Speedwell are better known for their extensive conversions to B.M.C. cars, such as the 110-m.p.h. Riley Elf we tried a few months ago, but the bulk of their business is in milder conversions, giving that little bit extra steam to the slower cars in the B.M.C. range. A popular conversion for the normal 850 Mini is one which brings the performance up to Cooper Mini standards, and we have recently been trying a de luxe Mini, perfectly standard except for the addition of the Speedwell Grand Touring engine kit. This kit consists of the Speedwell alloy cylinder head plus a twin carburetter layout and a Speedwell twin-pipe silencer. The alloy head, the patterns for which had to be made in Italy, is sold in two forms, the GT for the less powerful B.M.C. engines and the Clubman for the more powerful units. The GT head which was fitted to our test car uses standard-sized valves and single springs, and has a 10.5-to-1 compression-ratio, whereas the Clubman has larger valves, a different combustion chamber shape and a compression ratio of 11 to 1, which can be raised to 13 to 1 for racing purposes.
The three carburetter kits which are available with the alloy head are twin S.U. HS2, twin Amal, or single downdraught Weber with double chokes. The test car was fitted with the Amals, which give more performance than either the S.U.s or the Weber but have some idiosyncrasies which might deter some people from using them for road driving. The complete kit of cylinder head, carburetters, silencer, chromed rocker cover, spark plugs and all the necessary gaskets and so on costs a total of £70. The S.U. kit is cheaper at £65, and the Weber kit costs £72. The cylinder head can be purchased separately for £39 10s., and the carburetter and silencer kits can also be bought separately: £27 for the S.U., £33 for the Amal, and £35 for the Weber kit.
The test car was a normal de luxe Mini with about 6,000 miles on the speedometer and was standard except for the addition of the Speedwell kit, a Kenlowe fan, costing £12 15s., and stiffer shock-absorbers. The only unusual control for the driver to worry about was a motorcycle-type choke lever which was attached to the parcels shelf. The starting procedure with the Amals can be tricky but once the technique is learned, first-time starts can be made without trouble. With the engine cold it is necessary to have the mixture control at fully rich and then depress the accelerator when the starter is operated. The engine usually fires first time and then the mixture control must be closed fairly quickly to prevent stalling. In extreme cold weather it may be necessary to flood the carburetters by “tickling” the button on the float chambers, but the engine always started well with us without resorting to this technique. To start the engine when warm it is necessary to “catch” the engine by depressing the throttle just after the starter begins to turn.
According to Speedwell the advantages of the Amal carburetters, which are more normally used on motorcycles, is in their ability to hold their tune over long periods, although they can be difficult to tune initially. S.U. carburetters are easier to tune but tend to lose their tune quicker.
On the road the increased performance was immediately evident and the Mini charged along in an impressive way. Later on, the stop-watch confirmed that this car’s performance was well up to Mini-Cooper standards despite its less advantageous gear ratios, which leave it with a bigger gap between 2nd and 3rd gears, thus affecting the 50-m.p.h. time especially. As the attached table shows, the Speedwell car is slower to 50 m.p.h. than a Mini-Cooper because of the extra gear-change involved, but by the time 60 m.p.h. is reached it has more than picked up the lost time. It is also quicker to 70 m.p.h. than the Cooper, and on the standing-start 1/4-mile it is half a second quicker.
There is some loss of flexibility with the Amal layout and it is necessary to use the gears to get the best results, which is, of course, no hardship to the keen driver. If the throttle is opened wide at low r.p.m. in the higher gears there is a bad flat spot, but the person who lugs around in top gear will hardly be in the market for a Speedwell conversion, and in any case for the driver who wants a little more flexibility the S.U. and Weber layouts are available.
Due to the better thermal efficiency of the alloy head the Mini would run quite happily on 90-octane fuel but we used 97 or 100-octane during our test and obtained a fuel consumption of 32 m.p.g., despite a leak from one of the carburetters. Speedwell claim 34 m.p.g. and this does not seem at all unreasonable. The Supertone silencer does not raise the noise level appreciably, and in fact the exhaust note is most pleasant. Fitted to a Mini-Super de luxe which costs £493 the Speedwell conversion brings the price up almost to that of a Cooper Mini; fitted to the normal Mini at £448 there is quite a considerable saving. Many people prefer the drum brakes of the ordinary Mini over the discs of the Cooper range, and the standard Mini gearbox is a vast improvement over the wretched device inflicted on early Mini drivers. Another advantage of the Speedwell conversion is that the alloy head and attendant carburetters, etc„ can be transferred to a new car and the standard cylinder head and carburetter replaced when selling the car.