I want to start launching my STR S2000 at the start timer and to prepare for getting into prosolo. Diff/car has 140k, innovative diff bushings, 255 RE71R, oem clutch disk, ACT PP. I replace the diff fluid yearly with 75w140 amsoil.

What's the best way to do it without blowing diffs? Have asked a few different S2000 drivers and seem to get all sorts of answers. Some say preloading, some say just dump the clutch (AP2 owners) and others say there is no way to dump launch a stock AP1 diff without breaking it, you have to slip the clutch with that collar/sleeve being so weak. I've always done 3-4k aggressive slip starts but i don't to eat up the clutch life on 6k launches if i don't have to.

Just looking for any anecdotes or opinions on autocross diff abuse. I have a spare ap1 diff i got out of a buddy's parts car, so I'm covered there. Been considering sending this to Ben @ Puddy. Anyway I street drive many hours with a tire trailer to my autocross and track events so breakdowns are best avoided TIA

 

Most good courses nowadays eliminate the benefit of a hard launch but some dont and yes, for pro solo it will be a needed thing. Here is how I have done it and my take (Also filtered with other peoples inputs that I know).

You do NOT want it to just grab when you release the clutch. For one thing, it will just bog down, for another, this puts a lot more shock on the diff.

Some people vary on this point, but I do preload. So I get staged, then hold the brake and just barely let out on the clutch to preload the diff and then push the clutch back in. If the surface is level this probably helps, if the car rolls back at all then it probably does nothing.

For asphalt courses, I tended to launch (with str setup/255 tires) at about 5000 rpm. For concrete closer to 6100 rpm.

I do not side step the clutch, but pretty much release it as fast as I can lift my foot straight up off the pedal.

The idea is to get the tires to slip, so you do not bite and bog down/overload the diff

Once they are spinning, you can start feathering back out of the throttle so they start to bite. This keeps you closer to the power band as it starts to grip and go.

It takes some practice in this car vs some others. Pay attention that when you think the tires are spinning it is just not the clutch slipping!

I have had mine since 2014 and have done a fair number of hard launches. The previous owner had it for about 5 years and said it had plenty of hard autox launches. I have a spare diff but neither me nor the previous owner ever broke one.

The one part I have broken on launch was a CV joint at nationals the last year I ran the S2k there. Broke right on the line and broke the typical part of the joint (tripod.bearing holder broke and spun on the splines).

Keep in mind that temp and condtions play heavily into those rpm numbers. On first launch I would err on launching a little too hard vs not enough throttle. I think when I was trying to learn it I was launching closer to 6000 on asphalt, but was just making a big smoky burnout and started dialing down from there

Some try to slip the clutch a tad and then release it hard, but I found more often it would tend to want to slip the clutch badly when I released it that way.

 

Think about it this way. If you blow the diff, some hundred bucks plus 1-2 hours later you'll be back in business. If you roast the clutch trying to protect that diff, how expensive and time consuming would it be?

So give it as much rev as you need and release clutch quickly, as long as you can spin the tires it won't be that bad.

Watch this guy go at 6:26 and 8:47 (in-car)

 

Wife and I autocross an '03 AP1 and I've replaced the clutch 4 times in 6 years. We finally started dumping the clutch on starts to spin the tires on purpose. The tires are more expensive than a clutch but much easier to change. We're in Charlotte, NC this weekend for our first ever Pro Solo, so I'll let you know if the diff doesn't finish the weekend. We did the school today, with about 20 practices launches total and it didn't make any noise on the way back to the hotel. If it goes boom I'll leave it with a friend, take a rental home and come back later with my spare diff.

 

Not only are tires easier to change, but they can withstand slipping faaaaar better. Honestly you can spin the tires on every launch and you'll barely notice a difference in tire wear; the wear is going to come from pushing it through the corners. As you've learned though, slipping the clutch a bunch will wear it out very quickly.

 

Thanks for the inputs all. Seems like preloading and getting wheelspin will be the move.

Good luck this weekend. I'll be working Charlotte on Saturday. If you are in a silver car I believe we met at a Savannah event the week before Dixie tour.

 

I think the other variable that really is hard on them is heat. And with Pro Solo there is little to do about that. The back to back launches without the diff cooling is pretty hard on them. But as mentioned, slipping the clutch a lot just burns up the clutch... and a diff is still a LOT faster to swap than a clutch and you can probably get a used one for $400 or so. When I was still autocrossing the S2k (before codriving the ND) I would just haul my spare diff to Lincoln for nats just in case lol. And then only ever managed to blow up a CV joint! I was not running the pro's though in the S2k, just the tour. Would have swapped in the spare axle, but it was on the last run of day 2 and I trailored it down so just got it back on the trailer (Thanks to Alan Claffie and his wife and a couple of other folks that came over, who helped me push it onto the trailer!)

Also cool thing about that day, was that another drivver who also had a yellow Ap1 and did not know me, came over and said "if you dont do that to mine you can take your last run in my car!" which was super cool. Was not used to his setup and my mind was out of the groove by then so it was not my fastest run, but still very cool of him. Then the following year I get a message ... "hey Dave, its Ben ... blew up my motor, would you be open for a codrive?" so I had a codriver that year! One of the things I love about autox is the sportsmanship I see among the participants. I mean when Ben let me use his car, we were around .5 seconds apart for the combined two day event, so I was in striking distance of beating him, so pretty cool to let me drive his car on the last run knowing that! But alas, he still beat me I did at least manage to beat him when he was codriving with me so I didnt get beat in my own car at least!