EvilBoffin

It seems that many C6 Z06 autocrossers have given up on this car in the stock class, and have all but condemned it as 'uncompetitive' at the National level.

OK ... Why ??? - What is the primary flaw in the design that makes it such an underachiever?

Given that the typical, modern auto-x course tends to favor 'momentum cars', is the biggest problem that -

• The gear ratios are too tall (~61MPH in 1st, 91MPH in 2nd)
• The bodywork is simply too wide/bulky for most courses
• The car cannot effectively put its power to the ground
• The oversized rear swaybar is disproportionately large & out of balance with the front
• The available sizes of competitive racing tires that fit this car do not work well
• The spring rates are not optimal for low-speed events, making the car twitchy & hard to drive at the limit
• ... you tell me ...

Please understand, I didn't start this thread to insult anyone who is having success with a C6Z06 in stock class - in fact, it's quite the contrary...

I'm considering the purchase of a 2010 Corvette Grand Sport, that will share A LOT of Z06 suspension, body and brake components. However, it will have an LS3 (w/dry sump), Z51 gearing and slightly softer springs. I'm just trying to determine if the new GS has a Chinaman's Chance in Hell of being reasonably competitive in SuperStock before spending Big $$$ on this new model. This car will also be my primary street car and will see it's fair share of HPDE duty as well. But I still don't want to start out with a new car that has a major flaw that is not fixable without moving to a non-stock class (or maybe not at all).

TIA for your expertise...
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steve40th

I think it is too damn wide. When I run my C4, my times arent much slower than the C6ZO6, and he has Hoosiers A6. The C6 Z51 is about the same, and he with Hoosiers. I run BFG KD's.
The C6 ZO6 driver is an SCCA chief out here in HI, and is real good. Myself and the C6 Z51 (both C6's are 2007) and two years guys, with limited time on the track.
But when you see the ZO6 run, it just cant utilize its real attributes, power and braking. Just my 2 cents.

talon95

Foley is showing some speed this year in a C6Z. Pretty close at the last Pro to Braun and Salerno. From what I read, the start straight was very short at that even too, so no big advantage over any of the other SS cars.

http://scca.com/popup/raceresult.asp...13627&file=623

If the car does have problems, I'd say it's the gearing and overall handling balance with the large tire/wheel stagger and possibly the larger rear swaybar. It's only ~3 inches wider than the C5Z. That's not going to kill it.

Whether the GS will actually be better is hard to say. It will have better gearing, but it'll also likely be heavier. IIRC, they're basically taking a Z51 car and adding the bigger wheels/tires along some other stuff, but not the aluminum frame and CF body parts to make up for the weight gain.

Dave G.

acrace

I can't give you my assessment of a C6 Z06, as I haven't driven one in an autocross environment, but I can share my thoughts with regards to the Grand Sport, as I learned about the basics of this vehicle quite a while ago and thought that it would be the "best" C6 for low speed autocrossing.

I was put off by the 3310 lbs curb weight. There's stuff on there that you don't need for autocrossing, but does help for tracking, like the dry sump and the Z06 brakes. My C5 Z06 consistently scales around 3050 lbs in autocross trim (low fuel load, Hoosiers on stock rims, stock exhaust) and the car has done relatively well at the SCCA Nationals. No fancy shocks, though.

But the 3310 lbs curb weight of the Grand Sport, if I do a direct comparison to my C5 Z06, can be reduced to approximately 3180 pounds (subtract 100 pounds for taking out approx 15 gallons of gas [6.8 pounds per gallon], subtract 20 pounds for Hoosier tires, subtract 15 pounds for the removeable roof [although I don't have a good figure on the weight of the roof - anyone know what a C6 targa top weighs?]).

So, when I compare the 2010 Grand Sport to my C5 Z06, I would gain 30 horsepower at the expense of 140 pounds (4.7 lbs per hp). I would gain 1.5" of rear wheel rim width, so I can use the power more effectively. But I would have a wider car, as pointed out earlier. That may or may not be an issue depending on the type of courses that you run. From my perspective, given that the SCCA Nationals are moving to a large site, hopefully there will be less "thread the needle" stuff compared to the past couple of years. Gear ratios are the same as my C5 Z06 (which I think is a good thing).

I had a brief (very brief) glance at a spec sheet. It appears that the Grand Sport will have a softer rear spring than the C6 Z06. Didn't see the bar diameter comparison, though.

I'd like to get one, but I'm not sure that I want to lay out $55k (sticker).


Al Chan

Solofast

I don't see any compelling reason why the C6 Gran Sport would be measurably faster than a C5Z, so if you can find a nice low mileage Z you can put $30k in your pocket and have the same kind of performance.

One person campainging a C6Z on the SCCA forum was complaining of some of the nannies that can't be turned off, and that trying to get the car to work was a frustrating experience....

Sometimes having a really short gear (like the C6Z in first) results in a lot of slowing down when you lift off that really isn't necessary. It tends to make the car more jerky and less smooth and more upset on corner entry. Then the power comes in big on corner exit and you are out of control. You can only use so much power in the lower gears and it would appear that a C5z has almost enough.

ghoffman

This is not surprising. The basic system requirements for auto-x are different from road course work and in many cases are diametrically opposed.

LS3 Mike

Very true. The suspension alignment I use on the autox course is way to aggressive for the road course. The C6Z needs a more wide open course to go play. A Lotus Elise can run competitively with a Z06 on the autox course but would be walked away from on a more open road course.

ghoffman

That is a very good example, I don't even pay attention to Lotui with my car.

Datawiz

I think the C6 Z06 is great! I'm sticking to my story.

Cashmo

You might want to wait until the SAC/SEB class the GS before buying one. It's possible it'll end up on the Stock exclusion list depending on what actually comes on the car and how much it weighs. We may not know that until we get our hands on one.

Jeff

EvilBoffin

Thanks all for the responses, though nobody's really offered definitive evidence of the C6Z06's biggest weakness, so please chime in if you know something that might help ...

To clarify my situation :

I had a nice '08 C6 Z51 coupe - until I wrecked it recently. I had JUST installed a set of Pfadt adj. shocks & SS-legal front swaybar before it got smashed. The car is now in the hands of a salvage company...

The plan at this point is to replace it with EITHER an identical 2009 C6 Z51 or a 2010 Grand Sport - whichever (if I can figure it out) has more potential for balanced & overall performance in stock class form. The Pfadt suspension stuff was not damaged in the wreck, and will be re-installed on the new car.

I really have no interest in a C5 Z06, even though it's still generally accepted as the best Vette for SuperStock, and is obviously cheaper. I need the big hatchback of a C6 coupe for DD duty and to get me to/from HPDE's. Besides, I was very satisfied with the much nicer interior/exterior design of my '08 coupe, the better gearbox, steering, etc. Sorry - Just call me a undedicated poser, but I'm definitely a C6 guy...


So, the question stands - Which would be better for the stock class ???

- C6 Z51 coupe, with its 'narrow body' and skinny 8.5"-wide front & 10" rear wheels
. . OR . .
- Grand Sport coupe, with Z06 wide body & much bigger wheels/tires, stiffer suspension and ~75 lbs extra weight

.

talon95

Assuming the GS has same/similar bars/springs/etc.. to the Z51, the GS will likely be better than the Z51. The 1" wider front wheel alone is probably enough.

Dave G.

EvilBoffin

Some replies to comments made so far...

Hi Mr. Cashmore - This car is effectively nothing more then a steel-framed C6 Z51 with Z06-width fenders (NOT carbon fiber), brakes & tires. If the Z06 isn't excluded, then I really can't see how this heavier (GM reports 3310 lbs), less powerful (436HP) version could be considered for 'The List'.

That's understandable, especially given the extremely fast, open courses that the Gulf Coast Autocrossers crew is able to set up for you down there in FL (I attended some GCA events in Ft. Myers several years ago in a Boxster S - VERY fun stuff ). The courses I typically run are tiny & very tight in comparison...

Well, if the width really does turn out to be the primary culprit, then the GS is just as screwed as the Z06 - that's unfixable. However, since the gearing of a GS is the same as the old Z51 setup, and assuming most handling imbalances could be addressed w/properly-valved shocks & a bigger front swaybar, then that's a different story...

That's basically it. The question is, will the bigger wheels/tires and stiffer-then-Z51 suspension improve the car enough to offset the excess weight and body width?

The coupe's OEM removal roof is about 25 pounds. By itself, removing the roof gets a GS back down below 3300 pounds during auto-x runs (again, assuming the pre-production weight estimate is correct).

Supposedly, both f & r spring rates are right in the middle between Z51 and Z06 rates - so the end result is an unknown at this point. I think the engineers have a great opportunity to create a really nice compromise in rates, here. We'll see...

One of the forum dealers (Dave@Kerbeck) recently reported that a Vette program insider in Detroit said that the rest of the suspension is actually the same parts that are on the current Z06 - swaybars & shocks included. Remember that Z06 shock valving was changed to improve drivability in late '07, too...

.

talon95

If you're talking about Z51 vs. GS still, then absolutely. I think all of the cars (C5Z, Z51, C6Z) are limited by the front wheels for ultimate cornering grip. More front wheel will definitely offset the relatively small weight gain from the wider wheels/tires and some other bits that don't add too much (brakes, etc...).

The Z51 would likely compete fairly well against the C5Z if they would just put the 9.5/10.5 wheel combo on it. That's assuming there are no other limitations like electronic nannies, poorer spring/bar combinations, etc...

Dave G.

Pylons

The stock autocross difference between a Z51 and a GS is going to be very small, imo...if the GS isn't any heavier than a Z51, then I suppose it's probably the choice, but by a small margin.

EvilBoffin

When I got my '08 Z51 corner balanced/aligned, it weighed 3236 pounds with 1/2 tank of gas, transparent roof mounted (22 pounds) and no driver. That includes the big 3LT option package (HUD, heated seats, telescoping wheel, Bose stereo, etc.), stock runflat Goodyears and OEM forged 'Gumby' wheels.

The Grand Sport is supposed to come in around 3310 pounds, which I assume had a full tank of gas (typical of a manufacturer's published 'full wet' weight). Not sure which option package was included in that quoted weight - 1LT, 2LT, 3LT or the full-leather 4LT...

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acrace

The curb weight that GM lists (3310 in the case of the GS) is with fluids filled. So it is a full tank of gas. Also the curb weight listed by GM is for the base vehicle - so 1LT.

Z16 Autocross

Pretty new to autocross however my 2 cents would be gearing on the C6Z and the wheel combo. If you run most courses in first gear you are going to have way to much torque coming out of a corner and the smaller wheels are just going to make that worse. If you run the course in second gear you will not have as much torque as a C5Z running in second to pull out of a corner. Combine that with smaller tires and ....

Pylons

what smaller wheels?

Z16 Autocross

8.5" front and 10" rear on the C6Z vs 9.5" front and 10.5 rear on the C5Z