The amount of power offered up by the Mazda MX-5 is an ongoing petrolhead dilemma. The very point of the car is - arguably - simplicity, so it should be about low power and low weight. And yet, a more powerful version has always seemed an enticing prospect. It’s such a great platform to build on, with great balance, low weight and drive sent to the correct wheels, so a bit more poke couldn’t hurt, right?
It seems like we’ll never get that from Mazda, so as with previous versions of the MX-5, the current ‘ND’ generation is best looked at as a base upon which you build your ideal sports car. And that’s highly likely to involve adding a turbocharger.
To find out all you need to know before taking the forced-induction plunge, we headed to Oxfordshire Mazda tuning gurus BBR GTI for a chat and a drive in two of its turbocharged ND MX-5s - a 1.5 and 2.0.
Here’s what we discovered…
If you’re thinking you’d be better off using a 2.0-litre ND MX-5 as a starting point, you’d be right. There’s no price difference between the 1.5 and 2.0 kits, and if you’re going for the latter, you’ll end up with 242bhp as opposed to ‘just’ 210bhp. So what’s the point in the 1.5 kit? Well, it all depends on where you live.
Some markets only have the 1.5-litre ND available in the first place, so without a kit for the 1.5, owners would be waving goodbye to their turbo dreams. Then you have to consider markets where vehicle taxation is done by engine size, where it makes sound financial sense to go for the dinkier engine and slap a turbo on it, than pay over the odds for the sake of an extra 32bhp.
Conventional single-scroll turbo kits are available for the current MX-5, but as far as BBR is concerned, twin-scroll is the way to go, and not just for the reduced lag such a setup affords. It’s also to do with scavenging, which is - if you don’t know - all about a low pressure area following each exhaust pulse that ‘pulls’ it out, leaving the piston with less work to do.
“Putting a conventional turbo on it would be very inefficient because you’re introducing a lot of back pressure close to the cylinder head,” BBR boss Neil McKay told us, adding, “You undo a lot of the work Mazda has done doing a scavenging ‘4-2-1’ exhaust system. But with twin-scroll you have lower EGTs (exhaust gas temperature), and keep the scavenging effect.”
The BBR-breathed 1.5 is probably the least turbo-feeling turbo car I’ve ever driven. Neil describes driving it as “just like an MX-5 with a bigger N/A engine,” and I’d agree. There’s a slight lift at 4000rpm that’s just about perceptible, but that’s about it. It’s still revvy and responsive - it’s just that now, it doesn’t feel sluggish.
It’s much the same story in the 2.0-litre, although the switch to forced induction is more obvious here. You get the first clue as it starts to pull from 2000rpm, and this time it’s at 3000rpm that you start to feel the turbo coming on boost. But again, the inline-four’s eager character is still there, but backed up with some proper performance.
With a few choice chassis upgrades (we’ll get to those later), it’s well within what the MX-5 can take. It doesn’t feel excessive or encumbered by its newfound power. There’s still plentiful traction, with the added bonus of it being easier to get the rear-most tyres moving around.
In the wet the car does struggle a little to put its power down off the line and does spin up the rear wheels in both first and second, but we wouldn’t expect much in the way of wheel spin in the dry (so far, we’ve only tried it in damp conditions).
When we first talked about driving these cars on Instagram, more than a few of the comments were directed at the kind of forced induction employed. Why not go for a supercharger? It’s all to do with the inherently high parasitic losses of a supercharger, Neil explained. You’d need a hefty amount of boost to get a meaningful power gain using a supercharger, and that doesn’t sit so well with the high compression ratio of the SkyActiv-G engine. Plus, as we covered in point no. 3, the turbo setup used on these two MX-5s doesn’t spoil the party with a boosty, laggy delivery. So, a turbo it is.
The 242bhp belted out by the 2.0-litre BBR car is achieved with a fairly modest boost pressure. It’s actually good for much, much more power. “You can go further, but that’s where we’re comfortable,” Neil explains. Why? Because any more and you’re risking throwing a con-rod.
If you want an output exceeding 240bhp but lower than 300bhp, you’re going to want to look at stronger pistons and con-rods, plus a beefier clutch. BBR is working on a Stage 2 kit that’d allow this kind of power, but the contents of it haven’t been confirmed yet.
A Stage 3 kit going beyond 300bhp (350bhp is possible with the BBR turbo) would require all of the above, plus a new transmission.
So, if you can give your MX-5 an 82bhp boost, giving it the performance the chassis deserves while keeping its sprightly, revvy nature, one of these turbos is a bit of a no-brainer, isn’t it? Unfortunately, not quite.
While the aftermarket will happily provide us with the faster MX-5 that Mazda won’t, it comes at a reasonably hefty price. The turbo kit on its own is £4395 if you want to fit it yourself, or if you’d rather BBR did the work and you keep your hands clean, that’s £4995.
Doing the kit on its own isn’t exactly advisable either. You’ll be wanting to upgrade the brakes, which is £665 if you go for the four-piston front setup of the 1.5 we tried, and £895 for the six-piston front arrangement on the 2.0 test car.
Then you have the suspension to think about. Even the firmer ND MX-5s are soft and roll-happy little rascals from the factory, something which won’t gel well with an 82bhp power injection. The lowering springs on the 1.5 and 2.0 weigh in at a reasonable £195 if you’re fitting them yourself, while the Ohlins coilovers - as fitted on the 2.0 test car - are a wallet-busting £1635.
Using a 2.0-litre ND with the Sport chassis as a base might be the way to go, as the standard-fit Bilstein dampers should work quite nicely teamed up with BBR’s lower, stiffer springs. But even then, that’s nearly £6582 for the kit, the springs and the six/four-pot stoppers, plus fitting.
With the earliest 2.0-litre NDs now going for under £15,000 though, you’d be a little over £20,000 out of pocket having created the best all-round sports car I’ve ever driven. That’s how good a 242bhp Mazda MX-5 is - in comparison, something like a Porsche 718 Boxster feels over-powered, over-tyred and just too serious.
What’s interesting is a sizeable chunk of BBR’s customers aren’t interested in waiting for depreciation: many are happy to send their cars in for the turbo kit before they’ve even got behind the wheel. We totally get why.