Firstly, you'll need to understand the principals of supercharging, and then (hopefully) it all makes sense which makes it easier to build – if you already know, then skip this bit!

Supercharging, like turbo charging is a method of achieving forced induction, which is essentially blowing air into your inlet manifold (this is why superchargers are often called “blowers”) instead of letting the pistons suck air into the inlet manifold – this is referred to as being “naturally aspirated” or na. The more air you can blow into your engine (as long as you add more fuel) the harder your engine will run and the more power you can make – this is a crude explanation, but it will suffice.

Superchargers differ from turbos as they are driven from the crank (via belt, chain or gears), whereas turbos are driven by the exhaust spinning a turbine which then turns a shaft attached to a compressor wheel that compresses the air fed to the engine.

There are a few different types of supercharger, but I’ll only be referencing the roots-type from here on as that’s what an Eaton M45 is! The roots blower is basically an air pump consisting of two spinning, interlocking lobes or blades that draw the air from one side (inlet) and push it out of the other (outlet) – simple!

There are a few components to the build, so I’ll list the major ones I used here, explain what each do and where I got them (with an idea of cost):

Supercharger (SC) - £200

I chose the M45 because… it fits! The front suspension towers of the Exocet are a fair bit further inboard than the MX5, narrowing the space between the chassis and the engine, so fitting the bigger M62 while possible (and largely the same process) may require some custom work on the SC mounting, the chassis or both.

Note: the ’45 & ’62 respectively refer to the number of cubic inches each displaces per revolution. This being the case the ’45 has to spin faster than the ’62 to do the same job… more spinning = more outlet heat which is bad (more about that later). In short, the M62 is better but for me the M45 fits better.

The M45 is sourced from a Mini Cooper – the later ones (R53 & R52 JCW) have Teflon (black) blades and the earlier (R52 non-JCW) are graphite (grey) coated – The Teflon ones are considered “better” as they are meant to be more efficient (reduced heat?) but condition is the main concern when buying used. Check for wear on the rotor tips and any damage to the rotors & coating. There should be zero play in the pulley and it should turn freely by hand without any horrific noises. If there is any doubt, walk away as rebuilds can get expensive.

You don’t need the mini outlet ‘horn’ or the water pump (that goes on the back) but there are some minor modifications you will need to do:

1. Hack off the mounting bracket / tab on the outlet side of the body – this needs to come off to give you room to fit a custom outlet at an angle.
2. Fill the (red) hole at the back of the body with a bolt & some PU adhesive.

I got mine for £200 on FaceBook Marketplace – It’s best to source one locally so you can satisfy yourself you are getting a good’un before you hand over the cash. You can always buy a reconditioned one if the budget allows.

SC Reduction pulley - £50

The stock pulley on the M45 is 65.5mm in diameter which is a 1:1.93 ratio to the crank pulley on the engine. That means at 7k rpm (engine) the SC will be spinning at ~13.5k rpm, displacing ~608k cubic inches of air per minute. That sounds like a lot, but it’s not enough!

In order to increase the rpm of the SC in relation to the engine rpm you can either make the crank pulley bigger or (more commonly) the SC pulley smaller. It’s actually preferable to fit a larger crank pulley and keep the (larger) stock SC pulley – with a small pulley the belt is more likely to slip as it has less physical contact with the pulley.

The generally accepted maximum rpm for the M45 is 16k (Eaton say 14k, but they would…) – a 54.5mm (17% reduction) pulley has a ratio of 1:2.32 so at 6.5k engine rpm (top of the power curve of my engine) the SC is spinning at ~15k rpm… that’ll do!

I found an “as new” 2nd hand KAVS 17% pulley on ebay for £50 (retail is ~£120). The stock pulley is a push fit, but it’s on there tight… really, really tight! You can cut them off (not recommended) or use a special puller (borrowed from a friend). The KAVS pulley slides right on the shaft and grips as you tighten the bolts (don’t forget thread-lock).

Supercharger oil - £13

It’s a good idea to change the oil in the supercharger as it’s not an easy job attached to the mini, so it’s probably still the original stuff in it. It doesn’t take very much and you can get a kit on ebay for £13 which includes a syringe (which helps). Alternatively you can splash out on a 946ml tin of Mobil Jet Oil II which will last for numerous oil changes.

Injectors - £250

More air means the engine needs more fuel, the stock MX5 fuel pump is more than adequate to supply the volume to the rail but the injectors will be on the limit with a stock SC pully, add a reduction pulley and you’ll need to replace them with higher flow rate injectors.

RX8 injectors are the most popular (used) choice but the market has been flooded with “ebay fakes” which are fairly useless copies of the 440cc “yellows” – to be avoided.

I initially fitted some used RX8 350cc “reds” I found on ebay for £60 but they leaned out at 6k rpm so bit the bullet & replaced them with 550cc EV14's from DigiTune (https://digitune.co.uk/product/mx-5-89-05-550cc-ev14-injector-kit/) - £250

Intercooler (IC) & pipes - £150

The cooler the air pushed into the engine the denser it is (more oxygen) and the better it burns which increases power and reduces the risk of pre-detonation (aka ‘knock’).

It’s possible to run the outlet of the SC directly into the inlet manifold without an intercooler (and some people do…) but with the 17% reduction pully, my planned usage (track driving) combined with the relative inefficiency of the roots M45 blower, the inlet air temperature can reach a blistering ~90 degC – this would have a disastrous impact on my power goals (~200bhp) and would probably cause engine damage. There are (basically) 3 ways to cool the boosted intake air:

1. Water-methanol injection – injects a fine water/methanol mist into the inlet which can have an impressive cooling effect. Expensive & tricky to set up properly but can be used alone or in conjunction with #2 & #3 below.
2. Charge cooler – the boosted air runs through a pipe which is cooled by water – for best results use iced water. You’ll need to keep a ready supply of ice to get the best from it and also a 2nd radiator to cool the water – bit of a faff but it’s a more compact solution than an air to air intercooler.
3. Air to air intercooler – the most common solution. Runs the intake air through pipes (or flat channels) surrounded by aluminium fins which work to soak the heat out – works best when outside air is flowing quickly past the fins (works even better when the fins are wet). Biggest downside: it’s big.

I went for #3 and mounted the Intercooler (£100) in front of the radiator connected up between the throttle body and supercharger with 90deg silicone pipes from ASH (£50). I mounted the IC as close to the radiator as possible (~8mm) and filled the gap with some insulation foam (sticky back stuff supplied in a strip). The theory is that all the air passing through the IC will carry on through the radiator so I won’t overheat the engine. If there was a big gap between the two, the air would be disrupted and wouldn’t flow through the radiator as cleanly reducing its cooling efficiency.

When I built the Exocet I planned for forced induction in its future by lowering the upper front chassis support bar (cut off & re-welded lower). This allows me just enough room to route the IC pipes over the top of the radiator without having to cut the bonnet.

Airflow is important, particularly when introducing an intercooler in front of the radiator. Air will follow the path of least resistance, so given the choice it will flow around rather than through the intercooler / radiator. To combat this I mounted the IC as close to the radiator as possible (~8mm) and filled the gap with some insulation foam (sticky back stuff supplied in a strip). The theory is that all the air passing through the IC will have to carry on through the radiator. If there was a big gap between the two, the air would be disrupted and wouldn’t flow through the radiator as cleanly reducing its cooling efficiency.

Another modification was filling the gaps between the radiator and the bonnet (on all sides), with aluminium, rubber sheet, edging strips and self adhesive foam, again forcing all of the air to flow through, rather than round the radiator. Lastly, the Exocet grill is not large enough and about 1/3 of the intercooler was outside of direct airflow. This was resolved by drilling 5 large holes in the lower lip. For road driving the numberplate covers these holes, on the track the numberplate is removed.

Bypass Valve (BPV) – £20

When the engine is at idle, the supercharger will still be running (as its permanently connected to the crank via a belt), that means at 850 rpm the SC will be pushing ~87k cubic inches of air per minute at a closed butterfly valve (in the throttle body) – with nowhere else to go the pressure will either pop a hose off, or it will stall (i.e. try and slowdown / stop) the SC which will likely result in the belt slipping, shredding and/or coming off. The effect is much worse when you are going from WOT (wide open throttle) to braking (closed throttle) at high rpm – for example on track.

The solution is to release (blow off) or redirect (bypass) this pressure away from the closed inlet throttle body – the most common way of doing this is to re-circulate the compressed air from the SC outlet back into the inlet, so it’s basically just pumping it around in a loop. The bypass valve is located between the outlet and the inlet of the ‘loop’ and is controlled by a vacuum hose run from the inlet manifold. When the throttle is closed a vacuum builds up in the manifold and opens the bypass butterfly valve, re-routing the compressed air into the loop. As soon as the throttle is opened, the vacuum disappears and the bypass valve snaps shut and all of the compressed air routes towards the now open throttle body & into the inlet manifold.

The Mini that the M45 came from also has a bypass valve (pictured) - picked one up from ebay for £20. Remove the small black tube and cap off the fitting on the body of the BPV, run the vac line from the inlet manifold directly to the black plastic actuator.

Blow Off Valve (BOV) - £33

Like the BPV the BOV is also connected to the inlet manifold by a vacuum line and is opened by the build up of a vacuum when the throttle is closed, the difference is that the pressure is released to atmosphere (i.e. outside) rather than being recirculated.

I probably don’t need a BOV in addition to a BPV but the pipe was just the right size to bridge the gap between the throttle body and the intercooler, so I got it. In my mind it’s cheap insurance if the BPV fails for any reason, and it’s there if I go down the turbo route later on. If it causes me issues going forward I’ll just blank it off or disconnect the vac line so its disabled.

2nd Throttle body (TB) & cable - £50

The throttle body is a simple butterfly valve that opens when pulled with a cable attached to the accelerator. Once it’s open the air rushes into the engine, mixes with the fuel in the cylinders and is ignited by the sparkplug. The more you open the valve, the more air rushes in & the faster the engine rev’s. If you didn’t have a TB your engine would be at WOT (wide Open Throttle) redline and then (after some drama) would blow-up / give up!

There are three ways to control the throttle with this setup:

1. Single throttle body at the intake manifold (i.e. stock TB only)
2. Single throttle body at the SC intake (i.e. pre-charger TB only)
3. Two throttle bodies (both of the above)

All are valid and all work, I went with #3 and here’s why…

#1 – apparently getting the car to idle smoothly is challenging and the noise of the SC is significantly higher without a pre-charger TB.

#2 – when going from WOT to closed throttle, there will be a sizable vacuum build up in the inlet manifold and with nothing stopping it that vacuum will collapse the silicone elbow hoses connected to the IC! There is also the risk that if a hose pops off or the IC fails, the engine will effectively be stuck at WOT (bad). The 'fix' is to use only rigid aluminium pipework.

#3 – bit more of an effort as you need to connect the two TB’s together with a 2nd cable and set them up to work together nicely. Also, the accelerator is heavier, but that can be fixed to some degree by removing one of the springs from the 2nd TB. For me it was simple, I don’t want a massive embarrassing noise (and fail track limit DB tests) and I don’t want to risk a runaway throttle!

I bought a TB from a mk1 1.6 MX5 for £30 and a universal throttle cable kit for £20. I had to modify the stock TB by removing one of the crescent cable guides from the Mk1 TB and welding it to the stock one.

Mounting Kit, pulleys & belt - £280

There are a few M45 mounting kits out there for the MX5 engine, and like the cost of them, opinions vary. Because most of the kits available are designed for the MX5 engine bay (more space than an Exocet) and for a car with power steering (no PS for me!) and are relatively expensive (I’m on a tight budget), I went for the weld-your-own fitting kit from Garage42 (£190 delivered).

Additionally I needed to buy:

1. 0.5m of 60mm OD steel (1.5mm) pipe for the SC inlet & outlet- £15
2. 0.5m of 38mm OD steel (1.5mm) pipe for the bypass / loop - £15
3. 2x MX5 cam idler pulleys - £30
4. Mercedes Vito (large ribbed) idler pully (in place of the missing PS pump) - £15
5. Gates ribbed belt - £15

The wedge shaped outlet from the Garage42 kit wouldn't clear the Exocet chassis (as the suspension towers are a bit more in-board than the MX5 - green tape shows the interference) so after a bit of CAD (Cardboard Aided Design) involving a toilet roll initially, I made my own out of some 60mm pipe.

I dry-built the kit and once I was happy spray painted all the components with black engine enamel. Once that was dry I put it all together in anger using JB weld red high temp sealant on the mating surfaces.

I also made a belt tensioner with a length of threaded rod and a couple of M10 bolts (for the idlers) with nuts (for the treaded rod) welded sideways on the top of them – turn the rod to add tension.

Aligning the supercharger pulley to the aux belt / crank pulley is really important - just a few degrees of misalignment will wear the belt prematurely. Significant misalignment will throw the belt off! It takes time, patience and a bit of tech to get it right, but it's worth it.

The Garage42 kit I used is pretty much fixed in terms of adjustment but its worth checking with a straight edge on the crank pulley to see if the SC pulley is miles out - mine was spot-on but if I needed to adjust I would have enlarged the bolt holes to give a little wiggle room.

I used a digital inclinometer (~£10 on Amazon) to ensure the two pulleys were at the same angle. I put a straight edge on the crank pulley & zeroed the device, then put it on the SC pulley and added / removed washers from the front mounting bolt until I had it at zero (i.e. same angle as the crank pulley)

Finally, once everything is pretty much in place you'll need a belt. Get some string or wire and route it through the pulleys & idlers, measure it and that's the length of belt you'll need. Mine was 1240mm so I got a Gates 4PK1240 micro-v belt. Note:4pk is the number of ribs in the micro-V, the SC pulley can take 6 but the standard aux-pulley is only 4. You can get a 6 rib (oversize) overlay pulley which negates the need for a reduction pulley on the SC but it costs a fair bit more.

Exhaust Heatshield - £35

The mounting placement I used is the most common and is referred to as “hotside” because the SC sits directly above the exhaust manifold (the hot side of the engine). Heat soak from the exhaust to the SC casing is something to be avoided (it gets hot enough on its own!). Mounting it “coldside” is the obvious solution but adds complexity and expense. The simple solution is to add a heatshield between the manifold & the SC.

The stock MX5 heat shield is better than nothing, but it’s not really up to the job. You can buy a performance aftermarket heat blanket off the shelf (i.e. Track Dog Racing ~£170) or you can make your own by sandwiching some exhaust wadding between two pieces of 0.5mm embossed aluminium sheet and moulding it round the manifold. I made my own (£35).

Miscellaneous - £130

There are also a few other bits & pieces like hoses, clamps, paint, RTV, brackets & bolts I bought which came to ~£130.

Standalone ECU, wideband & AIT sensors & tuning - £825

It’s possible to run an M45 on an MX5 engine without a standalone ECU but only marginal gains will be available without a proper tune. Also, with a stock ECU you probably won’t get away with a reduction pully and bigger injectors without some timing and fuelling issues.

Electronics not being my strong suit I decided that I needed to buy new as the thought of trying to troubleshoot a faulty 2nd hand unit filled me with dread!

After some much appreciated advice I bought a plug-n-play DIY-EFI Core4 ECU with built-in wideband controller, complete with wideband & air inlet temperature sensors for £650 (https://diy-efi.co.uk/). The ECU is based on the Speeduino platform and is tuned with TunerStudio. I won’t go into any detail about tuning, AFR settings and timing adjustments because I can’t… I haven’t a clue! So off it goes to Tom Chappenden at Coldside Engineering (https://coldsideengineering.co.uk/) for a tune (£175).

So there you have it – how to supercharge your MX5 / Exocet to ~200bhp for ~£2k – enjoy!

Gareth Morgan - 2024