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Unread 01-25-2016, 03:43 PM
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Default Re: Vorshlag Budget TT Build: Project DANGER ZONE

continued from above

While we started out thinking about just an 18x11" wheel and 315 tire (which would have fit under the stock fenders), the 335s fit so well under the stock fenders we measured the car for 18x12" wheels at both ends and pulled the trigger. 18x11 and 18x12 wheels cost the same, and I didn't have any (free) 315 tires laying around. So the 335/345 tire combo was chosen for... budget reasons??



The easiest way to add a lot of performance to this TTC Corvette, since we had just had the engine rebuilt to painfully stock spec's, was adding mechanical grip. The easiest path I could think of was to add giant wheels and tires and dump some weight. So I started to do some TT1/2/3 calcs for this car late last season and it looked like TT2 would be a good goal. Here are our TT2 goals...
  • Goal Weight: 2850 empty, 3050 with driver
  • Goal P-to-W: 8.0 to 1 (TT2)
  • Goal power: 381 whp
So the 381 whp power goal? Yea, we're not close to that yet, and I will show our dyno tests below with the new motor. We have some things in mind but we figured it was worth testing this theory with the existing ~295 whp power level before we do any mods that could not be "undone", so we had a path back to TTC in mind if the first 2 events in 2016 were a total bust on the big tires.


This 2923 pound weight was with the full cage, 18x12" wheels, and half a tank of fuel. The car is still about 75 pounds too heavy

The TT2 class move was the reason for trying to lower the weight goal - and why we pushed ahead with the plexiglass rear hatch and snatched out the ballast box and multiple 45 pound weight plates. We still have some pounds to go on this 2850 pound goal (without driver), but there are plans to get there. I was just hoping we could get closer to this weight for the first event, and since we got the car down to 2840 pounds back with the 4-point roll bar, small wheels, and no rear hatch before.



If you have read my build threads over the years you know I like to use the biggest wheels and tires that can fit a car, for both road course or autocross use. One of the things our shop is known for is pushing the limits of wheel width under stock fenders on a number of cars: BMW E36, E46, E90, & 1M; EVO X, Subaru GR, S197 Mustang, C5 Corvette, and more. And now we can add C4 Corvette to that list. And maybe.... "fit" is a stretch here, I'll admit. But this car will get some flares soon to make it all look right and cut down on aero drag. We did learn what it takes to stuff a 315 and 18x11" under stock C4 fenders with this test, which is more usable data for most folks' cars.



So as you can see the 18x12" front wheel we spec'd "fits" up front with no mods using the 335/30/18 Hoosier. This could work with zero mods for a street car with 11" wide wheels and a 315/30/18, but we wanted to go for broke so I decided on a 12" wide wheel and 335 tire up front. Again, we had so many sets of 335s and 345s from before. This worked so well up front on a 3600 pound TT3 Mustang that I figured it would work as well or better on a 2850 pound TT2 Corvette.



The rear has an even bigger tire, the massive 345/35/18 Hoosier A7, used on an identical 18x12" wheel as the front. Yes, this tire should used a wider wheel, like a 13" or even 14" wide wheel. But the CF5 and F14 wheels from Forgestar top out at 12" widths in their 1-piece wheels. We wanted a less expensive set ($360/each) of wheels for this initial "big tire test", so we went with the same widths we used on the TT3 Mustang (18x12" front and rear). As you can see in the image above, the 18x12" wheel works with the 345 Hoosier... but it does have a hint of a squeeze. This is a 13.8" wide tire.

This was the fun "lets bolt the front on" test, before we had the wheels powder coated. Just one front 335 tire, tested quick, looked good. Dismount, take to the powder coater, and wait 6 days... Always waiting.

FIRST DYNO TUNE + MORE OPTISPARK WOES

Back on January 14th, the crew here at Vorshlag had finally installed the rebuilt wiring harness (after 3 days of chasing down "changes") and the engine would crank and run... for 2 seconds. The folks who sold us the "1995 Corvette ECM" were supposed to turn off the security system in the computer (VATS), since our 1992 Corvette's key would be coded differently than the used ECM they sent us. They charged us a pretty penny ($315) for a used ECM, but part of that was "installing your VIN # and turning off VATS", and they were one of only a handful of shops that said they had a manual trans 1995 Corvette ECM in stock, too.



Well of course that was bull. It was not in fact a Corvette ECM, but instead a 1994-95 Caprice or Camaro LT1 ECM, made for an automatic transmission. And they left the VATS system on, which was why the car won't run more than 2 seconds. But luckily the guys at True Street Motorsports know LT1s and using LT1edit they were able to convert the ECM to a 1995 Corvette manual trans unit with a reflash. They also reset the speedometer to work with the 345 tire we were going to use (not on the car at this point).



The True Street guys fought with the ALDL port (the OBD port) for 5+ hours, calling the new wiring harness supplier (who I will never use again) and attempted some trouble shooting - since this port now didn't work. It worked before with the 1992 harness, but the $1000 custom built, 1992-to-1995 engine conversion harness they supplied seemed to have nothing but problems.

True Street had to overnight the LT1 "bench harness" for this style ECM, and remove the 1995 ECM from the car for each tuning iteration. With no ALDL port they couldn't data log during a tune, either, which made it even more difficult. The Corvette was stuck at their shop for 2 days and my tuning bill reflected the added work they spent chasing the harness problems. Oh well, live and learn... next time I'll use my standard "stand alone" wiring harness supplier and just punk on the factory gauges. We still lost the factory oil temp connection with this new harness, which is something I really wanted to be able to monitor.



Once the ECM was reflashed well enough where they could make a pull on the dyno, the ignition was apparently breaking up above 4500 rpm. You can see the jagged red dyno curve above, which is with the refreshed LT1 and 1995 ECM + MAF. This curve was beating the old stock LT1 and 1992 ECM tune (in blue) by 25 ft-lbs all the way to 4000 rpms, then it started to tank, then the curve went berserk at 4500.

Both of those dyno graphs are uncorrected, and the "301 whp" line was actually the "284 whp", once SAE corrected. Which is the tune we ran at the 2 NASA events last year. The newly refreshed but still painfully stock LT1 engine was pulling strong in the lower revs, but something wasn't right up top. Over 4500 rpms it wouldn't run. By this time it was nearly 2 pm on Friday, the day before the MSR event. I had to take it like it was, hope it was just an Optispark or spark plug problem, and see if we could fix it back at Vorshlag. I loaded up the car quickly and blasted back to the shop. We hadn't even fit the rear wheels yet and we had another car (BMW E46) to finish, too. Rough, nerve wracking day.



As I raced back to the shop with the trailer full of Corvette, and I called our shop manager Brad and had him order spark plugs and water pump gaskets to be there in a few minutes. We unloaded the car and immediately started yanking the brand new OEM style Optispark, which was installed back in October when we were chasing the bad 1992 ECM problem. It was a vented, name brand, brand new unit with 0 miles on it. How could it be bad??



Ryan swapped the nearly brand new spark plugs for a fresh set of NGK V-power plugs, and I had hoped one of the old plugs had a visibly broken tip or insulator (which would explain the dyno issue). Nope. The MSD billet Optispark distributor was pulled off in October, but it wasn't the problem, so now it went back on. Ryan and Brad thrashed with this while Olof and I worked on cutting the rear fenders to clear the 345s (see below) and Jon and Steve finished up the BMW 330 prep. It was an "all hands on deck" kind of afternoon.



The old MSD unit went in, the water pump went back on, the cooling system was refilled with distilled water and a splash of Redline water wetter, the engine was run, and the cooling system burped. Moments later we were cutting the front fenders and by 7 pm the car was loaded into the trailer. Fingers were crossed - there was no way to "feel" the ignition flutter until it was in 3rd or 4th gear, and it was pitch dark outside. I was not going to go blasting down the road with no lights to test this. We'd have to see if the MSD Opti fixed the 4500+ rpm issue the next day on track at MSR... if it didn't rain. Or snow. The weather was beautiful on this Friday, but predictions of COLD, wind, snow and rain were in the forecast for the next day. Yay. And MSR-Houston NASA event was the next weekend, 5 hours away... we needed to test the engine right NOW.

MAKING THE WHEELS AND TIRES ACTUALLY FIT

While the Optispark was going in, after the car returned from True Street on Friday, all four of the 18x12" wheels and a scrub set of 335F/345R tires were fitted to the C4. The fronts looked a little close to the hood for bump travel and the rears poked out a mile. Time to cut some fiberglass.



Making the rears fit took a bit of fiberglass removal. First we laid down some green painters tape then I marked an "eyebrow cut" line, to give the rear some bump travel. At this point the tires stuck out about 1/2" but we would later see that we needed more like an inch of poke. Again, this is an identical 18x12" wheel as the front, and we planned up front to use a bit of spacer to make it fit out back. Long term, these 18x12's might all become front wheels, then we'll get the new M14.2 Forgestar 2-piece wheels out back in 18x13".



Fiberglass dust is nasty stuff so we had Brad cutting and me or Olof with the vacuum sucking up the dust, and dust masks. Below right is the initial fit, then I went back with a 2" sanding disc and cleaned up the wheel arch a bit.



The front wheel openings were also trimmed, very quickly. There was another "eyebrow cut" at the hood-to-wheel arch portion, just above the tire, to allow for bump travel. This happened right after an Optispark change was finished.



That's how it was loaded into the trailer at 7 pm Friday night. Yes, this looks a bit janky, but there was no time to make flares. We really just wanted to test this big wheel/tire combo on track to see if there was a prayer of TT2 competitiveness for later in the 2016 season.

SCCA CLUB TRIALS AT MSR CRESSON, JAN 16, 2016

I'm going to try to do this event coverage quickly, as it was only a 1 day event and we were just there to test the Corvette and BMW. I didn't really care how we finished, as this was really just an HPDE day that they were calling a "Club Trial" competition event. They had transponders but there weren't any classes, so they used a PAX factor to post "competition" times. Yes, an autocross PAX applied to road course times. I can't make this stuff up.



Anyway, we got there at 6 am, an hour before sunrise, and were working in the dark to have both cars ready before a 7 am driver's meeting. The C4 was unloaded, Amy and I did the wheel swap on her BMW, the AIM Solo lap timers were mounted, and we checked everything else we could under pitch black skies. The driver's meeting took a bit and after we got out, right as the sun was coming up, we greeted Brad and Olof from Vorshlag who had come to help. This became an eventful day of "track side car mods" so their assistance was most welcome.

The SCCA Club Racers had 25 entries spread across 5 run groups and Club Trials had 30 entries jammed into 1 run group. This is a 1.7 mile course and 30 cars spread out across a VAST array of skill and prep levels makes for a crowded track, but again - this was just a test for both cars.



They gridded the C4 in P1 and stuck Amy about a 1/4 of the way down the order. We went out into the first session and it was COLD with 40 degree temps and 20 mph winds all day. I could hear all sorts of tire rub on the out lap so I got the heck out of the way and dove into the pits before I held anyone up. But more importantly, I was able to test out 3rd and 4th gear under load. NO MORE OPTISPARK ISSUE! Engine ran strong all the way to 6000, so that's already a win. But getting no laps meant I'd be stuck at the back of the pack, which sucked.



Amy drove the whole session in the BMW with the rear tires rubbing the rear fenders badly, sending up plumes of tire smoke. Did she stop? No, the ran the whole session. Oh well, if she ruins her tires its her own fault. The Bilstein PSS coilovers were sprung too softly and allowed a TON of body roll, so we'd be doing a big spring rate upgrade immediately after this event.

continued below
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