Wednesday 12 December 2012

Porsche 911T Project - Propulsion conundrums

The 'B' series engine that BMC used in various MGs is famously a heavy old lump. Initially with a capacity of 1500cc, it used what might generously be called 'tried and trusted' technology, and with block and heads made of cast iron, 5 main bearings,  push-rods and siamesed ports, it weighed just under 200kgs and produced around 100bhp in standard 1800cc MGB form. That is, if you were lucky and the pair of SU carburettors were properly set up.


The old 'B' series engine; mainly cast iron.
The 'B' series was first lowered into the MGB in 1962, and ever since engineering geniuses have worked on improving the engine's output, to the point where a state of the art racing engine will now produce close to 200bhp. That's enough the propel a 830kg historic FIA specification MGB at a fair old lick. However, while a run-of-the-mill B series rebuild comes with a £2,000 price tag, that heroic racing engine will cost you, dear customer, £10,000.

I found myself musing over pushrods and cast iron when investigating my options for replacing my 911s tired old motor. In contrast to the B series, the 911 engine was made of the best alloys the foundry could provide, and a few years into its life, Ferry's old engineering-led Porsche company went even more exotic, casting the main engine and gearbox casings out of magnesium alloy. All the better to save a few kilograms of weight, cantilevered out behind the rear axle.

The weight of all this expensive German engineering? Well, close to 200kgs seeing as you ask. It varies depending on the precise variety of 911 (add a turbo or two and that's another 50kgs), or the same as the MGB's lump. And decades of work by just as clever engineers have resulted in historic 2litre 911s racing with a bit over 200bhp.

The prices aren't quite the same though. A straightforward 911 engine rebuild performed by someone who knows what they're doing will cost you the thick end of £7,500.  Should you desire more in the way of power it'll cost you twice that, even before you get to exotic induction set ups. A full house 911RSR race engine on the famed high butterfly throttle bodies can cost you £40,000, and I'm sure its possible to pay even more. 
300bhp of slide throttled, mechanically fuel injected 911 gorgeousness

The engine in my 911T was by marketing desire and engineering expediency one of the low points in 911 hierarchy. In the early 70's, meeting emission regulations in the critical US market was one of the biggest challenges facing Porsche. By 1972, the capacity had been bumped up to just under 2.4litres by increasing the stroke, and therefore mid-range torque. While the 'E' and 'S' models still had decent power outputs (165bhp and 190bhp respectively), the lowly 'T' was given cheap cast iron components and had its breathing constrained by narrow inlet and exhaust ports. 1973 also marked the introduction of the Bosch K-Jetronic fuel injection system that would underpin 911s for the next decade. Unfortunately, the first effort used in the 2.4T wasn't that unsuccessful - it also demanded a specific piston design that didn't lend itself to increased in compression ratio beyond the T's  8.5:1. I can still remember how contemporary journalist presented this as a tremendous advantage, allowing owner to stop at the 2 star petrol pump to fill their 911. In reality it was bad news for efficiency; the 2.4T produced only 140bhp, up a mere 10bhp from the smaller 2.0 the 911 was launched with a decade earlier.

The most desirable option for me would be to have my engine rebuilt to match the car's hotrod looks. The 2.4 can easily become a 2.7, achievable by changing the pistons and barrels for RS items. Unfortunately, this approach means most of the rest of the components joining the old pistons in the bin. And even the bits left would need some serious engineering to allow them to cope with the 200bhp that is my target. The estimates I've had for a proper job tot up to around £15,000, and while originality and the lightweight of the mag. alloy motor is clearly desirable, its difficult to see this making sense in a car that might sell for not much more than than on a good day.

I think I need to think about this again.

SS7







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