The Dreaded N Specification
Many people have difficulty with this when choosing replacement tyres, so this article sets out the position specific to the 993, although many aspects affect other models.
The N Specification is a Porsche system of approved tyres, first introduced in the 1988 Model Year "" it means that Porsche have worked with the manufacturer to produce an optimal road tyre for a particular model and wheel size combination, and tested it to their satisfaction at the time. Whenever this process is performed for an individual set of approved tyres, the N spec number will increase. Often this will coincide with a model change, but can certainly occur within a model's lifetime, and thus a bewildering chart of approved wheel and tyre combinations (for summer and winter use) is produced and published twice a year in Christophorus, the Porsche company magazine, and in TSBs issued to OPCs. Importantly, Porsche recommendation is matching brand, type and correct N spec ALL ROUND "¦..
Here's a quote from a TSB ..
If new tires are to be mounted or the tires of one axle are to be replaced, tires of the same make, the
same type and with the same specification code must always be used on each of the two axles. If tires
are replaced on one axle only, the different tread depth from that on the other axle can cause a noticeable
change in the familiar handling. This is especially the case if new tires are mounted on the rear
axle. This effect decreases with increasing tire mileage.
When replacing a tire on an axle, make sure that the tread depth of the new tire does not differ from
that of the other tire by more than 30 %.
They are also rather proscriptive on getting a round result when fitting tyres ..
In order to optimize smoothness of rolling, it is appropriate - and necessary in individual cases - to
mount the tire in a certain (favorable) position with respect to the wheel (matching). Matching
(uncontrolled and controlled) is explained below:
Uncontrolled matching: Turning the tire on the wheel by 900 or 1200 if necessary in order to achieve
an acceptable value with regard to rolling smoothness (true running, imbalance and weight distribution
of balance weights).
Controlled matching: With a balancing machine with matching program. In most cases, this produces
an even better result with regard to the rolling smoothness (true running, imbalance and weight distribution
of the balance weights) than can be achieved with uncontrolled matching. Maximum permissible radial
runout and lateral runout of the wheels < 0.7 mm. Maximum permissible radial runout and lateral
runout of the wheels with tires < 1.25 mm. Values < 1.50 mm - ideally approx. 0.5 mm - are
desirable.
So it's no wonder OPCs tend to work with one local tyre shop ..
For the 993 the variables come down to 16", 17" and 18" wheels, wide or narrow body, and manufacturers Bridgestone, Continental, Dunlop, Michelin, Pirelli, Toyo and Yokohama. Not all manufacturers produce (or did produce) all sizes, and, being in England, we generally ignore the winter recommendations, which are aimed towards use on packed snow. This may be a consideration for some users, of course! 16" wheels are also a rarity here, so I'll skip those.
The last list for 993 was produced for MY 1998 but subsequently a couple of 993 TSBs have been issued, updating the list. Manufacturers do continue to produce the same sizes in multiple N specs, although sometimes you have to search hard to get the correct one. Higher (and some lower) N spec tyres will not have been tested on the 993 (refer updates), although they may work well. Sometimes manufacturers produce identical size / models with and without the N rating. The N spec tyre will be different in some way, compound, tread grooves, sidewall construction, whatever, but it may not be apparent.
So here's the list by model and fitment "" "˜summer' use only !
Carrera Coupe 2/4, Cabriolet, Targa, Carrera S
17" x 7" 205/50
17" x 9" 255/40
S02 N3, Sport Contact N1, SP Sport 8000 N0, MXX3 N1, Proxes F1-S N0
Pirelli special case : Direcionale N2 Front, Asimmetrico N2 Rear (this combination seems no longer available, Dec 05)
The 2004 993 TSB recommends these in the same sizes ...
Bridgestone Expedia S-01 N1
Bridgestone S-02 N3
Bridgestone Potenza S-02A N4
Continental SportContact CZ 91 N0
Continental SportContact N1
Continental SportContact 2 N2
Dunlop SP Sport 8000 N0
Michelin MXX3 Pilot SX N0
Michelin MXX3 Pilot SX N1
Michelin Pilot Sport N2
Toyo Proxes F1 S N0
Yokohama A 008 P N1
Pirelli P-ZERO Asimmetrico N1
Pirelli P-ZERO Direzionale N2 5859/Asimmetrico N2 3
Pirelli P-ZERO ROSSO N3
Carrera Coupe 2/4, Cabriolet, RS
18" x 8" 225/40
18" x 10" 265/35
Sport Contact N1, Pilot Sport SX N0, Asimmetrico N2 or N3
2004 ..
Bridgestone Expedia S-01 N0 4
Bridgestone S-02 N1
Bridgestone S-02A N3
Continental SportContact CZ 91 N0
Continental SportContact N1
Continental SportContact 2 N2
Michelin Pilot Sport N1
Pirelli P-ZERO N0 4
Pirelli P-ZERO Asimmetrico N1
Pirelli P-ZERO Asimmetrico N2
Pirelli P-ZERO Asimmetrico N3
Pirelli P-ZERO ROSSO N
Carrera S, 4S and TT
18" x 8" 225/40
18" x 10" 285/30
S02 N2, Asimmetrico N2
2004 ...
Bridgestone S-02 N1
Bridgestone S-02 N2
Bridgestone Potenza S-02A N3
Continental SportContact N1
Continental SportContact 2 N2
Michelin Pilot Sport N0
Michelin Pilot Sport N1
Pirelli P-ZERO N0
Pirelli P-ZERO Asimmetrico N1
Pirelli P-ZERO Asimmetrico N2
Pirelli P-ZERO Asimmetrico N3 4
Pirelli P-ZERO ROSSO N4
Porsche did not do all this testing for fun "" if you want your car to handle the way the factory intended, there are good guidelines here to follow. If you change sizes at all, you (are on your own and) need to be aware of the change in the ratio of front to rear rolling diameters, because a significant change will screw up the ABS and/or the 4WD. The logical calculation of rolling diameter from the tyre section / profile is not accurate in this regard, so be warned, you may need to check with the manufacturer. FYI, the calculation for rolling diameter is Wheel Diameter (in mm) + Section*Profile*2, so for a 225/40 18, it becomes 18*2.54 + 225*40%*2, giving a logical RD of 637.2. As an example, an actual figure for a Bridgestone S02 in that size is 642.62 ....
People often ask about fitting 285/30 x 18s on the rear of narrow bodied 993s (Kylies!) ... it may well require 'rolling' the wing edges, and only gives you understeer anyway "¦ Don't assume bigger is better! Another thing, pre-96 cars need a steering rack brace fitted to run 18 inch wheels(refer to applicable TSB for non-approved VINs), while they are not recommended for Targa at all (Not quite as simple as that, check for details).
Worth noting that the same tyres in different sizes are often at different N specs .... a totally normal stage of the process.
That's pretty well it! HTH, cheers, Maurice
< Message edited by MoC2S -- 30/1/2006 19:33:40 >
_____________________________
This is the article
