merchauser

I swapped my 16" wheels on my 1990 for Carrera LLL 18" with Michelin Pilot Sport 2 low profile tires. for the past few months I have been using the factory recommended numbers.

factory spec is 36/44, but in my pea brain, that seems like too much for the lower profile??

I have contacted local tire stores and both Michelin and Continental and nobody is on the same page

some say you MUST apply the factory PSI regardless of tire size. a few had absolutely no clue, and others just told me I needed to play with the pressures.

I am not an aggressive driver and main concern is making the new tires last as long as possible.

Comments?

SeanR

Do 34/36 and you will be fine.

Jim M.

I used to run 34F, 36R with 18" wheels 255-30 F and 315-30 R. But your right you need to play around with the pressures.

LexVan

I would agree with this.

How is it now? Confort? Performance? Turn in? Etc.

merchauser

Hey SeanR, and others, 34/36 "sounds" better, but would like to understand more about the "why?"
is there some sort of formula for a plus 1 ro 2 upgrade? at 36/44, what are my risks with tire wear or poor ride?

merchauser

LexVan at 36/44, the car seems to be fine, but I really wouldn't know any better since I have nothing to compare to.

SeanR

Go drive around with 44 in the rear then drop it to 36 and go drive. Then tell us how it feels.

hb253

See my reply in this thread from last year.

https://rennlist.com/forums/928-foru...on-advice.html

There's a link to this PDF https://toyo-arhxo0vh6d1oh9i0c.stack...s_20170203.pdf that explains how to calculate pressures for optional tire sizes.

Hugo

Jerry Feather

My experience with 44 pounds in the rears was that the tires, Michelin AS Plus, 9 by 17 inch wheels, wore right down to the air in the middle of the tread (actually) in just a few thousand miles. I went to 36.

FredR

Take a look at the writing on the sidewall and you should see the design loading for that specific tyre type. it will probably say something like "rated for 600kg @ 50 psig. The 928 runs roughly 400kg per corner plus a total load of say 200kg [assuming a couple of androids and some luggage] which is 50kg per corner. Then compute a pressure based pro rata on a total load of 450kg relative to the rated load and pressure. Whatever these numbers work out to be they should be taken as the minimum you can run with and typically work out to be somewhat less than folks expect.

I work on the premise that wider tyres with lower profile will carry a higher load and thus need less pressure and I typically run my front at 33 psig and the rears at about 31 psig but I have a 255 section up front and a 285 on the rears.

The stock S4 pressures are fine for the front but 44 psig for the rear I found to be somewhat ridiculous and the only good reason for using it was to calm the RDK alarm system. I jumpered that thing when I ran with the stock 16 inch wheels on my late S4 I typically ran about 35 psig all round [hot]. I generally allow 2 to 3 psig for warm up.

Get the pressure correct and you will optimise tyre life - too little and the edges of the tyre wear, too high and the centre goes. Racers will measure tyre temperature across the tread to check for optimal inflation for the running conditions.

If all else fails a hot pressure of 35 psig will probably get you close to where you need to be.

LexVan

Technically, racers measure the temperture of the tire's core. Not the tread. As they pit, the tread starts to cool, while the core's stay accurate for a longer period of time, and that's what the tire engineers use. I learned this from the Michelin staff this year at the WeatherTech IMSA races at Road America. Michelin gave a technical session to us in the Porscheplatz tent.

worf928

Please post your exact tire sizes.

Tire width is as - arguably more - important in determining reasonable pressures than is wheel diameter or sidewall height.

merchauser

Dave C

I am running 235/40-18 up front and 295/30-18 at rear

Bertrand Daoust

Michelin Pilot Super Sport on Carrera III wheels.

235/40-18 Front, 265/35-18 Back.

32 psi cold, front and back.

Just perfect for me.

worf928

IMO, Folks should keep in mind that manufacturer's tire pressure recommendations are constrained by liability considerations. In the case of Porsche the recommendations are for sustained autobahn top-speed attempts with a fully loaded car. They do not want an owner experiencing a blow-out at top speed with the car laden with their family and a weekend's worth of luggage.

The newer Porsche sports cars equipped with TPMS have four different pressure goals depending upon load and maximum speed.

The physics of tires haven't changed since ... the invention of the tire. But, materials, design and manufacturing technology have undergone significant changes in the last 30 years.

Modern tires are able to operate at higher temperatures and dissipate sidewall heat faster than the old-tech tires that were extant at the time Porsche recommended pressures for 928s(*).

This means that pressures can be lowered thereby increasing grip, increasing comfort (and decreasing gas mileage...)

(*) GTS is an exception because better tires were available and thus, the recommendation for GTSs is 36/36 with - just about - the same size tires. 225/50x16 front for pre-GTS at 36 vs. 225/45x17 front at 36. And S4 rear 245/45x16 at 44 vs. GTS 255/40x17 at 36.

Bottom line: for modern tires the 44 PSI recommendation is nuts.


OK.

I would start at 34-35 front and 32-34 rear cold pressures for 'normal' driving (e.g. NOT auto-cross, NOT open road race, NOT track and for sustained speeds under ~140 mph...)

Depending up the sensitivity of your butt-o-meter, tire wear, etc., change pressures in ~2 PSI increments.