At first we were very interested when we heard about Specialist Component’s A-series conversion. A piece of lateral thinking that utilises the cylinder head and throttle body fuel-injection from the four-cylinder BMW K motorcycle engine.
However when we heard Specialist Components’ twin-cam A-series gunning for the redline, our interest ballooned into obsessive curiosity. How could two halves of two completely disparate engines come together and not only work so well together, but look so right too…?
Before we delve into the ins and outs of the conversion though, a little bit about Specialist Components…
John Kimmins started his engineering career as an engine-mount, suspension bush and chassis designer in Wiltshire before moving to Lotus in 2000 as a chassis design engineer, where he worked on chassis systems for Elises and customer projects.
“Specialist Components was started in 2005 and it was based on Minis – making them lighter, faster and better – because I’ve always had Minis since I first started to drive and I’ve never had an engine which I haven’t taken to bits and tuned… The electronics side, is the other big side of Specialist Components’ business, such as: engine management systems for the trade to run engine configurations in petrol format, which is more Simon Hill’s (John’s partner in the firm) area of expertise.
“So that’s where the background came from and I got some good contacts at Lotus, which is where I met Simon, and we developed the A-series to its ultimate format in five-port form. We then stumbled across a problem with the five-port – ie the power you can get from it economically and reliably – and we started looking for an alternative,” says John.
In addition to supplying its own bespoke components, SC has a fully equipped race-car engineering workshop which allows them to analyse competition cars in order to optimise their performance. Plus the company also writes and sells its own engine management systems and provides a mapping service.
“The work we’ve done for the company in terms of billet components and electronic engineering, sort of fell into place when we looked at the BMW K twin-cam conversion (prior to this, Specialist Components concentrated on selling components such as high-flow billet ram pipes for Weber carburettors, billet H-beam con-rods, billet crankshafts, various engine steadies, rear trailing arms, handbrake quadrants, etc). Originally, I looked at the K to get my personal car up to a level beyond everyone else’s and then we’d offer the proven design to the public. The K conversion is now the core of SC’s Mini business.”
However, with the presence of a fully-built twin-cam A-series lurking nearby, we can no longer avoid the issue, so we get down to nitty-gritty…
The origins of the K conversion…? “A chap called Gregg Temkin from Seattle, did a conversion to an 850cc Mini back in 1989. He took an 8-valve cylinder head and placed it on the block, realising that the pistons weren’t too near the chambers – so he offset bored the 850 to 996cc to create a short-stroke engine. He then managed to get the cylinder head on the block and then devised a system to drive the two camshafts, before fitting two motorcycle carburettors on the front of it. So he had a really nice little engine in a really ratty looking old Mini 850 and I bet it went really rather well…”
However, whereas Timkin used carbs, SC has been far more cunning and made use of the motorcycle’s OE throttle-body fuel-injection system. “We always advocate that people use the standard BMW fuel-injection because it’s a very nicely made, reliable piece of kit and it’s effectively free. The size of the throttle bodies are just about right for a Mini engine too and there aren’t any restrictions,” says John. “We’ve also got various maps which we’ve done on engines and we’ve got our own ECU and wiring loom which is a matter of plugging in just two wires.”
The main work you have to do to the A-series is on the surface of the cylinder block. If you’re looking at the engine from the front, the front three cylinder head bolts remain in the same position for the BMW head. So if you have a normal Mini block at home, you can take all the cylinder head studs out apart from the front three and the centre, and slot the head on to position and the head then stays there.
You then have to block up all the remaining head stud holes, all the tappet holes go in the top of the block and all the water ways get blocked up. Then you mark the positions for the new cylinder head studs and the new waterways – it’s all a vertical operation, so someone with a nice sturdy pillar-drill could do the work at home. It isn’t that complicated, it just takes time.
The head is converted in a few ways. There are a couple of areas at the back which you have to weld up (where the oil drains would normally be) and also at the front of the head there is a small area you need to weld up where the chain-drive used to be.
The cylinder head will now sit on the block, but the next obstacle involves the pistons. When the A-series pistons reach the top of their travel, they will touch the valves, so you have to pocket the pistons to make sure that they clear. This is a little bit more complicated, but luckily even taking into account all of the chamber combinations you get with aftermarket Mini pistons and stock ones, you can get a compression ratio to work because the BMW K’s chamber volumes are roughly the same as the A-series.
“Next, you have to drive the double overhead camshafts, so we’ve developed a belt-drive system which is based on the most heavy-profile toothed belt you can get. We’ve got a pulley on the end of the crank, we use the old camshaft to drive the oil pump (which we now call the idler gear) and then we’ve got the two pulleys on the BMW head. Using one of our conversion plates and some cam adaptors, you then have a drive system set up for the cams giving a nice triangular belt-run on the front of the engine and for a belt tensioner, we use a Ford Pinto component because it just happens to fit a Mini backplate in the right place.
The interesting thing is that the A-series is one of the few engines to still go clockwise in rotation. The BMW runs anti-clockwise, so the tricky thing would be if the cam profiles were asymmetric because the valve timing events would be completely wrong – but luckily the BMW cams are completely symmetric so you can run them in both directions.
Thinking I’ve spotted a flaw in the plan, I ask John about what exhaust manifold they use because the bike one is useless… SC offers a stainless steel 4:2:1 manifold which follows the same route as an LCB allowing pretty much any aftermarket Mini exhaust to be hooked up to it. It’s even been designed to suite both 8-valve and 16-valve engines and to provide a good compromise.
If you think you might be interested, SC offers a £20 manual which follows the A-series transformation from OHV old favourite to Bavarian-crowned howler and you wouldn’t be surprised to hear that these are selling like hot baked goods.
Unfortunately, this does mean that BMW K engines are no longer found languishing on Ebay, popularity now means that these engines are being rabidly picked – particularly the 16-valve variants for around £350.
“The manual is a step-by-step guide and it includes lots of technical info on the different heads – the flow figures, cam types, injection and all the rest of it,” says John.
Okay so what next? If you’ve bought a manual and you want to go for it – what are the options? “We sell conversion kits which consist of all of the parts to get the K head onto the block and get the drive system sorted – and these retail for £535 plus VAT.
“You get a drilling template (for DIY block adaptation), billet-machined bottom cover for where your old drive system used to be in the Mini, a billet-machined top cover which makes the end of the BMW head work, cam adaptors, top bespoke camshaft pulleys, the idler gear (the old cam) pulley, crank pulley, timing belt, belt tensioner and billet ram pipes for the throttle bodies.”
Available in both 8-valve and 16-valve configurations, most of SC’s customers stipulate the sexier 16-valve engine which does produce more power but it also shifts the powerband further up the rev-range. Given that the 8-valve is a cheaper option, if you’re buying the head for a road car, do not immediately dismiss this route.
“An 8-valve with a 1293cc block with 10.5:1 CR – which is at the budget end of the build – will produce 108bhp with the torque spread from 3000-7500rpm. So it is like a stock production engine and it drives like a 15-1600cc car, but you can rev it too because the dynamics of the A-series become very smooth-running. This is due to all the combustion cycles being equal, whereas a five-port head does give the A-series an inherent roughness. A budget build with a 16-valve head, will produce similar power – but if you fit BMW motorcycle RS-spec cams, increase the capacity to 1380cc – you’ll be looking at 130bhp and it’s still very tractable and drivable.
The graph does reveal how impressive the SC TwinK is, but alas, we’re more interested in the sound and the glimpse of the fuel-air mixture.
SC ENGINE PRICE LIST (plus VAT and delivery)
- 130bhp 1380cc 16-valve twin-cam £4995.00
- 110bhp 1293cc 8-valve twin-cam £4695.00
- 150+bhp 1380cc 16-valve twin-cam £6995.00
K Twin Cam Conversion Packages (plus VAT and delivery)
- 16-valve Twin Cam Package including cylinder head £2698.00
- 16-valve Twin Cam Package excluding cylinder head £2399.00
- 8-valve Twin Cam Package including cylinder head £2498.00
- 8-valve Twin Cam Package excluding your cylinder head £2299.00
“Beyond that, if you start porting cylinder heads and fitting Sprint (effectively semi-competition) cams you can get up to 160bhp and the engine will rev around to 9000rpm. So there is a real broad spectrum of builds between a bog-standard 8-valve and a full-house 16-valve. Oh and yes, turbocharging is a possibility too…” says John.
North Field Mill