Good morning/afternoon depending upon which side of the pond you reside. I recently acquired an 03 R53 for peanuts, motor had 80,000 miles on it but sounded awful hence the peanuts. I have put in a new motor from BMW/MINI, for a princely sum, and have started with some mild upgrades.
M7 cai, Canton expansion tank, 15% reduction pully, upgraded intercooler, magniflow header and milltek resonated exhaust system. Not sure what kind of horsepower this will give me, maybe someone here has an idea.
My intention is when the motor in broke in I would like to fit a big valve head, with a sprintex supercharger 15% pulley.
My question on this is, with the stock although new motor handle this without coming apart.
The brakes and suspension has already been done, so no worries there.
Thanks for your input in advance.

 

Welcome to the forum. I will let others to follow answer the technical question, however, I suspect they will want more information about the motor you installed.

We need pictures of your MINI!

 

I would expect that you've added approx. 25hp with your current modifications. Stock is 163hp so you should be running around 188hp full stop. A Stage 1 tune would net you another 12hp and get you to approx. 200hp. If you do go the larger head/supercharger route you will also need to upgrade to larger injectors.

 

Looks OK, not sure you want to invest in big valve head or sprintex SC 15% pulley if you already have generic 15% pulley. Getting correct upgraded ECU Map is important to make best power with reduction pulley. If you use 17% or 19% pulley you get more power but also more heat and wear on everything.

Engine sounds like new or refurb from BMW with no miles so follow break in, even have break in oil for first miles then change to synthetic after that. For your basic upgrades the engine should be fine.

When you installed new engine did you also change water pump? What about engine mounts, these get old, if mounts are OEM rubber then they are OK for street use, not as good for performance, if you use polyurethane mounts then get lots of shaking and rattling with old dash and frame due to vibrations.

Did you reuse old supercharger and just change pulley? if only added pulley then bearings and insides of SC are dirty or worn, sometimes need to monitor and get refurb SC.

 

Welcome

I'd stick with just the 15% pulley on stock supercharger and build other mods for more power.

 

Larger injectors would be the next step. The JCW uses 380 injectors, but add a cam and larger injectors with a tune, would really bring up the horse power, if you think you need it.
Make sure to use break in oil first! (pretty damned important)
The "S" engine has a billet crank and is pretty strong.
I have an 06 factory JCW model and it has very good smooth power increase. If you are using the car in the twisties, you want smooth power increase! The 210hp of the JCW model, is all you really need on the street, as it is easy enough to take the twisties to a happy ride. (much more will make it easier to take it past the cars ability to stay on the curves! Just a thought!)
Suspension upgrades would need to be done, so handling can be improved! Also better brake pads.
Run flat tires will not be a good option (several will disagree, but I live in the twisties and ride to a high level and runflats are scary at best!) Summer tires will bring handling during the warm months, to a high level of lateral grip!
How about some pics of your car and work on it.