushrod and damper, and comment on the strengths and weaknesses of each particular design – a true master of his craft.
[Editor’s Note: The cost of manufacturing parts & labor for this custom-built suspension alone, exceeds the list price of a basic Porsche 911.]
Video: Joe Scarbo Explains Bonkers Inboard Suspension in 1 minute
The Bump Stop
If you made it to this point in the article, you’re either as fascinated as I am by comprehensive technical detail, or you went into auto-pilot, your eyes glazed over, and read to this point while pondering how you’ll convince your significant other that your next project car is a great idea. Either way, I appreciate you sticking with me, and I hope you enjoyed all the nitty-gritty details of this incredible build.
I, for one, count ‘Bonkers’ as one of my all-time favorite cars, as it really has something for everyone; both automotive uber-nerds and casual enthusiasts alike cannot resist its charm. With this build, Akram and Scarbo managed to craft a package capable of mind-blowing acceleration, Newton-defying roadholding, and, most impressive of all, impeccable manners under real-world driving conditions. To take a car as special as the GT2, and turn it up to ’11’ without ruining the experience, is no small feat. Can you really improve on a unicorn? Akram and Scarbo seem to have found the secret sauce.
Thanks to both Akram and Joe for taking the time to delve deep into ‘Bonkers’ – and thanks to AlphaLuxe for the seat time – it’s been an honor and privilege to drive and write about this magnificent beast!
Author’s Biography: KevinB
KevinB is a 29-year-old auto enthusiast – a Mechanical Design Engineer by day, and serial side-hustler by night. By age 4, he could spout the make and model of any passing vehicle, and by 12 he was an expert in the inner-workings of the internal combustion engine. His love for all things motoring expanded into the two-wheeled world at age 17. He currently owns a 2014 Porsche 911 GT3, and a 2018 Ducati Panigale V4, and has somehow managed to retain his driver’s license.