Building a fast street car is a challenge. Building a fast race car is the next level. But to build a fast endurance race car, is a whole other thing.

Yost Autosport is a name that a lot of you will be familiar with. The Las Vegas-based outfit are renowned for building their own cutting-edge race cars, but without the multi-million dollar backing of a major manufacturer. On a relative shoestring budget, Jordan Yost and Mike Bonanni, have relied primarily on their own ingenuity and determination to build the the perfect endurance specification BMW M4, with the goal of winning the famous 25 Hours of Thunderhill.

The car itself is no stranger to Speedhunters. Dino managed to grab a visit with Yost Autosport last year before SEMA, when the car had just been finished. Well, for the first time anyways.

For its 2016 iteration, Yost Autosport’s primary goal with their F8x M4 was to prove the reliability and the potential of the base platform of the M4 in the 25-hour race. For 19 hours of last year’s race, they were successful, with the car running consistently and problem free. It was only after car to car contact that problems began to arise, which ultimately put an end to their race.

Not to be disheartened, Yost Autosport almost immediately set about tearing the M4 down to the shell and started again. I briefly spoke to Mike Bonanni about the motivation behind their M4 and he told me that “winning the 25 Hour takes the perfect storm of machinery, drivers, preparation, and luck. We can do our best with all but the luck part.”

He continued, “For 2017 we tore the car completely down to the bare chassis with the intent to rebuild it in a way to maximise its potential. Our big goals were to shed weight, implement aerodynamics, and stuff larger rubber under the new wide-body kit.” I guess that’s as good a place as any to start.

For all intents and purposes, the 2016 car was a standard-bodied BMW M4 with a wing and the requisite safety equipment. Things have obviously escalated since.

With the 2016 car tore down, Yost Autosport media blasted the chassis, made weight reduction improvements where possible/allowed, powder-coated the bare car and started to rebuild it once again from the ground up.

The addition of a wide-body by PSM Dynamic gives an obvious increase in girth. It’s not just an aesthetic addition though, as the kit allows a much wider tyre to fit inside the enlarged wheel arches.

A considerable amount of aerodynamic work has been carefully applied, too. While the same adjustable chassis-mounted rear wing remains from the 2016 car, Yost Autosport outsourced the revamped aero package to MFR Engineering who designed and built a complete flat bottom kit with side skirt extensions, rear diffuser and front splitter.

Air flow under the car is equally important as air flow over the car, but is often neglected or just completely ignored.

What might have caught your eye in the opening image is the removal of large portions of bodywork behind the rear wheels, in an attempt to evacuate turbulent air that would otherwise have been caught.

It also gives a perfect view of the Achilles GS 328 slicks from the rear, and shows you how much rubber is being put to the ground. I wouldn’t fancy driving behind it, mind.

The front fenders are custom vented items, while further weight saving has been achieved by the use of dry carbon doors with Lexan windows. The bonnet, too, is custom dry carbon fibre.

Mike told me that “the only things we didn’t mess with were the aftermarket parts we had already proven – our suspension, engine package, transmission, cooling kit and brakes.” As such, this was an intelligent overhaul. Nothing was changed unless Yost Autosport felt it needed to be.

Their suspension setup remains the same, with BC Racing Custom DR Series coil-overs suspending the car with the same company’s adjustable front sway bar end links also in the mix. The rest of the suspension componentry that has been changed from OEM is almost exclusively by SPL Parts.

Similarly, the power plant remains the same as before. The factory 3.0-litre twin turbo S55 motor still runs stock internals and stock turbochargers (wrapped in Heatshields Products turbo blankets), just with upgrades to the cooling and fuelling of the engine.

CSF Racing take the lead here, having supplied their top-mount, dual-pass charge cooler (now hydro-dipped in dollar, dollar bills), their front-mount heat exchanger and an adapted E92 M3 oil cooler which has been custom mounted.

With a Gintani custom tune for 100-octane fuel, the M4 makes 600hp at the wheels, along with 612lb-ft of torque. Outright power isn’t the goal, and while 600hp would be considered mild for what the S55 can achieve, the engine’s longevity and reliability are paramount over the course of a 25-hour race. Cooling is infinitely more important and beneficial than another 100hp would be in an endurance race car.

Again, in a similar vein, the same reason applies as to why the stock 7-speed dual-clutch transmission has been retained. It’s often very difficult to beat OEM reliability, purely because of the research and development that BMW can afford to put into their product.

The one weakness of the BMW dual-clutch transmission in the F8x is its tendency to overheat and go into limp mode, especially where lots of fast and hard shifts are taking place over a short period of time. To resolve this issue, Yost Autosport again turned to CSF Racing for their F8x DCT dual-pass cooler.

“One of the biggest projects on the rebuild was the re-engineering and gutting of the factory wiring harness and intricate computer system. Southwest Bimmers in Las Vegas spent countless hours removing unnecessary electronics and re-establishing communication with the vital parts of the car in a way that they all still function like they did from the factory. What sounds like a fairly simple task is unbelievably complicated with a car like the M4,” said Mike.

The factory gauge cluster has been custom mounted in the centre of the car, while a MoTeC C125 digital dash takes pride of place behind the customised OMP steering wheel.

“Lastly, a redesign of the cockpit area for simplicity, function, and comfort for the drivers rounds out the major areas of improvement for the rebuild. Of course a list of a million small changes were also performed,” he concluded.

Behind the front seats, resides some of the fuel system componentry. The fuel filler neck which stretches from the rear quarter window, leads to a custom FIA-specification 38-gallon saddle tank fuel cell. A Radium Engineering swirl pot and venturi system are also in play.

The aforementioned wide-body allows a comfortable fit of the BC Forged RS43 wheels which measure 18×11-inches all around, with a 30/65-18 Achilles slick tyre equipped. The silver wheels cover an AP Racing Radi-CAL Pro 5000R brake kit equipped with Hawk Performance DTC-70 brake pads.

The Radi-CAL range of AP Racing brakes features an asymmetrical caliper design with optimised air flow to evacuate heat and prevent fade, crucial over endurance length races.

Another considerable modification from the 2016 car, is the addition of a custom oval side-exit exhaust.

With the car now weighing 1,542kg (3,400lb), along with the aerodynamic improvements and increase in its foot print, Yost Autosport will be setting their sights high for the next 25 Hours of Thunderhill.

“We are still fine tuning the performance of the car after the rebuild, but overall the rebuild has been a huge success and we are anxious and excited to go back to the 25 Hour in December with this version of the M4,” said Mike.

“We are proud of the fact that from day one we have stuck to our guns of doing everything as properly as we’re capable of. There were many times where ‘the easy way’ seemed much more attractive an option than the proper way, but we never gave into temptation and impatience.

“Much of this build was spent venturing into unknown territory and we have put together an incredible team of partners and people who have bravely taken on the task of spending their time and money developing highly complicated parts that put up with the abuse of one of the world’s longest endurance races.

“It would be much easier for these companies to build parts that are geared toward street use and look good on paper but everyone on this car has the same attitude and drive to create the best products, not the best selling products. So if there’s one thing we want the world to know about it’s that every company that has their name on the side of this car is building parts that go far beyond the industry standards. Everyone involved with this project are truly passionate and determined to be the best.”

Paddy McGrath
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paddy@speedhunters.com