Posted: Thu Nov 20, 2008 3:06 pm
Making some assumptions - road use, maybe the odd track day, reliability, usability and fun
Engine - 1275 Sprite/Midget, or Marina/Ital
Bore - either +0.060" to give 1330cc, or offset bore 73.5mm to give 1380cc - any more and head gasket reliability can be a problem.
Head - 11-stud 12G940 with later Cooper S valve sizes, which are 1.4" inlet (~36mm), 1 5/32" exhaust (~29mm). Any larger and the head is very prone to cracking between the valve seats, and cannot be converted to unleaded. Obviously properly flowed, ported and 3-angle valve seats. CR minimum of 10:1, and preferably 10.5, converted to run with super unleaded fuel.
Cam - either Kent 266 for torque by the bucket full, or 276 for a bit more top end power, and still very driveable. More than a 276 may be great for a while, but for day-to-day??
Rockers - 1.5 ratio roller tipped - claimed to be worth going up 1 cam.
Carb - single SU HIF6 or HIF44 on flowed/matched Titan manifold, connect the water heating for easy 'warm-up' driving. Filter K&N with short stub stack.
Ignition - Aldon 'Yellow' distributor with vacuum (100AYV), Aldon Ignitor electronic trigger pack, high output coil e.g. Lucas 'Sports' and best quality silcone plug leads, e.g Magnecor.
Exhaust - large bore LCB (1 3/4" bore minimum) and enough straight-through silencers to give you a blend of 'nice noise', but not tiring on a long journey.
Crankshaft - ideally the EN40B version, but quite rare so more likely the EN16T, then tuftrided and balanced. Centre main strap.
Clutch - standard Spridget 1275 cover plate and driven plate to suit the gearbox being used.
Miscellaneous - full crank, rods, flywheel balance, maybe lightened flywheel (no more power, but better 'pick-up'), duplex timing with vernier adjustment (maybe belt, but no more power).
Result?
With 266 cam and everything else as above but the K&N filter/stub stack (used the MG Metro airbox cleaned up, and fitted with a K&N element) gave 100 bhp at 5,400 rpm with a very flat torque curve of >=100 lbf. ft. from 2,500 to 5,000 rpm.
With 276 and all above, 112 bhp at 5,800 rpm and maximum torque of 108 lbf. ft. Changing the air filter from the MG airbox with K&N element to a round K&N filter with short stub stack, increased the power from 104 bhp to 112 bhp! Same day same RR.
Fitting a Weber DCOE45 added 4 bhp, but the much reduced driveability and increased noise made it quite wearing on long journeys, so I put the SU back.
This is the spec of the 1380 Midget engine fitted in our Minor for the last 30,000+ miles. Is it the ultimate? Well it meets all of the 'assumptions' in spades!!!
Engine - 1275 Sprite/Midget, or Marina/Ital
Bore - either +0.060" to give 1330cc, or offset bore 73.5mm to give 1380cc - any more and head gasket reliability can be a problem.
Head - 11-stud 12G940 with later Cooper S valve sizes, which are 1.4" inlet (~36mm), 1 5/32" exhaust (~29mm). Any larger and the head is very prone to cracking between the valve seats, and cannot be converted to unleaded. Obviously properly flowed, ported and 3-angle valve seats. CR minimum of 10:1, and preferably 10.5, converted to run with super unleaded fuel.
Cam - either Kent 266 for torque by the bucket full, or 276 for a bit more top end power, and still very driveable. More than a 276 may be great for a while, but for day-to-day??
Rockers - 1.5 ratio roller tipped - claimed to be worth going up 1 cam.
Carb - single SU HIF6 or HIF44 on flowed/matched Titan manifold, connect the water heating for easy 'warm-up' driving. Filter K&N with short stub stack.
Ignition - Aldon 'Yellow' distributor with vacuum (100AYV), Aldon Ignitor electronic trigger pack, high output coil e.g. Lucas 'Sports' and best quality silcone plug leads, e.g Magnecor.
Exhaust - large bore LCB (1 3/4" bore minimum) and enough straight-through silencers to give you a blend of 'nice noise', but not tiring on a long journey.
Crankshaft - ideally the EN40B version, but quite rare so more likely the EN16T, then tuftrided and balanced. Centre main strap.
Clutch - standard Spridget 1275 cover plate and driven plate to suit the gearbox being used.
Miscellaneous - full crank, rods, flywheel balance, maybe lightened flywheel (no more power, but better 'pick-up'), duplex timing with vernier adjustment (maybe belt, but no more power).
Result?
With 266 cam and everything else as above but the K&N filter/stub stack (used the MG Metro airbox cleaned up, and fitted with a K&N element) gave 100 bhp at 5,400 rpm with a very flat torque curve of >=100 lbf. ft. from 2,500 to 5,000 rpm.
With 276 and all above, 112 bhp at 5,800 rpm and maximum torque of 108 lbf. ft. Changing the air filter from the MG airbox with K&N element to a round K&N filter with short stub stack, increased the power from 104 bhp to 112 bhp! Same day same RR.
Fitting a Weber DCOE45 added 4 bhp, but the much reduced driveability and increased noise made it quite wearing on long journeys, so I put the SU back.
This is the spec of the 1380 Midget engine fitted in our Minor for the last 30,000+ miles. Is it the ultimate? Well it meets all of the 'assumptions' in spades!!!