John Blunsden: The firmly-sprung Group 5 Imp becomes a three-wheeler through Kidney Bend. Smaller wheels and tyres cause this car to use up more road than the Club Imp.
John Blunsden: The Club Imp has 13 inch wheels, but still sits 1¼ inches lower than the standard car. Note the difference in driver height between this and the Group 5 Imp. The lower position was preferred.
The engine-compartment feature which immediately identifies the Group 5 Imp from the Club version is the standard radiator and water filler to the left of the generator. This can be removed under Club regulations.
Standard instrumentation is compulsory under Group 5, but this does not bar supplementary instruments. The Club car has a non-standard panel, and of course minimal interior trim.
The smaller-capacity fuel tank emphasises that the club Imp is normally involved in only short-duration races. Note the dual bracing struts across the front bay of both cars, and the shield over the radiator on the Group 5 version.
Group 5 and Club Fraser Imps / by John Blunsden. - Motor Racing 1966, September. - p.412-414. - [Track Test ; No. 54]
JUST over a year ago, in an earlier track test story, I was rash enough to comment that the Hillman Imp was 'a fundamentally non-raceable car' (because of its front suspension), and ever since I have suffered from chronic indigestion through having to eat my words. It's all Alan Fraser's fault!
This season, of course, the Fraser Imps have been operating with works backing, and have been cleamng up very effectively in club racing, as well as playing an increasingly important role in the 1 litre class of Group 5 (British Championship) events. Team drivers Bernard Unett, Ray Calcutt, Nick Brittan and Jacqui Smith are all either leading or challenging for the lead in one or other championship, and the team have put Rootes right back on to the racing map. You don't do that sort of thing with 'nonraceable' cars!
This track test covers the two 1 litre versions of the Fraser Imp -the Club and Group 5 cars - which Ray Calcutt has been using this year, the intention being to find out how the lighter but lower-powered car compared with the heavier but mechanically more sophisticated Group 5 model.
Calcutt's Club car is Fraser's 1965 development Imp, and of course is immediately recognisable from the Group 5 car by its flared wheel arches (modified to take widesection tyres) and by the absence of interior body trim. Some criticism has been voiced against the appearance of these works-backed cars in Club racing, but as anyone with the necessary cash can buy an exact replica of the Club Fraser Imp (which has been developed around components available to anyone) the validity of such criticism is highly questionable. At the same time should the opposition leap-frog ahead by 'using something like a Formula 3 engine, or similar exotic parts, the Fraser team intend to fight back with something equally effective... and probably just as costly!
The 1 litre displacement of the Club Imp engine has been achieved by increasing the cylinder bore from 68mm to 72.5mm. The standard crankshaft is crack-tested and balance-checked, a certain amount of handwork is carried out on the rods and the flywheel is lightened, but all start off as standard components. The cylinder head is a Rally Imp part, considerably modified of course, particularly in the region of the ports. Twin 38DCOE Weber carburettors are mounted on a fairly long four-pipe induction manifold with 'O' rings.
Four alternative camshafts are available, the car as tested being fitted with the 'B' cam, desiged for slow-circuit use, and giving a .36 inch valve lift. The inner valve springs are Rally Imp components, and the outer springs are a special Terry racing design. Flat-top Hepolite pistons, with Dykes top rings, are fitted, and the compression ratio is 9.7 to 1. The standard sump and bearings have been retained. but the exhaust is a special four-into-two-into-one system, the tall pipe emerging below the rear of the car on the right side. A German Reintz metal-and-fibre cylinder head gasket has effectively overcome earlier leakage troubles. Average power output for a Fraser engine in this trim is 92 to 93 bhp at 8,000 rpm, with 80 pounds/feet of torque at 6.000 rpm. The power starts to come in around 4.800 rpm, and 8.800 rpm is usable, although I was given a limit of 8,400 rpm for the test.
The Club Imp's gearbox is a standard unit complete with synchromesh, apart from two pairs of straight-cut pinions to give closer third and top gear ratios. The final-drive is also standard Imp equipment, and the drive shafts are the thicker Imp van pattern.
A Bendix pump draws fuel from a frontmounted 4 gallon glass-fibre tank, the lines passing through the bodywork of the car, whereas the ally water lines run beneath the body to and from a shallow front-mounted cross-flow radiator, the standard rear radiator being dispensed with. A separate oil cooler is mounted vertically at the front, on the right side of the water radiator.
The van-type wishbones of the rear suspension are strengthened by plating, and high-rate springs are fitted, lowering the car about 1¼ inches from standard, even when using 13 inch wheels with 5.00 tyres. Adjustable Armstrong telescopic dampers are mounted vertically in the normal Imp position. At the front, the wishbones have been modified to give a lower pivot point, the steering rack mounting also being altered to correct the geometry. As at the rear, higher-rate springs help to lower the car 1¼ inches, and smaller Armstrong dampers are fitted. A static camber change has been achieved from positive to 1½ degrees negative, with very favourable results, the hard front suspension helping to reduce camber change under heavy braking. The stiff suspension at both ends of the car is also aimed to cut down weight transfer due to body roll. An anti-roll bar, coupled between the front stub axles, is carried by a pair of bearings under the front body bay, which has been strengthened by two struts running diagonally forwards from the scuttle, and a further strut running backwards from the scuttle on to the floor, just ahead of the gear shift. (In a serious accident last year, this extra bracing proved an invaluable safety precaution.)
Both the steering box and brake assemblies are standard Imp equipment, apart from the VG95 brake linings. The Minilite wheels have 5½ inch rims at the front and 6 inch at the rear, and the Dunlop white spots were run at 42 psi all round for the test.
Apart from the ally outer skins of the doors, all the body panels are steel, and other than the laminated screen, all windows are in Perspex, with circular vents built into the door windows. Dry weight of the Club car runs out at roughly 11 ¼ cwt.
In describing the Group 5 car it is, perhaps, easiest to explain how it differs from the Club Imp. On the engine side, special intruder-type pistons are machined by Rootes from blanks supplied by Hepworth and Grandage, while a special cylinder head has been developed from a head produced for an experimental engine some three years ago. As a result, a great deal of compromise has had to be accepted in order to fit it to the Imp engine. For example, it will not accept larger valve sizes than those used in the Club head (these in turn are larger on the inlet side than standard Imp, and slightly larger on the exhaust side), but at least the Group 5 head benefits from straighter inlet ports, and is probably worth 5 bhp. (A completely new head, with larger valves, is on the way.)
Meanwhile, the interim-size valves are of special design, and the Group 5 camshaft is carried on five bearings instead of the normal three, but otherwise is similar to the 'D' Club camshaft. Another engine change from Club specification is the addition of a 'lump' on the standard sump to provide extra capacity. A special head gasket has been developed for this engine by Coopers Mechanical Joints, of Slough.
The compression ratio is approximately 11¼ to 1, and power output is 106 bhp at 8,300 rpm. Power starts to be felt around 5,500 rpm, and the upper limit is normally 9,000 rpm or a fraction above, of which I was invited to use 8,700 rpm. An improved exhaust system has been developed for this car, still a four-into-two-into-one system, but with more gentle curves, and a tailpipe extracting on the left side of the engine.
Group 5 regulations demand that a car's normal radiator should be fitted, but this does not prevent an additional radiator from being mounted up front, as on the Club car. The rear radiator is coupled into the system, but is not served by a fan. A recent modification is to run the fuel lines from the 10 gallon front-mounted tank through the structure of the car on the right side (a box member used on road Imps to carry the heater lines).
Unlike the Club car, the Group 5 runs on 12 inch wheels, and it is probably a combination of the smaller volume of air passing beneath the car and the slightly higher running speed of the output shafts which causes the gearbox of the Group 5 cars to run at a higher temperature. To overcome this, the box has been given its own oil cooler alongside it, with a Bendix pump collecting the oil from the drain plug and returning it through the filler. The engine oil cooler is mounted at the front, as on the Club car, the lines passing along the floor of the car to the right of the driver, while the water lines are also at floor level. to the left of the gear shift.
The Minilite wheels have 5 inch rims at the front and 6 inch rims at the rear. and carry Dunlop 4.50 tyres -the only size currently available. The car's track has to be adjusted so that the tyres do not protrude outside the standard bodywork, of course.
Regulations demand a minimum weight within 5 per cent of the car's homologated weight, and the Fraser Imp manages to get within 3 per cent. Full body trim is required, of course, as well as all the standard instrumentation. But apart from the inevitable rev counter, there is an additional battery of five dials, recording engine oil temperature out of the cooler, and in the sump, engine oil pressure, gearbox oil temperature, and water temperature. As on the Club car. an anti-roll hoop is built into the cockpit, with a pair of bracing struts through the rear upholstery.
Getting down to the test. I started off with the Club car while some final adjustments were made to the Group 5 version. Anyone who has watched the Fraser Imps in action will have noticed that they produce a fairly choppy ride, and it came as no surprise to find things set up on the firm side. For the first lap or two it seemed to be weaving quite a lot under braking, but I think this was all part of the settling-down process. and after a while everything dropped into line.
Under a constant throttle, the cornering performance was very predictable, with a fairly modest degree of understeer the overriding feature, a characteristic which considerable 'buck jumping' over the rougher sections failed to destroy. However, an immediate and at times fairly substantial oversteer would be induced when, for any reason, the throttle had to be feathered in the middle of a corner. In other words, because of certain engine characteristics, a steering reaction which might be expected from a power back-off could be induced by just neutralising the throttle. After a while I became accustomed to it and found that a short 'sideslip' half-way down Paddock was quite a useful way of getting round the corner.
Despite the considerable amount of 'hop and skip', the power seemed to get through to the road surprisingly well, and the Club Imp is without doubt a very 'diceable' little car. I was not all that happy with the selection of second gear from third -a certain amount of vagueness there- but otherwise the box was up to the usual slick Imp performance, while the brakes, which don't get used all that hard anyway on a car of this weight, were more than adequate.
Life gets pretty noisy inside, and on this car it gets quite warm, too, on a sunny day, the side 'portholes' being of limited value in deflecting air into the cockpit. But at least the sun did come out for the track test, and so I was not inclined to complain!
After 12 flying laps I pulled in, having covered the final lap in 62.4 seconds (Ray Calcutt had earlier lapped the car in 61.6 seconds, and in fact earlier in the year had been timed around the Club circuit in a fantastic 59.8 seconds!). I then climbed straight into the Group 5 version so as to get as clear a comparison as possible.
An immediate change was the seating position. The Group 5 seat had been mounted much lower, and I found this gave me a better feeling of control, even though the seat itself was less comfortable, due to a much too shallow cushion giving insufficient thigh support.
The considerably narrower rev band called for second gear at Clearways, whereas third had sufficed on the Club car, and there was no doubt that the Group 5 engine had more steam at the top end. It was interesting to compare the cornering characteristics of the two cars because the Group 5 had a thicker anti-roll bar at the front. and a thin bar had also been fitted at the rear.
The result, coupled with the smaller section tyres, was a car which revealed more pronounced understeer, and which definitely used up more road in a corner. On the other hand, the rear bar, far from inviting sudden oversteer, seemed to tie the back end down very well and actually gave a far more predictable and correctable breakaway when the rear wheels eventually let go. I definitely preferred the Group 5 car on this score, although I would have liked less understeer to begin with.
This car was even more firmly sprung than the Club version, and although it seemed just a little softer on initial deflection, the suspension quickly hardened-up, and seemed to get through to the bump rubbers -noticeably at the rear- more rapidly than on the Club Imp. I imagine it was cocking a wheel more frequently, and certainly the engine note was varying almost continuously over the rough stuff.
Yet in spite of all this, and of the considerable amount of wheel turning it involved, I think the Group 5 car was on aggregate slightly the more pleasant car to drive - or was this because I was getting more used to the Imp technique generally? The much improved gear selection of this car was another point in its favour - it is always comforting to know that you are going to get another ratio first time!
If I had to drive the car for any great distance, I should have liked to have tried it with a marginally thinner front roll bar, slightly softer damper settings, an improved seat and a lower steering column mounting: or even possibly a slightly smaller diameter steering wheel. I have a feeling I should have liked that very much indeed.
But after 12 laps (the same distance) I pulled in, this time with a fastest time of 62.3 seconds, or virtually the same time as in the Club car. and a full second slower than Ray Calcutt's best.
The time is approaching, of course, when, even on a tight circuit like Brands Hatch, the Group 5 Imps are going to outpace their Club stablemates (Bernard Unett's long-circuit Group 5 record is already well below his Club record time). Perhaps it is an exaggeration to say that the Alan Fraser team are still only scratching the surface of racing Imp development, but it is certainly true to say that they have a number of exciting offerings up their sleeve for the not-sodistant future.
There is that 'full' Group 5 cylinder head on the way. There is also a lighter-weight version of the Club car. And before the season is out, the team hope to move up the ladder and run Group 5 cars in the 1,300 cc class. Come to think of it, maybe they have been only scratching the surface at that!
The Imp Site
Article supplied by Robin Human, National Weekend 2011 (Bangor)