Cayman GT4: Where are we up to now?

[Motorhead]

Not my area of expertise Ralph but that makes sense: more pistons = greater pad area = greater clamping force; larger brake disc = greater heat dissipating area. And according to your table, the 991S front discs are both larger in diameter and thicker than those on the base car; so more clamping force and greater heat dissipation. Presumably the 991 and 991S have a different front/rear brake bias but at the end of the day overall braking efficiency is down to ultimate tyre grip. Jeff

 

[MrDemon]

You cannot lock a wheel at 100mph , so you need clamping force. Tyres don't come into it till later hence why one threshold brakes. You can hit the brakes as hard as you like at 100mph

 

[fbr]

ORIGINAL: ralphmusic Back on to brakes, I read somewhere on the web (so it must be true) that the total area of the front v rear pistons is as important as the size of the disks - the former being force and the latter heat management. So fitting 991S 6 pot calipers v 4 pot standard and retaining OEM 4 pot rears would imbalance front/rear braking but that retaining 4 pot each end but increasing disc size would retain normal brake balance. Any views from those who understand these matters?

Ralph Interesting discussion. Whilst not pretending to understand anything about brakes here's a [link=http://brakepower.com/]fun site[/link] to fill up the dark winter nights and maybe give you some answers. Can't vouch for the veracity of the calculations. best wishes FBR PS The current 981 has a larger diameter master cylinder than previous models at 25.4 mm and is fitted with a 8/9" tandem brake booster with a boost ratio of 5.5, if that helps .

 

[Motorhead]

ORIGINAL: MrDemon You cannot lock a wheel at 100mph , so you need clamping force. Tyres don't come into it till later hence why one threshold brakes. You can hit the brakes as hard as you like at 100mph

What about wet/slippery surfaces..? The coefficient of friction between tyre and road surface is critical for any speed. And remember that we're talking here about production cars with ABS, EBD, etc. designed primarily to be used safely by average drivers on our roads and not race cars in the hands of experienced track drivers familiar with threshold braking techniques despite the fact that Porsche's development drivers will fall into this latter category. Jeff

 

[fbr]

ORIGINAL: Motorhead ..... And remember that we're talking here about production cars with ABS, EBD, etc. designed primarily to be used safely by average drivers on our roads and not race cars in the hands of experienced track drivers familiar with threshold braking techniques despite the fact that Porsche's development drivers will fall into this latter category. Jeff

Dear Jeff Is this what you had in mind? FBR

 

[Motorhead]

Yes, that's what I like to see Frank - a Porsche on a typical Scottish road..! Jeff

 

[fbr]

ORIGINAL: Motorhead Yes, that's what I like to see Frank - a Porsche on a typical Scottish road..! Jeff

Jeff Sorry it's not the hatchback . Moving on. Some questions re the putative aerodynamic devices, increased engine power and heat management. Does the front vent on the upper edge of the bumper act to both reduce lift and improve cooling by relieving the hot air generated by the radiators? I assume the lower density of the hotter air generated within the front bodywork will tend to increase the lift especially if it is allowed to be vented downwards and hence the need/advantage of the vent on the forward part upper body surface? Does that suggest a third radiator? Similarly does the increased pressure differential created by the enlarged rear spoiler assist in the enhancing cooling of the rear engine compartment by increasing airflow beneath the car? How is this balanced against the cost of increased drag? Does that tell us anything about the possible increased engine power? I assume the large aerofoil is there to increase down-force and aid stability to better manage the increased engine output? FBR

 

[spyderman]

ORIGINAL: fbr Does the front vent on the upper edge of the bumper act to both reduce lift and improve cooling by relieving the hot air generated by the radiators? I assume the lower density of the hotter air generated within the front bodywork will tend to increase the lift especially if it is allowed to be vented downwards and hence the need/advantage of the vent on the forward part upper body surface? Does that suggest a third radiator? Similarly does the increased pressure differential created by the enlarged rear spoiler assist in the enhancing cooling of the rear engine compartment by increasing airflow beneath the car? How is this balanced against the cost of increased drag? Does that tell us anything about the possible increased engine power? I assume the large aerofoil is there to increase down-force and aid stability to better manage the increased engine output?

I don't think that grille is anything to do with downforce and all to do with cooling. The S and GTS will have centre rads anyway but it could certainly be that the GT4, with a larger capacity engine, will need more cooling. Chris.

 

[ralphmusic]

My 997 PDK GTS had a centre radiator but no vent so likely it is cooling coupled with some legacy styling.

 

[Motorhead]

No problem with the Boxster Frank - a really great roadster and all credit to you for choosing the ragtop over the tintop given the inclement weather often experienced in your part of these Isles. Unfortunately, my knowledge of vehicle-related aerodynamics is pretty slim so these are just my random thoughts. I think that the front vent is there primarily to promote airflow through the front radiators, and the two front air scoops are much larger than those on the standard cars, but it's possible that the GT4 will feature an additional centre rad to aid cooling under race conditions. Venting the airflow upwards will, of course, provide some frontal downforce but it's likely to be very a small contribution. The giant rear wing is obviously there primarily to increase rear downforce and is used in conjunction with a more agressive front air dam and modified rear diffuser to produce a balanced aero package. There may well be some improvements in airflow under the car and around the engine as a result of these modifications, but it's very difficult to say if this could benefit engine cooling given that the engine is positioned well clear of the airflow. What is clear though is that the wing will increase drag compared with the standard set-up at higher speeds, necessitating increased engine power. Will 380PS be sufficient? Jeff Footnote: I believe that the centre rad was only fitted to PDK-equipped 987-series cars but is not used at all on the 981.

 

[spyderman]

ORIGINAL: Motorhead Footnote: I believe that the centre rad was only fitted to PDK-equipped 987-series cars but is not used at all on the 981.

Thanks Jeff.

 

[Motorhead]

Back on the subject of brakes [again!], Paul Frere in his 2006 book "Porsche Boxster Story" describes the extreme requirements of the Standard Porsche Brake Test as follows: This consists of 25 consecutive retardations with a deceleration of 0.8g from 90% of the car's maximum speed down to 100kph [62mph] from where the car is immediately re-accelerated as quickly as possible , using the gearbox, to 90% of its maximum speed, and so forth. During the entire test procedure the deceleration must never fall below 0.8g and the temperature of the discs must never exceed 700degC. An impressive spec which sounds more like that for a race car rather than one for normal road use. Jeff

 

[dpoynton]

ORIGINAL: Motorhead An impressive spec which sounds more like that for a race car rather than one for normal road use.

Isn't this exactly what we buy into? D

 

[ralphmusic]

Except that the drilled holes get full of brake dust and cracks emanate from the holes and the discs need replacing. Slots would be better IMO.

 

[Motorhead]

Probably down to manufacturing costs Ralph. Cheaper to drill multiple holes than to mill slots perhaps? Incidentally, I've only just come across these relevant comments from the outgoing Porsche Motorsport chief, Hartmut Kristen: http://www.motorauthority.com/news/1026708_porsche-eyes-gt4-spec-cayman-race-car Jeff

 

[ChrisW]

At last, more light than heat ... 380BHP, 6 speed manual or PDK-S from the GT3, GTS Sport chassis -20m with no PASM, mechanical locking diff, brakes from Carrera S with PCCB option. According to the January edition of GTPorsche ...

 

[daro911]

ORIGINAL: ChrisW At last, more light than heat ... 380BHP, 6 speed manual or PDK-S from the GT3, GTS Sport chassis -20m with no PASM, mechanical locking diff, brakes from Carrera S with PCCB option. According to the January edition of GTPorsche ...

3.4 or 3.6 or 3.8. Did it say[&o]

 

[flat6]

With the -20mm sports suspension from the GTS (it's actually an option for any model and pre-dates the GTS), does that mean that the GT4 won't handle any differently to any other 981 (putting aside the downforce effect from its increased aero when you are driving fast enough for it to influence). I don't want to hang off every word of a magazine at this stage though, but assuming for a second that that is true, that would imply that no work has been done in that area. I find that hard to believe. Passive dampers, yes. Lifted straight out of the parts bin, I suppose they may be that good already. I appreciate that other changes such as a wider track would culminate in a package that handles differently from other models. Will be interesting to see the full list of upgrades over the other models.

 

[Motorhead]

GT Porsche haven't brought anything new to the table 6. LSD/PTV and Sport Chrono [featuring active engine mounts] are a given, I think, and Carrera S brakes have been discussed here for some time. Lowered/stiffer suspension is also bound to feature, most probably bespoke rather than the standard sports suspension. On the choice of engine, the current betting stands as: 3.6L [MrD]; 3.8L [RoW] Which probably means that MrD is correct, given Porsche's perverse choices when it comes to allowing its lesser cars of the leash and possibly overshadowing "King" 911. Plus, Porsche may well have a stock of 3.6L short engines left over from 997.2 days. Jeff

 

[daro911]

Will we see exclusive GT4 colours too like the Met Green Cayman R[&o]