3.45 BAR seems high to me

[ProfMark]

Hello everyone. I'm a new Porsche owner whose 996 C4S came with Pilot Sport (N3) tyres. 225/40 ZR 18 and 295/30 ZR18. They were manufactured in '16 and '20, respectively. They both have "Max 50psi" printed on them.

I checked pressures for the first time, and found them inconsistent, with pressures between 1.7 BAR and 2.4 BAR, so I filled them all to 3.45 BAR (50psi). But 50psi seems high to me. Should I be running them that full? My car is a daily driver.

It's important to know that I have never had low profile tyres before, so this kind of pressure is new to me.

Thanks in advance.

 

[Magic919]

Start with the Porsche recommendation of 36/44 and work down from there.

 

[ProfMark]

Start with the Porsche recommendation of 36/44 and work down from there.

36/44 is recomended for the stock tyres though. These are different sizes. Doesn't that make a difference?

 

[Magic919]

No.

 

[Parsley]

Start with the Porsche recommendation of 36/44 and work down from there.

This :thumb:

 

[ProfMark]

Ok. I'll refill them to 36/44.

But it still leaves my question: why? Don't different tyre profiles require different pressures? The 225/40 ZR18 and 295/30 ZR18 arent stock tyre sizes.

 

[Ratattak]

Ok. I'll refill them to 36/44.

But it still leaves my question: why? Don't different tyre profiles require different pressures? The 225/40 ZR18 and 295/30 ZR18 arent stock tyre sizes.

Because tyre pressure is dependant on the vehicle loading not the tyre size

 

[ProfMark]

Interesting. So if I ran monster truck tyres or racing slicks, they should still be filled 36/44?

 

[Ratattak]

Interesting. So if I ran monster truck tyres or racing slicks, they should still be filled 36/44?

Why would you do that ? Anyway they wouldn't fit !

 

[Magic919]

Profile is the least significant bit as it's just the ratio for the sidewall depth. Width changes, if significant, could affect pressure required.

If you could measure the contact patch of your tyres and compare with a typical standard size tyre, you'd have a clue what might be required.

Back in the real world, the suggestion above works.

 

[Rincewind]

Pretty sure the tyre sizes you mention are the standard C4S tyre sizes anyway.

 

[ProfMark]

Profile is the least significant bit as it's just the ratio for the sidewall depth. Width changes, if significant, could affect pressure required.

This is exactly what confused me! I assumed that low profile tyres required far more pressure, which is why "50psi max" is printed on them, and ultimately why I filled them to 50.

I'll get around to refilling them tomorrow afternoon. My car drives fine with them at 50, and the tyres can be run at 50, so there is no huge rush to correct them. By the evening they'll be 36/44.

thanks everyone :thumbs:

 

[infrasilver]

It say 50psi max on them because that is the maximum pressure you should ever put in that tyre due to the possibility of over filling and the tyre failing. Stick to factory spec or sometimes a little lower if you want a comfier ride?

 

[deMort]

Tyre pressure for different tyres will be in the hand book .. should also be a label under the bonnet or inside the fuel cap lid depending on model .


2.5 F bar
3.0 R bar.

A lot of people run a few psi lower for a better ride .

 

[gizlaroc]

Porsche is a bit weird in that they only ever put the max pressures, which presumes fully loaded car and max speeds.
The reality is most 911's are 36F ad 44R for that, however, for road use more like 32-34F and 36-40R is far better.


You can always use the old 4psi rule if you don't know what pressures.
This is slightly less accurate with modern rubber, but goes like this.

From cold set your pressure, then after a long fast run, ie properly hot, measure the pressures again. You are looking for an increase of 4psi.
If less your pressure is a bit high to start, if more than this the pressure is too low.

Probably more like 3, or even 2psi rule these days, but pretty good way of getting pressures close to where they should be.


50psi is way too high, I would run them too far on that pressure.