tire pressure changes

[pgackerman]

While reading the latest issue of Fiero Focus I learned Pontiac changed tire pressure and the V6's HP (140 to 135 HP) in '87s & '88s  to meet noise mandates.

What was the original recommended tire pressure?  FYI, 30 psi on my '88 GT.

[Drewbdo]

30 PSI ? That seems low for a factory setting. I usually see 32-36 PSI.

Was the PSI also lowered for the noise mandate, or only the HP?

[pgackerman]

PSI was also lowered for the noise mandate.

Sticker on the door panel says 30 PSI.

[Fierofool]

Both my 86 and 87 list 30 psi cold inflation and add 4 psi if the tires are hot.  I assume that means they should read 34 psi on hot tires that were cold inflated to 30.  Both 88's have the same specs.

The 86 and 88 coupe came with 14 inch tires and the 87GT and Fastback Formula had 15 inch tires, but inflation pressures are the same across the board. 

I have heard that the horsepower rating was changed due to insurance ratings for the car.  The 135 HP rating is actually at a lower rpm than the years that were rated at 140. 

I normally run 35 psi in my tires, front and rear, and it seems to give me good tire mileage.  I also do that to stiffen the sidewalls, reducing the flexing when taking hard curves.  Too much pressure will balloon the tires causing them to run on just the center of the tread.  That reduces traction.  Reducing pressures will allow the sidewall to flex too much causing the tread to roll sideways in hard turns. 

[pgackerman]

I usually run around 32 PSI, but sometimes decide to go by the book/decal and run at 30.

Have you noticed any differences other than cornering at 35 PSI?

Anyone with an '85 have different recommendations on their decal?

[Fierofool]

I can't say that I have or haven't noticed any difference.  From time to time I've heard that factory ratings are slightly under-inflated.  Most of my tires wear evenly at 35 psi, except that GT's seem to go through 2 pairs of rear tires to 1 pair of front tires.  It could be due to the extra 200 lb of weight on the fastback. 

I've owned 2 85's but I don't remember what the pressure recommendations were.  I always ran 35 in them, too.  One had 14's on it as long as I owned it, but the other got converted to 15's.  The 86SE is the same as the 85GT, except it has a 3rd brake light, so I wouldn't expect any difference in recommended tire pressures. 

[Fierofool]

The 85 and 86 V6's were rated at 140 hp at 5200 rpm and 170 lb torque at 3600 rpm.  The 87 and 88 V6's were rated at 135 hp at 4500 rpm and 165 lb torque at 3600 rpm. 

I have an owner's manual for the 1985 model year and it doesn't give a tire inflation pressure.  It only says to refer to the door placard.  It also says that there could be from 4 to 8 kilopascals increase in pressure after driving at highway speeds.  Never heard that word before. 

[TopNotch]

To see how much of your tires contact the road, do the paper test...

Place sheets of paper under your tires like in the picture, so that the sheets are tight against the tires. The distance between the sheets is your contact area. Higher pressure results in a smaller area.
I run 35 PSI rear, 32 PSI front, which gives close to equal contact all around.
ps. Yes, I know I need to get new tires.

[scottb]

you can also use the chalk test...... run a line of chalk across the tread, drive around the block. Where the chalk has rubbed off tells you were the tire is making contact. Just in the middle means pressure is too high. Adjust pressure until the line rubs off evenly.

[Fierofool]

I love those ideas.  They verify how cheap us Fiero owners are.  I'm sure there's a much more expensive way to get someone else to determine the best psi. 

We could sprinkle some flour around in the parking lot at the next RFTH and let people drive through it the night before to decide if they need to adjust tire pressures before the run. 

[GTRS Fiero]

We used water.  Cheap, easy to use, no cleanup, easily transported, etc.

[pgackerman]

To see how much of your tires contact the road, do the paper test...

Place sheets of paper under your tires like in the picture, so that the sheets are tight against the tires. The distance between the sheets is your contact area. Higher pressure results in a smaller area.
I run 35 PSI rear, 32 PSI front, which gives close to equal contact all around.
ps. Yes, I know I need to get new tires.

Thought they were racing slicks...

[GTRS Fiero]

Ha, ha.  Racing slicks may be flat, but slick is the one thing they aren't.  No tread.  They are like a rubber version of a wire wheel.  Rub your hand with the grain, and they're rough; rub your hand against the grain and you'll take the skin off.  They don't shed water worth anything.  My brother had stockers on his Camaro, and could light them up at will.  He tried Hoosier slicks, and tore up something in the drivetrain.

[scottb]

I love those ideas.  They verify how cheap us Fiero owners are.  I'm sure there's a much more expensive way to get someone else to determine the best psi. 

I prefer the term "frugal"

[pgackerman]

If there's a cheap way to do it, a Fiero owner knows.

FrugalFieros.  Sounds like a website.