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

911 Porsche World - - This Month -

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

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