991 Race Track Drivers - PSM On or Off?
12-10-2013, 10:29 AM
#17
Burning Brakes
A bit of a threadjack, but a few years back, our local group was invited to brand new facility (H2R in San Marcos) for an all Cayman day to drum up some business. Since this was basically an exhibition day, the three rules were PSM on, NO passing, and if you run off the track you're out for the day.
It was my first time to drive the 987 Cayman S on the track, but my "old school" thinking was how could I possibly go fast with the pesky PSM on? Well...granted we were probably only running 7/10's that day but my PSM light never came on! I think in general, Porsche's systems are more of a safety net than nanny compared to those of other marques. The only time I've seen the light flash on my 991 on the street was very briefly in some wet conditions. If and when I get the 991 to the track, I'll leave it on until I absolutely feel it's holding me back.
It was my first time to drive the 987 Cayman S on the track, but my "old school" thinking was how could I possibly go fast with the pesky PSM on? Well...granted we were probably only running 7/10's that day but my PSM light never came on! I think in general, Porsche's systems are more of a safety net than nanny compared to those of other marques. The only time I've seen the light flash on my 991 on the street was very briefly in some wet conditions. If and when I get the 991 to the track, I'll leave it on until I absolutely feel it's holding me back.
12-10-2013, 02:16 PM
#18
Rennlist Member
Sorry for the delay. Yesterday was pretty hectic with us finishing up the 981 roll hoops and moving onto the 991.
I wanted to get back to you though because you want to order tires. No apologies for thread-jacking. I'm here only to help so, let's see if I can.
I failed to look at tire math last night but I would think that if you went to a 19" tire with the same rollout, just like you identified, you will be fine. As I think about it, the 19s I was on were fairly low profile. I don't think you can ruin the cars handling with more sidewall as much as you can hurt it without running enough. I think you are spot on as to why the car felt different. It drove like a car in the wet with too much sway bar. It felt too stiff because I had shot the relative spring rate of the tire through the moon by changing the profile. It needed more compliance in the wet, which is why we run soft springs and bars when it's wet.
I see so many people with 20" wheels and tires and a "ride" or "balance" that sucks because they don't keep the big sidewall. I don't think I would be concerned about adding sidewall as much as I would be concerned about not providing enough.
Make sense? And thanks for finally helping me understand why that went down like it did. Sometimes some mild discussion can shed light.
Cheers!
I wanted to get back to you though because you want to order tires. No apologies for thread-jacking. I'm here only to help so, let's see if I can.
I failed to look at tire math last night but I would think that if you went to a 19" tire with the same rollout, just like you identified, you will be fine. As I think about it, the 19s I was on were fairly low profile. I don't think you can ruin the cars handling with more sidewall as much as you can hurt it without running enough. I think you are spot on as to why the car felt different. It drove like a car in the wet with too much sway bar. It felt too stiff because I had shot the relative spring rate of the tire through the moon by changing the profile. It needed more compliance in the wet, which is why we run soft springs and bars when it's wet.
I see so many people with 20" wheels and tires and a "ride" or "balance" that sucks because they don't keep the big sidewall. I don't think I would be concerned about adding sidewall as much as I would be concerned about not providing enough.
Make sense? And thanks for finally helping me understand why that went down like it did. Sometimes some mild discussion can shed light.
Cheers!
The way to avoid this, of course, with a 19" setup is to just maintain the stock tire aspect ratios, and run an overall smaller diameter wheel/tire setup.
You say that people run into more trouble by going too low on the aspect ratio/sidewall height. But if Porsche did their chassis design/tuning around 20" wheel/tire combo with a 35-series aspect ratio, wouldn't it seem to make sense to maintain that 35-series ratio, only in a smaller overall diameter 19" setup?
Here are the calculations for the sidewall height--in order to maintain the overall diameter of the wheel/tire package when going down to a 19" setup, the sidewall increases by about 0.5":
Front
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
245/35-20 3.4in 13.4in 26.8in 84.0in 754 0.0%
255/40-19 4.0in 13.5in 27.0in 84.9in 746 1.0%
255/35-19 3.5in 13.0in 26.0in 81.8in 775 -2.7%
Rear
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
295/30-20 3.5in 13.5in 27.0in 84.7in 748 0.0%
295/35-19 4.1in 13.6in 27.1in 85.2in 743 0.6%
305/30-19 3.6in 13.1in 26.2in 82.3in 770 -2.8%
Thanks a lot for your comments--great to have a voice of knowledge from the business/industry on here.
12-10-2013, 07:04 PM
#19
Three Wheelin'
I run the smaller diameter with the 19"s and it runs quite nice. I haven't tried it on a track in the rain, but on the track in the dry I really don't notice any difference other than my pocket having some more money it from not having to buy 20" track tires.
12-10-2013, 07:25 PM
#20
Rennlist Member
Mayor West, can you please post your tire and wheel specs? Brands, sizes, etc. This is the first car I've purchased where getting a track setup that works and is "relatively" price efficient per session is such a pita. Thanks
12-13-2013, 08:07 AM
#21
Basic Sponsor
Rennlist
Site Sponsor
Rennlist
Site Sponsor
Thread Starter
Guys,
Don't get lost in the "analysis paralysis." Regardless of how the suspension was tested or designed, if minor changes in sidewall radically changed the handling, then they would have 1 spec 20" for the car. There is no way the Pirelli and the Michelin have the same relative spring rate. Different tires will affect the car differently.
Just don't set the car up on 19" low profile rubber bands.
Don't get lost in the "analysis paralysis." Regardless of how the suspension was tested or designed, if minor changes in sidewall radically changed the handling, then they would have 1 spec 20" for the car. There is no way the Pirelli and the Michelin have the same relative spring rate. Different tires will affect the car differently.
Just don't set the car up on 19" low profile rubber bands.
05-23-2015, 10:29 PM
#22
Three Wheelin'
The other major problem with not being able to defeat PSM completely is - using stock OEM brake pads is like throwing money in the fireplace. With PSM off I can barely get 2 days out of a set of OEM rear pads. Front pads are fine, but rear pads are toast after two days on track. The pads disintegrate into the rotor and cause a warped rotor feel upon heavy braking because their is so much material stuck in the rotor. The OEM pads are absolute garbage at high temperatures because the ABD is constantly engaging the rear pads when detecting the slightest slip.
05-24-2015, 12:09 PM
#23
Rennlist Member
The rotors are likely not warped, but have pad material deposits. Should go away after a few days of street driving. Stock Rear pads will be gone after 2-3 track days whether you leave PSM off or on because of the torque vectoring "feature". Yes it works very well but makes track days that much more expensive. I shudder to think what running a diffless McLaren regularly on the track costs.
05-25-2015, 11:35 AM
#25
Off.
05-25-2015, 10:15 PM
#27
Individual DE driving styles vary greatly. I've had instructors obviously annoyed that I wasn't pushing my car at full throttle all the way down the straights right up to the point I'd have to jump on the brakes into ABS. It would not take too many laps driving like this or even worse, over driving the car to burn through a set of tires and brakes.
A far as tire sizes, along with the new larger overall tire diameters, Porsche has optimized the 991 suspension to handle 19's and 20's pretty much equally well. The choice of what to use is ours to enjoy and becomes a matter of personal preference.
A far as tire sizes, along with the new larger overall tire diameters, Porsche has optimized the 991 suspension to handle 19's and 20's pretty much equally well. The choice of what to use is ours to enjoy and becomes a matter of personal preference.
05-26-2015, 07:47 AM
#28
Rennlist Member
Regarding 18's, aren't most of the 991 GT3 folks running those?
Is our rear suspension different than theirs?
Been sort-of considering an autocross set-up & would make a huge different if could run some A7's in 18's vs. 19 or 20's.
Is our rear suspension different than theirs?
Been sort-of considering an autocross set-up & would make a huge different if could run some A7's in 18's vs. 19 or 20's.
05-27-2015, 10:11 AM
#29
Addict
Rennlist
Lifetime Member
Rennlist
Lifetime Member
https://rennlist.com/forums/991-gt3/...-prep-diy.html
TT
05-27-2015, 10:38 AM
#30
Basic Sponsor
Rennlist
Site Sponsor
Rennlist
Site Sponsor
Thread Starter
Marc Lisner - How goes it?!? I hate to be the bearer of bad news but you can't run the 991 on 18s. There is a knuckle at the bottom of the rear upright that sits less than a few millimeters from the inner lip of the wheel. One of the bigger issues we had trying to get the car classified was the fact that it needed 19s and no 19s existed in our series and then the project died. With the amount of rim flex known, its just not safe to pursue. I saw it done by another team at Daytona at a DE and it definitely rubbed up in the banking. IF you had widebody quarter panels and could therefore space the wheels further outboard, then you would be in business. It is really close but on a regular 991 C2S, if you want the wheels further from the upright you don't have enough quarter panel to stop the tire from protruding beyond the bodywork in the rear.
