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Unread 06-29-2015, 12:01 PM
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Fair! Fair! is offline
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Default Re: Vorshlag Build Thread - Mark C's 2002 C5 Corvette Track Rat

Project Update for June 29th, 2015: We've been working on Mark's C5 Corvette since the Optima event and have made big strides. We have some new products we're testing on this car which are somewhat significant, too. Mark finally got to race this car at an autocross, where I co-drove with him. This was to be a test of the new set-up, but we had some weather issues, which I will detail below. Let's get caught up.

Unintended Weight Loss

If you have ever read one of my posts you know I'm a freak for weights... we tend to weigh cars and parts more than other shops, because we know how important weight is in racing. After tires, weight matters most. So let's look at what we started with...

Left: The weight of Mark's C5 after Round 1 of mods (3114); Right: Weight of a stock C7 Z06 for comparison (3570)

The 3114 pound weight above was after Round 1 of mods were added - it had almost no fuel, full length headers (lighter than cast iron manifolds), 18x11" Forgestar wheels (the 18x10.5" TSW wheels it started with were even heavier), two composite Sparco race seats, harness bar, and a Borla exhaust. That's not bad, considering what a stock, carbon fiber C7 Z06 Corvette weighs (3570 lbs) - which is about the same as a 4 seat 2011-14 Mustang GT with a giant DOHC V8, stick axle and steel bodywork. Mark came from a 2012 Mustang GT before this car.

We haven't really done any real weight saving tricks on this C5 in Round 2 of the mods, just component upgrades to make it faster. But we always keep an eye on weights and pick parts that are "weight neutral" or lighter, whenever we can.

And this picture above is where we are now, with similar fuel levels (almost none), which is 3050 pounds. This is almost exactly what a C5 Z06 weighs (I've weighed one at 3048 lbs, which was stock and low fuel). The Z06 is a lighter car than a coupe from the factory, as it doesn't have the giant glass rear hatch, no removable roof, narrower wheels than this car has, and a titanium exhaust. And while a 64 pound loss might not seem like a big deal, we weren't even trying to remove weight here. The car still has full interior, air conditioning, and we even went with heavier front rotors (Centric 1-piece vs Stoptech 2-piece). So a semi-stripped C5 Coupe could easily get under 2900 pounds or less.

A Brief Detour To Talk About Time Trial

This new weight of 3050 + a 200 pound driver with the new 425 whp power number (up from 400) puts power to weight at 7.64:1 (down from 8.4:1 when it got to us), which is a tick better than TT2 territory (8.0:1). This is good - so with a little ballast or bit less power it could get right at limit of the TT2 class, when Mark takes the TT plunge. Sometimes it is tough to fit in one of the three numbered TT classes (TT1/2/3), but this one works out perfectly.

Does this TT and power-to-weight ratio stuff sound confusing? I talk a bit more about TT classes in this thread on the S197forums. Yes, this is a Mustang forum, but the sub-section (corner carvers) where we read and post there is pretty focused, and we have had some decent tech discussions there. In post #23 there I talk about the pros and cons of autocross vs HPDE vs Time Trial vs W2W racing. Posts #28 and #33 is where I talk a bit more about Time Trial, which Mark is building this C5 for.

Time Trial rules might seem confusing at first glance by racers from other racing groups or non-racers, but it's really quite simple: measured wheel horsepower and measured weights (with driver) are combined to form a ratio (power to weight). This is the CORE basis for almost all NASA racing (W2W) and TT classes. It works really well - better than "engine displacement to weight" ratios and "turbo modifiers" that the SCCA likes to use (which was dropped in the 1970s by almost every other racing organization on the planet). Let's look at some numbers...

Relative Ranking of NASA Classes
TT1 5.50:1
TT2 8.00:1
TT3 (stock aero ) 9.00:1
TT3 (modified aero) 9.4:1
American Iron 9.0:1 to 9.5:1
TTB 10.50:1
Spec Iron 11.75:1
TTC 12.00:1
CMC ~12.7:1
TTD 14.25:1
TTE 16.50:1
TTF 19.50:1

AI, SI and CMC are three popular "pony car" wheel to wheel (W2W) racing classes within NASA, in case you are unfamiliar with those names. There are small power-to-weight modifiers to all of these classes, especially the numbered classes, but these base numbers are pretty much the goal you shoot for. Not all "letter class" TT cars can hit the power to weight max (with the limitations on mods, you are lucky if you can!) but the number class cars can (its just costs money!).

I ran our TT3 Mustang at 8.8:1, even with the -0.4 "non-stock aero" hit (we picked up an even bigger ratio bonus for running over 3801 pounds, +0.6). We run our TTC Corvette at 11.3:1 with a bonus for only running a 245mm tire (+0.7). And when it comes to the modifiers, "minus equals plus and plus equals minus"... I know, it's wack, just don't ask.

Suspension Development + Hoosiers

During the past 7 or so weeks since Optima we have done a bit of development. Lex from MCS stopped by our shop and we discussed shock options for the C5, as well as the two big spring choices - monoleaf vs coilover springs.

The C5 uses a single transverse mounted monoleaf spring at both the front and rear of the chassis, of course, and the Y-body Corvette chassis has done this for generations. Mark's base C5 Coupe was fitted with the two springs above. There is some associated hardware that goes with mounting the monoleafs, which we removed, shown below.

We swapped the C5's factory transverse springs for a quartet of 2.25" diameter steel coilover springs from Hyperco, which I have weighed above, top right. So if you add up the monoleaf spring and hardware weights (9.7 + 11.9 + 3.6) that's 25.2 pounds, replaced by 8.6 pounds of coilover springs. Now we didn't weigh the MCS TT2 aluminum bodied monotubes, but they are probably another 4-5 pounds lighter on each corner than the steel bodied Konis that came off (which are already sold). So a good bit of the weight loss in Round 2 came from the suspension changes, the rest was in the exhaust and airbag delete + race steering wheel.

Coilover springs (left) are more common, cheaper and come in more rate options than monoleaf springs (right), which are built to order

And yes, we could have used custom built, higher rate monoleaf springs (see above right) instead of coilover springs, and that's what we did on our C4 Corvette for TTC. For the C5 we could have sourced monoleaf springs from the same place as these coilover springs - Hyperco. Why didn't we? Three reasons - cost, availability/options, and time to change springs track side.

Lex summed it up best: "You can take a dozen coilover springs with you to test at the track - easy to change, cheap to buy, tons of rate and length options. Are you going to buy an equal number of ($500/each) monoleaf springs to test with?" He has a point. At $80-100 each for Hyperco coilover springs, this gives you a lot more flexibility and options to test with, plus they are a lot easier to change track side. Plus they are lighter and don't have any "unintended swaybar effect" like transverse monoleaf springs do.

After deciding on using coilover springs (which moves the load path of the suspension from the leaf mounts to the shock mounts) we knew we wanted to make some spherical shock mounts for this chassis different than what other companies have made. With these two pieces of info we ordered the MCS TT2 shocks (above) in different lengths and configurations than normal. We definitely didn't want to end up with dampers that were "too long", like the set that came off this car.

The comparisons above show the front shocks (left) and rear shocks (right), Koni 3000 series vs MCS TT2. The new fronts are shorter and have a little more bump travel. Couple that with our raised height spherical mounts and they had even more travel. The rear was the nightmare with the Koni's, and with ride heights 1/2" lower than stock we were left with only 1/4" of bump travel. That ain't enough. The new MCS body length is slightly shorter, and non-inverted. But the big space savings was moving the adjuster to the top of the shaft instead of the one built into the lower fork. That saves almost 3/4" of an inch in body length alone, which equals more stroke in the bump direction. After you look at the motion ratio of the shock it makes for about 2 inches more bump travel = plenty.

Here's a close-up of the inverted front MCS monotube double adjustable, which we call the "TT2" shock series (TT1 = single adjustable non-remotes). Like I said, we didn't have the rear shock inverted to save room, but left the fronts inverted (how they normally make them for a C5/C6). That means the rear shocks' 2-stage adjuster knob would be buried inside the "blind hole" that the factory shocks mount into.

So we cut a big hole above the shock to gain access to the knob. This was a relatively easy fix, and makes for MUCH easier shock adjustments than before with the Konis (jack up car, insert tool, rotate and hope to make both sides the same). Now the knobs can be reached quickly - pop the rear hatch, lift the carpet, and turn the knob. Pull up for compression, push down for rebound, nice positive clicks.

The C5's rear TT2 shocks us the traditional MCS 2-way knob, shown in the video above - down for Rebound, up for Compression

The trunk access holes above were made by first removing the rear shocks, looking at the blind hole, then approximating the center of this hole and drilling a 1/8" pilot hole up through the fiberglass rear deck. Once the center of the shock opening was located by this pilot hole, a 2" hole saw (with an 1/8" pilot drill) was used to cut the openings from the top side. We cleaned them up after these pics were taken and temporally stuffed a foam koozie down inside the cavity through this 2" opening, to make a water tight seal around the shock shaft. There is nothing really structural here, and the pilot hole kept the 2" hole saw centered over the blind shock cavity and away from the steel frame rail. We will install a better cover next time the car is at our shop.

The upper shock mount itself is something we designed and machined in-house, to test on Mark's car. This eliminates a big "double rubber bushing" mount in the upper part of the shock, which becomes more crucial when you move the suspension loads to the shock mounts (which only see "damper loads" in stock form).

continued below

Last edited by Fair!; 06-29-2015 at 04:32 PM.
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