BMW E21 3-Series with Mazda 231bhp rotary engine


E21 Beemer with Mazda RX-8 rotary engine. Home built on the cheap


Contrary to the laws of car improvement, you don’t have to replace your small engine with a larger one to gain a performance advantage. Sometimes you just need to think of a different type of powerplant. Dan Hargrave bought this 1981 BMW 316 back in 2013, ‘I was commuting a short distance to work, didn’t need a daily driver, so fancied an interesting car on a budget of £1500,’ explains Dan. ‘I was really after an E30 BMW but then saw this E21 on the Retro Rides website for £2000. I phoned but the seller was firm on his price. Sometime later I was visiting friends in London and out of the blue the guy called offering me the car for £1200 but I had to collect it that day.’ Result.


The 316s had a 90bhp, 1.6-litre engine until 1980 when, although they were still badged 316, the cars were actually fitted with 89bhp, 1.8-litre engines. ‘I drove it about for a year and tried to get a bit more power out of it with a swap to a Weber carb. Then I took it off the road for winter and dropped in a 140bhp 1.8-litre M42 engine from a 1997 BMW E36. That was more fun, but it was still quite tame. The following winter it came off the road for Polybushes and I decided that January it needed a different engine.’ Dan didn’t fancy going any tried and tested routes though.

‘The 2.5-litre BMW six-cylinder conversions have been done to death on the E21 and once fitted there’s virtually no room under the bonnet and about an inch of space around the radiator.’ The switch to a rotary powerplant came about because Dan and his father John had decided to retire their track car ‘54 plate Mazda RX8 in favour of a MINI Cooper. ‘We’d had the RX8 for three years and couldn’t find anyone to buy it, even for £600. So I paid my dad half the Mazda’s value and started to strip it.


‘People ask how hard it was to swap in a 1308cc, 13B-MSP rotary engine and I usually say it was pretty straightforward,’ admits Dan. ‘But since I’ve thought about it all again for this feature I’ve remembered a lot of little things I had to work out.’ The first of those things being engine mounts. ‘The RX8 block was located in the Mazda using mounting arms, one worked fine in the BMW but the offside mount fouled the exhaust manifold, so I had to custom-build one. The Mazda gearbox mount is a beam style that connects to the rear diff which has flexible mounting bushes, I couldn’t make that fit the BMW floorpan when I offered it up, so I’ve solid-mounted the RX8 six-speed gearbox.’


‘Then the Mazda sump fouled the subframe,’ says Dan, ‘the sump was about two-inches deep so I cut it in half, deepened the front to four-inches and made the rear half sit flush against the frame. It’s the same volume but there’s only about three-inches of clearance under it. I had to shave a little off the transmission tunnel to clear the clutch slave cylinder too.’

The RX8 exhaust manifold touched the BMW’s steering shaft so Dan moved the shaft across 20mm. ‘Then I wound one trackrod end in 10mm and the other out 10mm, it’s now got loads more lock in one direction than the other… No – it’s actually not that bad,’ he laughs. ‘I used the RX8 electrics, put in the entire engine and interior looms with the Mazda fusebox, then added a little of the BMW’s wiring to it for items such as headlights. I removed the Mazda air conditioning and power steering – it was tempting to keep them, but at that point I’d been working on the car for months and really just wanted to get it on the road.


‘I added D585 coil packs, K&N filter and Magnecor ignition leads. Mazda optimistically said it had 231bhp but, in truth, you were lucky to get 200. I put the BMW onto a rolling road and it measured 175bhp. Probably because the exhaust is only a two-inch diameter custom system built before I did the rotary conversion.’ The pipe was custom-made by Jack Tonkin (see J’s Garage on Facebook.) ‘The standard Mazda pipe was 2.5-inches so it’s now likely strangling it a little. There’s a modest silencer and back box, although no catalytic converter, making it a bit noisy but not overly loud.’

The fuel pressure regulator feeds to an aftermarket Bosch 044 fuel pump since the standard Mazda unit sat in the fuel tank – and that was thrown away. ‘I suspect the fuel pressure regulator is also zapping power; it was a £15 no-name one from eBay and the pressure gauge flickers around so the engine is either getting starved or over-fuelled. In the near future I’ll get it mapped to adjust the ignition timing for the D585 coil packs. There’s room for improvement since I built the car with parts I had – not necessarily the best bits for it. I might rebuild the engine sometime, it’s always been in decent condition but it’s done over 80,000 miles now and bits are pretty expensive – I’m looking at over £1000 just in service parts.’

The radiator is from a BMW E36 since it had a useful side-mounted expansion tank. ‘There’s only a single oil cooler; the RX8 had two but the BMW has far more airflow around the front and so far it’s managed fine with just one. I made the coolant and clutch pipes as I went along.’


The RX8 gauges went into the stock dashboard on slightly trimmed mounts, ‘I just wedged it in there,’ admits Dan, ‘the fuel gauge fits into a plug on the left hand side, it sort of dangles so I usually only connect it when I need to check the fuel level. The BMW tank is smaller than the Mazda’s so I just fill it up every time I go out.’ Those Recaro seats probably came from an Escort or Capri, ‘they’re the only thing in there that isn’t a weird dark green colour. I could reupholster them to match the rest of car but then they’d be unsellable later.’ The steering wheel is a Momo and the gas mask is there because for some time it was over-fuelling and there was a strong petrol smell. ‘It does freak people out if you pull up next to them wearing the mask…’


The Mazda’s fly by wire accelerator pedal was used, as was the RX8 gearknob. ‘The gearstick came out about 50mm further forward. I made up a propshaft by cutting the Mazda and BMW ones in half and having a friend – Patrick Banks who can TIG weld anything – put them together. I’m young and had no idea what I was doing in that respect. Once on the road I had a vibration around 40mph, I checked all the mounts, wheels and so on before I thought to get the propshaft checked by an expert. ‘He phoned and I thought he was ringing to say it was ready to collect,’ laughs Dan. ‘Instead he said, “Who the hell built this propshaft?” I learned then that a universal joint of some sort is vital to prevent driveline vibration, so the prop went back to Patrick who cut it in two and welded up some flanges to accommodate a rubber donut bush from an E36 BMW.’


Dan had better luck with his suspension designs. ‘It’s got camber adjustable top suspension mounts. The front shock bodies are cut down by about three-inches, I cut off the spring seats and used Bilstein B6 BMW E30 shocks with coilover sleeves welded over the top. The rears are Gaz gold universal coilovers with rose joint bottom mounts.’ The springs are 300lb front and 350lb rear. ‘My wife Emma would probably disagree but it’s a comfortable drive – or at least it’s not uncomfortable for something this low. It’s probably three-inches lower than standard, I’d like to go further just for the looks; it still looks high to me, but that front spoiler takes a hell of a beating – the ride height is a decent compromise for the overall driving experience.


‘There’s probably room to fit nine-inch wide wheels on the rear although the rear arches were pretty much made of filler so I don’t know if I could hack them about. The wheels are currently 15×7, I think they’re the Melber alloy wheels that were optional on the E21 and E30s; they’re similar to the BBS Mahles.’ The front brakes are E21 323i discs and calipers – which naturally bolt straight on – with EBC Greenstuff pads and standard rear drums. ‘For a road car, they do the job. If I ever took it on the track they’d need an upgrade.’

Out back the standard differential is holding up fine, ‘with the skinny tyres it breaks traction very easily, it’s scary pulling away quickly on anything other than a wide, flat road. To be honest I’m kicking myself for not pulling the complete rear end out of the RX8 before I scrapped the remains of the shell. It would have been a lot of work to mount it in the E21 though. I’ve looked into fitting a BMW limited slip diff but can’t find any; I’m after the small case LSD from a BMW E36 if anyone out there has one…?’

Small changes include retro chrome mirrors with the standard door mounts blocked off, the addition of keyless central locking – ‘I was fed up of scratching the doors’ – and a ‘how to kill’ sticker on the inside of the door. ‘That doesn’t really relate to anything except I thought it was funny since I thought the car was probably most likely going to try to kill me…’

The tailights are now completely red, ‘a coat of green glass paint lets the red indicators continue to shine orange.’ The screen washer bottle and battery were moved into the boot and Dan performed a conversion to a single screen wiper. ‘It’s a bit of a bodge, I extended the radius of the arm that pivots on the wiper motor to enlarge the throw using a Peugeot 306 fuel pump throttle arm. Then removed the driver’s side wiper and fitted a slightly larger arm on the passenger side. Like most of the car I just gave it a crack to see if it worked.’

Most impressive is that this BMW is Dan’s first real build. ‘In the past I did the normal things like swapping wheels and lowering my cars. I had a turbo diesel Peugeot 205 that I cranked up the turbo on, actually my first time on a track was with the 205 at the PPC 2011 event at Mallory – where it suffered fuel problems and broke down near the hairpin…’

Was building the BMW worth the effort? ‘Definitely. You can feel how much lighter the front end is now and the power delivery is even better than the RX8. The Mazda was about 1200kg compared to the E21’s weight of 950kg. The only thing I’d change would be the amount of time it took to build it since I was out in the garage for about four months. I’m totally happy with though. The only downside might be that it used to get 28mpg, but now returns about 15mpg – but that’s when I’m taking it for a blast. There’s no point in driving it conservatively, is there?’

Thanks to my dad John for his help and advice, and to my wife Emma and sons Cody and Jesse for putting up with all my hours working outside in the garage.


This is Dan’s first build and he’s rightly proud of it. Mazda RX8 engine in BMW E21 3-Series. Mazda ECU still runs everything. Mazda loom spliced into BMW. Mazda gearbox. Green interior is retro-funky. ‘I’d like to lower it further, but this is a good compromise.’ Shark-nosed mini Beemer has extra negative camber. BMW 316 rad keeps the rotary cool. Mazda gauges squeezed into BMW pod. Seats are Capri 2.8i, we think. US spec tail lights clean up the rear view.

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