HOWTO: IMPROVE THE SHIFT ON YOUR 915 GEARBOX
Pre-g50 911s famously have a gear change often described as ‘awkward’, ‘challenging’ or even ‘impossible’, but Us-based Stomski Racing reckons it has the affordable and effective solutions – in the shape of a precision coupling for the main longitudinal s
Fitting an improved 915 gearbox linkage from Stomski Racing
We began last month’s technical feature – an analysis of Classic Retrofit’s ingenious bolt-in, no-drilling airconditioning system – by suggesting that effective cabin heating and ventilation are not high among the earlier 911’s attributes. And sadly – without wishing to sound as though we have a vendetta going here – the same must be said of the pre-964 cars’ gear-shift quality, too.
The post-1986 Carrera 3.2, with its G50 transmission, is a vast improvement over the previous 901 and 915 gearboxes (although still not as good as either the 964 or the 993), but in cars with those previous units any modest performance gain that might accrue from increased engine power is more than likely to be largely negated by the time – and the concentration – it still takes to shift cleanly and precisely from one ratio to the next.
Proponents of those earlier gearboxes will (correctly) argue that they are strong and reliable, with a proven world-class competition pedigree, and (incorrectly, this writer believes) that anyone who can’t cope with their idiosyncrasies ought to stick with a contemporary 944. (Which, paradoxically, has a by and large exemplary gear shift. But then I would say that, wouldn’t I?)
Maybe so, but the fact is that these are meant to be high-performance sports cars we are talking about, not some antediluvian lorry. And, even if you are not overly concerned about 0–62mph times, they ought to be capable of delivering a more fluid, a more efficient and ultimately a rather more rewarding driving experience, without the constant fear of damaging either the gearbox itself or, worse still, ‘buzzing’ an expensive air-cooled engine.
And they can be that good, too – perhaps for the sake of just a few hundred pounds, and barely more than two hours’ relatively easy work. There is, understandably, no guarantee that you will achieve a miraculous improvement merely by the addition of a few after-market peripheral components – however well designed and engineered they may be, they cannot overcome possible problems within the gearbox itself – but based upon our own hands-on experience with this 1981 911SC, it certainly has to be worth a try.
So, what are those peripheral items that we are dealing with here? Primarily the universal-jointed coupling at the rear end of the primary longitudinal shift rod, which is accessible beneath a metal cover on the central tunnel, behind the front seats. And after that – a little more ambitiously, but not impossibly so – the four metal-and-rubber bushings via which the engine and the transmission are suspended beneath the body shell. More on these in a moment.
The logic is simple. That coupling, pulling, pushing and/or twisting hundreds
or perhaps even thousands of times during any one journey, is the vital single link that transmits the movement of your hand, via the gear-shift lever, to the shift rods inside the transmission. Any free play within the coupling, which could be the result of either manufacturing tolerances and/or wear and tear, will necessarily dull the essential sharpness of that connection.
And the fact is that these hard-working devices do wear out. The original Porsche component extracted from this car was showing a substantial gap between the central pin and the nylon outer bushes, and naturally the resulting free play would have been magnified at the top of the gear lever, both longitudinally and laterally. Bigly. (Or big-league, as Donald Trump almost certainly actually said in that TV debate, even if that is not nearly as amusing.)
New couplings are available from Porsche, at around £72 plus VAT, but there is nothing in their rather basic design to suggest that they will retain any additional precision for long. Far better, then, to spend a little extra on an after-market device, such as the one shown here from USbased Stomski Racing (US$198). Inside its sealed-for-life rubber boot is a small but robust pin-and-block universal joint, and that should remain as-new more or less indefinitely. Fitting and adjustment could easily take little more than half an hour.
The science behind the same company’s symmetrical, semi-solid polyurethane engine and transmission mounts is a little harder to grasp, but is based on the none the less sound premise – espoused by Porsche itself in the electro-magnetic ‘active’ mounts it later fitted to ultra-hard-core track cars such as the 997 GT3 RS – that by limiting the (small) movement of the power unit against the pushing and pulling of the shift lever, you will transmit more of said lever’s input, more quickly, to the internal mechanisms that actually do all the work.
Installing those, currently priced at US$376 per set of four for this 911SC (see the panel on the opposite page), is naturally a little more complicated – you need good, safe access to the underside of the car, and unsurprisingly to support the powertrain with either a jack or a transmission stand for the duration – but, even so, the job should no more than around 90 minutes from start to finish.
This car’s owners were understandably
happy with our handiwork. (Which, so far uniquely, we had carried out as something of a ‘blind tasting’, in order to attempt to determine whether any improvement in the shift quality was genuinely that, rather than the mind telling the hand that it had to be.) And so, too, were we.
There was a marked improvement in the precision and overall feel of the shift, even after the coupling alone had been installed, and although the engine and gearbox mounting bushes (initially the middle of the three grades supplied in the kit) brought about only a marginal further improvement in what remains essentially a fairly ‘soft’ car, we have no doubt that for even occasional track use – and the odd cross-country thrash – it will be a far more confidence- inspiring and certainly enjoyable set-up.
In truth, the owners later had those middle-grade bushes replaced with the softest ones, but profess themselves big fans of the coupling’s greater ‘connectedness’ with what is going on inside the gearbox, and we have heard nothing since. And in that context – as in most others – no news is good news. PW
The gear shift in our guinea pig 911SC was tolerably mediocre – but still offered scope for improvement. Uniquely, the car’s situation would allow us to fit the uprated items above, and see if the owners noticed any difference: a sort of blind tasting. Key to the process was undoubtedly the shift-rod coupler: original was quite noticeably worn, but crucially this one is not only more precise to start with, but should resist any future deterioration. Engine and gearbox mounts offer three easily swopped levels of firmness: details on next spread
Original mounts are secured to the plate by ordinary M8 nuts, bolts and washers; new ones go on the same way – but make sure you fit them the right way up, as here. Engine mounts, using the same beautifully machined aluminium blocks, are mounted with the large central boss facing down. New Stomski mounts have a slightly thicker outer flange, but length of all fixings is more than sufficient to cater for that. BS Motorsport’s Rob Nugent used new M8 fixings, and crucially with a so-called wave washer (arrowed) in place of the original Porsche spring washer: see
in this issue for an explanation. Aluminium grease will make sure everything comes undone again, far into the future. Red intermediate bushes selected first, later changed to softest yellow items. Either way, don’t forget spacer and so-called Schnorr washer (both supplied) to help lock main through-bolt
Transmission mounts are built in to a plate secured to the floor at the front end of the gearbox, and which needs to be removed. But that is possible only after the rear anti-roll bar has been partially detached. Loosen nut and bolt at the lower end of each ARB link; this will allow the bar to swivel downwards far enough once the mounting brackets have been removed. With transmission supported, undo and remove the four nuts and washers securing plate to the underside of the gearbox. Remove the two large bolts securing the plate to the floor – one at each end – and it should drop clear. One of the four studs unscrewed from the gearbox, but that’s no big deal: either treat it as a bolt when you refit it, or first remove the nut and refit the stud alone, perhaps helping it stay put with Loctite
Engine mounts work on exactly the same principle. Car needs to be on the ground this time, but again with some suitable support beneath the engine while the mounts are removed. A trolley jack beneath the crankcase is fine; just make sure that its saddle is padded to avoid damaging the relatively soft alloy. Arrow in photo top right highlights the point where some previous mechanic has casually levered the transverse engine-mounting bar against the engine lid’s slam panel, presumably to get the eye at the end to line up with the main through-bolt: don’t make the same elementary mistake. Again we started with the red intermediate blocks, but later changed these to the softer yellow items at the owners’ request