SPE­CIAL­ISTS Corten-miller

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Renowned for its mk3 su­per­charger con­ver­sion, the Lincs­based tuner has many other strings to its bow

Best known in MX-5 cir­cles for its 300bhp su­per­charger con­ver­sion for the mk3, Corten-miller puts great store in thor­ough­ness and ex­cep­tional qual­ity tun­ing work Brett Fraser

For a sec­ond or two, we’re caught off guard. From be­hind us comes a slightly sin­is­ter snarling and whoosh­ing sound, rem­i­nis­cent of a wild an­i­mal, and it star­tles us. There is, how­ever, noth­ing to worry about. It’s just Corten-miller’s mk3.5 demon­stra­tor be­ing backed into the work­shop by the fam­ily-owned com­pany’s Sales Direc­tor, Tom Corten­miller: the pri­mal sound­track is cour­tesy of the engine’s Rotrex C30-94 cen­trifu­gal de­sign su­per­charger hun­grily suck­ing in air and com­press­ing it, and while the Rotrex blower is one of the qui­etest avail­able, when it sneaks up on you the noise is quite men­ac­ing. Men­ac­ing in a good, strangely ex­cit­ing way.

This is the very car that we at To­tal MX-5 drove back in our Win­ter 2016/17 is­sue. Man­ag­ing Ed­i­tor Ben­nett, our man be­hind the wheel for that test, was much im­pressed by the Corten­miller su­per­charger con­ver­sion, which in the case of the demo car pro­duces 300bhp. Yes, it’s true, we’d all be pretty stoked to be driv­ing a mk3.5 with 300bhp un­der the bon­net, but it wasn’t just the 85% in­crease over the stan­dard power out­put that Ben­nett found so ap­peal­ing, it was the qual­ity of the in­stal­la­tion.

As he said at the time: ‘The big­gest com­pli­ment that can be given to a mod­i­fied car is to liken it to driv­ing a stan­dard fac­tory ma­chine. And that is ex­actly how the Corten-miller su­per­charged MX-5 feels.

There’s no lumpy idle or strange blips in the power de­liv­ery. The idle is rock steady and the clutch, de­spite be­ing stronger, is still easy on the leg. What is dif­fer­ent, how­ever, is the power de­liv­ery and how it’s de­liv­ered.’

To date Ben­nett has been the only one of us to visit Corten­miller, so we’ve de­cided to rec­tify that sit­u­a­tion with a daytrip out to the flat­lands of Lin­colnshire. Much like the coun­try­side of the Suf­folk/nor­folk border where sev­eral of the To­tal MX-5 team are based, CM’S Friskney head­quar­ters – north of Bos­ton and south of Skeg­ness, right out on the east coast – isn’t re­ally

on the way to any­where, so it has to be your fi­nal des­ti­na­tion and not a place you pop into on a jour­ney else­where. Which means you have to have a rock solid rep­u­ta­tion for qual­ity in or­der for non-lo­cal cus­tomers to make the ef­fort to come.

Amus­ingly, most sat­navs con­spire to make the fi­nal leg of the trip more tax­ing than it need be. Rather than tak­ing you along the ob­vi­ous route, the

A52 be­tween Bos­ton and Skeg­gers, you’re di­verted across the Fens and over nar­row roads sur­faced like a rum­pled du­vet. Only harder. And with big­ger pot-holes. In our some­what poorly sus­pended mk1 we have to weave around as though we’re steer­ing through in­vis­i­ble cones to miss the worst of the creases and crevasses, oc­ca­sion­ally emer­gency brak­ing for big­ger ob­sta­cles. Keeps us on our toes…

When we re­gale Tom Corten­miller with this tale he just laughs. ‘Most sat­navs seem to be­lieve that that’s the quick­est way here, even though there’s a main road very nearby,’ he tells us. ‘On the other hand, we’ve sold a lot of our sus­pen­sion kits by get­ting cus­tomers to nip back along those fen roads in one of our demo cars to com­pare against their own.’

The Corten-miller fam­ily has al­ways lived lo­cally, and the per­for­mance tun­ing busi­ness was set up by Tom’s fa­ther, John, in 1979. ‘Next year will be the 40th an­niver­sary of my fa­ther es­tab­lish­ing his work­shop on the same site that we’re still on,’ Tom ex­plains. ‘Back then he was pre­par­ing rally cars, plenty of mk2 Es­corts, and he was into high per­for­mance cars all his life. Sadly he passed away in 2004, but both my brother Ben and I in­her­ited his car-lov­ing gene. Ben has al­ways been in the car in­dus­try, and although I went away to univer­sity to study busi­ness in­for­ma­tion tech­nol­ogy, it wasn’t too long be­fore I was back here. Our mother, Lynette, works here, too, as man­ag­ing direc­tor.’

Although CM is known amongst en­thu­si­asts as a na­tional tun­ing out­fit, it’s also run as a lo­cal garage. ‘There are two el­e­ments of the busi­ness,’ re­veals Tom, ‘the tun­ing op­er­a­tions and the ser­vic­ing and MOT side. With the lat­ter we cover about a 30-mile ra­dius from here and it brings us in a good, steady flow of work.’

Sure enough, on the day of our visit there’s an el­derly Mit­subishi Shogun in for at­ten­tion, and a Porsche

Boxster up on one of CM’S three ramps that’s hav­ing a cool­ing is­sue sorted. But there’s also a very se­ri­ous-look­ing Honda Civic Type R race car grac­ing the work­shop, to­gether with an im­mac­u­lately re­stored and up­rated mid-en­gined Re­nault Turbo 2 rally car. ‘De­spite what it might look like to­day, we don’t re­ally do much mo­tor­sport prep,’ re­veals Tom.

‘How­ever, the Civic rep­re­sents the ab­so­lute peak of what we’re ca­pa­ble of here. We’ve taken it from a stan­dard EP3 Type R into a 500bhp fron­twheel drive beast that com­petes in the Civic VTEC Chal­lenge and have done ev­ery­thing our­selves bar the

paint­ing. We also pro­vide sup­port at the race meet­ings.’ Even if you’re not a great mo­tor­sport fan, spend a few min­utes ex­am­in­ing the stan­dard of work­man­ship on this car and you’ll have no hes­i­ta­tion in hand­ing over your own road car to CM for a few up­grades.

‘We’ll tackle pretty much any­thing from, say, a set of up­rated springs and dampers, to a full-on high per­for­mance con­ver­sion with an ex­treme and be­spoke spec­i­fi­ca­tion,’ says

Tom. ‘In sup­port of this we have our own ded­i­cated cylin­der head shop and engine build­ing fa­cil­ity, a Hunter sus­pen­sion align­ment ma­chine, and we also have, in its own pur­pose-built room, a Dyno Dy­nam­ics rolling

road with four-wheel drive ca­pa­bil­ity. And crit­i­cally, in this age of ad­vanced elec­tron­ics, we have Toby, our full-time ECU cal­i­bra­tion en­gi­neer.

‘We work mainly on Ger­man and Ja­panese mar­ques, and the lat­ter, of course, in­cludes the Mazda MX-5. That said, we’ve never been much in­volved with the mk1 or mk2 mod­els: be­cause of their age, you can start work­ing on one el­e­ment of them, the brakes, say, and then un­cover a whole other can of worms in the process. They’re won­der­ful lit­tle cars, don’t get me wrong, but they’re best left to the guys who spe­cialise in them. The way we pre­fer to work is to give our cus­tomers a fixed price to carry out a su­per­charger con­ver­sion on a mk3, for ex­am­ple, where we have a pretty good idea of how much work is in­volved – in­clud­ing in­stal­la­tion and VAT it’s £4995 for the Kit Pro 230250bhp con­ver­sion, £6195 for the Kit Club Sport 280–300bhp ver­sion. With the older cars you keep find­ing more prob­lems and the price keeps go­ing up: it can be up­set­ting for cus­tomers on a tight bud­get, and from our per­spec­tive can un­ex­pect­edly tie up one of the ramps, pre­vent­ing us from get­ting on with other jobs.’

But that’s only part of the rea­son that CM picked the mk3 for its su­per­charger con­ver­sion kit, as Tom ex­plains: ‘Early on we re­alised the po­ten­tial for ECU tun­ing, so when we were look­ing around for a new project we’d al­ways look at a car’s ECU and work back­wards from there. We wanted to ac­cess the ECU and tune in­side it, rather than de­velop a pig­gy­back sys­tem, and the mk3 MX-5 gave us that ca­pa­bil­ity di­rectly through its di­ag­nos­tic port: in some ways I still find it in­cred­i­ble that you can tune a car in this way.

‘And the rest of the mk3 is a great plat­form to work on. Com­pared with ear­lier MX-5S it’s a proper car that drives well and ben­e­fits from lit­tle niceties such as heated seats. Sure, it’s a bit over­weight, but it han­dles well and in stan­dard form sim­ply lacks enough power. In other words it is the per­fect base for our su­per­charger con­ver­sion.’

And why a su­per­charger, rather than the more pop­u­lar tur­bocharger? ‘Both types of forced in­duc­tion have their pros and cons, but we think that the su­per­charger’s very lin­ear power de­liv­ery bet­ter suits the char­ac­ter of the mk3.

‘And com­pared with a turbo, a su­per­charger has a very low ther­mal im­pact on the engine bay. We spent a long time de­vel­op­ing and test­ing our kit, mak­ing it a bolt-on in­stal­la­tion but to OEM stan­dards, and as a con­se­quence we’ve had no re­li­a­bil­ity is­sues.’

For a while, a few years back, mk3s were pil­lo­ried by some as be­ing too big, too porky, and in­suf­fi­ciently like an MX-5. Val­ues tum­bled. But Tom has wit­nessed a change in at­ti­tudes more re­cently. ‘Some cus­tomers may have bought their car quite cheaply but they’re pre­pared to in­vest in it quite heav­ily be­cause they now re­alise its po­ten­tial. We find that many peo­ple don’t sim­ply go for the su­per­charger kit, but they also up­grade the rest of the car, too – clutch, brakes, sus­pen­sion, ex­haust, wheels, tyres, etc.

‘And we can fit what­ever brands a cus­tomer wants. Although we’re a sup­plier, and some­times dis­trib­u­tor, for many of the ma­jor tun­ing com­pa­nies, we’re not tied in ex­clu­sively to any one of them, so can be flex­i­ble about spe­cific re­quests for com­po­nents.’

Tom sees plenty of po­ten­tial in mk3 tun­ing. ‘Although peo­ple are start­ing to re­alise that it’s a much bet­ter car than por­trayed in ear­lier re­ports, mk3s re­main very good value and gen­er­ally are still in good con­di­tion and healthy sup­ply.

‘Our su­per­charger kit – es­pe­cially when com­bined with some sen­si­ble up­grades of other com­po­nents – turns them into an awe­some lit­tle sports car. The mk4 is next on our radar.’

Corten-miller’s geo­graph­i­cal lo­ca­tion might not be con­ducive to a ca­sual visit, but Tom and the team’s pas­sion for top qual­ity tun­ing may well mean that after a bit of phone time with them, you could be jump­ing into your car to make a spe­cial trip.

Corten-miller’s 300bhp su­per­charged demo car poses in the Lin­colnshire work­shop along­side a high-tech Honda Civic racer and poorly Porsche Boxster.

Tom Corten-miller, sales direc­tor

In 2019 Corten-miller will cel­e­brate be­ing based at its cur­rent site for 40 years. As well as a tuner, the com­pany is a gen­eral garage and MOT cen­tre.

CM’S re­cep­tion area is com­fort­able and lined with temp­ta­tions…

Show­cased in the CM re­cep­tion area is the com­pany’s su­per­charger con­ver­sion for the mk3’s engine. In top tune it gives 300bhp, which is nice.

The Rotrex su­per­charger looks as though it might have been placed there by Mazda it­self.

CM likes to make its mark on the parts it fab­ri­cates.

Pol­ish­ing the pipe’s in­te­rior for swifter gas flow.

Good­ies rang­ing from gifts to, er, ‘es­sen­tials’…

CM not tied to any brands, but does stock good ones.

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