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The Plus 8 Morgan was quite a performance car when it was launched back in 1968, a fact that’s often overlooked these days.

The Plus 8 Morgan was quite a performance car when it was launched back in 1968, a fact that’s often overlooked these days. With the evergreen Rover V8 engine shoehorned under the bonnet, this retro roadster weighed less than 1000kg, making it a serious sports car that could match the very best for pace – 0-60mph in just over six seconds was terrifi c going back in ’68. It still is, 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, to show those MX-5s a thing or two!

Get One Now

Mechanically this American-derived V8 is almost burst-proof, so long as the oil has been changed regularly – annually, irrespective of mileage, we’d say – to avoid sludging, which will hinder and hurt the engine’s hydraulic tappets. The only other issue is in the retention of the cylinder heads;Buick engines had 18 retaining bolts for each head, whereas on ‘SD1’ motors of the late 1970s one outer row of four bolts is not used, and this creates uneven stress on one side. This does not necessarily cause the head gasket to blow, but allows unspent petrol and exhaust fumes to ‘weep’ into the centre of the Vee, and contaminate engine oil, resulting in bore wear and cam and crank bearing failures.

Amazingly, Rover did not finally solve the problem until 1994, 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 engine that Rover inherited from Buick was already old when it arrived on UK shores. Earliest versions had rope oil seals on the crankshaft and mains bearing caps that were apt to ‘float’ 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 – with neoprene oil seals and a stiffer alloy block with cross-bolted mains caps.

‘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.

Hotting One Up

The V8 has enjoyed a long and legendary service life and, even in standard tune, kicked out a 155bhp. If still in good nick, perhaps with a thorough decoke, electronic ignition and setting up on a rolling road, it may well still suffi ce.

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 235lb.ft 3.9-litre unit achieved by using 94mm bore. When the engine went to a 4.2-litre, the crank was changed to one with a 77mm throw and the 4.6 litre unit is obtained by further stroking to 82mm. 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 – but remember, Morgan offered a 220bhp 4.6-litre engine for two years in 1997.

Whilst upping capacity is sure way to up power, there’s plenty of scope for modifi cation of the heads. Careful attention to the shape the ports and matching to the appropriate manifolds is worthwhile, as is cleaning out the shallow dish-shaped combustion chambers. Valve guide bosses can also be shortened and smoothed to improve gas fl ow, and valve throats opened out. Standard 39.9mm inlet and 34.3mm exhaustvalves are fine for road tune engines, whilst 41.4mm inlet and 35.5mm exhaust are about as far as you can realistically go.

A 9.75:1 compression ratio is best for most purposes; with so little chamber in the cylinder head, this usually obtained by the correct combination of pistons and the type of gasket used, either tin or much thicker composite type. With valve seat inserts used in the alloy heads, damage due to the use of unleaded fuel is not a problem no matter how hard you drive.

A camshaft change should be pretty early on the list of modifi cations, as the engine benefi ts from better breathing. If you stay on hydraulic tappets (there’s not much point in swapping to solid lifters on a road engine) the engine is limited to just over 6000 rpm and so tuning for good mid-range torque is the best way to go. 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 engines.

Hand in hand with any camshaft change must come an overhaul of the ignition system. Most original distributors are likely to be well worn by now and so (as far as tuner RPi is concerned) the only way to go on an early (pre GEMS management system) engine is to fi t a Mallory distributor. This operates as a six-volt system (retaining 12 volt for starting) and so needs its own coil and ballast resistor. Having said that, many other companies, such as Aldon and H& H Ignition Systems, can provide a similar performance system.

Early SD1 engines were fitted 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 offeredin 1983 (Lucas L-Jetronic) are fairly crude on the electronic front and are not responsive to usual performance chipping. Really, the best course of action for anyone with a pre-GEMS engine looking for a decent increase in power is to fi t a fourbarrel Weber carb conversion, which sits neatly in the centre of the Vee. RPi’s Weber kit mates the carburettor to an Offenhauser inlet manifold. A Lucus fully electronic ignition was utilised on the 3.9-litre model, incidentally.

Other carburettor installations have been tried with the engine; notably BL fitted 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 a reasonable enough cost, the four barrel Weber can’t be beat still.

As an indication, 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 is available – which, given the performance of even a basic Plus 8, is plenty for road and mild competition use!