17% more Ooomph

Smaller Pulley, more pushey.

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We are on lockdown,so whilst we haven’t even had Ermy on the road, we have time. And with time comes fettling. And painting things that don’t really need painting.


(Removing all the sensors, bolts and plastic fittings and gaskets. Clean up the cover by using a stiff brush to polish the metal, spray the cover black, inject enamel paint into the lettering on top, replace gaskets and sensors)

Don’t worry we haven’t gotten away from whats important. It’s time to get more oomph out of a car we haven’t even driven since it was supercharged.

The supercharger is on and spinning away happily. We don’t control how fast the charger spins as it is connected directly to the crank pulley which, when idling is spinning at 900-1000 rpm.
This connects via a belt to the pulley on the supercharger.

For every rotation of the pulley, the belt moves the circumference of the crank pulley, in turn pulling the supercharger pulley. . The pulley on the supercharger is smaller than the crank pulley so one rotation of the crank means multiple turns on the supercharger.

Our original supercharger pulley had a circumference of 21.5cm. And the Crank a circumference of 41.5cm. That means 1 crank turn is 1.93 turns of the supercharger.


The new one (On the right) is smaller. 17% smaller, so 18cm of circumference. (17.85mm really but our method of cutting and trimming a bit of timing belt isn’t that accurate)

Belt lengths

This means for every rotation of the Crank, we get 17% more rotation, instead of 1.93 turns, we get 2.32 .The speed of the supercharger at idle thus goes from 1800-2000 up to 2200 - 2400. No real suprises, it goes up by about 17%.

Changing the pullet is “Easy” in that it involves just pulling the old one off. The old pulley is “Interference fit”, i.e. it was made to fit so tight onto the spindle that it wouldn’t rotate. These are fitted by making the pulley slightly smaller than the shaft, heating the pulley so it expands, fitting it, and letting it cool down and shrink to fit.


As you can see there is no fixture or fitting. So to remove it, apply a lot of force evenly over the whole pulley so that it doesn’t deform the pulley.

Specific tool

Enter a very specific tool. This clamps to the outside of the existing pulley and pushes a bolt through the centre. As you apply force to the bolt, it pushes the tool and pulley evenly away from the supercharger.

This one was £60 off of ebay. Rentals are available though, which is very sensible given you probably only need the tool once.

With the spindle exposed, the new on fits on with a mount which is tapered. Our pulley is not tapered, so as the pulley is screwed on, it lightly crushes the mount against the spindle so that it grips.


We then need to put a shorter pulley belt on, as our belt tensioners cannot adjust enough to take up the slack or the smaller pulley.

And thats it, our charger is now spinning faster, making more pressure and adding more oomph.

This change can also be applied on the original Mini Cooper R53 which our charger came from, albeit all the fitting is a little more effort as the bay is a bit more cramped than Ermys!


Welcome to Twits with Spanner. A blog following two chaps as they attempt to build an Exocet kit car with bits from a donor MX5, spanners and lots of hope. Read more about us