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In this article, we’ll be exploring how you can transform your G8x M car into a BMW G82 M4 GT4 car.
Fortunately, the chassis of the M4 GT4 is a modified and upgraded version of the chassis used on the normal G82 M4. No special fabrication or extensive body shell modifications are required.
We’ve put together this short guide on the changes you can make to your G8x M car to transform it into effectively a GT4 class racing car. Now it would be easy to spend a huge sum of money turning your road car into a racing car, but we’ve decided to approach this in a (relatively) cost-effective manner.
We’ll explore the M4 GT4 in stages, and explore how you too can make your M car as capable as a full-spec GT racing car.
Chassis and Suspension
Front Axle
One of the most prominent changes between a road going G8x M car and the G82 M4 is the springs and dampers. The road cars use electronic dampers and high-performance progressive-rate springs, whereas the M4 GT4 uses 2-way independently adjustable motorsport-spec coilover units with fixed-rate springs.
Whilst it is possible to spend thousands and thousands on 1,2 or 3-way adjustable coilovers, a much more cost-effective solution is to retain the OEM dampers and to instead fit a height-adjustable spring kit. A height-adjustable spring kit will allow you to lower the ride height of your M car, for a lower centre of gravity whilst also improving body control and grip.
The height-adjustable springs can then be combined with camber plates and front caster bushes to transform the suspension geometry at the front axle.
Camber Plates
The front camber angle on the G8X series M cars is fixed as standard. As a result, camber plates are required to increase the negative camber at the front axle, and this can only be achieved with camber plates.
Camber plates give you full control over the front camber axle of your BMW and allow you to unlock a huge amount of additional cornering grip, whilst also helping to ensure even tyre usage and to combat unwanted understeer.
Caster Bushes
The top mounts on the M4 GT4 are specially designed to give as much positive caster as possible. Caster angle impacts high-speed stability, steering feel and cornering grip, making it a massively important aspect of suspension geometry. BMW chassis really relish additional positive caster, and the M4 is no different.
To achieve similar results on a road car, an offset caster bush is required. These bushes improve steering feel, steering accuracy, braking stability, high-speed stability and cornering grip due to increased dynamic camber gain, making them very worthwhile upgrades. The additional steering feel also helps to eliminate the ‘glassy’ feeling of the G8x M cars’ steering.
Rear Axle
It is very important to also pay attention to improving the performance of the rear axle.
The G82 M4 racing car has a fully-adjustable rear axle, where every single control arm of the multi-link suspension system can be adjusted to fine-tune and optimise the handling of the car. In comparison, the road going G8x M cars have limited camber and toe adjustability that is controlled by using adjuster bolts.
These bolts are fine for making road-biased setups, but we are looking to match the performance of a racing car, so more advanced hardware is required.
Rear Toe Arms
Whilst the rear toe and rear camber on the G8x M cars is adjustable as standard, the range of adjustment is limited. The result of this limited range of adjustment is a compromised chassis setup. Ideally, rear camber would be increased for improved cornering grip and the rear toe angle would then be optimised for traction. However, on the G8x M cars, it is possible to either achieve optimum rear camber OR optimum rear toe – achieving the optimum settings for both aspects of the rear geometry is not possible as standard.
This is where an adjustable rear toe arm is required. The wider range of adjustment offered by an adjustable rear toe arm means the rear toe and rear camber angles can both be optimised.
Adjustable rear toe arms also feature high-precision rose joints which ensure the rear toe geometry remains fixed during high performance driving, as opposed to the OEM toe arms which feature flexible rubber bushes which cause the rear toe angle to change dynamically, leading to instability during driving.
Rear Camber Arms
The G8x series of BMW M cars feature two upper rear camber arms as standard. These arms are not adjustable on a road-going M2/M3/M4 but they are fully-adjustable on the M4 GT4. This is because an aggressive semi-slick or slick tyre requires higher levels of camber in order to work effectively.
Adjustable camber arms are required to increase the rear camber beyond approximately -2 degrees as the OEM rear camber bolts simply do not have a large enough range of adjustment to provide the camber needed for track tyres.
Adjustable camber arms also use high-precision rose joints which do not flex. This ensures the rear camber angle remains fixed during a corner, resulting in more consistent levels of grip and improved stability during cornering.
Traction Arms
The final adjustable arm at the rear of the M4 GT4 is the trailing arm or traction arm, as we like to term it. This rear arm is fixed as standard with a rubber bush that flexes during high-performance driving, leading to instability at the rear axle. This arm can be replaced with a rose-jointed adjustable arm for improved stability during acceleration.
As this arm is adjustable, the length of the arm can also be increased for improved traction. By altering the length of the arm, the arc angle of the rear arm is changed and this causes the rear suspension hubs to rotate forwards slightly during acceleration. This movement physically digs the tyres into the tarmac, resulting in increased grip at corner-exit.
Solid Rear Control Arm Bushes
The final change at the rear axle of your BMW when transforming it from a road car into a racing car is the solid rear control arm bush. This bush removes the flexible rubber bush in the lower control arm of the M4 resulting in a more responsive chassis whilst also preventing the rear camber angle from being shifted during cornering.
By preventing the rear geometry from changing, the performance of the car is made more reliable as the car is more able to use its tyres more effectively. More importantly, the confidence of the driver is boosted as the car is accurate and responsive to their inputs, with no second-guessing whether the car will behave as intended.
Body
The single biggest difference between the M4 GT4 and the road-going G8x M cars is weight. Through use of sustainable flax-based composites, the M4 GT4 is significantly lighter than the road-going version. The interior is also fully stripped-out in order to save weight. Whilst this would be possible on a road car, stripping the interior would compromise the usability of the car as a road car, so isn’t the best option.
When trying to match the performance of the road car with the race car, another solution emerges: tune the engine for increased horsepower. We calculated that in order for a road-going G82 M4 to achieve the same power-to-weight ratio as the M4 GT4 it would need to have approximately 580 bhp.
Tuning
The S58 engine used in the M4 GT4 and road-going G8x M cars is a robust and powerful engine which can be tuned quite easily to generate significantly more power than standard. Interestingly, the M4 GT4 uses a slightly detuned version of the S58, but this is so the car remains competitive within the GT4 ruleset it competes under.
Road going G8x M cars do not suffer from the same regulations, so with tuning it is possible to achieve far more power than standard. For example, a Stage 1 map from Tom Wrigley Performance will give your G8x M car 620 bhp without the need for any supporting modifications. This level of horsepower will actually give your M4 a greater power-to-weight ratio than the M4 GT4.
The automatic gearbox used in the M4 GT4 can also be mapped for improved response during gear changes, and the gearbox itself is more than capable of handling increased levels of horsepower without the need for additional modifications.
Tom Wrigley Performance also offers the potential of using multiple maps, so your BMW can have different amounts of horsepower for different circumstances. For example, 620 bhp may be too much for a smaller circuit such as Oulton Park, so a less powerful map could be used. When driving a wide-open circuit such as Spa-Francorchamps or Silverstone, the more powerful map can be used as the longer straights suit high-horsepower cars better.
Aerodynamics
The final aspect of transforming your G8x M road car into a GT4 equivalent racer is the aerodynamics. The M4 GT4 uses a large rear wing alongside a pronounced front splitter and front canards (dive planes) to generate far more downforce compared to the road cars.
It is possible to source aftermarket wings, splitters and canards so it is possible to also achieve a similar level of increased aerodynamic efficiency and performance.
To conclude
As we’ve seen, it is possible to turn a road-going G8x M car in a GT4-rivalling track car, whilst not compromising daily usability without breaking the bank.
Please get in touch if or explore our shop of upgrades.
