Going to Autocross my 84 928S tommorow
02-09-2014, 01:42 AM
#1
Pro
Thread Starter
Going to Autocross my 84 928S tommorow
I am going to run my 84 928S at the local SCCA autocross tomorrow. I normally run a 1986 944 NA. I have wondered since I bought the 928 what it would be like. I/m running 225/45-17 fronts and 255/45-17 rears, Dunlop DZ101's. Any suggestions of what tire pressures to run would be appreciated. Ive checked and these sizes were run on the later models. All I found looks like street pressures were 36 front and rear. I've been able to find some suggestions from Corvette guys who say to run 39-40 in front and 34-36 in the rear. Please give me some suggestions.
02-09-2014, 02:39 AM
#2
Rennlist Member
Autocrossed 18 in Dunlop Direzza Star Specs on my GTS. Have a cheap IR temp gauge. Found the pressure across the tread was even at about 34psi both front and rear when hot. That meant I started my first lap at 30psi cold. Main thing was to monitor the middle of the tread with respect to either edge when the tire was hot immediately after a lap. If I let the pressure get higher than 34 the tire would ballon, the middle of the tire tread would heat up and even blister. The 18 inch Direzza Star Specs had a very narrow temp range to be sticky, the 17 inch DZ101's might be different.
There was a Honda S2000 that rand the Star Specs and also found 34 hot was the stickiest.
Without a temp gauge I would start at 32psi cold and monitor the pressure gain after each lap. If they don't heat up to 36 I would let 2 psi out so they would heat up more. If they heat up to over 36, I would add 2 psi.
When I was running 16 inch wheels on my 944 it would take a 3 psi difference to make any difference in the handling. It liked 36 psi hot front and rear using IR readings to see that the tread was heating evenly across the tire. Started at 33 cold. That was on several brands of high performance street tires and the old goodyear R1s as well.
Most autocross guys run with too much pressure and their tires never heat up enough over the short run to do them any good. It sounds like your Corvette guys are fighting rear tire spin and aren't using the fronts very much. If I ran that much pressure in my front tires I would blister the middle of the tread.
With today's radial tires and stiff sidewalls the tire pressure has a LOT to do with the aspect ratio of the tire And the rim width they are mounted on. The wider the rim and lower the aspect ratio the more sensitive the grip is going to be to pressure to maintain a flat contact patch.
There was a Honda S2000 that rand the Star Specs and also found 34 hot was the stickiest.
Without a temp gauge I would start at 32psi cold and monitor the pressure gain after each lap. If they don't heat up to 36 I would let 2 psi out so they would heat up more. If they heat up to over 36, I would add 2 psi.
When I was running 16 inch wheels on my 944 it would take a 3 psi difference to make any difference in the handling. It liked 36 psi hot front and rear using IR readings to see that the tread was heating evenly across the tire. Started at 33 cold. That was on several brands of high performance street tires and the old goodyear R1s as well.
Most autocross guys run with too much pressure and their tires never heat up enough over the short run to do them any good. It sounds like your Corvette guys are fighting rear tire spin and aren't using the fronts very much. If I ran that much pressure in my front tires I would blister the middle of the tread.
With today's radial tires and stiff sidewalls the tire pressure has a LOT to do with the aspect ratio of the tire And the rim width they are mounted on. The wider the rim and lower the aspect ratio the more sensitive the grip is going to be to pressure to maintain a flat contact patch.
02-09-2014, 12:25 PM
#4
Race Car
Mind your fuel level.
I had fun on my outing last year, despite being on ridiculously unsuitable tires.
The participants were fairly curious/accommodating, though I didn't care for the attitude they heeped on a participant with a blacked out Mustang GT....who summarily handed most of the field their walking papers.
I went on a 'training' day...and the instructors rotating through the passenger seat spoke authoritatively.....but without adding much value...."Interesting car, this thing is going to oversteer like a bear..", "wow, don't see too many of these, it's going to understeer like mad if you aren't careful..."..."OK, this should be interesting it's hard to tick off a good with these because they are so prone to body roll"...
Regardless, hearing the Cayenne GTS, and the suspense about whether or not the Smart car was going over were added benefits.
I had fun on my outing last year, despite being on ridiculously unsuitable tires.
The participants were fairly curious/accommodating, though I didn't care for the attitude they heeped on a participant with a blacked out Mustang GT....who summarily handed most of the field their walking papers.
I went on a 'training' day...and the instructors rotating through the passenger seat spoke authoritatively.....but without adding much value...."Interesting car, this thing is going to oversteer like a bear..", "wow, don't see too many of these, it's going to understeer like mad if you aren't careful..."..."OK, this should be interesting it's hard to tick off a good with these because they are so prone to body roll"...
Regardless, hearing the Cayenne GTS, and the suspense about whether or not the Smart car was going over were added benefits.
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02-10-2014, 01:40 AM
#9
Rennlist Member
LOL, reminded me of the comments from the last two instructors i had on ride along.
First one was completely silent. Then said..."All you need is some competition rubber on this thing."
The second Yelled "BRAKE!" 10 ft before my brake zone going into the first 2 turns, then said, "You're doing better than me."
Neither were much help. Biggest help was reading a LOT, and seat time, seat time, seat time. AND only changing one thing at a time, then see if it was faster or not before adopting and making it a habit.
The thing that made the biggest difference in lowering my lap times was working on corner entry. Idealy braking very late and turning in as I lift off the brakes. If speed and entry is correct the car goes into a 4 wheel drift with the rear rotating. If you turn in too much to quickly the car understeers. If you turn in too late after you have lifted off the brakes the car understeers also. Well, unless you hit the brakes to early and are going too slow. It's about timing and speed. Almost as tricky is getting back into the throttle as you come off the brakes. You want just enough throttle to check the rear rotation and maintain the speed to maintain the 4 wheel drift, but not enough to stop the rear from drifting or worse put the car into power oversteer. The idea is to brake as late as possible and carry enough speed into the corner to 4 wheel drift/throttle steer into and through the apex.
First one was completely silent. Then said..."All you need is some competition rubber on this thing."
The second Yelled "BRAKE!" 10 ft before my brake zone going into the first 2 turns, then said, "You're doing better than me."
Neither were much help. Biggest help was reading a LOT, and seat time, seat time, seat time. AND only changing one thing at a time, then see if it was faster or not before adopting and making it a habit.
The thing that made the biggest difference in lowering my lap times was working on corner entry. Idealy braking very late and turning in as I lift off the brakes. If speed and entry is correct the car goes into a 4 wheel drift with the rear rotating. If you turn in too much to quickly the car understeers. If you turn in too late after you have lifted off the brakes the car understeers also. Well, unless you hit the brakes to early and are going too slow. It's about timing and speed. Almost as tricky is getting back into the throttle as you come off the brakes. You want just enough throttle to check the rear rotation and maintain the speed to maintain the 4 wheel drift, but not enough to stop the rear from drifting or worse put the car into power oversteer. The idea is to brake as late as possible and carry enough speed into the corner to 4 wheel drift/throttle steer into and through the apex.
Last edited by RKD in OKC; 02-10-2014 at 09:30 AM.
02-11-2014, 01:39 AM
#14
Rennlist Member
If you are having loads of fun autocrossing expect to replace the rear tires annually. If REALLY into it you will be rotating the front tires across the car mid season because you are wearing the outside corners off and replacing them annually as well.
Had a second set of rims with autocross compound tires for a while. Idea was to save the wear on the street tires. Ended up having to buy a new set of tires every year anyway so went to just running softer street tires all the time and not changing tires back and forth. Typically get new tires in the fall to have the most tread for winter driving.
Had a second set of rims with autocross compound tires for a while. Idea was to save the wear on the street tires. Ended up having to buy a new set of tires every year anyway so went to just running softer street tires all the time and not changing tires back and forth. Typically get new tires in the fall to have the most tread for winter driving.
Last edited by RKD in OKC; 02-11-2014 at 01:56 AM.