The Eaton M112 superchargers fitted to 4.0 Jaguar models can be easily upgraded by changing the pulley, increasing the driven speed of the supercharger.
This is a very popular conversion proven by long term success on thousands of cars.
On the Jaguar M112 supercharger there is a choice between 6% and 10% supercharger upgrade pulley.
The 6% or “1.5lb” M112 supercharger pulley is easier to fit, needing no material removal from the supercharger snout (except in rare cases where the snout has been factory machined off-centre in the casting, in which case a high spot may have to be trimmed slightly), and offers around 25bhp/ftlb midrange increase. That is a difference you can feel, especially as it gives a more instant response reducing the apparent ‘lag’ these engines are often criticised for. The majority of pulleys on the market, including those provided by other suppliers in the UK and worldwide, are the 6% variant. Our stainless steel 6% pulleys are available in our online shop.
The 10% or “2.5lb” M112 supercharger pulley does require aluminium removal from the supercharger snout to reduce diameter and allow clearance for this smaller pulley. The task is quite achievable with DIY skills for the keen home tinkerer (I changed my first one on the drive at home). Some garages are wary to attempt the job until they realise it’s actually quite straightforward, if still a bit of a pain! The extra work with a die grinder or mini belt sander pays off, as these pulleys give 35-40bhp/ftlb midrange increase.
We have had more than one customer ring us when they return home from having this job done, and say “no way that is only 40bhp!”. The improvement is a black and white difference and gives these cars a new lease of life. Our stainless steel 10% pulleys are available in our online shop.
Concerns over belt slippage with smaller pulleys are a legacy of other manufacturers using aluminium for upper pulleys in the early days of modifying supercharged Jaguars. Aluminium has limited friction qualities when hot, which can be an issue on the small upper pulleys on these cars, but our upper pulleys are made from high grade stainless steel and do not suffer such issues. That said, if your belt is old & worn, we stock a range of high quality brand belts.
While snout trimming for fitment of 10% pulleys can be done in-situ using hand tools, a much tidier finish (to the ideal size retaining maximum strength and free of stress-raising flaws) is achieved when the snout is turned down in a lathe, giving a perfectly machined finish. If you need your snout upgrade with 10% pulley completed to the highest standards, send your snout or full supercharger to us to be machine finished for peace of mind.
We can also provide upgraded and rebuilt snouts on exchange basis to minimise the time your car is held up in the garage. Please email tuning@powerhouse.uk, or ring us on 01246 260665 – Option 3 to discuss.