After reading a few posts here it seems the consensus is that the recommended 36 F & 44 R tire pressures are way too high, and that most people go with 33 & 36. Anyone know why Porsche recomends such high tire pressure, and is it safe to go lower?

 

I use the higher pressure for my 1996 C4S which has 18" P-Zero Rossos front and rear.

 

not sure on the recommendation... but I do know my guy at Orbit Racing recommends more like 33 & 34 in my '97 Cab with 18's and Bilstein HD's and Eichbach Springs.
Says that tire pressure can drastically effect the ride in a car with a setup like mine.

 

Yes, Ken, and I'm sure that it works just fine for you, but judging from the photo of your car in your avatar, those pressures work best for you because you drive mostly on the grass.

After much experimentation, 33/37 worked well for me in my C4S.


[on edit - to answer Sean's original question]

Porsche recommends such high pressures for the following reasons (in no particular order): longer tread life, to ensure that the car will always understeer, to better protect the wheels from damage.

Is it safe to run lower pressures? Certainly. I personally ran my C4S for 5 years with lower pressures, never had any blowouts, adverse tread wear, or damaged wheels. Of course, my car was dangerous, as it didn't understeer.

 

For maximum performance and best ride, lower pressures will be best.

For maximum rim protection around town, I usually run the higher pressures.

Chip

 

FWIW, Porsche recommends 36 all around (if I remember correctly) for my 17" tires, which is where I run them.

 

DJ,

When not driving on grass, I do find the ride harsh, particularly on the less than perfect Irish roads. I'll give 34/40 a try. A bit reluctant to go less because of possible pot hole damage to my precious rims.

 

To add to DJ's list - higher pressures yield better gas mileage & generally less noise. I run my 18"s at 34/38 in an urban setting complete w/potholes, etc. & have had no problems.

 

dumb question: Shouldn't we try to maximize the contact patch of the tires? and that means the pressure has to be low enough to use the outermost treads (assuming the camber setting is within range) but high enough so that they wear out evenly with the center's. At 33/35 the outer half of my outermost treads still not in contact with the ground.

thx

 

Based on what? Eyeballing or do you see more wear inside? Never mind what the tire looks like but if you have more wear inside, you either have too much negative camber for your driving style or your toe is off.

 

Strictly on eyeballing and my camber is conservatively set at -1 (total toe in 3/32"). Didn't check the inside treads though.

 

Then, you should quit worrying about this. Minus 1° camber in front is just about right for good street handling and even wear assuming your drive a bit on the aggressive side.

 

Viken,

Do you have an opinion regarding Porsche's recommended 36 front and rear with the 17" tires? I am interested in performance over ride comfort (fairly aggressive street use). The tires seem to be wearing evenly.

 

IMO, tire pressures can be the same for 17" and 18" plus or minus a pound or two. Also, there should always be three to five pounds difference between the front and rear tires. Both to compensate for the extra weight and even out the handling characteristics. I never understood Porsche's logic behind the 36/36 and 36/44 pressures and couldn't get anyone to explain with any validity.

 

Viken,

Thanks for your input. I always thought it odd that Porsche recommended different pressures front/rear for the 16s and 18s, but equal front/rear for the 17s. I wondered if it's because the 17" rears are 50mm wider than the fronts, while the 16" and 18" rears are just 40mm wider than their respective fronts (if I remember correctly): 16s are 205 front, 245 rear (40mm difference); 17s are 205 f, 255 r (50mm difference); 18s are 225 f, 265 r (40 mm difference).

I would think that the higher pressures for the rear would increase rear grip. A rear tire that is comparatively wider than the front (that 50mm vs. 40mm difference) would do the same. Perhaps because of this, the 17" rears are specified with the same pressure as the fronts in order to maintain the same handling balance as with the 16s and 18s.

What do you think of this theory?