I thought traction (or mpg) was the goal

My TPM reading on my 2016 Boxster at 50 degrees reads -3 PSI on all four tires when I first start up. If I drive long enough to warm up the tires it can read -1 on all four but it varies. The manual says to fill to 29 PSI cold 'at 68 degrees'. What should I fill to at 50 degrees or lower. In the past I always filled to the recommended pressure in the winter. regardless if it was 68 or much lower.

Sorry for the dumb question.

Jeff Benson:

2016 Boxster, See it here: www.greatvalleyhouse.com/Boxster

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First car: 1952 Hudson Hornet

First roadster: 1962 Austin Healy 3000 III

Still have: Computer free 1974 MGB

Previous Porsches since 1982: 924, 944, 944S, and four Boxsters; 97, 03, 08 & 2012

Well, I would reply but - living in New England - people never believe what we say about air pressure and deflation

Excellent question. There is a conundrum in tire pressure. Truthfully, you only care about hot (operating temperature). But in reality, we have to fill them cold. So we guess at temperature and therefore pressure rise, which are proportional (since PV-NRT).

So, i would fill such that under normal driving, they are right in the middle of the spec range, or your preferred range. For example, on a 986, porsche recomends 29/36. They do this specifically to make the car under-steer ( safety). I, rather, run about 31/34 (cold).

Sound s like you at minimum need to add 1 pound.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Grant

gee-lenahan-at-gee-mail-dot-com

I can easily imagine that the ideal pressure for optimal traction varies with pavement temperature. It's also possible that the max tire temp (and hence pressure and traction) depends on the ambient temperature.

... two factors, but maybe more of a factor in the final temperature and hence pressure is how hard the car is being driven.
You can drive the car gingerly and your tire pressures may rise 1-2 psi even in the summertime.
But compare with a pressure increase of 15-20 psi or more when at the track (even with track and abbient temps in the 50s).
What rises the temperature in tires is mostly the flexing of the sidewalks when braking, accelerating and turning.
Happy Porsche'ing
Pedro

Pedro Bonilla
1998 Boxster 986 - 314,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Grant

gee-lenahan-at-gee-mail-dot-com

Don't know if it is right but it has worked over the years for me and that is to just inflate the tire to the recommended cold inflation pressure regardless of the temperature, within reason. If the "cold temperature" is warm or even hot as it can be with the car parked outside in the summer sun and heat I don't bleed air pressure out.

However, as a concession to the higher ambient temperatures in effect in the summer months I check tire pressures in the AM after the car has sat overnight and make sure the tires have the right "cold" tire pressure. For the Boxster this is 29psi front and 36psi rear. Thus the tires get inflated to these pressures at 68F, 78F, or 58F, 48F, 38F or even lower if I'm on a road trip and venture into an area where it is colder. (In the case of road trips, I can leave home with temps approaching 100F and be in Flagstaff that night and wake up the next AM to 19F.)

and watch out for sun that's been hitting one side for a while, thus heating the tire(s.) I mention "cold motor" because heat radiating/convecting to the tires can heat them up, too, and, well, Boyle's Law and all that.

Funny thing, spec for 981 19" winters is 32 all around, but the TPMS readouts show +1. Also, I hate that 31 psi, and at least one other 30-something is skipped. I ask each time the car's serviced if there's been a software update.

I suspect this is in all Porsche manuals:

"Pressure increase as the result of temperature increase

In accordance with physical principles, the air pressure changes as the temperature changes. The tie pressure increases or decreases by around 1.5 psi (0.1 bar) for every 18 F (10 C) change in temperature.
The Tire Pressure Monitoring takes this relationship between tire pressure and temperature into account."

I am not sure what "into account" means, but there it is.

Peace
Bruce in Philly

In my 2016 Boxster manual the cryptic sentence "The Tire Pressure Monitoring takes this ..... into account" has disappeared .