Chaz Posted February 2 Report Share Posted February 2 Warning, this is a really nerdy and ultimately not particularly real-world relevant post... but since I have thought about it and failed to find an answer online, I thought I would ask. I was doing my usual tyre pressure check and forgot that the recommended pressures for 18 inch winter tyres are not the same as the 19 inch summer ones. At the same time, my pressure gauge failed and I went hunting to buy a new one. As a result, I was checking the Bar to PSI conversion and came across a note on Black Circles, which said that measuring in Bar with a digital meter that can report to a decimal place is more accurate than PSI. I realised that was nonsense, since there are 14.50774 pounds per square inch pressure to every 1.0 Bar (also stated on the Black Circles website!) an therefore a single PSI is 1/14th of a Bar versus 1/10th. Besides which, the digital gauges I've used for ages reads at 0.5 PSI. All of that took me back to the numbers in the manual and they are given in Bar, with the PSI in brackets. Trouble is that the conversions to PSI are not accurate and that made me think that Porsche must "think" in Bar, being metric/European. Here's what I mean: The manual gives 2.0 Bar (30 PSI) front and 2.1 Bar (31 PSI) rear for 18 inch wheels. Using the correct conversion, I make that 29 PSI and 30.5 PSI (30.466 in fact). For the 19 inchers, 2.2 Bar (32 PSI) front is fine, but "2.3 Bar (34 PSI)" rear, is closer to 33 PSI (33.3678). There is a note in one of the books about developing the 987 that remarks on the improve tyre pressure balance front and rear, compared to the 986 - about which I have no idea if it matters or is just marketing puffery. * * * So, when we say or when you read 32/34 as standard pressures for 19 inch wheels, should we really be saying 32/33 PSI? Like I say, totally not at all relevant, but I thought I would get my maths checked at least! Cheers Chaz 1 Quote Link to comment Share on other sites More sharing options...
½cwt Posted February 2 Report Share Posted February 2 (edited) They are German, so SI units i.e. Bar (or even Kpa) not psi for pressure, Nm not lbft for torque, KW (or PS at a push) not bhp for power. The only things that are truly imperial on a Boxster that I am aware of are the wheel sizes and the thickness of the original dual row IMS bearing. To your point, I set my tyre pressure gauges to Bar when doing the Porsche (and The Better ½'s Golf). Edited February 2 by ½cwt Quote Link to comment Share on other sites More sharing options...
Paul P Posted February 2 Report Share Posted February 2 My car was serviced at OPC back in April - 20,000 mile service interval - they delivered it back. Because I like to check I looked at "miles to next service" - approx 18.4K miles - huh? Questioned OPC - "wheres my 1600 miles gone" - answer - "PIWIS is sh*t, service interval is 30K Km ....." - I have a PWIS - they turned out to be right - its how it translates it, so yeah, they think in Km, Bar, Litres, Bratwursts .and cabbage. And.... tyre pressures are "at 20 deg C @ sea level" - so the numbers they quote are a bit approximate, but at least the newer cars have temperature compensated values in the TPMS display. Quote Link to comment Share on other sites More sharing options...
iborguk Posted February 2 Report Share Posted February 2 11 minutes ago, Paul P said: at least the newer cars have temperature compensated values in the TPMS display. Which read in Bar (at least by default). Quote Link to comment Share on other sites More sharing options...
BBB Posted February 2 Report Share Posted February 2 I think that although your calculations are correct, they imply a degree of accuracy that is illusory. We would all have to own calibrated pressure gauges, with an accuracy of +/- 1 psi for it to have any relevance. So, the manual figures are “good enough” for the purpose we use them for. Quote Link to comment Share on other sites More sharing options...
½cwt Posted February 2 Report Share Posted February 2 1 hour ago, BBB said: I think that although your calculations are correct, they imply a degree of accuracy that is illusory. We would all have to own calibrated pressure gauges, with an accuracy of +/- 1 psi for it to have any relevance. So, the manual figures are “good enough” for the purpose we use them for. And know what the ambient temperature adjustment for pressure needs to be compared to the control conditions that Porsche use to set the 2.0 or 2.5bar measurement. Quote Link to comment Share on other sites More sharing options...
½cwt Posted February 2 Report Share Posted February 2 1 hour ago, Paul P said: My car was serviced at OPC back in April - 20,000 mile service interval - they delivered it back. Because I like to check I looked at "miles to next service" - approx 18.4K miles - huh? Questioned OPC - "wheres my 1600 miles gone" - answer - "PIWIS is sh*t, service interval is 30K Km ....." - I have a PWIS - they turned out to be right - its how it translates it, so yeah, they think in Km, Bar, Litres, Bratwursts .and cabbage. And.... tyre pressures are "at 20 deg C @ sea level" - so the numbers they quote are a bit approximate, but at least the newer cars have temperature compensated values in the TPMS display. Not unique to Porsche that one. My VAG daily is coming up for second service at around 38k miles. Quote Link to comment Share on other sites More sharing options...
Chaz Posted February 4 Author Report Share Posted February 4 Thank you all. Good to know that the thinking is correct, even if the minor variances are too small to care about. I was also trying to get my head around the recommended pressure at 20 deg C and what you should set if the ambient temperature was 0 deg or 10 deg, in broad terms. On the basis of 0.1 Bar difference for each 10 deg change, you'd try to set at 2.1 Bar when it was 10 deg (front for 19 inch) and 2.0 when the temperature drops to zero. Trouble is that temperature never does drop in convenient 10 degree leaps! On the question of a calibrated tyre gauge, the Etenwolf (https://www.etenwolf.com/products/digital-tire-pressure-gauge?variant=44165534351574) is the only one I've found that says it is calibrated. Cheers Chaz Quote Link to comment Share on other sites More sharing options...
½cwt Posted February 4 Report Share Posted February 4 3 hours ago, Chaz said: Thank you all. Good to know that the thinking is correct, even if the minor variances are too small to care about. I was also trying to get my head around the recommended pressure at 20 deg C and what you should set if the ambient temperature was 0 deg or 10 deg, in broad terms. On the basis of 0.1 Bar difference for each 10 deg change, you'd try to set at 2.1 Bar when it was 10 deg (front for 19 inch) and 2.0 when the temperature drops to zero. Trouble is that temperature never does drop in convenient 10 degree leaps! On the question of a calibrated tyre gauge, the Etenwolf (https://www.etenwolf.com/products/digital-tire-pressure-gauge?variant=44165534351574) is the only one I've found that says it is calibrated. Cheers Chaz Doing a bit of research I found info from Continental and Kwik Fit which says maintain the listed pressure to avoid under or over inflation which then affects the contact patch and wear patterns. So adjust to say 2.0bar or 2.5bar per the manufacturer's data. I guess the logic is that as tyres are effectively air cooled or water cooled by the environment they run at ambient plus x°C. Hence the advice to regularly check your tyre pressures, and adjust if required. Quote Link to comment Share on other sites More sharing options...
Chaz Posted February 5 Author Report Share Posted February 5 When I was looking at this a few months ago, out of idle curiosity, I found the online advice a bit confusing and impractical. I think the difficulty arises from theory versus practice. The advice not to increase your tyre pressure because the gauge reads under the "correct" pressure when it is colder than 20 deg, is based on the assumption that your tyres are correctly inflated and have maintained that pressure before you check. Same for the advice on not letting air out of the tyre when checking in hot conditions (mainly found in Californian-based tyre advice websites!). Since tyre pressure is relative to ambient temperature, what you would need to do is deflate the tyre to less than the temperature-adjusted pressure and reflate to hit that target. What I can't see any of us doing is waiting until the air temperature is 10 deg and then adjusting tyre pressures to 0.1 Bar less than the manual's stated numbers! In practice, if we are checking and adjusting tyre pressures weekly or so, it makes sense to adjust to the manual's recommendation (or your preferred, experience-tested settings). At worst (for the UK) you'd be 0.2 Bar over-inflated when warm (assuming you adjust at zero degrees), which is 10%ish. Would that matter? Cheers Chaz 1 Quote Link to comment Share on other sites More sharing options...
Chaz Posted February 14 Author Report Share Posted February 14 In case anyone is interested, I have now tried the Etenwolf calibrated tyre pressure gauge. It reads Bar to two decimal places (PSI to one), which is actually a bit disconcerting. After a few wheels, getting a good seal is relatively easy and easing off that to let some air out is also OK; I've had several gauges where I find that (rather essential) task to be nearly impossible. The gauge does make you feel daft, trying to get .01 Bar accuracy, but hey. Overall, the tool feels robust and long-lasting (with the two AAA batteries being easier to change than the CR2302 on my Halfords gauge, which gave up the ghost a few weeks ago, new battery or not). Cheers Chaz Quote Link to comment Share on other sites More sharing options...
BrianJ Posted February 15 Report Share Posted February 15 I guess whatever the answer to the OP question, they still measure wheels in inches!! Quote Link to comment Share on other sites More sharing options...
Chaz Posted February 15 Author Report Share Posted February 15 Umm, 48.26cm wheels, just doesn't sound as good as 19in, does it! Chaz Quote Link to comment Share on other sites More sharing options...
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