KorbenDallas

My differential has bit the dust and I need a new one. I have 3.90's in my C5 MN6 right now and like them, but I'm wondering if I should go 3.73's instead.

There are several tracks I could possibly go to, but Roebling Road is probably the closest road course. My dyno was 411rwhp, but I trap 122-123mph on the strip which is more in line with a 430-440rwhp car. IIRC my rev limiter is set at either 6600 or 6700 and I shift at either 6300 or 6400rpm.

What type of speeds would I see at Roebling? How about the other tracks in FL?
Do you guys with similiar gear ratios/powerband have to shift into 5th (H/C Z06's with 3.42's comes to mind)?

thanks

AU N EGL

Roebling road is a 4th gear 3.42 diff track. No shifting one you out of the pits and up to speed.

so say 80 - 90 thought curves and up to 135 entering in the front straight brake zone to slow ( no down shift) to take T1 and T2

Other tracks in FL are Sebring ( east of Tampa) Moroso and Homestead, down Miami way.

Sebring is a very fast track with 5th gear speeds near 160 mph on a few straights. Cant say about Moroso and Homestead.

Most guys on road course find the 3.42 perfect. 4.10s and 3.90s are out of the question and over heat on road courses. 3.73s might be ok but you may still shift too much.

Almost no one uses 5th gear on most road courses. Red line the 4th and hold it for 50 or so feet then brake and down sift.

KorbenDallas

So 3.42's, maybe 3.73, with my MN6 transmission? Or were you saying 3.42's with a Z06 gearbox? (Sorry I had to ask since I mentioned a Z06.)

Also, would the DTE strut brace provide any traction benefit for this type of racing? How about a hardened output shaft? I don't want to spend money where I don't have to.

AU N EGL

YES. the MN12 is almost geared to low, the MN6 has an advantage here. but no need to change. I use the MN6 with 3.42s and ( at one time) just north of 400 to the ground.

Most track or road racing uses only 3th and 4th gears. Never go slow enough to downshift into 2nd and most straights are not long enough to use 5th gear

If you have harded out put shafts great, if not no need to get them. Many of us run 400 to 500 rwhp with stock shafts and no problems

the strut brace could help, again if you dont have it, dont really need to get one.

what does help are two things, 1 - oil cooler, seperate or radiator combine and 2 - trans cooler. Both are the hardest part of the corvette, overheating.

Now here is a hard part, a road race / track day car and drag racing car are TWO differnt cars. One car does not do well for both events.

A few other minor up grades, Fresh DOT 4 brake fluid EACH time you go to the track, far better brake pads, if not road racing brake pads, and brake cooling ducts, to cool those rotors and calipers down.

20 to 40 min session are hard on the brakes. Harder then most ppl know.

AU N EGL

Here are some in car videos of Roebling

http://www.bigpinekey.org/roeblingroad.htm

Dont let Falcons smoothness fool you. He is driving a lot faster then it looks

sgsvette

AU N EGL: hate to disagree with you, but I race at Sebring probably 20 times a year with a 3:90 gear and run 2:17's and the fastes national T-1 car has never even seen a 2:18 with a 3:42 gear. I use the same gear at Roebling (where I run 12 schools a year) and if you show up with a 3:42 gear I'll eat you for lunch. Why do you think it's ilegal to use a 3:90 gear in T-1? unfair advantage! Also holding 4th gear for 50 feet or so instead of shifting to 5th is not the fast way around any track.

RAFTRACER

I would disagree with that. If you have to hold a gear briefly longer and spin the motor a little higher, it is faster typically to do this instead of making an upshift ( uncoupling the motor) and then an immediate associated downshift ( also unsettles the car, espeically on a high speed corner entry). 50 feet is really short at 150+. I'll carry a gear longer than that if I have to, and my motor is stock just like a T1 motor

Additionally this is probably why the fastest T1 guys spin their motors 7,000 + , just like I do. GOT VALVE SPRINGS ???

KorbenDallas

What do you do about your rear end heat buildup? Do you have a cooler? Do you change fluids more often?

RAFTRACER

I dont think that I agree with this either, although I dont have a whole lot of Roebling experience. I have ran there however , and with the 3.42 /M12 combo that I ran there during OLOA, I was 4th gear on the straight, back into 3rd in 1 through 2 , 3-4 on the straight after 2, back into 3rd before the next left hander, 3rd gear all the way onto the straight leading to the last corner, 4th gear though last corner(man is that a fast corner) and repeat. Of course this was on real street tires (Michelin PS2's). That was a bad day at Roebling, at least we made it. Engine swap and a 700 mile drive in 24 hrs. . Then throw a belt on the 1st session and take a 10sec penalty on the 2nd.

Tom, with the MN6 and 3.42, 3rd gear should be even more useable , even with real sticky tires.........

Maybe Mr. Gilbert can shed some more light on Roebling gearing..

KorbenDallas

Thank you for the info, yet again, AU N EGL! I was a fan of the 1/4mi for a little while; mainly to try to guage my car's performance when I did H/C. I've always wanted to do road racing, but unfortunately my differential problem is keeping me back from starting when I wanted to.

sgsvette

If you can spin your motor to 7000+ then I agree with you because then you really aren't holding back, but I assumed FroDaddy had a close to stock engine. Personally I find it faster to even short shift a little and go into top gear, but that also depends on horsepower.

I do have a cooler but have never noticed any difference in heat build up due to gear ratio. Generally without a cooler you get about 5 laps at high RPM before you start to smell the fluid. It's your alarm that it's time to back off or pull in the pits to let it cool down. I would recommend using synthetic gear oil.

I agree with your gear selection at Roebling on street tires, but if you run a 3:90 gear that would change because of the acceleration difference. You would use 5th gear down the front straight and have better use of 4th around the track. You can loose 2 seconds off your lap times with 3:90 gears.

FroDaddy stick with what you like, go back to the 3:90

AU N EGL

each time you shift, up or down, you loose a 1/10 sec. so the less you shift, carring more momenteum the better. those are the trade offs, to get more speed. plus 7000 red line.

I do believe T1 rules require stock diff, which is 3.42. Not to say some guys dont use something else Does not mean the T1 cars are the fastest cars, but the drivers are some of the best corvette racers around.

The WCGT vettes run 4.10s and 3.90s and spin 74-7600 rpms too

at Roebling:
Been a while since I have been down there, severy guys running slighty faster lap times when not shifting. But then again traffic does tend to hold one up for a bit. I have always shifted down to 3rd going into the brake zone at T1.

AU N EGL

Of course you would eat me up, its your track and I am there once a year in Jan when its rained ever time. plus your experience level is a weeee bit more then mine too.

sgsvette

I didn't mean to imply that I am faster than you, just that the gear selection would be a great advantage over the stock 3:42 gear.
Roebling has better grip and is much smoother now that they repaved the track surface, so the lap times have come down by about 1.5 seconds. It would not be unusual for you to carry 4th through turn one now.

AU N EGL

No worries. That brings up another question? ( I will start another thread) Low gears & Higher RPMS vs Higher gears and Lower RPMs.

varkwso

I run Roebling - an awful lot- several days on the new surface already. MN6 with 3.42 (FRC) and MN12 with 3.42(ZO6) - I also ran it with a 94Z28 with 3.42 (no question 5th was required on the straight with that car). Some people can eat me up there - lots cannot. I would not run a 3.90 there with either trans but I dislike swapping gears anyway and 5th is a shift I like to avoid if I can (that 5th to 2nd downshift oops is dang expensive). I have grip problems with a 3.42 and a 3.90 would cause me pain there. I would like a 3.73 or 3.90 at Barber or CMP though.

With the new surface in a C5 we are at the rev limiter in 4th (~143mph) at the start/finish line at Roebling now in both cars. Shifting to 5th gets you up to maybe 150 and change at the brake zone - unless I am chasing somebody or the clock I breathe the car on the front straight - I can eat up most folks on the back half and/or T1 brake zone anyway. T1,2 & 3 are always 4th gear turns with 3.42. If you make a mistake in 2 or 3 I will pass you in 4.

T4,5, & 6 have been faster every time I segmented the course in 4th than 3rd - but third feels faster and definitely allows you to set the nose better with throttle modulation - depends on the condition of your tires I think more than gear - I know the car runs cooler in 4th. If you are not at the rev limiter coming out of T7 in third you are too slow - I am flatfooted in 4th from mid-turn T7 to brake zone T1 or shift to 5th on the front straight. The entry to T6 is the beginning of the front straight at Roebling. T8 is a late apex as fast as you have the courage, or the tires, for. With 3.42 we are apexing at ~100-110 and exiting at ~120-125 at T8. Avoid the urge to lift entering T8 and you will pick up speed - but it is based on being able to stick in the corner.

I have ridden with over 100 folks at Roebling and been on track with maybe 2000 more. I have seen just about all the lines and been on most of them (sometimes even when I meant to). I have been in many C5s there. I cannot see a rear gear change advantage there - it is a high speed track (as opposed to Barber or CMP) - but I am always willing for some one to show me the light. Having run Sebring not sure what 3.90 will do for you there either - must get you into 5th a lot sooner? Are there any corners you can run in 5th there in a C5 with 3.90s? Thanks!

Going to check out the other thread now!

jeff

racerwannabe

i have read with interest this entire thread..now i'm really confused..i have run roebling many times with 3:42's ..this time ,,next week ,i'll be running 4:10's...should i expect to go faster or not???

rev limiter is set at 7000..i expect to stay in 4th till i get straight going to the last turn and get 5th real early....my other car..the one at rippies,isn't a good guage since it has so much hp..this is our stock motor with headers..and we'll have some good 710's all around....
Johnny

sgsvette

I think you can expect to go faster, but with the 4:10 you might be close on RPM at the end of the front straight.
Post your thoughts after you run.

racerwannabe

based on my calculations ,i can't run out of revs in 5th gear..at savannah..car won't accelerate that fast..if 5th is .75 overdrive that comes to 170 or so at 7000...400 hp won't push us that quickly,,but i should be really in a sweet spot in 4th the rest of the track ..we'll see next week...