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Unread 07-21-2015, 10:59 AM
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Default Re: Vorshlag Build Thread - Mark C's 2002 C5 Corvette Track Rat

Project Update for July 17th, 2015: Short and sweet update this time, after another autocross test event (where we got more than one run) and a slight change in priority of the build, but its pretty cool.

Mark's C5 has been driven in anger four times since he bought it - at Optima on street tires, at the June autocross on Hoosiers, another June event in Oklahoma, and now at this July event. The last 3 events were on the new suspension we built and installed and there are some good comparisons I will show below. The last 3 events were also on the same (used to begin with) set of A6 Hoosiers, which began to fall off a cliff and lose performance due to heat cycles and age. It is always tough to test on less than ideal tires, so we didn't make radical changes. I'm still waiting to see this car on a road course - which is what we all agreed that it would be intended for in the first place.

Let me just start out with the obvious - summers are HOT here in Texas. We had a bit of a reprieve from our normal super hot June-July-August months with unprecedented rains from early Spring into July this year, but this past weekend was back to "normal" for us - sunny, hot, and humid.

It was perfect weather for a Vorshlag pool party the day before this autocross event, which we had at Brad's pool. Started at 3 and I wasn't home until midnight, and we all ate and drank our fill. That 5 am wake up alarm on Sunday for this autocross was brutal. Once again Amy left me to my own devices, and wisely passed on autocrossing at this event herself.

Texas Region SCCA Event, Aug 12, 2015

I arrived at the Texas Motor Speedway event site at 7:05 am, way too early and way too bright. Felt like garbage but managed to help Mark unload the Corvette off the trailer and walked the course, then slept a bit. This region likes to mix up the run-work order every event, so in June we worked 2nd heat and ran 4th (last), this time we ran 1st and worked 3rd.

Mark offered another co-drive so we both entered in Super Street Mod (SSM) class again, which is where the car falls due to the relocation of the spring from the transverse to coilover location. Its not really a SSM car, not hardly, but its just "where it fits" and these events are just being used for some initial testing of the new shocks and springs. By 9 am we had the driver's meeting and then things got underway for Heat 1, when we were set to run.

We planned a little better this time and had the "tire sprayer" loaded with water and ready in grid. The temperatures quickly climbed into the mid 90s and track temps were 145F, as measured with an IR gun, so we needed the water this time. We lined up with the two-driver cars. I ran first, and that first run was a mess. Mark rode with me and we had a lot of wheelspin, but much of that was a dirty course - I was the 4th car to drive on it.

We noted throughout the day that while the car would launch pretty hard from the standing start (3000 rpms) we still couldn't use 1st gear much, as the rear tires would start to break loose in a straight line at around 4500 rpms in that gear. So we short-shifted to 2nd at 4500 rpm every run and just used 2nd gear for the entire course. There were three VERY tight 180 turn-arounds and the revs were a bit low coming out of these, but it now has more torque and we didn't want to just shred the rear tires trying to use 1st - unlike the June TMS event at the same site, where we only had one run each before the thunderstorm hit, and both of our runs were terrible.

This run group was a bit small and made the event rushed for us. After each run we barely had time to get the tires bled down, switch the numbers, reset the video cameras (one on roof, one mounted inside), spray the tires down (they were overheating badly, especially the left side), and then switch drivers and get belted in and the steering wheel back on. This left very little time to watch other cars, change the suspension settings, etc. In the end we only made a couple of small shock rebound changes, as the car seemed to be handling pretty dang well. The track was cleaning up and also getting hotter, and these outside changing variables kept us from doing any effective shock testing.

Back in March at Optima, before the exhaust and tune, the car was "running out of gear" in 1st in the autocross, but 2nd was very sluggish because of the lack of mid-range torque. Looking at the before and after dyno charts, with the new exhaust and updated tune now, there is actually a 50 ft-lb increase in the mid-range, and it was noticeable at this event. Instead of trying to do the 1st-2nd-1st-2nd gear change dance, we could just leave the car in 2nd gear and let the low RPM torque pull us out of the 180 turns without feeling like the engine had no power.

What was not working so well were the tire compound and brake booster. The 295/315 A6s Mark got for the June event were used scrubs, and there have been another 60 runs runs put on them since at the June TMS event, the June Oklahoma event with lots of runs, and this July TMS event with two drivers. Not only were the tires extremely HOT (after a run we couldn't touch them for a full second) they were heat cycled out. Dead, kaput, done. I am not a fan of doing testing on used tires, and then there's the fact that autocross competition events don't necessarily make for good tests. But its what we got, so be it.

We both noted that the brake booster seemed to run out of assist after each short straight. I have noticed this before in our TT3 Mustang, running at this site back in May, where that car's booster was "running out of vacuum" after two long straights. I had to press so hard on the brake pedal then that I sheared the retaining pin that connected the brake pedal to the booster pushrod on that car. This same thing was happening to the C5 - either there is a vacuum leak in the booster or the big camshaft it has, with lots of overlap, is not producing enough vacuum after ~5 seconds of Wide Open Throttle. We were both overshooting the braking zone on the three 180 turn-arounds on our first two runs. On my third run I calmed down and just braked way early for each stop, which helped me drop two seconds and made for a 48.5 second run. I told Mark to try the same thing and he matched that time on his 4th run (48.6+1). By my 4th run I realized that these tires were blazing hot and weren't gripping like they should, so I slowed down all my inputs even more, which led to a 47.48 second run - the quickest time of the first heat.

In-car Video of Terry's (first segment) then Mark's (second segment) best runs of the day.

There's some video above that Mark shot with his GoPro of our best runs. Admittedly these runs don't look all that impressive, and at the end of the day the PAX placings weren't very good. Mark had cone trouble on his best run and seemed to be trying to use more steering angle than I was - and the tires just couldn't take it. It felt like we were on very heat-cycled tires, and that's what it boiled down to. I had to baby the throttle and temper the steering inputs or the tires would just run out of slip angle and slide. It felt a little like driving in the rain, to a degree - any abrupt input (brakes, steering, throttle) would over-tax the tires quickly. Driving here in May in my TT3 car on fresh 335/345 Hoosier A7s was a breeze - and had noticeably more grip!

But still, even as low as the grip felt and the issue with the brake booster pushing our braking points back in the C5, that 47.4 run was 3rd quickest of the day (out of 134 cars) and the fastest of any "car with fenders". Our best time was only edged out by a tenth of a second in heat 3 by a shifter kart, and again in heat 4, when the surface was cleaner and rubbered in, by an FMod car. Would we have gone quicker in later heats? Mark maybe, but not me - lack of sleep was catching up to me, and I was glad to be done working after heat 3, to head home early!

Jon (left) took CAM-C in his 2006 GT - also on MCS TT2s. The S550 (right) is looking quick in FStreet class

It was a great event, a fun course, and well run as always. Thanks again to Mark for letting me co-drive and for doing the video, which saved me some time from my normal post-event routine of video editing. It was a fun day, even with the heat and from being pretty tired and dehydrated all day, I was glad to still be able to get some decent laps in. We took 1-2 in SSM against some more heavily prepped Corvettes, so that felt good as well.

Before and After Cornering Images

Its hard to convey how much better the car handles now than it did before, when it had the Konis and stock springs. One of the easiest ways to improve handling is to reduce roll, dive and heave by increasing spring rate - and when you do that, you can also lower the ride height for a lower center of gravity. Those go together - if you can compensate for shock lengths (like we did with the MCS set) and raise the rates enough to eliminate bottoming, a lower CG is a good trick to improve lateral grip all on its own.

Based on a our experience we like to start off with at least least triple the front springs rates and double the rears on a 4 wheel independently suspended car like this one. As grip levels go up even more, so will the need for more spring rate. Of course when you increase spring rates dramatically you always need to improve the dampers to deal with that as well - and high quality monotube adjustables is the way to go. The Koni 3000s are technically monotube adjustables, but both the hassle and "randomness" of their infinitely adjustable valves is what makes them less than ideal, not to mention their excessive length leading to bottoming out the suspension when lowered. So the MCS is a good fit here. We could have used custom high rate transverse springs, and that would have been easier, but again - we like the low cost of coilover springs (Hyperco) vs $500/each custom built high rate transverse springs (Vette Brake Products).

OK, no more theory, lets see some data. To me one of the best ways to show the handling improvement, outside of lap times or data logging on a fixed road course (we haven't gotten there yet on this car) is to look at images of the car loaded up in a high-g corner, which we have below.

BEFORE: Koni Monotubes, stock springs, 295/315 BFG Rival street tires

AFTER: MCS TT2s, Coilover spring upgrade, 295/315 Hoosier A6 R-compound tires

As dramatic of a difference as these pictures show, it feels even MORE improved driving the car back to back. Mark didn't get to race the car on the old shocks/springs, but I did - and its a night and day difference. The new setup doesn't have the excessive roll and dive as the old, nor the "infinite spring rate spikes" when the shocks bottomed out, as they did before. This makes the car more precise at turn-in and gives the driver more confidence when pushing the car hard. Sure, the A6s were "compounded out" but it still had some decent grip and a good feel.

The improvements are doubly impressive when you consider we have also upped the mechanical grip used with the MCS shocks. Going from (worn out) BFG Rivals to (worn out) Hoosier A6 tires, even in the same size, is a big step up in grip. But the C5 just seems to WANT MOAR GRIP! So we are working toward that next... new tires, and bigger tires.

Having run probably 50 or more autocross events at this TMS site over the last 15 years, I feel like I can say with confidence that fresh Hoosiers in these same sizes would have been worth ~2 seconds that day, and on a hot day like this not having 2 drivers running back to back to back would have been a benefit as well. Moving up to wider tires like a 335/345 A7 would be worth even more. I cannot emphasize enough how important tires are in racing!

continued below

Last edited by Fair!; 07-21-2015 at 12:37 PM.
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