911UK said:
Jamie
One final question, regarding a great area of debate on 993 18" rear tyre pressures
993's run most of the range with front and rear tyres pressures at 36psi
the 17"s tyre combo is regarded as the best set up with 255/40/17 at 36psi
How when you run 18's on the back the Porsche advised tyre pressure is a whopping 44psi which had a significant impact on the ride quality, which become very hard in comparison to running 17's more than just the swap up to 265/30/18 or 285/30/18 rubber and a lower profile tyre
Many owners find the 44psi setting creates a ride firmness that is far too harsh
The question is multi-part, why such a high PSI level just from going up from 17's on 255/40/17 on 36 PSI to 18's on 265/30/18 or 285/30/18 on 44 PSI ?
We know that running tyres under inflated can cause problems, but can the rears be run safely and comfortable at pressures down from 44psi, towards maybe 40psi or 36 PSI as run on the 17's?
What is the solution to this ? as a recent survey of 993 owners running 18's found that their average PSI was set on average at around 40PSI or lower than the OEM 44PSI as the ride setting was too harsh
Also for Track Days, you normally recommend running tyres HOT about 3 PSI higher, but taking the 44psi up to 47psi then puts it VERY close to the Tyre max of 50 PSI (stamped on the side of the tyre!)
So what's the Tyre pressure advice for 18s 265/30/18 or 285/30/18 on a 993 ? keep it at the OEM COLD 44 psi or are there other safe options ?
thanks
Hi
firstly, the tyre pressures on car are set for two reasons, firstly being to carry the load of the car, and also to give the tyres the appropriate settings for the balance of the car to handle well.
So breaking this down..
On 18" tyres, most of which will have been original spec on quicker 993's primarily the settings will be based upon the load and force potential generated by these cars, so naturally the 18" tyres will have a higher pressure setting. On a larger wheel with a smaller sidewall, it is often needed to run a slightly higher pressure simply as the forces are concentrated more in the junction of the sidewall and tread, meaning to reduce any fatigue potential, a little bit of pressure can improve this.
The pressure settings will reduce comfort, but equally an 18" tyre will always be less comfortable than the 17" one.
Unfortunately I can't endorse legally running lower pressures. It's the drivers choice and the drivers risk. While I'm not saying it won't work, it wont have been tested by Porsche or most tyre suppliers. Hence you will increase the tyres rolling resistance and possibly the wear will get worse.
If it works, then thats fine, but sadly I can't endorse it.
On track day pressures, then again if you are driving hard, you need the right pressures to make sure the tyres are fine under high stress. But Always bleed them down hot after a couple of steady runs, and this will stop them going through the roof and rising too high.
the 50 psi on the sidewall is a cold starting pressure, which would be higher hot. So don't worry they wont get to 50 psi and burst
