antoniodelgad68

Porsche should at least give a tentative release date to slow down all the rumors, I know wishful thinking. TD

Nizer

Interesting that tire sizes are same as 918 Spyder, though the RS load ratings are slightly higher.

918 Spyder:
Front - 265/35ZR20 95 (Y)
Rear - 325/30ZR21 104 (Y)

GT3 RS:
Front - 265/35ZR20 99 (Y)
Rear - 325/30ZR21 108 (Y)

antoniodelgad68

GioF that is one sick video, well at least we no it is not turbo charged. TD

Macca

Paris early October gets my money.

Can some one tell me why a GT3 needs a 1" larger rear wheel than front wheel? I can understand lower profile can assist feel and turn in on the track with a front wheel but rear wheel? It cant be the need for a wider rim as the 991 GT3 already has a 12" rim fitted "only" with a 305 (could easily accommodate 325).

I dont get it. It adds cost to consumables and makes the chances of getting a decent R comp tyre for track days even less likely than the 20" on the standard GT3. As for the load ratings Id say there is nothing indicative in that to the cars performance...

stujelly

I don't see this as a good track car compared to the 997.2

tires selection is going to be a challenge.

can't wait to be proven wrong

I hope I can save me from myself.

Last RS was beautiful then the track bug hit.....

Hope I can keep this one off of the track

Hoopumpers

Really? Have you tried tuning new turbo cars from manufacturers such as BMW or Mclaren recently? ECU is entirely locked down through encryption. No success in cracking them for years. Only options for "tuning" is through fairly arcane piggyback trickery. Additionally, with the complexity of hybrid systems, I would not trust some home brew mechanic to muck around with the cars.

Nizer

Load rating has nothing to do with performance. it relates to the load carrying capacity of each individual tire.

What's interesting is that the 918 Spyder is likely to end up being 500-600lbs heavier than the upcoming RS yet is spec'd with a lower load rating tire.

Macca

^^^ my guess is the 918 tyres were designed long ago and Michelin have come up with a newer more load capable design in the passing years. Just a hunch.

Mike in CA

The only performance advantage I can think of is that a larger diameter wheel allows for a larger contact patch, all else being equal.

http://www.roadandtrack.com/motorhea...t-tire-patches

Macca

Thanks Mike. Assuming rolling radius is the same as 991 GT3 rear wheel on 20" and again assuming you put a 325 on teh rear of the GT3 to make it comparable (it does afterall have plenty of space being 12" wide) I really cant see any measurable benefits. Certainly nothing that would make a worthy difference assuming a capacity increase/suspension geo improvement and PDK-S software changes.

Im getting the distinct feeling the 991 GT3RS will be more design over substance (i.e. very agressive looking with its extra wide rear end and gils over its wider front fenders, double bubble composite roof etc). Im sure it will move the game on - of that I have no doubt but many of these features I am seeing like larger rear wheel, wider front and rear fenders, double bubble roof and scooped hood I am scratching my head to understand what exactly they are there for from a purely practical/performance perspective. I was never an RS customer truth be told - but in any case where I can see design behind purpose on my 991 GT3 on the RS Im getting lost in design signatures and wow factor. After all the 991 GT3 already has the C4S wide bidy and would be no issue to stuff some 325s under teh rear on the existing rims. Inafct with some extra negative camber my geo guy was pretty sure a 265 front would work fine with a 325 rear - beyond teh stuff we cant see (spherical lower/upper control arms, monoball inserts etc) and capacity/HP/Torque increase Im thinking much of the eye candy works against the "fit for purpose" brief (i.e. heavier body with lower drag etc). These are just observations - Im sure this will be an absolutely killer car for a track junkie and be very coveted due to its lower volume and higher price. maybe we will all be surprised at the launch. Im 100% sure we are looking at an atmo engine with PDK only and 525 bhp with 330lbft tq. My guess on performance numbers are 7.17 Ring Time and 3.3s to 100 kmph with vMax only 1 mph more than the GT3 (to keep the marketing boys happy!).

Mike in CA

Theoretically, a larger diameter tire will give a larger overall contact patch, something that can't be achieved just by making the tire wider. But as you so correctly point out, whether it provides enough of a measureable benefit to outweigh the disadvantages is an open question. I'd like to think Porsche is using a 21" rear wheel for a sound engineering reasons and not for bling, but it does seem like they might be looking for "improvements" and "wow" factor over the GT3 at the expense of practical issues. But then who ever said an RS was practical!

Macca

Mike.

Here is the data from tirerack on the two tyres.

The new 991 GT3 has a reasonable increase in front contact patch with the 265 over the 245 at the front. So along with the slightly wider track we can expect reduced understeer and more front end grip. But you will notice the front rolling radius is quite a lot more than the 245. I have done something similar on the 993 for track use (235/245 fronts).

Now look at the rear. the rolling radius is larger too. The tyre contact patch is considerably wider at 1.1" over the 305/20 tyre!


How ever the revs per mile have fallen dramatically (almost 10%!) and are totally out of alignemnet withthe front tyres (not an issue as such).

When you increase the rolling of the drive wheel on a car you effectively make its gearing higher. This is how they will pull out the 1 mile per hour increased top speed. I dont believe its much to do with grip. The track is already increased. The rear engine configuration already provides in a 911 more grip in a straight line than you need! If the car was packing 800 bhp I could see the advantage iut not at circa 500.

So the front tyre is wider for more grip and less understeer but the rear is only slightly more contact patch for higher gearing (often not so good on track but good for Vmax runs)

The fact is you can order 265+325 on 20 combo right now as they will fit fine on the 991 Gt3 9+12J rims and benefit from some of this same theory.

It will be interesting if the car looks visually odd for the larger front wheels. Chances are the 21" rear rims will hide this. You could notice it with my 993 if you were fussy - but it doesn't affect dynamics. However increasing rear rolling radius does make a noticeable difference to the on track gearing has I have learned and smaller rolling radius wheels are generally accepted to be "faster"....so....

Heres my prediction - they will shorten the ratios in the box and make a big plus point from this (when actual fact uit will just be accommodating the lower rev per mile effect of the larger diameter rear rolling radius hardware!).

P.S. Im not an engineer!

<3mph

IIRC, there were rumours the RS would be available with optional magnesium wheels direct from the 918. If so, it would make sense to keep all wheel options on the car of the same dimensions.
Also, I wonder if there may be some benefits in terms of RWS by going with a wider tire. Maybe someone with the technical smarts could comment on that theory.

nash

It will be a joy to be locked in to the OEM tires at those sizes with no other options.... let's start the guessing at $850/rear tire.

I'm looking forward to the car, but it's going to be unrealistic to track it with those wheel sizes.

Macca

Its a possibility but given the price of those wheel options on the 918 youd probably need a lazy 20G to buy them!

If you look at the chart I posted youll find the 991 GT3RS factory tyre fitment in those sizes is 10lb heavier alone than the stock GT3. Then add the extra weight of a 21" rear wheel x 2 and Im sure you will talking close to 18lbs. The PCCB on the GT3 is about the same weight as the steelies due to the larger thickness and diameter of the ceramic discs so no real benefits there. +18lbs of un sprung weight is probably no big deal to handling but along with a heavier body shell you can see small stuff like this adds up and it will need a very big effort indeed to get this car to the sub 1400kg published weight it ideally should be (given differences with previous gen RS cars)....

A set of tyres for the 991 GT3 will be $2500 USD plus shipping according to their site - around $360 USD a set more than the 991 GT3 factory sizes and over $500 USD more than most "other brand" R comps should run to when sizes are available....

By was of comparison the correct size fitment Michelin Super Sport tyres cost $1400 USD a set.....Im going to be running these after the MPSC2 on my car are finished to see what the overall time difference is at my local track. Im betting less than 2 seconds on a 2 mile track with much better wet track/road performance and 30%+ longer tyre life in mixed usage so for a saving of almost $800 USD a set given a mixed usage if only 1-2s in it I will save the money and go with the non N rated MPSS and bank the difference!