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Morgan Plus 8

Want a little bit more out of your classic for road or track work? Here’s our top tips – with the aid of marque experts! Published: 4th Jan 2013 - 0 Comments - Be the first, contribute now!

Morgan Plus 8
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Morgan’s Plus 8 was a real performance car when it was launched back in 1968. With that now evergreen Rover V8 engine shoehorned under the bonnet it made this retro roadster (which weighed less than 1000kg back then) a serious sports car that could match the very best for pace. It still can for that matter… But such is the tuning potential of this versatile V8 that many owners are tempted to give their cars a bit more zip for road and competition use. And why not, as it makes this great classic even better.


Mechanically this American-derived V8 is almost burst-proof so long as the oil has been changed regularly to avoid sludging, which will hinder and hurt the hydraulic tappets. It was already an old design when it arrived on UK shores. Earliest versions had rope oil seals on the crank and mains bearing caps that were apt to ‘fl oat’ and knock out the bearings. Rover’s development of the engine was somewhat reluctant, but by 1982 most of the major problems had been ironed out – including using neoprene oil seals and a stiffer alloy block.

‘Stiff’ blocks can be identifi ed by wider and more uniform strengthening webs in the centre of the Vee between the cylinder banks, and the cross-bolted type by the bolt heads low down along the sides of the block.

The only other issue is cylinder head retention; Buick engines had 18 retaining bolts for each head, whereas on ‘SD1’ motors, one outer row of four bolts is not used and naturally this creates uneven stress. This does not necessarily cause the gasket to blow, but allows unspent petrol and exhaust fumes to ‘weep’ into the centre of the Vee and contaminate engine oil, resulting in wear.

Rover did not fi nally solve the problem until the mid 90s would you believe, when the other row of four bolts was deleted and the heads were attached to the block by just 10 equally spaced bolts. A ‘fi x’ for 14-bolt heads is to torque down the centre two rows of fi ve and simply leave the outer four fi tted but un-tensioned and held in place by Loctite.

The rest of the car is sturdy. The stout ash frame can deteriorate but treated replacements are available as are the steel chassis it sits on. The quirky sliding pillar front suspension needs regular lubrication as do the track rod ends or the steering will be very heavy.


Even in standard tune the V8 kicked out a healthy 155bhp. If still in good nick, perhaps a thorough decoke and setting up on a rolling road, it may well still suffi ce for your needs. Post SD1, Rover upped the power by increasing its capacity. The original 3528cc engine has 89.5mm bore and 71mm crank throw, with the later torque-fi lled 235lbft 3.9-litre unit achieved by using a 94mm bore. When the engine went to a 4.2-litre, the crank was changed. The 94mm bore is as large as the can be safely obtained within the block because some of these engines have been known to crack behind the cylinder liners.

A lot of gain can be had from improving the heads. Valve guide bosses can also be shortened and smoothed. Standard 39.9mm inlet and 34.3mm exhaust valves are fi ne for road tune engines, whilst 41.4mm inlet and 35.5mm exhaust are about as far as you can go.

A 9.75:1 compression ratio is best for today’s fuels but thanks to valve seat inserts used in the alloy heads, damage due to the use of unleaded fuel is not a problem.

Better breathing means a camshaft change. Stay on hydraulic tappets for a road unit as it’s low rev torque that counts more. The timing gear and chain on early engines is likely to wear and warrants fi tting a Duplex set-up, although this is not practical on post 1994 powerplants.

Early SD1 engines were fi tted with either SU or Zenith-Stromberg carburettors, and with these new needles and some fi ne tuning on a rolling road can accommodate mild stages of tune, but the fi rst fuel injection systems offered in 1983 (Lucas L-Jetronic) are fairly crude and are not responsive to performance chipping. The best course of action for anyone with a pre-GEMS engine is to fi t a four-barrel Weber carb conversion, which sits neatly in the centre of the Vee. 

Other carburettor installations have been tried with the engine – notably BL fi tted both Pierburg fuel injection and four Weber DCOE carbs on the rally TR8’s – but for a good power hike, increased response, and simplicity of installation at reasonable cost, the trusty four barrel Weber can’t be beat.

Any signifi cant modifi cations should be complemented by an uprated ignition system; electronic if not fi tted but best of all twinned with a performance distributor.

So is it worth it? Well, a pair of modifi ed heads, fast road camshaft, four barrel Weber carb and some decent extractor exhaust ‘headers’ will see the basic 3.5-litre nudge the 200bhp mark; up it 3.9-litres and 250bhp and above is available – which given the performance of even a basic Plus 8, is plenty enough for road and mild competition use, don’t you think!


Peter Mullberry Fabrications is one of the top Morgan tuners that even the factory consults although he says that the majority of owners only alter their cars to gain a better ride. In the mid 80s Morgan switched to rack and pinion which was a great improvement over its sloppy predecessors; sadly converting a car is involved and only for the dedicated. Mullberry markets a steering bearing conversion costing £160 which helps, in terms of arm effort.

Unless you want to spend £8000 on Librands’ front suspension kit, you’re stuck with the sliding pillar design – but it can work effectively enough. Adjustable dampers are essential and Peter markets AVO types but with his own valves. As the Plus 8 rides high, lowering is the next step and front springs are £66 a pair.  Rears sag once leaf springs have worn. If new are needed, they’re £70 each. More signifi cant are worn dampers but many will have swapped them for telescopic units by now, to improve the handling. A standard conversion kit costs around £200.

Mullberry Fabrications has developed a fi ve-link coil design; too exotic for the road but for track work it’s another matter.

Brakes are barely adequate! Mintex F4R pads is the fi rst step. Tyres? The Morgan uses a rim that was also fi tted to the Rover P5B, Vauxhall Ventora FE and the Jensen Interceptor. A 205/60 is a good compromise for road use. For more details call Peter on 02476 397666 or click on



  • Decoke and set up on rolling road
  • Gas-fl owing cylinder heads
  • Larger engine (they are inexpensive)


  • Larger engine (over 4-litres available)
  • Racier camshaft
  • Carburettor and inlet manifold swap
  • Super/turbocharger installation
  • Five-speed transmission




  • Uprated, adjustable dampers
  • Telescopic damper conversion
  • Steering bearing conversion
  • Mintex or EBC brake pads


  • Rack and pinion conversion
  • Lowering the ride height
  • Dedicated adjustable AVO dampers
  • Mullberry front/rear suspension upgrades


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