Can the PTU in the FF & Lusso handle the power of a 700hp V12

Ferrari's 4-wheel or PTU system is totaly different from a conventional 4-wheel or 4X4 system that is based on a single gearbox with a central differential that gradually regulates the power from one central engine to each wheel individually, allowing all wheels to handle the same forces and are equally dimensioned.

The Lusso & FF are actually classic 2WD vehicles, a front-mounted engine with a rear-mounted gearbox that handles the full power to the rear wheels, noting that the rear is more strongly dimensioned than the front.
The difference lies in the fact that the front wheels are now additionally driven by a 2nd gearbox called 'PTU' which is mounted at the front on and in the engine. This gearbox has at the front only 4 gears " executed in a two speed gearbox" and from the 5th gear it is disengaged and drives the Lusso or the FF as a normal 2WD.
Both gearboxes are mounted in the same line on the crankshaft of the central engine, the PTU is directly on the front side of the engine and the rear gearbox on the other side or rotated 180° via a classic drive shaft or universal joint shaft. Both do not know mechanically about each other and function standalone. Ferrari's rear gearbox is a proven concept and is clearly an oversized design to handle large forces, but the front PTU is a different story and is remarkably smaller in size than the rear.

If you drive the Lusso in 1st, 2nd, 3rd or 4th gear on a straight track at a constant speed, you won't create any slip, virtually no power is sent to the front wheels, you drive as if you were driving a 2WD. If you take a sharp turn and you accelerate the PTU gets in action, the front wheels get more power so they turn faster, you get more grip and the car is less likely to skid. But once in 5th gear you must realize that you are driving back in a classic 2WD because the PTU will be switched off and you will have to make sure that the car does not understeer or break out in the opposite direction of your rotation. You will then have to make sure that there is enough grip on the front wheels by taking back the throttle or by making less steering angle.



According to Ferrari's technical director Roberto Fedeli, the PTU "in this picture" of which the internal gears between the 2 front wheels are visible, can only handle 20% of the engine power. It is small in size "4 to 5 times smaller" because there is hardly any space in the engine compartment. It controls the speed of the front wheels based on two wet oil clutches for each side of the car. They decide if there is slippage and therefore more or less power has to be sent to the left or right front wheel.

According to Fedeli, the PTU can run the front wheels at the same speed as the rear wheels. This means that a large amount of power, or perhaps almost all of it, can temporarily be transferred to the front wheels.

Suppose Ferrari would have limited the Lusso so that only 20% of the power can be sent to the front wheels then we can't speak of a real 4 X4 like an Audi quattro where 70% to 80% of the power can be sent to one of the 2 axles. So it is to be expected that more than 20% of the power can be pushed to the front axle to get enough grip in slip situations.

And since the PTU of the Lusso can rotate all wheels at the same speed, this means that more than 20% of the power can be directed to the front wheels.

So if it can handle only 20%, Ferrari should have a safety device built in if the power on the front axle is more than 20% of the torque. But this is not the case because then this system would have to be disabled at high loads and this is not done because this would be visible by driving behaviour or the instrument panel. The following situation shows the Achilles tendon of this concept:

Suppose you drive at 200km/h in 6th or 7th gear and you want to take a left turn, you slow down before the turn and are now doing probably 4 to 5000 rpm, you want to add throttle to get back to speed. You are driving manually and you put your Lusso in 4th gear because your want to increase rpm as you want to leave the corner faster. Shifting into 4th gear activates the PTU and suddenly an important part of the power comes on the PTU. The PTU will make the right wheel turn faster than the left one because it has to travel a longer distance than the opposite wheel. Driving quickly into a corner will cause immense horizontal and vertical forces to be applied to the left front side of the car, which tends to slow this side down. The wet clutch detects slippage and orders more power to the left wheel. The power demand on the front wheels is then significantly more than 20% and probably closer to 50% as we are now easily making 6000 rpm's and from the power graph of the Lusso we can see that we are moving almost closely to full load. A conventional 4X4 now brings more then 50% of the power to the front wheels and for the Lusso that will make no difference because as long as the clutch detects slip it will increase the power to the front because it wants the left front wheel to spin as fast as the rear wheels.
It then looks like you are driving a monster V12 that is shifted by a mini gearbox in close proximity comparable to an Alfa 156. In addition, the drive shafts of the front wheels "see picture" are lighter than the rear wheels as in a classic 2WD Ferrari where the full power is only put on the rear wheels which always process the full power. However, at this time, more power or the same forces may be applied to the front wheels as those applied to the rear wheels. It is highly doubtful that this small mini PTU can handle these monstrous forces.

Since there is no protection to limit the power on the Lusso's PTU, it occasionally gets into situations where it is overloaded. And if there is any power limitation, this would be very unsafe because your 4x4 would fail in stress conditions where you need them high. For the PTU this situation is very stressful and moreover, these forces suddenly come through shifting from 5th to 4th gear. A classic 4x4 differential does not have this problem as it gradually builds up the distribution.

Today there are reports of dozens of broken PTU's because the seals and forks have failed due to high pressure and stress factors on the forks while driving as in this situation.

PTU failure is related to the use of the car. For example, the PTU is more prone to tarmac than snow. The FF/Lusso is originally presented in a snow environment where it has to work much less than on asphalt. In a wet or modulable surface the forces to be transferred to the PTU are smaller. The daily use of the Lusso in city traffic and cornering by shifting to lower gears is much more stressful than cruising at constant speed or long distances. It is doubtful that the concept can cope with this. Driving on freeways puts almost no strain on the PTU unless one accelerates and decelerates continuously. This also explains why some cars with many kilometers have escaped PTU failures where cars with very few kilometers that are only used to make short trips in city traffic are much more likely to have PTU failures.

The current PTU concept is too fragile and cannot cope with the high forces at these heavier loads or with people who drive sportily. The Lusso is a combination of a mouse and an elephant. I you ride it like a mouse with an elephant you will have no problems, but if you ride it like an elephant with a mouse, the PTU will probably fail. The problem is that as a driver you don't always know in which mode you are driving.

The first thing to go are the seals, which are under very high pressure. If you are saddled with a defective PTU it is advisable to grade it up with better seals and stronger forks as one of these specialists recommends.

https://www.ssautotech.co.uk/ferrari-ff-ptu-rebuild.html

It is clear that the current PTU concept is a dead end street story for Ferrari. Despite the fact that the Lusso is a fantastic car to drive or own, you should be aware of this Achilles tendon of your car if you want to keep it in good condition. Entering the circuit with such a fragile concept does not seem to me a good idea. It is equivalent as asking for PTU failure given the high stress your PTU will have to endure unless you manage to complete your lap only in 5,6 or 7th gear as if you were driving a classic Ferrari.