A K-Series is Reborn – Part 4

Finally we come to the last part of the K-Series is reborn saga and I’ve left the fun stuff till last.

The Supercharger chosen for the job a Rotrex centrifugal supercharger, this supercharger has many advantages over the other designs.  The first of which is size, space in the Elise engine bay is at a premium so having a supercharger this is not much bigger than a alternator is a great advantage.  The second thing is because of the design it is almost silent.  I have driven a few of the latest Lotus Exige supercharged cars with their twinscrew design superchargers, like most things they sound great to start with but the constant whine does get wearing after a while – yes I realise that comment makes me sound old!

Rotrex supercharger

The supercharger setup is pretty basic really but the attention to detail is what makes it work.  As mentioned in previous articles some of the parts came from the old Turbo Technics kit.  In fact Greg, at Hangar111, decided to throw most of the kit away and redesign it.  The bits retained were the rather nice supercharger bracket/alternator bracket and one of the tensioners.  The charger/alternator bracket was primarly designed to hold the TT supercharger which used to bolt directly to it.  The Rotrex obviously didn’t do this so we set about designing an additional bracket that would fit the Rotrex and bolt cleanly to the TT charger/alternator bracket.  This was one of those jobs that seemed a quick one on the surface but the positioning of the supercharger is critical to ensure that the belts run true and of course the bracket provides a positive location.  In the end the bracket we designed is probably a bit OTT, however we are confident that the we could hang the car off it so it will be more than up to the job.

rotrex supercharger fitted

With the supercharger fitted to the brackets it was time to look at the tensioners.  One of the tensioners from the old TT kit was retained, this provided manual tension of the belt near the crank pulley.  The auto-tensioner was the one we had to re-engineer.  We wanted as much wrap of the belt around the supercharger pulley as possible.  This would ensure that the belt wouldn’t slip and it also meant we wouldn’t have to run a massively strong spring.  The tensioner was redesigned to bolt straight onto the new supercharger bracket, this had two effects.  One was it would be a lot closer to the supercharger and provide almost 300degrees of wrap and the second it would eliminate the need for engineering a arm to mount the pulley on.

With the tensioners all done the belt was fitted to the car for the first time.  It was a bit of a special moment to see everything linked up with the belt, and the good news is the care and attention in designing the brackets and tensioners meant that the belt ran perfectly.

Due to the positioning of the supercharger it means we have to move the inlet from one side of the engine to the other.  The disadvantage of this is the inlet has to run above the exhaust manifold.  To counter act the heat soak I wrapped the exhaust and also built a rather substantial head shield around it.  The final thing was to wrap the inlet pipe in reflective foil.  Thankfully using all this heat management seemed to work and heat soak was kept to a minimum.  The last big change to the inlet was on the actual inlet manifold itself.  As I mentioned the throttle body was now on the other side of the engine and also the air no longer came in via the left of the engine bay but by the right.  To deal with this a new top half of the inlet manifold was used that had the entry on the opposite end to the original item.

K series supercharger inlet manifold

There was now one vital part of the supercharging system missing.  You’ve got all this air coming via the throttle body, heating up as it goes over the exhaust and finally it gets squashed by the supercharger.  The upshot of this is the air leaving the supercharger is over 90degrees in temperature.  This isn’t good for performance or, more importantly, reliability.  What we needed here was a way of cooling the air down before it enters the engine.

There are a number of ways to do this but we decided on a charger cooler.  A charge cooler is basically a radiator in a box that the air from the supercharger passes through.  Water is then pumped through this radiator to the front of the car where there is another radiator.  This radiator is cooled by the moving car and thus the water is cooled and this in turn cools the air going through the charge cooler…..hope that made sense.  The issue we had was making the charge cooler big enough to do the job but also to fit under the wing of the Elise.  With some creative design, shoe box with cardboard tubes attached to it, we got a fabricator to create the charge cooler.  The pre-cooler radiator came from Hangar111’s own supercharger kit they do for the later Toyota powered Lotus.

Pre-cooler in the front of the car –

Elise pre-cooler installed

Charge cooler

Elise K Series charge cooler installed

With everything fitted to the car it was finally time to start the car.  This has to be the worse moment of any engine build.  It wasn’t helped by the engine refusing to build oil pressure when I was cranking the engine with the plugs removed.  After a lot, you can read 30 minutes of messing about, I finally got a reading on the oil pressure gauge.  I fitted the plugs back in and primed the fuel pump.  Standing well back I turned the key, barely a whole rotation of the engine occurred and she barked into life.  Excellent.

Once the engine was up and running I arranged for the car to go upto Emerald to get mapped.  Due to work commitments I was unable to take the car myself so Greg came down and collected the car and took it up to the guru that is Dave Walker at Emerald.  Dave was under instruction to run the car in on the rollers then do a complete map for it.   All through the build of the engine I had the figure of 220bhp as my ideal output for the engine.  This was based on what TT used to get out of their K series cars.  I know we had improved on most things on the kit but I wasn’t going to get to ambitious with my expected figures.  The prime focus of the project was to produce well mannered, reliable engine with enough power to enjoy without losing the ethos of the Elise.

So there I was sat at work on the day the car was at Emerald wondering how my engine was doing and if anything had tried to escape from it yet….like a conrod or a piston…or both!  When I got a text from Greg saying “Car’s done, is 268bhp enough?”  Of course I know what a piss-taker Greg can be so I didn’t take the text seriously and instead I picked up the phone and rang him.  Well he wasn’t taking the piss the engine was producing a staggering 268bhp and 196ft/lbs!  I was speechless.   What’s more the fueling had spare capacity, the charge cooler was reducing the inlet temps of 90 degrees to a very cool 32degrees and the boost was a mere 8psi.  This all meant that the engine was relatively under-stressed and if one day I fancied doing something stupid with a wind up of the boost and 300bhp would be more than possible *rubs chin* For now I have limited the engine to 7000rpm in order to retain reliability of the VVC mechanisms so I’ll have to live with just 260bhp for now.

Elise VVC Supercharged power graph

So a remarkable result and way above expectation.  When I went to collect the car I was afraid that it wouldn’t feel like an Elise anymore.  How wrong could I be, keeping the K was the best move because the dynamic of the car was exactly the same as it had always been.  The power, oh the power!  Yes there is loads on tap but the engine behaves just like it used to, only now you get from point A to B a tad quicker.  Over all the project has been a real learning curve for me, the K is a great little engine and with the right bits and sensible approach it can still hold its own.  Right that’s enough waffling from me, I am off for a drive!