michaelkrelina

I have a 2003 C5Z. I'd like to have opinions / votes for most suitable final drive ratio for autocross. I'd like to stay in 2nd gear and minimize shifting. The available courses include a large parking lot, max speed < 60 mph, usual speeds <50 mph (or an airfield used infrequently, max speed <70mph)
1. 3.42:1
2. 3.73:1
3. 3.90:1
4. 4.10:1

mountainbiker2

3.42 would be my vote. You could try 3.73, but you would have to raise your rev limiter to 7000rpm to reach your desired mph.
http://www.f-body.org/gears/

dbratten

Bill Dearborn

The C5Z diff and tranny ratios are pretty close to what you want. It is a great second gear car. Get into second as soon as you can and keep it there. The regular C5 transmissions are geared higher and second isn't as useful.

Bill

TedDBere

What class did you want to play within? Changing the diff gears will bump you into class that you will not be competitive within without major $$$$. It's hard to find better gearing for autocross than the C5Z has, fwiw.

BFoster

3.42 IMHO! I raced this year with a 4.10 rear and was using the rev-limiter much more than I wanted to. Research Brian Foster Corvette on youtube to see a few autocross course videos and road course videos to see what I mean.

michaelkrelina

Ted
In our local competitions in Ottawa I'm in the "A" class. This is the top class here and there are no limits to the modifications of the car. I have started to do autocross just for fun but the competition bug is getting to me. I have not risen to the Nationals level in Canada and have not tried to figure out how my car would fit into that set of rules.
If I keep the current diff. gears, I will have to shift into first in the slower sections (I prefer this) or pull away slower in the second gear.
Mike

michaelkrelina

Thanks for sharing.
Your driving is faster than what we typically do in Ottawa. I appreciate the limitter problem.
Keeping the current gears and just shifting more may be a good idea fror me.

http://www.youtube.com/watch?v=4QV3atzQBtE

At the Picton Airport things get faster but I expect to go there infrequently.

michaelkrelina

Thanks everyone for your input
MK

TedDBere

I understand your situation, but if you get better and really like autox'ing and want to leave your "local pond" then you'll regret changing the gearing because of the class it'll land you in.

You are traction limited coming out of the corners with the current gears. That is to say, you can lay rubber in second gear coming out of just about any autox corner you'll see, so quicker gears are just going to make throttle modulation harder and even more important.

That said, if I were going to move to a class where I could switch the rear end I'd move to the 3.90s, but I'd also put huge, sticky, 335s on the rears to gain grip, soften the rear bar, stiffen the fronts and move to coilovers on a drastically lowered car. FWIW

Have fun and good luck.

TedDBere

BTW, with regards to money, here's the car you'd be up against nationally. You could just buy this one:

http://sccaforums.com/forums/thread/382198.aspx

michaelkrelina

I've started using A6 285 // 315 and will probably move to A6 315 // 335 sometime next year. I've also put in Pfadt coils and sways and probably will do bushings next year. I have not done any engine tuning and thought that the diff gears would be less invasive (and more reversible if necessary)

JVetthead

When I was doing autox I went with 3.90... Stayed in 2nd gear for most courses, worked perfect.
When I started doing road course with 3.90 it was quick but way to much shifting so went back to 3.42 and have not autoxed since.

michaelkrelina

Does 3.42 vs 3.90 make a lot of difference to the time? In autocross, if you shift between first and second with 3.42 gears, can you not overcome some of the problem? I think it can be busy in very technical courses to do a lot of shifting, so that at times one loses the feel for vehicle dynamics.

waktasz

The only time I would ever shift back to first would be like a 10 mph or less corner. Are you really seeing a lot of these? As has been said, the stock c5z gearing is about perfect for autocross. Tops out at the rev limiter with 315/30-18s at 73mph.

Greywolfe

3.42

TedDBere

If you shift you lose time. At the top levels thousandths matter and the 3.90 will give you extra time, but I'd also raise the rev-limiter to compensate for the lower speeds at the limit.

gkmccready

What ruleset is MCO using now? Used to be A-Stock, A-Super Stock, A-Street Prepared, and B-Modified was the progression and there were plenty of limits. The classing used to change based on points you accumulated for various modifications. Given that's decade old information now... I don't know what lots you guys have these days, but when we ran the Corel Center the speeds could get up there and the courses were a blast! And unless MCO adopted SCCA rules you can pretty much throw out all the comments about rules and classing in this thread unless you plan to hit a few SCCA events and then SCCA Nationals...

michaelkrelina

We have A, B, C, D, E scale
A: high performace sports cars such as C5Z, C6, C6Z, GT 3; modified S2000, M3, STI and others
B:
C:
D:
E: lower performance sedans on street tires

I have not studied the details of our classification but they are posted on the MCO / Autocross Forum. I don't think we follow SCCA rules and I'm stuck in Class A, no matter what I do. I am trying to tune the car in proportion to tuning me. I'm getting better but not ambitious enough to leave the home turf.