sjfehr

Damn, that's a big difference! Was that with sport+?

mopar bob

I only have sport no +. It was just that I was over driving and it was being my nanny. I was still over driving just 1.5 seconds faster.

majormojo

Well, this is embarrassing. And annoying. It turns out that my Longacre tire pressure gauge is reading 4-5 PSI too high. So the measurements I've been making are all off. It's going back to be recalibrated. On the upside, my car's TPM system, which I was accusing of reading too low and throwing false warnings, is actually pretty accurate.

I had the PDK system checked and I'm told I already have the latest firmware update (T082). Dealer did perform a "throttle adaptation and torque loss adaptation" per a tech bulletin. I have two days of autocross this weekend so I will see if the bogging re-occurs. I did find another thread discussing a similar problem with a similar solution.

http://www.6speedonline.com/forums/9...-traction.html

sjfehr

Yeah, that'll throw numbers off! 5psi is worth a recalibration, but it doesn't matter a lot for autocross if your gauge is a couple pounds off as long as it's consistent and you always set your pressures by that gauge. No two gauges are ever going to agree.
That's odd... in that situation, I'm usually much faster with the nannies on! I played with this a bit more (in sport+) this past autocross; all my competition runs were with PSM off, but I took two fun runs with it on, and I was a whole lot more consistent, bracketing my fastest competition run +.1 and -.1. On vs off is a tough decision- if I've only got 3 chances to get it right, do I prioritize consistency and confidence or rote speed?

Earlydays

I had s similar problem last year with my gauge (an expensive one) that was reading 7psi low, which really threw off the balance of the car!

majormojo

sjfehr, I agree. As long as 1) I measure with the same gauge, and 2) the gauge itself is consistent, then I can get consistent results, regardless of the indicated pressures. Knowing the exact correct pressures is really only important for communicating and comparing to others or reproducing using a different gauge. Given the choice, I'd rather be consistent AND correct. Just as in wristwatches, the man with a tire gauge knows his tire pressures, the man with two is never sure.

Another thing I have learned is that the TPMS uses temperature compensation, which means that the indicated pressure is not necessarily the actual measured pressure, but the measured pressure compensated to some standard temp. So a reading of "40" for example on a very hot tire might actually be an actual pressure of say 48 or so. 40 is what the tire pressure will be when it cools. I don't intend on using TPMS to measure and set tire pressures so it's a moot point, but interesting info and useful to know when setting pressures for the street and a good reason to NOT rely on it for measuring true hot tire temps.

btwyx

As others have said, the calibration of the gauge really shouldn't make a difference as long as its consistent. You work out what the correct tire pressure is, and use that consistantly. It doesn't matter what the pressure is (except as noted for sharing on forums).

Interesting about the temperature compensation. I bought a compensation pressure gauge thinking it'd be useful, the gauge is useful but I never use the compensation aspect. It compensates to 80°F, I'd prefer to be able to set my own standard temperature (which is the temperature of my garage in the morning).

majormojo

Both! Two runs on, then turn it off and go for broke on the third.

I have not tried an autox with PSM on yet, maybe I will get a chance this weekend in fun runs. So far, I've been using the opportunity to play with the traction limits at the slower speeds of autocross. Kept it on for track days.

However, last track day I came off a bit early due to a brake pad sensor warning that turned out to be false. Pads are just fine, but the rear sensor wires were overheated. Likely that caused a resistance change that triggered the warning. That rear brake overheating (calipers discolored too) is almost certainly due to PSM intervention - magic 1-wheel braking as mentioned earlier.

I swapped in a set of Pagid Orange pads, I'm hoping they will get enough heat to be effective in autox. So far even cold on the street they have a much better initial bite and more precise modulation than the OEM Textars, so I'm hopeful. The feel is close to what I got with Hawk HPS and SS lines on my Miata. And yes, they squeak as expected.

majormojo

Got some more data from this past weekend. The first 5 sets are after individual runs on Sat. The last one is after ~6 runs at the end of day 2 (didn't take measurements each run).

What I found subjectively is that I liked the handling balance best with even pressures F/R. Seemed to make the rear a bit less loose. The sweet spot seemed to be at around 42 psi. I will play with reducing rear pressures another 2 psi lower next time out.

The other thing I've learned is that the RE-11s are just plain loud, and start making noise well before they start breaking loose. So initially I was backing off much too soon, I need to keep pushing and make 'em howl.

I did try a few runs with PSM on and while the resulting run times were OK, I really found the "help" to be distracting. My driving style is mostly smooth and precise (probably not aggressive enough), I'm not a "toss and catch" style driver so the feeling of the interventions was intrusive. I think if I tried a more aggressive flat-out style and let PSM do its thing, that could work but wouldn't be comfortable for me. Although I suppose if I could prove to myself it was faster, I could learn to become comfortable with winning...

The PDK issue I experience previously was not present. Good smooth shifts all weekend long. I did try a few launch control starts but those seemed to result in more wheelspin and corresponding PSM intervention that was not as smooth as just doing a normal start - release brake, floor the throttle. Seems like the normal launch did a great job of minimizing wheelspin while still launching aggressively. So far, LC doesn't seem to have any advantage on autox starts.

majormojo

Just a quick update, I've had a few more days of Autox since last post and experimented with PSM on/off some more (always in Sport+ mode). Basically confirms what I stated previously, I can't see any noticeable time difference with PSM on, but the intervention is noticeable and unsettling (to me more than the car, but both). The feeling is very disruptive and not smooth. That is, in a slip situation, it intervenes quite aggressively where I would intervene (via steering or throttle) more gently. On the fence on this one, but PSM off feels better to me.

Tried a day running 1 - 1.5 lb lower pressure in the rear and the car felt more pushy. Definitely better balanced for me at even pressures around 42 psi.

Have not experience any recurrence of PDK "bogging" in 1-2 shift and had some opportunity to do a few straight launch control starts with no issues either. On autox courses where the start is LC friendly with no quick turns before/after the 1-2 shift point it definitely helps. Otherwise a normal start is faster. A launch control start with turning will almost always induce some wheelspin and throttle modulation that is actually slower.

mopar bob

When I did the Speed Train class earlier in the year, and I'm new at this. I saw a very bad throttle bog on one course setup. It was a triangle that was timed to teach you about braking and hitting the gas and boy it was fun. What I saw with PSM on was, I would be vary hard on the brakes then make a hard left turn and punch the gas and nothing for like 2 seconds then hang on. I didn't think much of it at the time as I was the second fastest of our group behind a very well driven Cayman S. Next time that I'm out PSM will be off but sport will also be off. I'm just looking for the slower throttle.
My 911 also feels better with 42 lbs in the back

jabo

For those of you with Caymans and Boxsters, you might try running with less pressure in the back than the front. This will give you a little more oversteer than the normal case where the rear has a pound or two more pressure. This has worked well for me. I usually run high 20s cold which after a few runs gets into the mid 30s and the sweet spot of around 36psi front and 34-35 psi rear. Any higher and the grip tends to fall off a bit.

Tom Tweed

Which tires are you running? I have found that pressure discussions like this are very much dependent on the specific tire construction and compound, along with ambient temp and track surface considerations. For instance, I would usually associate dropping pressure in the rear with less oversteer, not more, but there may be other factors at work in your case.

TT

jabo

Tom,

You have probably actually seen my results from when I ran AX in SDR at Qualcomm.

I was running Nitto NT-01s and also had pushed the tops of my shock towers as far in as they would go, giving me perhaps -1.3 deg camber up front.

I see that more and more folks in SD are acquiring Caymans and Boxsters for AX work!

Cheers,

JB Smith

Tom Tweed

Hi JB,
Ya, the mid-engine component of SDR autocrosses is increasing all the time--more than a third of the 100 cars that ran the last event on Jan. 3 were Boxsters or Caymans. I remember your Spyder--back in 2013, no? That was a dark year for me. I sat out the whole season with health problems, but I did show up at most of the events to work tech and I watched the results. You did well--anytime you can get in the top-10 in this region, you are peddling pretty fast. Lots of quick drivers here. During my break from racing, I sold all the air-cooled 911s and went over to the water-cooled dark side.

I haven't run the NT-01s yet, but I hear they're very similar to the RA-1s that I ran for a season on one of my old 911s. They seemed to like higher pressures than the Kumhos I usually ran, maybe from a softer sidewall construction? Most R-comps have a pretty narrow window of pressure that generates max. grip, and either above or below that ideal pressure, slip angles increase. What kind of camber did you set in the rear? With the stock suspension bits, I was only able to get -1 in the front and -1.8 in the rear on my Cayman S.

I'm running it in SS class, so no mods allowed to get any more negative, and no R-comps (140 treadwear minimum). I tried the P-zeros the car came with for 3 events, just to figure the car out, and then threw a comp alignment on it with some Dunlop ZIIs and gained a couple of seconds. The Dunlops seem to work well starting with 29F 30R cold. I end up with 34F 36R after 4 runs. I scored a ninth top time at the last event, and was the second fastest on street tires (Mike Avitt got me by around a second in his 991S w/powerkit). Results here.

The stock Cayman is about as fast at AX on street tires as my modified '68 911 was on V710s, I think. It weighs 1,000 lbs. more, but it has 100 more HP. At a big track, it would walk away from it.

How do you like Austin? Must be nice to be close to COTA.

C'ya,
TT