STU Build Phase 2, Part 2

Couple more things to cover before we get to the first events of the season.  Control arms are a big one- the stock bushings seem to have been tuned for the stiff sidewalls of the run-flat tires the C5 came with originally, which means they had to add some compliance into the suspension somewhere besides tires.  I think the bushings were a big part of that, and when you put on tires that aren't crappy 90s run-flats and crank up the roll stiffness with a big front bar you really notice the bushings deflecting every time the steering wheel is turned.  I decided to send my front arms off to Van Steel in Florida for their Delrin bushing install since they quoted a 1-2 week turnaround and they also replace the lower ball joints with new ones.  Phoenix Performance out of Pennsylvania is another company that does Delrin bushing installs but they don't replace the ball joints.  Both of those outfits press out your old bushings, clean the arms, and then jig them up and ream the bushing bores to precise diameters to install the new Delrin pieces.  I was very happy with how the Van Steel arms turned out, let's take a look:

Niceeeee.  Pretty easy install too, everything lined up well.  

Next up, Sparco Evo install.  Another big negative with the stock C5 is how crappy the stock seats are for driving around corners.  They might as well be bench seats for how much lateral support there is, so a new seat was required for serious driving.  The Sparco Evo seems pretty ubiquitous so I figured it was hard to go wrong with it.  I paired it with a set of Marrad machined aluminum fixed brackets to bolt it to the floor as low as possible with as little weight as possible.  Adding sliders and those huge bent sheet metal side mount brackets can add 10-15 lbs on top of the seat weight, and when the typical fiberglass bucket weighs 18-20 lbs you can quickly get way over the 25lb minimum ST-class seat+bracket weight.  I ended up at 27 lbs with the seat+brackets and also I am very happy with the seating position.  Only took a couple tries at seating position before I came up with one that worked.  I also replaced the hardware that came with the brackets with some nicer stuff from McMaster- now I only need one wrench size to pull the seat out instead of 3 and the nuts are nylocs for vibration resistance instead of normal nuts. 

Stock seat with electric motors was 40 lbs, so 13 lb weight savings. Passenger seat has manual controls so less weight to be lost there, might get around to that at some point... Lower priority since I don't sit in the passenger seat while I'm driving.

Speaking of weight saving, one other big area that is popular in ST classes is the battery.  C5s have 32lb AGM batteries stock so there isn't a 50 lb battery to remove like in some other cars but still that is some low hanging fruit.  I went with a 14lb Deka ETX-18l AGM powersports battery for now, it was $80 or something.  I found out later that Battery Tender sells 3lb LiFePO4 batteries for $250, so if I want another 10 lbs out of the car I know how much it will cost me.

Mounted it flat for a lower CoG.  Also added a tie-down later, don't freak out.

Also added an SLP Blackwing intake.  Supposed to be good for 15hp, I couldn't tell a difference really but every bit helps I guess.  The stock LS1 filter housing is definitely restrictive so it has to be better than that, and it looks cooler to have that big filter sitting there.

That's really it for Phase 2- HPT front spring, Delrin front CA bushings, seat, battery, intake.  Next we will have impressions from the first round of events.

Meanwhile, some more pictures from the front suspension overhaul:

STU Phase 2 Upgrades, Part 1

Here we go, the real STU build is beginning.  But first, gratuitous shots of my car on Daytona Beach during the 24 hour race weekend:

Good times.  It was a long drive from Asheville but it was the last time it'll have stock seats and stock front spring rates so we wanted to enjoy a nice roadtrip in it.  If you're not using Waze on roadtrips you're doing it wrong, by the way.  You'll also notice the Hankook RS3 tread pattern in the 2nd shot, I ordered those the week before the 24 hr and got them mounted up since the Rivals from last year were toast.  Good thing too since it rained most of the drive down there.  These will be the tires I run at least until July, then I will either get more Hankooks for Nats or try a different tire if there is a new hot option- lots of interesting things in the pipeline right now.  These are the generally accepted best option for C5s at this point in time so it seemed like a safe bet for me.  There was also the 285/30-18 Dunlop Z2 SS, but I wanted more sidewall height to be able to put down power better.  I was considering the Z2's on the front wheels and RS-3s on the rear but then you lose the chance for Hankook contingency.  It turned out that I got probably the last or 2nd to last set of these in the country, since Tire Rack was sold out of them only a few days after I received mine.  Very glad I didn't wait to buy them since the next batch doesn't come in until April 30th.  Hankook is always spotty on tire availability so if you know you want some and they are available now, buy them now!

Moving on, I also installed an MGW shifter and billet lower box before Daytona.  Wow, what a nice upgrade.  Before, the trans never really wanted to go into 1st or reverse and the throws were long and somewhat vague.  Now the gear engagement is very positive and mechanical.  Takes more effort but it is a rewarding experience to shift.  It's like shaking hands with Optimus Prime if you can imagine that.  

Finished off the beautiful MGW shifter with a machined Delrin shift knob in the C5.R style, big improvement over the awkward stock shift knob. 

Once we got back it was time for the serious work to begin.  Put the front up on jack stands and removed all 4 front control arms to send to Van Steel for Delrin bushings.  They press out the old bushings, clean the arms, and then jig them up and ream out the bushing bores to receive precisely machined Delrin pieces.  Comparable to spherical bearings in terms of stiffness, but Delrin is not metallic so it's allowed in the rules for ST classes.  Why not polyurethane?  Poly bushings, while stiffer than the stock rubber ones, still allow the bushings to deflect some amount which causes unwanted toe and camber changes throughout a corner.  They also need to be greased fairly often, whereas Delrin lasts several years (at least) without requiring any service at all. Some cars need to run poly or rubber bushings at some joints to allow for multi-axis rotation but the Corvette doesn't need it.  You can't adjust caster easily with Delrin bushings but the stock caster is set to 7-8 degrees which is plenty.  The Delrin allows you to take advantage of the well-designed suspension geometry while limiting deflection under load for better performance.  You can also run less static negative camber since the rubber isn't moving around causing your wheel to go further towards positive camber.  Less static negative camber means better braking performance with the front wheels and better acceleration with the rear. Here's the front all taken apart, was surprisingly straightforward to do:

Also with the front control arms off, the leaf spring is completely exposed and easy to swap.  I took the opportunity and replaced the stock fiberglass Z51/Z06 526 lb rate leaf with a Hyperco 634 lb spring (20.7% increase).  They call it the "HPT" spring.  You can also tell it's higher rate than the Z06 spring since it has so much less arch:

Another cool thing are the ride height adjusters on the Hyperco spring- Delrin pads (can you tell I like Delrin) that have a lower stack height than the rubber pads on the GM springs, allowing for a lower ride height (if you can run that low and not bottom out your shocks or go into a bad bump steer range).

I should probably also talk about why I chose this spring compared to some other ones on the market like VB&P.  First, VB&P rates their springs differently from GM so you can't really tell what your rate increase is.  Also, they don't come with the lower height Delrin adjusters, and lastly they are more expensive and you can't get them from Sam Strano (of stranoparts.com).  As far as the HPT vs the slightly lower rate HPS spring, my philosophy is that at this stage it's okay to make big changes since I am going to be retuning and relearning everything anyways.  Plus the car was very loose with square tires and I like a car that is biased more towards stability (understeer), since I can be more aggressive with the throttle.  I usually underdrive corner entrances so if I can stand on the gas at or before the apex it means I don't lose as much time to people that carry more speed into the corner.  Plus with a powerful RWD car you want most of your roll stiffness to be up front in order to put down power better.  Think of a trophy truck, how the rear end just squats and hooks up and the truck catapults off even on very low grip surfaces.  Speaking of trophy trucks, if you haven't seen the BJ Baldwin video (Recoil 2) where he drives through (over?) Ensenada, Mexico, you should check it out, good lesson on well setup long travel suspension setups.  The combo of the 33mm sway bar and the 20% front spring rate increase might mean I went too far, but I can easily go to a 30mm front sway bar or a size higher rear sway bar to balance things out.  Strano makes a nice tubular 1" 3-way adjustable rear sway that could end up being just the trick. 

 Speaking of trophy trucks:

How's that for a roll stiffness distribution!!!  I think Robby Gordon set up his Stadium Super Trucks to lift front wheels that high because it looks cool, but still I bet it has great traction since that inside rear tire is extremely planted.

Continuing with the spring rate discussion, it is also worth considering that the venerable Danny Popp sets up C5Z's for SSP autocross with higher rate front leafs and stock rears.  Not sure on what sway bars he uses though.  I'd really like to know his suspension setup that he won the Optima USCA last year with since he was on street tires, but I have been unable to find anything other than the fact he was sponsored by JRi so was probably running their shocks.  

Should have the Delrin-ified control arms back next week so we can take a look at those, and hopefully some good news on shocks as well.

NCAC 2014

NCAC.  It didn't turn out as well as I hoped, ended up 2nd behind a mildly prepped WRX.  I can place the blame squarely on my driving since on Day 2 I got my head in the game and beat the WRX, but it wasn't by enough to come back from Day 1 and I still left time on the course on Day 2.  I was getting faster each run by huge chunks of time (2 seconds on one of the runs!) so I was nowhere close to how fast the car was.  Even with pretty used up 275 width Rivals, the front end grip is tremendous.  Of course the rear of the car wants to slither around with any sort of throttle input but it is a Corvette on street tires after all.  It's a lot easier to drive fast when you don't have to keep catching the rear end of the car, or at least that's what I prefer, so that will be my primary focus in setting up the car for the 2015 season.  It was disappointing overall though, since I had success at NCAC last year in the paxed Tire class, taking the win against 23 other drivers, and Zmax is the site I am most familiar with.  Good motivation for next year I guess. Results: http://www.auto-x.com/forum/showthread.php?t=5502

Here's my two fastest runs, Day 1 and Day 2.  I'm pretty sure Day 1 I was just overslowing for pretty much every corner except one near the end.  I was hemorrhaging time in the high speed sections for sure.  Day 2 was better about carrying speed into corners but there was plenty more time out there.

I also distinctly remember being in the middle of a run and thinking to myself, hm, I am basically sitting on one of the side bolsters of the seat here.  The base C5 seats are so awful for autocross it's hard to fathom- I was sliding all over the place, having to use the steering wheel to hold myself in place, which is obviously not great for feeling the grip of the tires and what the car is doing underneath you.  Let's see, what other excuses can I find... No but that was really the biggest one, I am very glad I get to install a race seat in STU.  

As far as car setup for NCAC, I was running the Z51 front spring which is the same rate as a Z06 spring, along with an actual Z06 rear spring, Z06 rear sway bar (tiny bit bigger than Z51), Strano 33mm front bar, C6Z06 shocks, C5Z06 rear wheels all around with pretty beat 275 width Rivals at 29psi, maxed out front camber (2.2ish), no front toe, 1.5 rear camber with 1/8" toe in, and zero rake.  Otherwise bone stock, so basically a B-Street C5 with wider wheels.  2001 and up C5s got the Z06 rear sway bar and the Z06 rear spring is very close to the Z51 rate- I couldn't even tell a difference when I put it on, so the suspension setup is basically a 01+ C5 setup.  I was 0.4 seconds total (both days combined) behind a true BS '04 C5 on Penskes and ZIIs, driven by an accomplished driver, so I took a bit of solace in that.  The wider wheels make a big difference but he has been running that car for two years now so I felt it was a good benchmark.  Still lots of weight to pull out, race seat, a much more responsive and less mushy suspension setup to dial in, better shocks, race seat, more power, did I say race seat?

Also AutoxPix was there and my car made it into the NCAC highlight reel, check it out:

  Interesting to see how much the front end lifts up on throttle.  I think I'm turning a bit to the left there too so that lifts up the inside more, but you really can tell how soft the front spring is, and how little rebound damping there is in those Z06 shocks.

Next post: Winter 2015 full-tilt-ish STU build.

1. Heitkotter, 2. Borg, 3. Lugod, 4. Stuart

First post- we'll start with my prediction for the STU class at Nationals this year.  Out of the people I know that will be competing in the class at Nats that's my pick.  Pro driver Bryan Heitkotter bought Brian Peter's 350Z which won the class last year, Lane Borg in his usually prepped to the gills yellow '04 C5, Lugod in the OS Giken USA Red FRC, and Jeff Stuart in a 350Z which I don't know much about.  He won a 55-deep C-Street class last year so I can't see him doing worse than 4th.

As for where I think I will end up?  My goal is to bag a trophy.  Last one available would be fine... I have my doubts about how possible that may be but we will see. First step is to bag a contested trophy at a Tour this year.  I "won" A-Street in my Solstice GXP last year at the Atlanta Match Tour but... I was the only one there.  Four other tours attended in my autocross career with competition but no hardware.  It is a fun car though, I still drive it to work most days, and it was great to learn the ropes with.

Ok, now for the car this blog is supposed to be about. Well, maybe I should start with some more background, why am I building an STU Corvette?  Easy!  Why wouldn't you build an STU Corvette?  But really, my first real autocross car is/was (I still have it) a Solstice GXP I bought 3 months out of school.  I wanted something turbo, RWD, manual, and sporty.  Didn't consider much class competitiveness or practicality.  Found one without A/C and I thought, cool, less work to remove the A/C when I am turning it into a crazy fast racecar!  Eventually learned about National level SCCA autocross, got hooked, got destroyed but Junior Johnson in a C5Z06 in A-Street at the Wilmington Tour, drove 7 hours back home in a cramped uncomfortable hot loud car and decided enough of that.  I knew I didn't want to keep running the Solstice in the beginning of 2014 so I hadn't been doing any work to it and saved up some dough to buy a new competition car.  I wanted to run an ST class I knew, since I like modifying cars but don't have room for a trailer/tow vehicle or even a tire trailer and I'm also too lazy to swap tires at events.  Something that I could take to track days would be fun too.  They classed the C5 and 350Z into STU at the beginning of 2014 and that immediately caught my eye.  A Focus ST or Fiesta ST in STX also would have been interesting but I wasn't sure of how competitive they would be.  Someone should try a FiST in STX though, it's very narrow and can make a TON of torque.  That 1.6l Ecoboost is a heck of an engine. I ran some thrust curves and estimated how much weight you could drop from a C5 in ST trim, compared it to the reigning top dog GR WRX STi and the 350Z, and was pleased with the outcome.  Easy to see how the STAC classed the car in STU since the 2nd gear thrust curve is so much lower than an STi and it is something like 2-3" wider than a Z, but I think the C5 has a lot of untapped potential along with an extremely healthy performance aftermarket.  Easy to find Delrin bushings for control arms, huge selection of quality long tube headers, easy to find and fit wide wheels, engine responds well to tuning, lots of choices for fancy dampers, etc.  The Corvette forums used parts for sale area is also a treasure trove of killer deals so I could build up a nearly fully prepped car with much less headache and capital outlay than some other platforms.  I also like GM sports cars and LS1 engines so it was an easy choice.  Let's find one with A/C this time too.

August 2014, I've been spending quite a bit of time searching Autotrader and craigslist for suitable candidates.  Found some that look good but didn't quite have the money yet or they were far away, but then one pops up on craigslist in Shelby, NC.  A '99 FRC with 59k miles, few options, pretty much bone stock.  Turned out to be a pretty old guy that was having a hard time getting into the car anymore since it was low and didn't drive it much.  He was super anal about cleaning it and never took it out in the rain. I told him I'd take good care of it, didn't want to go too much into all the cone marks and track slag it would be collecting shortly... Picture below is the car right after I got it.

I picked it up on a Thursday or something and there was an autocross at zMax in Concord the following Saturday.  It still had Michelin run-flats on it with date codes from 2004 on the front and 2008 on the rear... yeesh.  Needless to say they didn't work very well.  I don't think I learned anything about driving it.  Nowhere to go but up from there though.  The North Carolina Autocross Championships were coming up in 2 months or so and I wanted to have a good showing in STU.  First step was to ditch those awful run-flats.  Enter ebay and the autocross forums, and I was able to grab a set of 4 18x10.5" C5Z06 rear wheels and 4 275-35-18 BFG Rivals with decent life left in them, all for a very reasonable dollar amount.  Car looked pretty good on them too, and the difference in grip and steering feel from those run-flats was simply astounding.

Picture above is from a drive I took in the NC mountain back roads with some buddies.  The M5 was having a hard time keeping up...

I did a couple events on the stock suspension with just the wheels/tires and it worked okay but the amount of grip was overwhelming the soft shocks and small anti-roll bars.  Enter the forums and ebay along with some guys from Charlotte with an SSR C5Z06, and I grabbed some C6Z06 shocks and C5 Z06 rear sway bar and leaf springs.  I also ordered Sam Strano's famous 33mm front sway bar to top it off.  I wasn't too impressed with the C6Z06 shocks but the front sway bar definitely made a good improvement.  I think the stock C5 Z51 FSB is 25.4mm diameter so going to 33mm was a huge change in front roll rate- before the bar the car would take what felt like multiple seconds to take a set in a corner, and after it was much more planted everywhere.  Stock bar vs Strano bar:

Better view of how much beefier the Strano bar is:

The big bar really highlighted to me how soft the bushings are in this car though, since now the weight was transferred nearly instantaneously through the bar once you turn the wheel, but then you could feel all the rubber deflecting, and then the car would turn in.  Coming from the Solstice with higher ride frequencies, quicker steering rack, and stiffer bushings, the turn-in response of the C5 is disappointing.  Most of that can be addressed under ST class rules though so I am not that worried.   I also played around with lowering the car around this time, and for one event at zMax I went too far in the rear and even on that smooth surface the car was bottoming out and losing traction abruptly multiple times on the course.  Learned my lesson there as I slowed down significantly.  Quick tip: aftermarket Corvette websites sell rear lowering kits but you can order 6 or 6.5" long 1/2" Grade 8 bolts and Nylocs from Mcmaster for much cheaper than those lowering kits.  Plus you can get bolts with a finer thread pitch for more precise adjustments.  A pain when making big changes but could be nice when corner-weighting the car.

 6" bolts seem like the right way to go for stock rear spring rates to avoid bottoming out all over the place.  I think the photo below is from when I was running it too low in the rear.  I had it set at zero rake under static conditions so on throttle it was very low in the rear.  It made sense to me that you want the rear low if you are struggling for grip, since there is less weight transfer off the inside tire, but it was too much.

That's good for now.  Check back in next time for how the 2014 North Carolina Autocross Championships went.