Can someone look at their 87 or 88 S4 for me?
02-07-2007, 01:19 AM
#91
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Chaad ... if you're not supposed to exceed 44psi under any circumstances .... then hiow comfortable are you that when you jack that tyre up to exactly 44psi, you are safe? Again I say ... the other side of the coin is, with 44 rear, you WILL lose traction compared to lower pressures.
02-07-2007, 01:33 AM
#92
Three Wheelin'
Heinrich...Falken GR Beta FK 451s, 225/50/16 and 245/45/16, both 51 max psi as indicated on sidewall. So, I guess I feel safe.
And to your other point, I understand that traction could be maximized at lower psi than 44, but was Porsche trippin' with that rating?
Also, aren't rear drive cars usually running higher recommended psi in the rear?
And to your other point, I understand that traction could be maximized at lower psi than 44, but was Porsche trippin' with that rating?
Also, aren't rear drive cars usually running higher recommended psi in the rear?
02-07-2007, 02:37 AM
#94
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The '87 manual has the same numbers.
Tire pressure, front and rear with cold tires
36 psi or 2.5 bar front, 44 psi or 3.0 bar rear for summer and snow tires; 36 psi or 2.5 bars for collapsible spare tire
Tire pressure, front and rear with cold tires
36 psi or 2.5 bar front, 44 psi or 3.0 bar rear for summer and snow tires; 36 psi or 2.5 bars for collapsible spare tire
02-07-2007, 02:44 AM
#95
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Originally Posted by Joe '87 S4
The '87 manual has the same numbers.
Tire pressure, front and rear with cold tires
36 psi or 2.5 bar front, 44 psi or 3.0 bar rear for summer and snow tires; 36 psi or 2.5 bars for collapsible spare tire
Tire pressure, front and rear with cold tires
36 psi or 2.5 bar front, 44 psi or 3.0 bar rear for summer and snow tires; 36 psi or 2.5 bars for collapsible spare tire
02-07-2007, 03:05 AM
#97
Rennlist Member
OK, I'll play -- One thing that has not been mentioned is that these cars hit the market at a time when the thumbscrews were being tightened on EPA regs. Essentially all manufacturers did stupid things to try to squeeze every MPG out of the EPA tests. One of the most egregious example that I can recall is the 10W oil in the differentials of S-10 trucks when they first came out. Most recommended tire pressures went up during this time too. My '69 Datsun 510, recomendation was 28F?R. my '74 Datsun Z, same weight, same tire size, recommendation was 32F/R.
There is no "correct" tire pressure. It's always a compromise, depending on your driving style, car, wheels, tire construction, etc. In my case, after extensive experiments with chalk across the tread progressing to IR temp readings after miles of straight highway at high speed, readings after intense cornering at the limit of traction, etc., I arrived at 30F/28R as giving me the best cornering and braking traction while keeping even temps across the tire without the sidewalls folding under or squirming. Stock size 17" Yoko ES100s.
Just to illustrate the point about "it depends", the same tests(except cornering @ limit of traction) showed that on my Dodge truck(~4500 lbs, 35" mud tires), the ideal contact patch is at 20F/18R.
I have seen statements from tire manufacturers about increasing pressure for sustained high speed driving, but I have found that the pressure I use is suitable for any sustained speeds I've been able to get away with -- temps no higher after sustained triple digits(~1hr) than after a 10 mile blast down the highway at 70mph.
To sum up, for a particular car/tire/wheel combo, there will be an optimum pressure for traction -- usually much lower than recommendations -- yet in cases where ultimate cornering performance is less important than rolling resistance, of course the pressure can be increased. That optimum pressure for traction can only be arrived at through experimentation, unless your setup is identical to someone else's -- and they have done the experimentation.
There is no "correct" tire pressure. It's always a compromise, depending on your driving style, car, wheels, tire construction, etc. In my case, after extensive experiments with chalk across the tread progressing to IR temp readings after miles of straight highway at high speed, readings after intense cornering at the limit of traction, etc., I arrived at 30F/28R as giving me the best cornering and braking traction while keeping even temps across the tire without the sidewalls folding under or squirming. Stock size 17" Yoko ES100s.
Just to illustrate the point about "it depends", the same tests(except cornering @ limit of traction) showed that on my Dodge truck(~4500 lbs, 35" mud tires), the ideal contact patch is at 20F/18R.
I have seen statements from tire manufacturers about increasing pressure for sustained high speed driving, but I have found that the pressure I use is suitable for any sustained speeds I've been able to get away with -- temps no higher after sustained triple digits(~1hr) than after a 10 mile blast down the highway at 70mph.
To sum up, for a particular car/tire/wheel combo, there will be an optimum pressure for traction -- usually much lower than recommendations -- yet in cases where ultimate cornering performance is less important than rolling resistance, of course the pressure can be increased. That optimum pressure for traction can only be arrived at through experimentation, unless your setup is identical to someone else's -- and they have done the experimentation.
02-07-2007, 04:06 AM
#98
Race Director
Originally Posted by Tahoe Shark
I think everyone must remember that in 1988 tires were a lot different than they are today. What started this thread is that I have changed the size of tire on my ORR car and wanted a starting place for air pressure at various speeds. I won't try to second guess why Porsche rated the pressure they did it 1988 but they did it in conjunction with the tire manufacturer.I would also venture to guess that there are no 1988 928's around that still are shod with original tires, original sizes perhaps but not the tires that were produced in 1988.
So this is the info Michelin gave me about my new tires as cross referenced from the ORIGINAL manufacturer data. Tires are PS2's in 245/45/18 (Y) 26.7" front and 275/40/18 (Y) 26.6" rear .
Speed Front Rear
0-143mph 32 30.5
143 -149 33.5 32
149-155 35 33.5
155-161 38 36.5
161-170 39.5 38
170-???? 39.5 38
Personally I would not run the tires that low at the 0-143 mph speed, but that is my decision. Michelin says it's OK. I'm quite sure that Michelin would not have given the same recommendation in 1988.
So this is the info Michelin gave me about my new tires as cross referenced from the ORIGINAL manufacturer data. Tires are PS2's in 245/45/18 (Y) 26.7" front and 275/40/18 (Y) 26.6" rear .
Speed Front Rear
0-143mph 32 30.5
143 -149 33.5 32
149-155 35 33.5
155-161 38 36.5
161-170 39.5 38
170-???? 39.5 38
Personally I would not run the tires that low at the 0-143 mph speed, but that is my decision. Michelin says it's OK. I'm quite sure that Michelin would not have given the same recommendation in 1988.
So you are going to run PS2's instead of the Yokohama's? This is quite a step up in tire width for the beast too....didn't you run 225-255's last year? Very tall tires too...but you have more than enough power to turn'em too.....I wonder what negative effect the slightly wider tires will have on the beasts aerodynamics?
Wow this turned into a serious flame war!!! All from a simple ??
02-07-2007, 04:18 AM
#99
Drifting
Should tyres squeal under hard cornering? I tend to increase pressure a little if they do, and then squealing reduces - I'm assuming that means I have more grip. Am I wrong?
02-07-2007, 09:26 AM
#100
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Paul, tyres can squeal, well-inflated tyres do squeal depending on the compound. I wouldn't go by squeal. Sharkskin has the scientific approach. One way to test your car for pressure is to inflate to 44 in the rear, go do a bunch of twisty turns back-and-forth in an open parking lot. deflate to say 35 and do the same, then deflate to 29 and do the same. You WILL feel the grip difference.
02-07-2007, 10:40 AM
#104
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The topic of why 44 PSI was specified was debated several years B.F. (before forum) on the e-mail list. If I recall correctly the accepted explanation was that the tire technology of the time (~1978) was such that the higher pressure needed to be specified in order to keep the tire from exploding at sustained autobahn speeds. A lower pressure might have offered better traction, but at high speeds the extra sidewall flex was too much for the tires (of the day.)
I have no idea if any of that is true or not - its just the way the debate ended on the e-mail list 'round 1999.
So, why didn't Porsche change the spec when tires got better in the early or late 80s? I don't know. Maybe because they didn't give a **** about the 928. They did change it for the GTS when they ran out of 16" wheel inventory and had to get 17-inchers TUV approved for the 928.
As for the rest, my $0.0928 for street driving:
- Do not exceed the cold inflation temperature shown on the tire sidewall when the tire is cold.
- If your CIT is 44 PSI then you can safely set your cold tire pressure at 44.
- 44 is ridiculously high. If it ever made a lot of sense it doesn't now.
- If you want to throw out everything "a dude said on the internet" then run 36/36 with 225/245 f/r (on 16" or 17") and you'll be running a factory-approved (more-or-less) tire pressure.
- If you are not running factory tire widths then you'll have to pick out your own favorite pressures (unless you get expert help like Tim did.)
I find that 33f, 32r on 255/40, 265/40 (17") works well for me on the track using the 'chalk method'. I increase the pressure by 2 PSI when doing normal highway driving. With these pressures I get even tire wear on all four corners (right down to the belts
) with my tires,wheels, sizes.
I have a set of GTS w/RDK 17" wheels (thanks Greg) that I'm going to put on my '91 once the snow melts. I'll run 36 PSI to keep the RDK happy and drive with few thoughts of tire pressures in my head.
I have no idea if any of that is true or not - its just the way the debate ended on the e-mail list 'round 1999.
So, why didn't Porsche change the spec when tires got better in the early or late 80s? I don't know. Maybe because they didn't give a **** about the 928. They did change it for the GTS when they ran out of 16" wheel inventory and had to get 17-inchers TUV approved for the 928.
As for the rest, my $0.0928 for street driving:
- Do not exceed the cold inflation temperature shown on the tire sidewall when the tire is cold.
- If your CIT is 44 PSI then you can safely set your cold tire pressure at 44.
- 44 is ridiculously high. If it ever made a lot of sense it doesn't now.
- If you want to throw out everything "a dude said on the internet" then run 36/36 with 225/245 f/r (on 16" or 17") and you'll be running a factory-approved (more-or-less) tire pressure.
- If you are not running factory tire widths then you'll have to pick out your own favorite pressures (unless you get expert help like Tim did.)
I find that 33f, 32r on 255/40, 265/40 (17") works well for me on the track using the 'chalk method'. I increase the pressure by 2 PSI when doing normal highway driving. With these pressures I get even tire wear on all four corners (right down to the belts
) with my tires,wheels, sizes. I have a set of GTS w/RDK 17" wheels (thanks Greg) that I'm going to put on my '91 once the snow melts. I'll run 36 PSI to keep the RDK happy and drive with few thoughts of tire pressures in my head.
02-07-2007, 10:44 AM
#105
Three Wheelin'
Originally Posted by SharkSkin
OK, I'll play -- One thing that has not been mentioned is that these cars hit the market at a time when the thumbscrews were being tightened on EPA regs. Essentially all manufacturers did stupid things to try to squeeze every MPG out of the EPA tests. One of the most egregious example that I can recall is the 10W oil in the differentials of S-10 trucks when they first came out. Most recommended tire pressures went up during this time too. My '69 Datsun 510, recomendation was 28F?R. my '74 Datsun Z, same weight, same tire size, recommendation was 32F/R.
There is no "correct" tire pressure. It's always a compromise, depending on your driving style, car, wheels, tire construction, etc. In my case, after extensive experiments with chalk across the tread progressing to IR temp readings after miles of straight highway at high speed, readings after intense cornering at the limit of traction, etc., I arrived at 30F/28R as giving me the best cornering and braking traction while keeping even temps across the tire without the sidewalls folding under or squirming. Stock size 17" Yoko ES100s.
Just to illustrate the point about "it depends", the same tests(except cornering @ limit of traction) showed that on my Dodge truck(~4500 lbs, 35" mud tires), the ideal contact patch is at 20F/18R.
I have seen statements from tire manufacturers about increasing pressure for sustained high speed driving, but I have found that the pressure I use is suitable for any sustained speeds I've been able to get away with -- temps no higher after sustained triple digits(~1hr) than after a 10 mile blast down the highway at 70mph.
To sum up, for a particular car/tire/wheel combo, there will be an optimum pressure for traction -- usually much lower than recommendations -- yet in cases where ultimate cornering performance is less important than rolling resistance, of course the pressure can be increased. That optimum pressure for traction can only be arrived at through experimentation, unless your setup is identical to someone else's -- and they have done the experimentation.
There is no "correct" tire pressure. It's always a compromise, depending on your driving style, car, wheels, tire construction, etc. In my case, after extensive experiments with chalk across the tread progressing to IR temp readings after miles of straight highway at high speed, readings after intense cornering at the limit of traction, etc., I arrived at 30F/28R as giving me the best cornering and braking traction while keeping even temps across the tire without the sidewalls folding under or squirming. Stock size 17" Yoko ES100s.
Just to illustrate the point about "it depends", the same tests(except cornering @ limit of traction) showed that on my Dodge truck(~4500 lbs, 35" mud tires), the ideal contact patch is at 20F/18R.
I have seen statements from tire manufacturers about increasing pressure for sustained high speed driving, but I have found that the pressure I use is suitable for any sustained speeds I've been able to get away with -- temps no higher after sustained triple digits(~1hr) than after a 10 mile blast down the highway at 70mph.
To sum up, for a particular car/tire/wheel combo, there will be an optimum pressure for traction -- usually much lower than recommendations -- yet in cases where ultimate cornering performance is less important than rolling resistance, of course the pressure can be increased. That optimum pressure for traction can only be arrived at through experimentation, unless your setup is identical to someone else's -- and they have done the experimentation.
Bigmac already brought up the fuel economy issue, and I already said that inflation pressures are always a compromise between grip and fuel economy.