GETTING TO GRIPSWITH 911 MFI PUMPS

A little knowledge can be a dangerous thing, we are often told – and surely few who have seen Donald Trump in action this past year would argue with that. Sometimes, though, knowing only a little – but just enough – can be all that you need to avoid a costly problem, or even an expensive disaster.

So-called MFI 911s – that is to say, the cars with a beltdriven mechanical fuel-injection pump – have a clever little cylindrical device on the top of the pump body that, in very simple terms, alters the fuel delivery to compensate for changes in atmospheric pressure. Or in other words the density of the air being drawn into the combustion chambers. Those changes might come primarily from altitude – there is a roughly 0.012 bar drop in atmospheric pressure for every 100 metres that you ascend – but also from something as mundane as the weather. Average sea-level pressure is 1.0 atmosphere (or 1.0 bar; that’s 14.7psi), but in extreme circumstances this has been shown to fall as low as 0.85 bar, and to increase to as much as 1.05 bar.

In neither scenario – the first in the eye of a hurricane, the second around the shores of the Dead Sea, some 430 metres (1400 feet) below ‘normal’ sea level – might it trouble you how this affected the running of your 911 engine, but for anyone living in, say, the high plains of the American Midwest, or planning to tackle the Panamerican Highway, it could be a very significant consideration. And the fact is that even here in the United Kingdom, where weather alone can see atmospheric pressure vary between around 1.05 bar and 0.93 bar, there would otherwise be a noticeable effect on the car’s performance and/or exhaust emissions.

Inside the aforementioned cylinder is a small ‘bellows’, for want of a better term. Made of ultra-thin and thus highly flexible brass sheet, and carefully sealed during manufacture to maintain its essential internal vacuum, it works in exactly the same way as an old-fashioned aneroid barometer, with a pin at its lower end acting on the fuelmetering side of the pump. By and large it is remarkably reliable, too, with many still functioning perfectly after nearly 50 years (the MFI engine, you will recall, was used between 1969 and 1977), and countless thousands of minuscule but stress-inducing expansions and contractions.

Eventually, however, it may well stop working. In many cases there might be only a relatively small effect on performance and economy, especially here in generally low-lying and only occasionally stormy Britain, but the engine would certainly not be running at anywhere near its best. New replacements have long been unavailable, with a consequent rise in the scarcity (and price) of good second-hand items, and repair is more or less impossible. Until now, anyway.

Neil Bainbridge, the driving force behind Bsmotorsport, and arguably the only UK specialist genuinely able to overhaul and crucially to calibrate Bosch MFI pumps, has had made for him a batch of these so-called aneroid units’ casings and top covers, to all intents and purposes indistinguishable from the original parts. These allow Neil to remove the old lids – which are inevitably destroyed by the process – and while the bellows inside are neither repairable nor at this stage replaceable with new (although needless to say Neil is working on that at this moment), at the very least to inspect them closely for damage, and then to refit them inside original but suitably replated casings.

That is most certainly not a DIY procedure, of course, but there is still much that you can do to prolong the life of your MFI fuel pump’s atmosphericpressure compensator. It doesn’t take kindly to vibration, for example, so make sure that your engine is always tuned to run as smoothly as possible – and perhaps avoid solid or even semi-solid engine mounts unless you really do need them for, say, limited-enginehours track work.

Overheating can be a major problem, too. The air-cooled engine itself may be famously robust in that respect, but thin and delicately soldered brass sheet is not. And for the same reason don’t allow the engine to run for any length of time with the hot-air pipe to the cold-start enrichment device disconnected. Chances are it will quickly roast the adjacent bellows, as per the one in the photograph below, in which the soldered joint between the bellows and its own top plate appears simply to have melted.

Perhaps above all, though – and I say this for the benefit of anyone who might have cause to pick up an MFI pump that is not attached to an engine – never, EVER, be tempted to use that appealing, brightly coloured cylinder as a convenient handle. The weight of the pump beneath it is such that the casing will inevitably become invisibly but quite significantly distorted, and then the pump will require either a full recalibration in order to compensate, or quite possibly to have the ‘aneroid’ removed and fitted inside one of those refurbished casings and lids.

Should you be in the – relatively – unfortunate position of requiring that kind of service, then the good news is that Bsmotorsport (bsmotorsport.co.uk) will charge you around £350 to open up and overhaul the bellows and its housing, plus £550 for a basic pump check and calibration on its state-of-the-art Bosch testrig. If you need a new bellows housing, reckon on around £450 for a rebuilt unit.

That may throw up other issues within the pump that need to be addressed, such that ideally it undergoes a full rebuild, but even at £1500 or more (all those figures exclude VAT, by the way) that has to be better than soldiering on with the fuel system working below par and, in these days of ecoawareness to the point of religious zealotry, potentially giving all of these iconic machines a bad name, and so hastening their demise.

Porsche’s water-cooled flatsixes have been a regular topic in these pages over the years – often when there is bad news to impart. It’s nice, then, to be able to redress the balance with some genuinely good news about them – or at least some good news about what one independent specialist has managed to achieve with them.

Three years ago Hartech – long pre-eminent in the M96/97 world and, fair to say, often controversial, too – disbanded its own racing team, and decided instead to ‘incentivise’ those drivers using its engines and carrying on their cars ‘Powered by Hartech’ decals. ‘It’s very straightforward,’ Barry Hart told me during a recent conversation. ‘The drivers earn points for first-, second- and third-place finishes throughout the season, and double points for a Championship win. Those build up to earn them valuable discounts against the labour cost of their next routine rebuild.

‘Our top three runners this year were Mark and Jake Mcaleer in their 3.4-litre 996 and Boxster “S”, respectively, and Ed Hayes in his 3.2-litre Boxster “S”. All three have won their individual 2017 Championships, and have also earned labour-free full engine overhauls this winter. It all adds up to something like £8000 worth.

‘All three engines have been 100 per cent reliable, despite running at the front of the field throughout a fiercely fought season. We also built the engine for the Cayman “S” driven by Mike Price and Callum Macleod in the 750MC Club endurance race at Spa on 1st October. They finished second, close behind the winning Caterham.

‘I don’t think that any other engine builder has managed to achieve such a consistently good performance covering different power units, classes and events in sports-car racing. It’s a great achievement for our business, showing the quality of our workmanship, expertise and technology.

‘It also underlines the scale of our work with ordinary roadcar engines,’ added Barry. ‘We machine, modify and rebuild hundreds of those each year – over a thousand since we started, and again with 100 per cent reliability – and with several larger-capacity models currently under test, further developments under way, and not least the purchase of a brand-new CNC lathe to add to our existing CNC milling capacity, you will see that we are not standing still!’

I couldn’t agree more. I think Barry and I would both further agree that Porsche itself should – and certainly could – have done rather more development work on these fundamentally excellent pieces of machinery. But equally there can be little doubt that without his genuinely tireless efforts to iron out their weaknesses there would not now be such a healthy market for the huge number of cars they power. It is, indeed, an ill wind that blows no good.

911 & PORSCHE WORLD