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Unread 05-20-2015, 09:05 AM
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Fair! Fair! is offline
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Join Date: Jul 2004
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Default Re: Vorshlag Build Thread - Mark C's 2002 C5 Corvette Track Rat

continued from above

After making that video, I've learned a lot more about this particular car. It was built for a certain class where some of the parts sort of make sense. Some parts on this car aren't as terrible when used in "ideal conditions" - the shocks might work fine on a smooth track and at stock ride height, and the other parts fit this points based letter class in NASA (TTA). So now the choices make more sense to me: using these aftermarket shocks but not upgrading the springs (more points), doing the headers and other exhaust bits (used to be +5), cold air (+1), and the BIG cam upgrade (+6) to get to a target power number. It probably stacked up well against the old C5 Z06 entries that used to rule TTA class. The part selection was also partially budget driven, and that's OK.

Left: Up through the 2012 season, TTA class was alive and well. Right: By 2013 TTA was dead and TT3 was a whole new ballgame.

The previous owner raced this car in NASA's TTA class at Hallett from 2010-12, and did fairly well in the car. The lap times (1:24s) were pretty good and he won plenty of contingency tires. TTA was a popular and competitive class that had a LOT of variety, but the C5 Z06 pretty much dominated at the top levels there for years. There are loads of posts about the loss of this class, but by 2012 it was dead and not coming back.

Since 2013, all C5 Corvettes go straight into TT1/2/3, so "points" mods restrictions are no longer a driving factor in parts selection. Instead of picking selective mods around a class that no longer exists, living with big compromises on the street, and being stuck with stock ride heights, the new owner Mark is going to build it with a bigger budget and NASA TT2 class in mind. So again, sorry if I was too harsh in my video above, and I kind of get it now, but there were still some big compromises to the set-up.

My "8 minute bitchfest" video also showed some of the driveability items that were above and beyond the problems uncovered during competition at Optima, which included:
  • A glitch which kept the car from running properly or having enough power to exceed about 60 mph with the factory Traction Control turned on (I figured that out the morning we went to TMS, within 5 minutes of driving the car for the first time on the highway)
  • The loud droning of the Borla exhaust was ear splitting at highway speeds
  • The rough tune wouldn't idle or cruise under 1500 rpms; you had to use a lower gear to keep the revs up or it bucked and stumbled, which made the drone even worse
  • The ride quality on the long-ish Konis suffered on the street - any real bumps would bottom the rear shocks, sending the spring rate to infinite
  • The clutch engagement was not smooth and made stop-and-go traffic a hassle. The lack of idle made the engine die when stopped, so you had to drive with 3 feet.
Long story short, it was a chore to street drive, which was a big ask for only 400 whp. Something wasn't right, and I knew our friends at True Street Motorsports could work their magic and tune out these driveability issues. But first, we needed to so something about this exhaust before they retuned the motor.

What's Next for the eBay Corvette? UPGRADES!

So after we finally had a few days to catch up after Optima in March we spoke with Mark and he even come by the shop to talk to us in person about plans. The general consensus was that there were several issues we found at Optima that needed to be rectified, and he had some choices to make regarding the suspension, exhaust, engine tune, and brakes. We also needed to know what color to powder coat the Forgestar CF5 wheels, since we had to rush order the set "raw" to make them in time for Optima.

So let's get to the fun part of this post - the upgrades! First, here's the former dyno graph from the previous tune done when the cam was added.

Yes, the image is hard to read but the basic numbers are: 338 whp / 356 wtq before the cam, and 400 whp / 381 wtq after the mega-cam and a tune were added. Big add for the cam, obviously, but it came at a cost of smoothness, idle, and other issues. From what I am told this power level was how the car was purchased by Mark this Spring. Something wonky happened to the tune after this old dyno chart was made, as I've had LG tune our cars in the past and they did a great job. Who knows, but whatever the reason, it ran like crap when Mark received it.

New Axle-Back Exhaust

The first item we all agreed needed to change was the exhaust. It was too loud and after looking at it up close, I felt it was also restrictive. The weird routing and tubing sizes looked all wrong to me. Did I mention that the tiny Borla mufflers + full length headers made the car EAR SPLITTINGLY LOUD? Yea, it was unpleasant, and I noted as much in the in-car video - where you can barely hear me describe the issues with the car over the monotonous exhaust DRONE.

I've built a lot of custom exhaust systems and installed dozens of off-the-shelf performance exhaust systems over the past 28 years of being a hobbyist fabricator/racer, a mechanic at a tuner shop, and then the owner at Vorshlag for the past decade. We have two really good fabricators here (Ryan and Olof) and two engineers that understand flow (Jason and I) and exhaust sound. We are also fanatics for lower weight, higher quality materials, and adding simplicity when possible.

The Borla axle-back system above isn't great, but it isn't 100% terrible. This C5 already had that installed, plus some nondescript long tube headers, all of which is just rusty enough to be recognized as 409 stainless. The header collectors, cats and mid-pipe section are 2.75" OD but the Borla axle-back section is only 2.5" OD. We know how to measure pipe and have a dozen calipers around the shop, so you're gonna have to trust me on those numbers.

The routing that the OEM system, this Borla system, and seemingly ALL aftermarket exhaust systems seem to take on a C5 is torturous and needlessly compromised. Its ALL ABOUT THE QUAD CHROME TIPS, yo. There are lots of bends and not a lot of room for a properly sized muffler case. And in the case of exhaust tubing and muffler case, bigger IS better.

We've done lots of before/after measured sound tests and lots of before/after dyno tests and after years of doing that we have figured out what works and what doesn't matter. When excessive NOISE is a problem we found that a LARGER case muffler of a certain type knocks the sound levels down. DRONE is handled in another way, and PERFORMANCE is all about maintaining a smooth and larger inner diameter and minimizing bends. So we spec'd out the biggest Magnaflow 304 stainless mufflers we could fit in the rear and got those coming, plus a lot of 3" mandrel bent tubing and some real stainless V-band clamp assemblies.

My new routing idea was fairly simple but not something we'd seen before on a C5 - exit out the side. This let us avoid a pair of 180 bends and a pair of 90 bends as well. This also gave us more ROOM for a bigger muffler case. This isn't revolutionary and since we built this I've had people show me other examples of this routing on C5/C6 Corvettes. Not surprising - its the only routing that makes sense.

Exhaust leaks are also a pet peeve of mine, and I find any audible leaks unacceptable. There's a half dozen ways to make a multi-piece exhaust system join together, and the "common" ways are the cheaper, most leak prone joint types: slip-fit and ball-and-socket joints are the worst, with bolted flanges with gaskets coming a close second. The proper way we've found, which is the most costly and complicated to install, is a V-band flange connection. Done right, these never leak.

We also build exhausts to be lighter, and can often save 40-80 pounds over OEM systems. But this car didn't have the stock exhaust, so we didn't weigh the changes. Since I also hate rusty exhaust components, we always stick with 304 stainless steel exclusively. Titanium is a real bear to weld and EXPENSIVE, and the less dense material makes for more noise. So we keep a lot of mandrel bent 2.5" and 3" 304SS tubing around.

Like all race shops would build it, we piece the systems together from the mandrel bends - slicing, fitting and tacking them together as we go. After tacked we remove the finished assembly and final welding everything on the fab bench. This is the right way to do one-off exhaust systems, but its time consuming and the materials are expensive. Spending 12-20 hours building an exhaust is not unheard of, and at our current shop rate ($105/hour) it can add up. This isn't the right choice for many folks, but when the options are all compromised (like the C5) or its a custom race exhaust where every ounce matters, or you need a quieter set-up than the aftermarket offers - this way works. And we stay pretty dang busy.

Not shown well is the layers of heat shielding we added on the inside of the rear "fenders" (bumper cover). This includes a composite aluminum/fiberglass mat on the inside surface with DEI gold foil reflective barrier on the top of that, near the muffler case.

The outside of the body has hand formed (to match the curve of the body) 304SS heat shields shown above, bolted in place with stainless button head M6 bolts. The exhaust system "grows" (front to back and laterally) when it gets hot, so there's a necessary gap around the tail pipes that changes slightly with temperature. If the stainless plates get covered in soot they can be hit lightly with steel wool and be shiny again.

The new sound (see video below) is a lot more pleasing, still having a bark on the outside but MUCH quieter inside. Oh, and this change along with a re-tune gained 25 whp...

Sean at True Street dyno tuned it this week and as you can see (above) it now makes 425 whp and 398 wtq. Not spectacular, but its still just a stock 5.7L LS1 with a cam, headers and a cold air. It picked up 25 whp and 23 wtq from the exhaust changes and their conservative "road course" dyno tune (STD corrections). We had them tune it for 91 octane (the best at most Oklahoma gas stations where Mark lines) and for road course use, so this isn't some ragged edge drag race tune, either. In a 3100 pound C5 with 425 whp it goes plenty well, and it would put the car near the limit for NASA TT2 as it sits... (nominally an 8:1 power to weight ratio class, with driver on board).

Click for video (tested on a closed course in Mexico):

Why has nobody made a C5 exhaust system "kit" like this? It isn't easy to install nor replicate for an off-the-shelf system that is easy for a DIY installer to put on these cars. There's cutting of the bodywork and the V-band clamps don't have any "slop" to allow for alignment. It was made to fit THIS car. People have already asked me what it would cost to build an axle-back set-up like this and ship it to them, and I said "first, ship me your car". This is a custom exhaust set-up that we have to fit to a given car. Not only are the large diameter tubes tough to fit around things like transaxle, suspension and halfshafts, the exhaust outlets need the heat shielding inside and out.

Wheels Get Color

The color Mark went with is a Blood Red powder coat that we had done at a local powder coat shop we've worked with for years. Yes, it was baked under 400F, yes it is safe, and yes virtually all wheels you can buy are powder coated. Please don't chime in with your internet stories of cracked wheels from powder coating. That was a scare from the 1990s and any decent powder coat shop knows how to coat aluminum wheels without altering the temper. Please don't propagate old internet wives tales, thanks.

Anyway, the finish came out beautifully and the pop of color really helps this otherwise bland silver Corvette.

NEXT: Suspension Plans

We have spec'd some MCS TT2 dampers for the car, which are being built and should arrive soon. We will install these with some other parts we want to make, which I will discuss in the next post. We will be removing the transverse springs in the process and go to coilover springs. Why go to that trouble? Why not spec a custom VBP transverse spring for both ends? Well we almost did that, and had really good results from the 1170 pound VBP (above left) custom front spring on my TTC prepped C4 Corvette.

After talking with Lex from MCS about this car when he was at our shop recently, he made a simple point: how can you tune the spring rates at the track if you only have $500/each transverse springs? A coilover spring is a whole lot easier to change and a lot cheaper to test several rates with. We can carry a dozen springs to the track and let the clocks show us what works best. Good idea.

Moving the springs to the shock (coilover) and away from the OEM mounting locations is no trivial matter on this car, though. You are essentially moving all SPRING loads onto places on the chassis and control arms that were designed for DAMPER loads. So things will be altered and beefed up to hold these higher loads. We will share more next time...

What's Next?

We have done a lot of events since Optima in our other cars, but the next event scheduled for the C5 is below:
  • May 30th - Five Star Ford @ ECR (HPDE event)
Originally scheduled for May 9th but was moved due to heavy storms, which gives us time to knock out some work on the C5. This time Mark can make it down to drive the car on proper dampers that don't bottom out after 1/4" of travel. He will never know the joys I had at Optima!

We're also working on a number of other interesting projects...

This Week at Vorshlag video for May 8, 2015

Click above our my latest "This Week at Vorshlag" video, linked here. In that I cover many of the other projects going on in our shop that week, so watch that or check out our Facebook page or Blog to see what we're working on outside of the Mustang world. I write insanely detailed forum build threads for all sorts of other car chassis and types...

Thanks for reading!

Last edited by Fair!; 05-20-2015 at 09:20 AM.
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