Sunbeam Stiletto
Ducted auxiliary front radiator means heater can stay off in summer
Is every classic car a project? Or do you reach the stage when you can say, ‘I think it’s done,’ and you can just relax into driving it, maintaining it, polishing it, enjoying it? My Stiletto is perilously close to that position. Everything works. It runs, handles and rides beautifully. The engine’s rebirth you know about from my last Saga, but there were other tweaks on the road to motive sweetness.
There’s a certain fast dual carriageway I sometimes use, with a ripply surface. The first time I zoomed along it in the Stiletto, I thought a rear hub was loose or a driveshaft Rotoflex was breaking up, such was the insistent vibration which made the engine/gearbox unit shake. But neither was the case. So at the Imp National weekend I bought a pair of new old-stock transaxle mounts. Fitting these with the gearbox in situ is a battle but just possible, and the vibration did lessen slightly.
But then I noticed what I should have noticed earlier, which is that a former owner had committed a cardinal sin of Imp assembly and fitted the Rotoflex couplings back to front, which causes the driveshaft spiders to cut into them. I had one spare competition-spec coupling, with extra reinforcing leaves, and Imp Club Spares supplied another in a Lotus bag – they’re the same as an Elan’s. They are fitted and the vibration has gone.
Gradual improvements
Another leap towards a refined drive came with ditching the Spax dampers, whose oversize mounting rubbers were too unyielding and whose ability to smooth broken surfaces was poor. Imp Club Spares came up trumps again with NOS Woodheads, and they are just right: supple when they need to be but controlling big body movements. Road roar and harshness have been slashed, too. And now, the big one: the ultimate civilised engine-cooling solution. Even with my rebuilt radiator and its superefficient core, the Stiletto ran slightly warmer than I’d like on a fast cruise with the heater off. So I have re-created, with a touch more finesse, what I devised for my last warmed-over Imp: an auxiliary front radiator mounted in the box below the front bumper that normally houses the horns. Quite a lot of speedy Imps have a similar-sounding arrangement, but mostly they gain airflow through the auxiliary radiator by
cutting holes in the horn box and venting the air into the front boot, from where it escapes around the bonnet sides.
It’s a nasty system for several reasons. It pressurises the boot at speed, so the bonnet lid lifts and wobbles. It heats up the petrol in the underbonnet tank. A boot full of luggage further impedes the already tortuous airflow. And the boot gets wet in heavy rain.
The ultimate compromise
My solution is to duct the air out of the nearside of the horn box, via a large-bore convoluted hose, and through the nearside front inner wheelarch that is a low-pressure area. This way, the auxiliary radiator is part of a self-contained, sealed system and the boot’s usefulness is unaffected.
The radiator is a Ford Granada heater matrix (about £22 via ebay), which is exactly the right height and depth and has pipe stubs at exactly the right angle. I chopped out the nearside end of the horn box and extended it with a construction in sheet aluminium, bonded to the horn box with strong seam sealer and featuring a removable lid to make installation of the matrix easier. A pair of jumbo-size airflow meter duct orifices, one on my aluminium box, the other on the wheelarch, were joined by the duct hose, and high-density foam around the edges of the matrix ensured that air would go through the aluminium core rather than round the edges.
The water supply was taken from the heater hoses via three-way junctions, and there’s a tap to turn the matrix off in cold weather given that it’s in a part of the water circuit not controlled by the thermostat. I may modify this with an electric valve and a switch inside the cabin. As for the displaced horns, they now live on top of the airbox. It works superbly. Cruising at 70mph on a hot day, with just cold air entering the cabin, sees the temperature gauge’s needle stay put at half way. After a cold start on a cold day, the needle takes an age to stagger up even a third of the way with the system turned on, an experiment – obviously I don’t do this normally – which shows how much cooling the auxiliary radiator contributes even before the thermostat has opened and the regular radiator comes into play. The ultimate no-compromise Imp cooling system? I’d say so.
‘It works superbly. Cruising at 70mph on a hot day, there’s cold air in the cabin’