Putting a Rover V8 engine in a Mazda MX5

R. Carter xmas 99



The engine and associated drivetrain mods fit nicely in the standard bodyshell. There’s plenty of work required but it’s mostly within the scope of an amateur with a garage with a pit & a hoist who has reasonable general metalworking and welding skills. And access to lathe miller etc (model room at work?..)



To avoid punitive emissions legislation and the necessity for fuel injection and catalytic converters you must start with a pre 95 donor mx5 (or eunos). My plan to treat the thing as a kit car just did not wash with the DVLA. Best bet is to start with an already registered vehicle of the right age (eunos could well be better as it is more likely to be exempt from the emissions legislation than a standard mx5 in the 92 to 95 region ). Alternatively go for a later injection V8 engine – but I don’t know where you’ll put the cat(s).


Prepare the donor vehicle

Find buyers for engine & gearbox. And you’ll want to fit a different diff too – standard diff is <19mph/1000 revs; trust me this would be a nightmare with the V8 regardless of whether it can take the power. So flog the diff and PPF too. The radiator in a 1.6 is probably not big enough – I have the 1.8 rad & this appears not to be either. Keep the power steering pump (don’t uncouple any of it). Flog the aircon – there is not enough room for it. Flog the ECU. Before you flog the engine, relieve it of the engine temperature transducer, the rad fan thermoswitch and the oil pressure transducer. You’ll put these on the V8 so the instruments work correctly.


Slice off and patch two corners of the parcel shelf/ chassis in the engine bay.  Dig a hole in the front subframe & patch that to allow room for the starter motor. On the nearside the upper of the 3 holes will be used for the engine mount, on the offside a new mounting piece must be welded & bolted on to avoid the starter motor. Most of the bottom seam (flange) on the subframe must be cut off to make room for the sump & seam welded.


The radiator brackets must be modified so the water pump shaft isn’t IN the radiator and some judicious hammering will later be done on the offside transmission tunnel to avoid the starter motor “bump” on the bellhousing.


That’s all that was done to the shell. At the end of the project the front brake pipes were repositioned to avoid the exhaust under the front wheelarch.


The greasy bits

Get the engine of your choice. If I were starting again I’d go for a short (or long) engine from real steel or rpi-engineering  http://www.rpiv8.com Check your heads are suitable for lead free gas. Get the weber (carter/edelbrock) carb on just about any manifold. All SU or stromberg options are too high. Mate this to a borgwarner T5 gearbox via an auto bellhousing with a 12mm aluminium adapter plate. If you do your own adapter plate, turn up a jig to get it properly central, or you can buy one ready made from magnum engineering. You’ll need to fabricate a clutch release mechanism, the photos show what I did – you may decide you can improve on it. The T5 box off a RWD sierra cosworth has the same spline as the rover box so you can use a stock rover clutch & cover plate. I retained the cosworth thrust bearing & that end of the release arm welded to my own release arm with rose joints for pivots.  I used a metro slave cylinder. I made my own spigot bearing out of phosphor bronze with 15mm ID for the gearbox input shaft. This was not up to scratch. I changed it for a standard ford part when I replaced the gearbox.


The fan extension on the water pump shaft must be cut off as short as possible so that the internal shaft is basically flush (easier than it sounds). the threaded fan extension sticks out miles & would foul the radiator.


If you use a sierra diff then a shortened sierra propshaft suits nicely. It’s better to replace the rubber doughnut joint at the gearbox end with a hooke joint however because it doesn’t end up quite straight and the hooke joint is much smaller & less likely to catch something.


Use a  large type sierra diff with a high ratio (I chose 3.14:1 but the choice is endless) & get an LSD put in. Any transmission firm will do this. Fabricate a frame to bolt onto the diff to mate to the mazda mountings. Engineer two adaptor plates so that the mazda CV joints connect to the sierra diff outputs. I got shortened drive shafts (half shafts) made through WGT in Northwich & everything slotted together nicely. A small portion of the rear subframe needs to be sliced off and patched to avoid the very back bolts at the top of the diff. (note mine STILL clunks here). The rear subframe is held on by just 6 bolts so it’s easiest to take the lot off (leave the brakes attached to the shell). On a UK car fit a rear chassis brace – eunos’s come with one.


Other Parts to be Fabricated

With no PPF you have to provide a gearbox mounting and front diff mounting(s). I fabricated a triangulated box section subframe to go under the transmission tunnel. This bolts on to longer seatbelt anchor bolts at the back, front subframe mounting bolts at the front plus I drilled two holes for two more bolts in the middle on the line of the gearbox mounting. So the subframe takes differential torque reaction at the back – supports the gearbox in the middle (on a void bush) – acts as a chassis brace at the front – and massively torsionally stiffens the transmission tunnel. Handy. Plus it gives you something to tiewrap the wiring loom to now the PPF has gone.


Obviously I had to make engine mounts. I’ve done these out of bits of 3mm mildsteel plate and box section tubing. For the rubber I used the disc type cortina mounts with M10 studding pointing out of each side. I was a little concerned that the engine mounts were too close together to take serious torque (remember the diff ratio is 30% higher) so I’ve added a further void bush bolted to the back of one cylinder head  connected to some diagonal braces between the strut tops and the top middle of the transmission tunnel. The engine seems to be held very firmly. Fortunately V8s are inherently smooth and well balanced (assuming you don’t have the racing flat plane crank!)


Wiring up the engine

Drill & tap the blanking plug supplied with the recon oil pump so that the mazda oil pressure transducer screws in ( I think its M10x1). The mazda rad fan thermoswitch is nearly the right thread to go in the engine temperature transducer hole in the manifold. Just be a brute and screw the bugger in, wrong thread and all! Alternator wiring is easy. Engine temp sensor – drill & tap a hole for it at the back of the manifold near its rearmost bolt on the drivers side. The thread is M10x0.8 or something very strange like that. If you think its M12x1.25 you’ve made the same mistake as me and have the ECU temp sensor instead of the instrument one. Doh! You’ll need to put that plumbers PTFE tape on the threads because the surface is rough cast here.

I retained the V8 electronic ignition (although it is apparently not very good). I connected to the wires at the back of the connector to the transistor module for the juice for the sparks and the tacho connection. Oh yes – there is a pot in the tacho instrument that allows you to re- calibrate it for the V8 – it really does adjust that far.

Extend the reverse switch leads round to the reverse switch on the box.


I tried to engineer a “shunt regulator” system retaining the original fuel pump but this resulted in dreadful flooding and was ditched in favour or an external electric fuel pump with the original “retired” and a series pressure regulator under the bonnet.


Clutch Master Cylinder

The Mazda master cylinder is too small and fouls the rocker cover. I put in a 7/8inch bore lockheed master cylinder with separate reservoir & it fits & functions perfectly – I had to knock up an ally adapter plate to bolt it to and add to its clevis fork to mate correctly to the mazda pedal.



The heater pipes are soft flimsy brass – I got a plumber to solder in some 90 degree copper bends & I piped to these. Radiator top hose is a corrugated flexi thing. Radiator bottom hose – where do I start… I got an SDI bottom hose & cut this up for bits. There’s a 32mm copper pipe with no less than 3 90degree bends going from the water pump to near the radiator – the pictures describe it best. Actually turned out OK in the flesh.

I used a remote oil filter and an oil cooler from a fiat uno (£5 scrappie) in the “smile”. The oil filter is difficult to reach & I plan to reposition it on the nearside cylinder head at the front. I mounted 2 metro radiator fans (£5 scrappie each) on an ally framework ahead of the radiator. I had to reverse the fan blades to “push” I wired the 2 fans to the original wiring – I guess that as I had aircon the fusing has coped OK


Now the warmer weather has arrived it's clear I have to improve the cooling. At present I'm thinking of a big aluminium "race" radiator from dealer alternative & some bonnet vents (probably off an escort cosworth or see what's in the scrapyards).



If you need cats – you’re on your own mate! I spent 1600 quid at JP exhausts & they earned it! I’ve taken 4 pipes to the back where they join in one big box & a nice fat chimney. The 4 pipes are for cylinders 1+5 3+7 2+4 and 6+8. These pairings are all firing on a 3/5 beat so I should get reasonable extraction on top overlap. As far as I know this has not been done before. The pipes go to the back just inside the sills so 4 thin pipes make more sense than 2 fat ones in speed bump land. The headers on the drivers side have to avoid starter motor & steering shaft. Helluva job



Adjustable ride height is handy as weights have changed. Watch out for lowering too much as things hang down more underneath now – the sump, the subframe under the gearbox and the exhaust pipes behind the sills. I used adjustable spax and these have stayed resolutely at maximum height fore and aft!

Front anti roll bar tries to go through front of engine – I modified an opel manta bar and modified some rover drop links and made some extra links (with yet more void bushes) & that all seems to work quite nicely. Note that the manta bar is 1mm greater diameter (thus about 15% stronger) than standard. With the lower engine this equates to very flat cornering and should make the car understeer more – time will tell.


I finally scrapped the spax springs and replaced them with stock springs but with the spring seats lowered about 3/4 inch. This seems to be an excellent compromise & I can now get over speed bumps!



You have wrong gearbox & diff & the wrong cable coming up from the gearbox. None of your standard kludges work (new dial faces -–2nd hand speedo) I made a wee gearbox from RS components bits & silver soldered the sierra cable and the mazda cable to the gear shafts. The 18/25 ratio gave PERFECT results on the SVA speedo accuracy tests retaining the kph speedo but now reading in mph. Actually the speedo has km/h written on and it is a VERY simple operation to black out the k!


Carb controls

Use the throttle cable that comes with the carb. I made a little aluminium cable clamp that goes in the hole at the top of the pedal.

I put the choke control in the innermost spare switch slot near the ignition keyhole. Choke cable also goes through speedo cable rubber bungy thing. Note this bung comes out easily if you squeeze across the rectangular bobbles on it (I broke mine…)


Legal niceties & insurance

If you modify a car which has previously been registered in UK you only need to tell your insurers. Privilege seemed quite happy with a 100% power increase & I’m paying them £500 plus for fully comp. You will need to get MOT’d in the normal way. There is no engineering appraisal to see if your vehicle is a danger to yourself or others.



My cooling is insufficient. My gearbox was knackered – I didn’t know that when I bought it.  My differential clonks into the rear subframe on torque reversal.  I’ll fix these eventually.  I’m looking into bigger front brakes too. My starter motor gave me trouble - turned out to be a badly plugged plug on the ignition switch.



I don’t like to think about this too carefully… but;

Starting with a MX5 – you’ll get up to a grand or so for the bits you don’t want. A fully recon’ed V8 will set you back over 2grand depending on spec. Gearbox say £300 differential the same. Driveshafts around £325 prop £150 (scrapyard + engineering). Clutch master cylinder £100. I don’t know what value to put on the bits I made myself. I’ve spent 8 to 9 grand & put in a lot of hours. Anyone who does the same has the benefit of the certain knowledge that IT CAN BE DONE and that it actually works rather well.


So, what’s it like mister?

So far, excellent. Handling appears to be exemplary. I seem to get just over 20mpg. The standard rover V8 is about 150 bhp. With my cam, carb & exhaust I have 220bhp. The rover block can be bored and stroked to 5.2 litres (latest range rover) and over 350bhp. The V8 is very smooth & pulls reasonably from under 1000 revs so it’s easy to potter. When you want to hurtle 2nd catapults you to 70odd & if you want 3rd does it again up to 100! You get the wonderful TVR noise (only a V8 can sound like this). At 4krevs+ it snarls in a most agreeable way. A pukka gearbox improved things vastly, the T5 box is notchy but precise & feels good with a very short lever. I’m impressed by how solid and unflappable it feels. It actually feels tighter (less sloppy) than a standard mx5, I put this down to slightly lowered suspension, lowered centre of gravity and a slightly higher rate anti- roll bar at the front. I’ve had a go in the TVR Griffith 500 & that is such a frightening machine; by comparison this is easy to drive & altogether a pussycat (nowhere near as fast mind)


What would I do different if I were doing it again?

Well I would actually go for a larger engine option! To get 200+ bhp out of the 3.5litre it ends up quite peaky, a 4.3 with a more or less standard cam would give the same power but you wouldn’t have to gun it as much so it would be more responsive in real life situations & probably use less juice. The chassis copes easily with a 100% power hike & asks for more (apart from the front brakes that is).


Ongoing jobs

The cooling has to be fixed and the differential clonks, then the motor should be OK for a while till I decide exactly how I'm going to beef up the front brakes.


Update 12 sept 2000

Recently spent £400 on an aluminuim race radiator from dealer alternative. Excellent  quality piece of kit, no cooling problems now. Mind you the old 1.8 radiator was ok but for some reason it clogged up & large sections stopped working. Anyone else doing this job need not assume the expensive radiator is necessary. Prior to buying the big rad I sorted all the undertray and added some stonking bonnet vents.

My brand new alternator died & I couldn't find the bloody receipt - so £40 on an exchange mitsubishi unit which is smaller but allegedly higher output.

The 2 metro rad fans finally tatered the 10A relay in the fusebox. This is largely due to the continuous operation of said fans. I got the car going again by swapping with the horn relay (passenger wing) and since then I've replaced the thermostat with a 82C one (original is 88C). Now "ram" airflow is sufficient and the rad fans only come on in traffic jams - as it should be.

Today 10000 mile service (oil, filter & sparkplugs). Car's going fine but I'll need a beefier battery before winter (one failure to start last week). This may be down to running with a dodgy alternator for a while, or possibly one should expect difficulty for a standard mx5 battery cranking a 3.5ltr V8!

So 10000 miles with some fairly minor trouble, not bad for a big project like this. My diff still clunks, I think I'll have to get it looked at as there seems to be way too much backlash in the diff anyway & the car kangaroos a bit as a result.


Update 12 Nov 2000

Recently the sump got holed by a lump in the road. This was always going to happen & I got WGT in Northwich to sort it out. They shortened the sump and the oil pickup pipe by a couple of centimetres and while they were under it the put some packing in to stop the differential from wobbling.

A new battery was fitted a wee while ago. This is 50% bigger than the outgoing one and cranks the engine like billyo - should be okay for the winter now. I've had a couple of stalling episodes which may be down to an HT fault (only happened in diabolical rain) so I may have some new leads to buy.


Update May 2004

Lots of miles (30k) since last update! The 3.5l engine valve guides were shot - plenty blue smoke - so I got another "sensible" car and put a bigger engine in the mazda; a 4.6 short engine and heads from rpi. This was done over christmas 2002 & the car run in. Then I rebuilt the spare T5 with a 0.63:1 5th gear & new synchro everywhere and new bearings, and got a TVR bellhousing. WGT put this in for me - £1200notes, bloody hell - I certainly can't afford to let garages work on this car. Furthermore a vibration which was what I really wanted them to sort out is still there as bad as ever. Anyway used the car a bit over summer, took it to Nobles motorsport in chesterfield to an mx5 dyno day (210 horses with 230lbft torque (210+ from 1500 to 5000rpm)) and a couple of days later the 5th gear broke; synchro stopped working & it didn't like to come out of gear. 

So in November i took it off the road while I sorted it out. Now the nice weather is here I've got my finger out to mend it: The gearbox is out at Glebe transmissions to be fixed, the flywheel is in my kitchen balancing on a spike - it's out of balance to the tune of 1.25oz at the rim - and I bought a "fully balanced with cover plate" lightened flywheel. You can see where it's been drilled to balance it - you can also see it's still a country mile out. I'll try moving the cover plate to the other 2 possible locations then I'll get my black& decker out......

Furthermore, once under the car it's clear that the starter motor bulge on the back of the bellhousing has been touching the transmission tunnel, with witness marks on both. So I've sawn 4mm of aluminium off the bellhousing - now it's clear. This "touching" alone completely explains the vibration. I should be back on the road in a nice smooth car next week!

The other job is to build inlet manifolds for the 4 weber 44IDF carbs I bought. I have steel plates made for the cylinder head and carb mating surfaces and a stack of mild steel "weldable elbows" to make the pipes. But it's a bit of a bugger of a job & may take ages to finish. But I've just got to swap out that offenhauser manifold.....


Update May 2005

Summer again so the '5' is back on the road. The vibration's been much better since I balanced the flywheel. Glebe didn't fix the 'box so in spring I pulled it out again to have a look myself. And the problem was glaringly obvious as soon as you pulled 5th gear off the layshaft - the WC version of the box must have an extended boss on 5th gear, the NWC one doesn't, so 5th axial position was being fixed by rubbing against the layshaft bearing cover plate bolts. So I turned up a spacer out of mild, reassembled with a new synchro assembly & hey presto - a perfect 5th gear.

I've finished the manifolds for the webers (here's a photo of the left bank) The system all looks really good, but I think it will hit the bonnet when an airbox & rampipes are put on. Whatever - I'll get it going without to start with - the box is only needed so a filter can be engineered. I'll mount the filter behind the left popup headlamp using the FM2 kit.

The car is starting to look a bit battle-scarred so if the bonnet does need modding that will be a good time to get the front & back bumpers re-sprayed, which should make it look new again - the rest of the car is in excellent nick. 8th June carbs & manifold fitted & 1st run out - seemed pretty good but definitely needs tuning - under bonnet picture below.


And Finally

At the end of 2005 I sold the car to a new owner in the south of England. I needed room for the next project. I got the front and rear bumpers resprayed and a couple of minor knocks tidied up in the front wings. The carbs were set up professionally so it sounded and went very well, but did seem to use quite a lot of juice! I've seen an album of photos on the net (titled 'Andy's V8 mx5' or something like that) with the car looking great, taken soon after the sale from the look of them. I know Andy was thinking about fuel injecting the car: good luck to him, I hope he enjoys it as much as I did.


 leftbank.jpg (106855 bytes)newcarbs.jpg (189192 bytes)



 TheBeast.jpg (101123 bytes)engine1.jpg (121968 bytes)engine2.jpg (125621 bytes)plumbing.jpg (105722 bytes)backend.jpg (94754 bytes)Underneath.JPG (87590 bytes)inside.jpg (92434 bytes)



Sorry about the lo- fi, I recorded these with a psion 3c flung in the passenger seat. One is me tootling down the road (2-3 gear change at 65mph) the other is the thing starting.




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