Interesting extract below from an AROnline 'Mini Development in the 1970's' article obout BL's research into the OHC A-Series engine:-
"In April 1982 the Mini was given higher gearing to make it more economical, but this did nothing for acceleration, and the car lost a little of its buzzbox characteristics. The fact that the ancient A-Series engine could cope with up-gearing was a tribute to its remarkable torque characteristics, even in its tamest form.
And that brings us to the subject of why the A-Series engine was never radically re-engineered to modern standards. Was it a matter of cost or were there other reasons?
Back in the 1970s BL had embarked on projects to develop overhead camshaft versions of both the A- and B-Series engines. The reason for this was that MG used both engines and as they exported to the USA where stringent anti-emissions legislation was in force, it was paramount that BL’s existing engines were made more efficient to enable them to remain both legal and competitive by using the more efficient emissions friendly overhead cam layout.
In the end the OHC B-Series evolved into the O-Series engine first seen in 1978, while converting the A-Series to the overhead cam layout proved to be a technical cul-de-sac. The immediate problem of continuing to sell the MG Midget in the USA was solved by replacing the 1275cc A-Series engine with the 1493cc Triumph engine also used in the rival Spitfire. But an OHC A-Series could be used in the ADO88 and other BL cars. In the event eleven prototype overhead cam A-Series engines were built in 1975 using Cooper S blocks in capacities of 970cc, 1097cc and 1275cc. The engines used aluminium cylinder heads.
The engineers obtained the following performance figures for OHC Minis:
970cc: OHC 59bhp @ 6750rpm, 51lb ft @ 5250rpm
1097cc: OHC 72bhp @ 6500rpm, 64lb ft @ 5000rpm
1275cc: OHC 84bhp @ 6750rpm, 80lb ft @ 4500rpm
The above figures look very impressive, and a bluff-fronted Mini Clubman fitted with the OHC 1275cc engine and twin HS6 SU carburettors could reach 100mph. It would be easy to accuse BL of another missed opportunity. But a look at the comparative figures for the existing overhead valve engines reveal another story. For a direct comparison first we will use the three Cooper S engines and then the standard tune single carburettor engines seen in the mainstream production Minis.
Standard OHV Mini engines:
970cc: OHV 65bhp @ 6500rpm, 55lb ft @ 3500rpm
1071cc: OHV 70bhp @ 6000rpm, 62lb ft @ 4500rpm
1275cc: OHV 76bhp @ 5800rpm, 80lb ft @ 3000rpm
998cc: OHV 38bhp @ 5250rpm, 52lb ft @ 2700rpm
1098cc: OHV 45bhp @ 5250rpm, 56lb ft @ 2700rpm
1275cc: OHV 54bhp @ 5250rpm, 67lb ft @ 2500rpm
Converting the A-Series engine to overhead camshaft simply pushed the peak torque further up the rev range and actually caused more problems than it solved. One of the great virtues of driving a Mini is its remarkable ability to pull top gear at low revs, something that was exploited by Austin Rover and later Rover when the car was up geared.
An overhead cam A-Series engine would have been torque shy where it mattered in everyday driving and that was indeed the case with the 100mph OHC Mini Clubman. The modern solution for the lack of torque where it matters would be to add more gear ratios, but BL’s engineers decided that a reliable revised transmission in sump gearbox was not feasible. Therefore the decision was taken to opt for the A+ programme. BL had already burnt its fingers with the OHC E-Series engine. The E-Series had been intended to supplant the B-Series, but its lack of torque resulted in the old B-Series going into the Marina and Princess, and even the O-Series was a disappointment. BL would not make the same mistake again."
http://www.aronline.co.uk/blogs/cars/mi ... ry-part-2/
