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FZP
Indianapolis


Joined: 18 Jan 2015
Posts: 2414
Location: Cheshire


PostPosted: Tue Jul 10, 2018 7:45 am    Post subject: Tyre Pressure at Petrol Station Reply with quote

So the other day I noticed that the handling was feeling a bit custard, so checked pressures and sure enough they were down. According to my gauge they were 31/36. I was expecting it to be off as it's probably been 2 months since I last checked. Warmer weather plays havoc with pressures.
Went to the local petrol station and requested 36/42 as I know they can be a little off. This morning I've retested pressures and I'm still low at 33/38.
Don't trust the machine team, have your own gauge and check them regularly Thumb
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alex yates
Brands Hatch
Brands Hatch


Joined: 06 Mar 2014
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Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Tue Jul 10, 2018 8:18 am    Post subject: Reply with quote

Thats cos you warmed your tyres up driving to the fuel station! When they cooled back to ambient the pressures dropped.
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alex yates
Brands Hatch
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PostPosted: Tue Jul 10, 2018 8:20 am    Post subject: Reply with quote

Tyres should always be checked cold in an ambient environment.

Have you not read your handbook? wack
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FZP
Indianapolis


Joined: 18 Jan 2015
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Location: Cheshire


PostPosted: Tue Jul 10, 2018 8:21 am    Post subject: Reply with quote

Fair point but it's only down the road so don't think it would be enough time to generate any heat. I'm calling dodgy psi gauge at pump
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alex yates
Brands Hatch
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2000 Porsche 996 Carrera 4

PostPosted: Tue Jul 10, 2018 8:23 am    Post subject: Reply with quote

Seriously, doesn't take much to get your tyres a few degrees higher. You wanna check the pressure after a hard drive when they're up nearer 50.
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Zingari
Donnington
Donnington


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PostPosted: Tue Jul 10, 2018 9:06 am    Post subject: Reply with quote

You're best over inflating on those auto machines to get your money worth. Then using a gauge at home to let the air out to the right pressure Thumb

All to do with Boyle's law apparently what Big Joes (JCX) thread on getting the optimum pressure on his 964 is the stuff of legend Grin
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FZP
Indianapolis


Joined: 18 Jan 2015
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Location: Cheshire


PostPosted: Tue Jul 10, 2018 9:22 am    Post subject: Reply with quote

I thought going 2 over would be plenty for the let down this morning. I'll have to go searching for that thread Thumb
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FZP
Indianapolis


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Location: Cheshire


PostPosted: Tue Jul 10, 2018 9:42 am    Post subject: Reply with quote

Zingari wrote:
You're best over inflating on those auto machines to get your money worth. Then using a gauge at home to let the air out to the right pressure Thumb

All to do with Boyle's law apparently what Big Joes (JCX) thread on getting the optimum pressure on his 964 is the stuff of legend Grin

I'm assuming this is the thread? I also found that reading the below thread, I needed to up my pressure to 1.21 jiggawatts at a constant 88mph to ensure I didn't cross the streams....and as we know crossing the streams is very bad.

http://911uk.com/viewtopic.php?t=95813&highlight=tyre%2A
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Disco
Estoril


Joined: 13 May 2008
Posts: 3874
Location: Hertfordshire

2010 Porsche 997 GT3

PostPosted: Tue Jul 10, 2018 1:11 pm    Post subject: Reply with quote

FZP wrote:
Fair point but it's only down the road so don't think it would be enough time to generate any heat. I'm calling dodgy psi gauge at pump


Dodgy gauge at the pump is generally assumed to be ubiquitous, but beyond that - think about the environment around the rears. Between the engine and the side boxes of the exhaust system on the 997 your rears can actually go up a couple of lbs if the car is just sitting idling for a while. Thumb
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jonno_
Spa-Francorchamps


Joined: 29 May 2014
Posts: 267
Location: Oxfordshire

2005 Porsche 997 Carrera 2S

PostPosted: Tue Jul 10, 2018 5:38 pm    Post subject: Reply with quote

Just to add another factor, esp. in summer, if you park outside you can easily get differential heating from the sun - I found it added c.5psi compared to the shady side.
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FZP
Indianapolis


Joined: 18 Jan 2015
Posts: 2414
Location: Cheshire


PostPosted: Tue Jul 10, 2018 6:16 pm    Post subject: Reply with quote

After reading JCX thread about pressure, I've inflated to 40/46 and do the let down later tonight and see what happens. I'd not thought about the heat soak effect of both exhaust and engine in and around the rear wheels, but obvious when pointed out. Still, unsure how much heat can be generated in a mile though to make that much difference to psi.
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alex yates
Brands Hatch
Brands Hatch


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Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Tue Jul 10, 2018 8:06 pm    Post subject: Reply with quote

Alot.
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FZP
Indianapolis


Joined: 18 Jan 2015
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Location: Cheshire


PostPosted: Wed Jul 11, 2018 7:56 am    Post subject: Reply with quote

Ok so tyres were set at 40/46 at the same petrol station. This morning and they were 36/41. I've dropped the pressure down to 34/40.
The one thing I can say about Michelin tyres is that they're highly sensitive to pressures. When the pressure is correct they're on another level. When they're off, it makes my car feel like it needs an alignment or something is broken.
Might go find some roundabouts later to test Grin
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alex yates
Brands Hatch
Brands Hatch


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PostPosted: Wed Jul 11, 2018 8:18 am    Post subject: Reply with quote

Never check your coolant level until car has stood overnight and temp is ambient. Same rules of expansion apply Thumb
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Zingari
Donnington
Donnington


Joined: 25 Oct 2009
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1993 Porsche 964 Anniversary

PostPosted: Wed Jul 11, 2018 9:10 am    Post subject: Reply with quote

alex yates wrote:
Never check your coolant level until car has stood overnight and temp is ambient. Same rules of expansion apply Thumb


Breaking wind the morning after a mutton tikka vindaloo and 10 pints of Guinness can have the same effect.

I've also tried taking a huge gulp of air before getting on the scales but so far it's not made me any lighter?

Once you get the optimum pressure it might be worth getting them filled with nitrogen to control the heat v pressure change Dont know
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FZP
Indianapolis


Joined: 18 Jan 2015
Posts: 2414
Location: Cheshire


PostPosted: Wed Jul 11, 2018 9:33 am    Post subject: Reply with quote

Zingari wrote:
alex yates wrote:
Never check your coolant level until car has stood overnight and temp is ambient. Same rules of expansion apply Thumb


Breaking wind the morning after a mutton tikka vindaloo and 10 pints of Guinness can have the same effect.

I've also tried taking a huge gulp of air before getting on the scales but so far it's not made me any lighter?

Once you get the optimum pressure it might be worth getting them filled with nitrogen to control the heat v pressure change Dont know

In the topic of Nitrogen, from all the reading I've done on it over the years is that each Nitrogen molecule takes longer to escape the tyre vs air. Less fettling required. As for stability it seems like in the real world road conditions, it's negligible.
I've just come back from a spirited run to the shops and the run out was great, the inbound run was greasy. I'm trying to remember when CG set my TP as to whether it was the last thing they did or done after the test drive. It was Xmas time so rather fresh. Might set the pressure the afternoon when the temps at it's highest and see if that helps.
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