I posted this in the general discussion but thought I may get a better response here.

 

I'm considering exactly the same thing and so I did
these calculations which I'll share. These numbers all
assume Z51 gear ratios and the mph values are at
max rpm (6500).

Stock (3.42 rear):
1st - 49 mph
2nd - 71 mph
3rd - 102 mph
4th - 147 mph
5th - 186 mph
rpm at 75mph in 6th: 1900

3.90 rear:
1st - 43 mph
2nd - 62 mph
3rd - 89 mph
4th - 130 mph
5th - 163 mph
rpm at 75mph in 6th: 2200

4.10 rear:
1st - 41 mph
2nd - 59 mph
3rd - 85 mph
4th - 123 mph
5th - 155 mph
rpm at 75mph in 6th: 2300

The best way to think of this is that 3.90 gears give
you about a 14% torque improvement at the rear
wheels and you give up 14% of your speed in each
gear as a result.

Going from 3.90 gears to 4.1 gives you an additional
5% torque at a further 5% speed loss in each gear.
Note that, ironically, it isn't first gear which is most
affected but your top gears.

Pat

 

The Z51's awkwardly spaced gear ratios cannot be fixed by a rear axle change. Two key criteria for gear selection in a 6speed are the ability to hit 60 mph in 2nd and cover 100 mph in 3rd. The 3.90:1 rear reduces top speed in 3rd to below 90 mph (as shown by Pat's calculations) and would not be desirable. 3rd in the Z51 is just right with the stock 3.42 for quick freeway blasts from 60 mph to 100 mph, and increasing the rear end gear ratio would incur the penalty of a shift to 4th that just won't be desirable in that speed range.

The base MN6 tranny is actual a better gearbox and would work well in conjunction with a 3.90 rear end while still mainining the above two criteria and allowing much better acceleration in 4th than the stock Z51.

The reality is that the inflexibility of the direct ratio 4th in these trannies renders both the Z51 and the M12 gear boxes unoptimized and mismatched with any rear end!

 

For those of us that are woefully math deficient could you post some numbers on the 3.73? I have the base and am interested in where that would put me.

 

raj, you make some great points. Are you recomending staying with the stock 3.42? I understand your comments about hitting 60 in 2nd and 100 in 3rd, they are measurement bench marks and adding in an extra shift takes time, not good for reporting performance numbers. On the other hand, it looks like you'll need to shift into 4th in the qtr whether you have 3.42 or even 4.10. So, would the 4.10 get you through the qtr, quicker?

In reality, I do not plan on tracking the car or worrying about the benchmark speeds. I just want to have fun zipping around on the streets and given I'll have the Z51 tranny, I'm looking for a match in the rear that will give me the most grins. I'm concerned about going overboard which the 4.10 will probably do, but the 3.90 looks like it could be a lot of fun. What about 3.73?

Pat, I sincerely thank you for taking the time to post your gear ratio calculations. It was exactly what I was looking for and very helpful. Did you calculate the numbers for 3.73? I still don't know what I'm going to do so I might as well get even more confused with more data! What are your thought's, go with 3.90?

Thanks guys.

Dana

 

Here are some random thoughts on final drive ratios.
First, my plan (once I get my Z51 MN6 C6) is to swap
the rear gear for a 3.90 ring and pinion. Why 3.90?

A 3.90 gear set will give about 15% more torque to
the rear wheels in each gear (with 15% less mph in
each gear). A 15% torque boost is going to make the
car feel significantly stronger in 1st and 2nd. Any less than a 3.90 (like a 3.73) is going to be verging on not
being that noticeable and any more (4.10) is verging
on excessive rpm plus it'll make the car harder to
launch without slicks.

The 1/4 mile time won't change by much, a tenth or
two I would guess. Trap speed won't change at all
since this is largely a measure of the power to weight
ratio of the car and that hasn't changed.

Just for fun, I computed the speeds in gears for the
base C6 with the 3.42 rear and with a 3.90 rear and
I'll post those below. Give these values, I like the Z51
ratios best and you can compare against the Z51 3.90
speeds below. All speeds are at 6500 rpm.

Base C6 (no Z51), 3.42 gear
1, 2.66:1 - 55 mph
2, 1.78:1 - 82 mph
3, 1.30:1 - 112 mph
4, 1.00:1 - 147 mph

Base C6 (no Z51), 3.90 gear
1, 2.66:1 - 47 mph
2, 1.78:1 - 71 mph
3, 1.30:1 - 96 mph
4, 1.00:1 - 130 mph

C6, Z51, 3.90 gear
1, 2.97:1 - 43 mph
2, 2.07:1 - 62 mph
3, 1.43:1 - 89 mph
4, 1.00:1 - 130 mph

Pat

 

An excellent observation, Raj, that the base C6 with
a 3.90:1 rear gear set has, as a result, approximately
the same gearing as the Z51 with the 3.42 gear set
in the first three gears but that 4th gear acceleration
should be stronger for the non Z51 car.

The implicit suggestion here is that a base C6 with
a 3.90:1 rear gear would have higher trap speeds
than a stock Z51 without any sacrifice in the first 3 gears!

Pat

 

I recommend staying with the 3.42's. I would also give it a year as your tires age...you may find yourself spinning through 1st ANYWAY with the 3.42's and the MN12 as the factory tires get harder with age & miles-not to mention as your engine and driveline loosen up to make more power.

Plus, there is the warranty and cost of setting up new rear gears properly. I would think you could better invest the money in an intake, exhaust, and tune which should give you 425-450 FWHP

I've never been a big fan of gears, and if I had gone any lower than with my factory MN6/3.42 gears, my 335 michelin rears would be useless with my current H/C combo

 

We went with the 3.90 gears in our Z51 development car. The car averages 1.55-1.58 60’ on 17” ET Streets. We have run 4.10 gears in the past with the ZO6 and found better track results. The reason we went with 3.90’s is the torque. The LS2 makes more torque than the LS6. Also we are planning on H&C in the near future. I feel 3.73 will make most Z51 owners happy for street use, 3.90 gears for track use, and 4.10 for non Z51 cars.

 

my 02 ZO6 (with same tranny as Z51-C6) has 3:73 gears and i finish the 1/8th at 101mph in 3rd gear at about 6300rpm (?) my rev limiter has been raised to 6800....seems the C6 should be able to do about the same????

 

Guys, think the street-not drags...

 

My thoughts from the start was a little more gear and basic engine mods such as exhaust, intake, tuning. Im old and I want to keep it docile. No aggresive cams and that sort of thing. I do like to poke hard at the loud pedal now and then so whatever I can get without giving up day to day driveability. This figured into my decsion to get the base suspension cause I like the ride comfort and I figured I can get overall better gearing with the MN6 and more final. Im trying to figure out my upgrade path and I like a clear idea of what is possible and what the overall goal is before I start.

 

One thing I'm not clear about on the C6 is if the
rear gear set is swapped for a steeper ratio, what
else needs to change to correct the speedometer
and odometer? Do the instruments run off the transmission? Is there a change needed there?

Pat

 

It needs to be edited electronically...

 

Some very vry good thearotical discussions above. If I may add some personal observations of C5s, Coupe-MN6 and ZO6 -MN12 transmissions. Cars had approximitly the same rwhp at 400. the MN12 had 390 rwtq and the MN6 had 425rwtq. Both had a 6500 red line and both a 3.42 rear end.

On the road race track, VIR, the MN6 car physically shifted less then the MN12. The MN12 needed 2nd, 3rd and 4th gear compared to the MN6, which only used 3rd and 4th gears for approximlty the same lap speeds. The MN12 Z06 did have higher temps in the oil and trans.

There was one ZO6 MN12 with a 4.10 rear end. He needed 2nd, 3rd, 4th, and 5th gears to keep up with the other two cars. However his oil temp and trans temps here sky high which required him to slow down to cool off or pit

In short stick with the 3.42 rear end and add trans and oil coolers.

 

Are you sure? The C5 didn't need any programming for the speedo after gear changes, only for shift points on the A4.

 

Has anybody got a 3.90 Ring and Pinion in Stock for a C6? Is this the same gear set as the C5? From what I have read, it seems to only be a different housing, but the same gears.

Jersey

 

Hmmm...I know for tire size you have to correct the computer...

 

True, but for stock tire sizes, the speedo stays correct, cause the readout is on one of the output shafts. See MikeMercury's page on swapping diffs.