what tyre pressure are you using?

[panameras]

I have heard a lot of different opinions on this topic.

Tyres I have are 235/40/18 front and 265/35/18 rear.

My hand book says: 29psi front and 36psi rear.

Some say low profile tyres need higher psi. less sidewall requires higher pressure.

so standard 16's are prob ok with 32-34psi. but 18/35's or 19/30's need about 38-40psi.

My driver training instructor recommends 30psi all around cold, doesn't matter what size tyre or profile.

What is the real story for tyre pressure? Do different size tyres require different pressures? ie; 235/40/18 vs 265/35/18 vs 235/45/17 vs 265/40/17??

What about drivetrain layout? AWD require different tyre pressure to RWD? and FWD? and engine layout? In the case of 986, midship engine requires higher rear tyre pressure?

Thanks

Edited July 9, 2007 by panameras

[Westcoaster]

Assuming that you are running the tires sizes that came on your car...

Alway! check when the tires are cold, and put in exactly what the tire placard on the door jamb recommends.

[FrayAdjacent986]

I'd go with what Porsche says over what your driving instructor says. Unless of course he's a mechanical engineer with years of experience on suspension components and tire engineering....

You might tweak your pressures on the track a little... but for daily driving, go with the specs provided by the manufacturer.

[jmatta]

I set my tires as stated in this PCA Tech article; I agree the car feels much better with the higher pressures (my car is an'02 S with 8/10x18's running 225s and 265s currently). Keep in mind, factory recommended pressures will induce understeer to keep drivers safe from themselves. These are also street pressures, as I don't even recommend running "street" tires on a track; they were not built for the loads or slip angles. If you continue to track your car, purchase an R rated DOT rubber like a Pilot Sport Cup. You can play with pressures all you want to suit your driving style and ability. I've been tracking 911s for many years, but do not track my Boxster (though I have driven numerous cars as an instructor for the Chicago Region).

http://www.pca.org/tech/tech_qa_question.a...B-14C71E506D95}

Copied from the PCA Tech Website...

JM

[911-996]

I’m running the stock 17" alloys, Porsche recommend 29 psi front and 36psi rear

I generally put 2-4psi more than recommended to compensate for pressure loss until the next checkup + it feels more comfortable with a firmer ride.

Edited May 25, 2007 by 911-996

[nick49]

I go with 36 front and 44 rear cold on my '99 996 Cab as recommended in my Porsche manual for 18" wheels. I had run less but just got new rear tires so I thought I check what the manufacturer recommended and go with that. Because I had run less for a long time on my fronts and then filled them to the recommended pressure, my handling felt really darty and the car would want to turn way more than my input. This is because with the higher pressure the footprint is way too small and has adversly affected handling in a negative way. As the fronts are slowly wearing in and the footprint is getting larger the handling is slowly returning and will ultimately be terrific.

Underinflation will increase wear significantly and can actually cause the casing to become stressed and damaged making the tire unsafe for high speeds and much more prone to a blowout. These Z rated tires need high pressure to keep them cool and work as designed, Porsche and the tire mfgs know better than anyone what should be run. And the recommended pressures are generally good and safe for all conditions, but some knowledgable people can play with the pressures to fine tune their car to their individual requirements. Before doing this understand and know the negative effects that can result.

[panameras]

I set my tires as stated in this PCA Tech article; I agree the car feels much better with the higher pressures (my car is an'02 S with 8/10x18's running 225s and 265s currently). Keep in mind, factory recommended pressures will induce understeer to keep drivers safe from themselves. These are also street pressures, as I don't even recommend running "street" tires on a track; they were not built for the loads or slip angles. If you continue to track your car, purchase an R rated DOT rubber like a Pilot Sport Cup. You can play with pressures all you want to suit your driving style and ability. I've been tracking 911s for many years, but do not track my Boxster (though I have driven numerous cars as an instructor for the Chicago Region).

http://www.pca.org/tech/tech_qa_question.a...B-14C71E506D95}

Copied from the PCA Tech Website...

JM

I dont really track my car that much, so my question is to find the optimum tyre pressure for maximum grip and handling of street tyres on the road.

I agree that factory recommended pressures will induce understeer. As a mid engine RWD boxster tends to oversteer more than an AWD or a FWD car, therefore lower recommended pressure at the front (29psi) for more grip and higher pressure at the rear (36psi) for less grip which would result in understeer.

All for safety reasons then?

or is the factory recommended tyre pressure there for tyre conservation?

because higher tyre pressure will wear the tyre evenly where as lower pressure will give you more grip but reduce tyre lifespan.

The last car I had was a front engine rear wheel drive and I found it handles best will 32psi front and 30psi rear. but this setting could be ineffective on a boxster since the weight balance is different.

Edited July 9, 2007 by panameras

[911-996]

Interesting theory panameras

Im guessing its got more to do with inducing understeer, tire wear wouldnt be their main concern.

Porsches generally turn to this for the safety factor but honestly I find a little more oversteer to be more comfortable.

[k-vic]

Interesting theory panameras

Im guessing its got more to do with inducing understeer, tire wear wouldnt be their main concern.

Porsches generally turn to this for the safety factor but honestly I find a little more oversteer to be more comfortable.

Vehicle manufacturers don’t make tires and normally recommend the minimum tire pressures for a vehicle. The tire manufactures gives you the maximum tire pressure for a tire and sometimes you can see the recommended maximum pressure on the side wall.

Tire pressure is about the amount of air needed to keep the sidewall from collapsing on turns. Check out the load tire on a car turning. As far as the recommended tire pressure I go for 2/3 the maximum allowed for safety when driving hard and slow and to provided good driver feel and handling and for optimum tire wear .

[steve_wilwerding]

I have stock 17"s and I run 31F/34R - takes a little of the understeer out and suites my driving style. I've found also that on street tires, 31F/34R cold works pretty well on the track (not sure what hot temps are).

[986fan]

Your book is correct

[panameras]

I have stock 17"s and I run 31F/34R - takes a little of the understeer out and suites my driving style. I've found also that on street tires, 31F/34R cold works pretty well on the track (not sure what hot temps are).

exactly what I have found. I now run 18s with 31F/35R on the road.

[rocket625]

I got my car back from the wreckers and they put 40 in the back and 36 in the front, running Yokohama 17 inch on stock 986 wheels.  Might be riding a tad stiff. 

[Canada-Eh]

The lower the psi in the front compared to the rears the more understeer you will have. Porsche purposely tries to build in understeer these days as it is generally preferred over oversteer. I've had 2 Boxsters - my current one is 33 lbs in the fronts and 36 rears with 17" rims. I've found this setup more responsive.