Tires and pressure Options

[lsintampa]

Here goes, maybe I'm not understanding something and there is opportunity for me to learn.

Firstly, what happened to the "tire calculator"?

Now onto the discussion. Porsche 914's for the most part call for tire pressures to be about 23PSI front and 26PSI rear. I may be off some, but for "most" 914's I think those numbers to be "horseshoe" close enough.

BUT, wouldn't one think that the PSI for the car is based on the tire size that came on the car?

IDK, but I would put some big bucks on the fact that most of us are NOT running on stock wheels / tire combination.

So I wonder for my setup - I'm running Yoko Avid envigor's 195/65/15's on 15X6 cookie cutters - what tire pressure should I be using?

It's just hard for me to understand the relationship between the car / tire size / and tire pressure. IE, if I put that same tire on a different car - would that car's recommended pressure be different?

Enlighten me please.

Thanks,

Len

PS - I wish you all a joyous holiday season and may we all enjoy a healthy new year!


(IMG:style_emoticons/default/wreath.gif)

[brant]

23psi sounds way to low, even for stock

that is a pressure that a person would run on the race track with a tire specifically built to have a stiff side wall
its too low for a street tire side wall

I usually run 30 or 32 on street tires of non stock size
I'm not fully aware of how the car weight interacts with the contact patch and PSI as a variable

but take into account that modern tires are built for modern/heavier cars and that the sidewall construction of how the tire is supported by the air pressure is likely as important as the weight of the car or suspension design

23psi sounds close to dangerous. Hard cornering and a flexible/soft undersupported sidewall could lead to a bead failure (think ford explorer roll overs)

[mepstein]

Everything is a relationship. Size, type, weight, width, etc. use the recommendations as guidelines and remember our cars are light.

- if your car is a 4, start at the lower range
- if its a 6 or 8, bump up the pressure.

[Tom_T]

Hi Len,

There was some Java script issue with the tire size calculator on here the stopped it from working, but you can google for them. Here's a link to one of several which I'd found doing that.....

https://www.tacomaworld.com/tirecalc

As others have said above, the tire pressure is a function of the size & weight they're carrying, etc. - so it'll vary on non-stock tire sizes &/or resto-mod cars at different weights.

IIRC the 3 OEM stock 155(/80)SR15 or 165(/80)SR15 or ...HR15 size tires ran more like at 26 psi or more, & the spare on the 914/4 models were to be kept at about 40 psi IIRC while in the trunk because it supplied the air pressure to the windshield washer system, but you're supposed to drop it to the proper tire pressure when used on the car.

As for other sizes of tires &/or on -6 or -8 or Subie/WBX conversions carrying more weight - you'll have to do what any auto mfgr. does to get the recommended tire pressures.

That is, you'll need to weigh on a CAT scale/etc., calculate or look up the per axle weight to be carried with 2 passengers & full fuel/fluids & luggage capacity, then divide that by 2 per wheel - then go to the tire makers' websites for their psi per weight chart (or email them to request one to be sent to you). Then use that chart to get the proper pressure for the load each tire at the 4 positions require.

The tire pressure on the side walls is the maximum for the maximum rated load, so you typically don't run them at the max.

Happy Holidays! (IMG:style_emoticons/default/beerchug.gif) (IMG:style_emoticons/default/santa_smiley.gif)
Tom
///////

[walterolin]

FWIW the 1971 owner's manual and the 914 Porsche Technical Specifications book (May 1974) call for 26 front and 29 rear for both the 155 and 165 SR 15
tires. I've never liked this, as Brant says above 30- 32 works better - even for stock size.

[lsintampa]

OK thanks all,

I did learn something - so that's good. (IMG:style_emoticons/default/beer.gif)


Just reset all to 30psi - we'll go with that.

No mods to the car that would add any significant weight at all... Stock 75 2.0L

(IMG:style_emoticons/default/w00t.gif)

[Cairo94507]

Given the size of some of the pot holes on the roads these days, I would start at 32 PSI and go up in 2 pound increments from there until I found the pressure that worked for your given set-up. Nowadays, tires seem to like to be around 40 PSI, at least on my two daily drivers.

[lsintampa]

Given the size of some of the pot holes on the roads these days, I would start at 32 PSI and go up in 2 pound increments from there until I found the pressure that worked for your given set-up. Nowadays, tires seem to like to be around 40 PSI, at least on my two daily drivers.

I'm in Tampa....

Pot holes? We don't have no stinking pot holes!

(IMG:style_emoticons/default/piratenanner.gif) (IMG:style_emoticons/default/blink.gif)

[michael7810]

Sorry for the highjack...but just how does one tell what is best for a given setup? For normal street driving which is what I do on my mostly stock 914/4, I've tried various pressures from OEM (26/29) to 32/32 and the only difference I notice is in the harshness of the ride. I don't drive enough to see abnormal tire wear. I recently did a DE and the instructor recommended staying with the OEM pressure (26/29 cold) to get "mid-30 PSI" when warmed up.

[ConeDodger]

Sorry for the highjack...but just how does one tell what is best for a given setup? For normal street driving which is what I do on my mostly stock 914/4, I've tried various pressures from OEM (26/29) to 32/32 and the only difference I notice is in the harshness of the ride. I don't drive enough to see abnormal tire wear. I recently did a DE and the instructor recommended staying with the OEM pressure (26/29 cold) to get "mid-30 PSI" when warmed up.

Since we've taken the engineer out of the equation by going with non-stock wheels and new tire tech, those stock recommendations don't mean as much anymore except perhaps a place to start.
Your instructor is correct about the psi rising as the tire gets hot, unless you are using nitrogen. Nitrogen is more stable with cold to hot psi.
I suggest using a pyrometer. Drive enough to get the tires warm, then jump out, or have an assistant check your tire temperature in three places across the face of the tread. The goal is equal temps. If your hot on the outsides and cool in the middle, you're under-inflated. Hot in the middle and cooler on the outside is over-inflation. Just keep testing till you get close to even across the tread face. Don't obsess. 1-3 psi difference is ok. Given our 50/50 weight distribution, one tire should give you pretty close to the same as all the others so don't expect huge differences.

[jim_hoyland]

This is on my expaion tank, never went by it after going 205 x 16 x 55
Found 35# a little hard but 32 works well

[mepstein]

I settled on 28F, 30R with 195x65x15, 2.0 Fuchs on an early 4 cylinder just because it felt good for around town and highway driving. I have boge shocks. If I had bilsteins, it might make higher pressures a bit more cushy.

[michael7810]

Sorry for the highjack...but just how does one tell what is best for a given setup? For normal street driving which is what I do on my mostly stock 914/4, I've tried various pressures from OEM (26/29) to 32/32 and the only difference I notice is in the harshness of the ride. I don't drive enough to see abnormal tire wear. I recently did a DE and the instructor recommended staying with the OEM pressure (26/29 cold) to get "mid-30 PSI" when warmed up.

Since we've taken the engineer out of the equation by going with non-stock wheels and new tire tech, those stock recommendations don't mean as much anymore except perhaps a place to start.
Your instructor is correct about the psi rising as the tire gets hot, unless you are using nitrogen. Nitrogen is more stable with cold to hot psi.
I suggest using a pyrometer. Drive enough to get the tires warm, then jump out, or have an assistant check your tire temperature in three places across the face of the tread. The goal is equal temps. If your hot on the outsides and cool in the middle, you're under-inflated. Hot in the middle and cooler on the outside is over-inflation. Just keep testing till you get close to even across the tread face. Don't obsess. 1-3 psi difference is ok. Given our 50/50 weight distribution, one tire should give you pretty close to the same as all the others so don't expect huge differences.

Thanks, sounds easy enough to do and makes sense to my old engineer's brain.

[euro911]

...
I'm in Tampa....

Pot holes? We don't have no stinking pot holes!

(IMG:style_emoticons/default/piratenanner.gif) (IMG:style_emoticons/default/blink.gif)

... but you DO have sink holes (IMG:style_emoticons/default/poke.gif)

[mgp4591]

Sorry for the highjack...but just how does one tell what is best for a given setup? For normal street driving which is what I do on my mostly stock 914/4, I've tried various pressures from OEM (26/29) to 32/32 and the only difference I notice is in the harshness of the ride. I don't drive enough to see abnormal tire wear. I recently did a DE and the instructor recommended staying with the OEM pressure (26/29 cold) to get "mid-30 PSI" when warmed up.

Since we've taken the engineer out of the equation by going with non-stock wheels and new tire tech, those stock recommendations don't mean as much anymore except perhaps a place to start.
Your instructor is correct about the psi rising as the tire gets hot, unless you are using nitrogen. Nitrogen is more stable with cold to hot psi.
I suggest using a pyrometer. Drive enough to get the tires warm, then jump out, or have an assistant check your tire temperature in three places across the face of the tread. The goal is equal temps. If your hot on the outsides and cool in the middle, you're under-inflated. Hot in the middle and cooler on the outside is over-inflation. Just keep testing till you get close to even across the tread face. Don't obsess. 1-3 psi difference is ok. Given our 50/50 weight distribution, one tire should give you pretty close to the same as all the others so don't expect huge differences.

Thanks, sounds easy enough to do and makes sense to my old engineer's brain.

Don't forget to get a tread depth gauge. As your tires wear, make sure to measure them occasionally for equal tread depth then make adjustments accordingly. It's more important the wider the tire is than stock but if you're not going too wide, the difference should be minimal. Still, a gauge is a good thing to have and they're ridiculously cheap.

[My 914]

I think 30 is an easy number to remember and a good average pressure that will work on many tires. Tire pressure gauges are also inexpensive and I keep one in the car because you never know when you'll need to check the pressure, and the ones at gas stations (if they have them at all) are terrible.

[Ian Stott]

I run 28 psi all around, 205/60. Bridgestone Potenza's, handles well, not too harsh a ride, tire wear after approximately 10 thousand miles seems quite even.

Ian Stott
Moncton
Canada

[Geezer914]

Firestone Firehawk AS 195/60x15. 26 front, 28 rear.