tjZ06

Actually I'm looking for the best AutoX and occasional HPDE combo.

The rear gears on my '02 Z06 are going out (it's whining like mad and has a lot of slop) so now is my chance to change them. I'm also considering swapping to a MN6 tranny rather than my M12 for the closer ratios.

Here’s some #s I came up with a few online calculators…

335/35-17 = 26.23" tire
315/35-17 = 25.68" tire

7k RPM and 2nd gear:
3.42s and M12
335 = 77.2 mph
315 = 75.5 mph

3.73s and M12
335 = 70.7 mph
315 = 69.3 mph

3.90s and M12
335 = 67.7 mph
315 = 66.2 mph

3.42s and MN6
335 = 89.7 mph
315 = 87.8 mph

3.73s and MN6
335 = 82.3 mph
315 = 80.5 mph

3.90s and MN6
335 = 78.7 mph
315 = 77.6 mph

4.10s and MN6
335 = 74.8 mph
315 = 73.3 mph


I was thinking of 3.90s and staying with the M12 tranny. However, this would drop my top speed in 2nd down to 66-67mph compared to 75-77mph with the 3.42s and stock M12. The series I run has lots of FAST courses and I think I'd start having to shift to 3rd a lot more than I'd like. 3.73s with the stock M12 would give me right around 69-70mph at the top of 2nd. This might be just right since it'd give me a few more RPM coming off the turns.

Now my other thought was to swap to a MN6 tranny. I may have a straight-across trade setup with a standard C5 owner. The MN6 would give me tighter ratios which should be advantageous for Road Course use (shouldn't matter too much either way for AutoX since I won't be shifting much, if at all). But because 2nd gear is so much higher it'd take 3.90s in the rear just to get "down" to the same MPH at the top of 2nd. Actually it'd be 1-2mph higher than the stock setup. So I'd be losing some RPM throughout most of an AutoX course. I could go with a 4.10 and the MN6 though and end up with just a bit less mph (73-74) in 2nd.

The way I'm looking at it the tighter ratios of the MN6 don't really seem worth it for me not being an avid road racer. I'd have to go all the way up to 4.10s just to have a barely more aggressive overall ratio in 2nd gear. If I stay with the M12 though I'm torn between 3.73s or 3.90s. The 3.73s will still give me 70mph in 2nd which should allow me to stay in 2nd for all but the very fastest sections of courses. The 3.90s though would give me that extra jump out of the turns though... but I may find I'm shifting to 3rd more than I'm happy with.

For reference with the stock (M12 and 3.42) setup I only had to get 3rd on one course so far this year. On my 335/35-17 back tires and with a 7k RPM limiter I can make 77.2mph with this setup.

Also, I run OSP (forged stock displacement motor w/ heads/cam) so I'm not concerned with classes at all when it comes to the gears/tranny choices.

-TJ

tjZ06

40ish views and no thoughts?

I guess the post was sort of info. overload/rambling.

For what it's worth I think I'm going to stay w/ the M12 and do 3.73s in the rear. The way I look at it is I AutoX the car a lot but HPDEs will be rather rare and I don't really plan to ever drag race it again (I'm just not that into it and the damage it's already caused is totally not worth the minor fun-factor). So really the MN6 would be optimized for road course use (tighter gear spacing) but would be slightly less advantageous than stock for AutoX. Why optimize the car for what it rarely does? I might as well optimize it for what I use it for the most: AutoX.

-TJ

XPC5R

One things to do is to use the diameter specs from your tire manufacturer (I assume kumho or hoosier) both of those are around 25.6 for both he 315's and 335's.

With a H/C car, even with a 75+mph 2nd gear, you should still have way more torque than the rear tires can put down on course 95% of the time. I'd gear the car to never come out of 2nd on your fastest courses. My preference would be the M6 + 3.90. Best of all worlds, I think it also gives you a taller 1st gear, which is a nice bonus. 2nd choice just keep the stock Z06 gearing - spend the coin for more tires and race gas

tjZ06

Well the plan always "was" to keep the stock Z06 tranny/rear but since the rearend is going out (stupid me decided to go Drag Racing on the 19" street wheels = violent wheel hop ) I'm looking at other options.

3.90s and the MN6 would give me just about the exact same overall ratio in 2nd (where I spend 95% of my AutoX time) so I wouldn't really gain anything for all the effort.

4.10s and the MN6 I'd gain just a bit.

But 3.73s I'd gain a nice advantage (2.07 2nd gear x 3.42 rearend ~= 7.08 total gear vs. 2.07 x 3.73 ~= 7.7) of roughly 10% more RPM for any given MPH in 2nd. I think that should be a nice little extra kick and use my powerband more effectively. Yet it won't require too many shifts to 3rd (like I said the series I run makes some very fast courses so I'd like to still keep ~70mph in 2nd which the 3.73s and M12 would just barely do).

-TJ

XPC5R

That sounds good too. I've got an 80+mph 2nd gear (A4 w/ 4.10's @7k rpm +) which works well for me.