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Tyre pressure question

Kimbo

Paul Ricard
Joined
5 May 2011
Messages
3,483



Please mister,

How come recommended tyre pressures for 16" and 18" wheels are 36psi for the Fronts and 44psi for the Rears...but for 17" wheels they are 36psi all round?
 
Please sir, please sir, I know this one! :bye:

I found out the hard way when my car was handed back to me by a workshop on 18" inch summer tyres, but with the pressures it had previously had in the 17" summer tyres, i.e., 2.5 bar instead of 3.0 bar in the rears.

I spent several days wondering why my nearly-new car had suddenly started to bunny-hop on speed bumps or even at slow speeds on wavy road surfaces. I thought it was an engine management or fuel supply problem at first. Later experience with a gummed up ISV in my current car reminded me of it, in fact.
I realised the cause when I routinely checked the tyre pressures and the problem went away after correcting them. (And before making an appointment to take the car back to the workshop for diagnosis).

I suspect that this will not have been the main reason that those pressures were chosen though. :D It'll probably be down to the different relationship in the cross-sections of those tyre sets calling for a different "balance" of the front/rear pressures in order to optimise the car's handling under usual road conditions.
 
I would say from this photo evidence (bottom pic), your 18" Turbo twists are running about atmospheric pressure all round (no pun intended, as they are definitely flat, but luckily, only at the bottom):

 
Red993C4 said:
Please sir, please sir, I know this one! :bye:

......It'll probably be down to the different relationship in the cross-sections of those tyre sets calling for a different "balance" of the front/rear pressures in order to optimise the car's handling under usual road conditions.

So, Red, you don't know this one. Go and stand in the corner.

.. no I don't know either, but whilst looking just now I found a Tech Info sheet from Porsche (number 4440) which states you can run 5mm spacer on the front with ET55 Cup2s - you wanted to do this didn't you Kim?

205/50 ZR 17 7J X 17 55.0
front 5 mm spacer ring per wheel approved, P/N: 993 361 605 00**
 
orangecurry said:
.. no I don't know either, but whilst looking just now I found a Tech Info sheet from Porsche (number 4440) which states you can run 5mm spacer on the front with ET55 Cup2s - you wanted to do this didn't you Kim?

Yes, thanks I did. I opted for a 9mm shim in the end just for the fronts, which doesn't require any extended bolts and pushes the wheels out just enough to fill the arches a bit more.

£40 from Porscheshop I think.



Now Red, you've been told about sticking your hand up before you know the answer.
 
Kimbo said:
Now Red, you've been told about sticking your hand up before you know the answer.

Sorry, sir. I had it written down but the dog ate my homework. :eek:
 
Red, I've just about had enough of your (lame) Dog excuses.

Curry, a hundred lines.





Very funny. No Rugby for a week.




Where's Uncle Exel.
 
These pictures proves 18" Twists, do look the best on a 993 :D
 
Not personally keen on that look Neil. The Hollowspokes got sold pretty quickly.

I like to see rubber.
 
Kimbo said:
I like to see rubber.

Uh oh, I think we know where this could go next... :hand:









Yes, a heavy discussion on tyres you filthy minded boys.
 
I can't answer the question, but I do know from experience that 44 feels far too high a pressure for the rears on 16s and 18s.

39 or so feels just right.
 
Leosayer said:
I can't answer the question, but I do know from experience that 44 feels far too high a pressure for the rears on 16s and 18s.

39 or so feels just right.

I agree. 44 is far too high. I've been running 37.
 
The Porsche 993 wheel bulletins specify wierd tyre pressures for the 17" wheels.

Could it be a typo and it should also read 44psi at the rear? :?

It is even stranger when you see that the winter tyres specifies 44psi at the front and 36 psi at the rear??

I've attached pics of the relevant portion of the bulletins - hopefully the've come out ok.
 

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  • porsche_tyre_pressures_113.jpg
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  • porsche_tyre_pressures_winter_156.jpg
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award said:
The Porsche 993 wheel bulletins specify wierd tyre pressures for the 17" wheels.

Could it be a typo and it should also read 44psi at the rear? :?

It wasn't a typo. Those pressures were also the ones shown on the stickers on the cars as they left the factory when new.
 

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