I own an MN6/F55 and I 've been considering swapping out my rear end gears for 3.90 or 4.1. I went through some numbers and found out some things that I felt may interest others. Of course the general concensus is that if you swap out the rear gears, you'll get more "oomph" in your stomp but you'll have to change gears more often, so the effect is roughly a .1 or .2 second gain in the 1/4mi time.

I'll be using the following stats:
C6/Z51 - 1/4mi: 12.7@113mph (9/04)
C6/F55/Base - 1/4mi: 12.9@111 (Extrapolated from Z51 time. I can't seem to find a magazine that tested a non-z51 vette)

Using the following stats (Taken from here http://www.corvettemuseum.com/specs/2005/specs.shtml):

F55/Base
Tire Size: 285/35/19
Rear: 3.42
1st: 2.66
2nd: 1.78
3rd: 1.30
4th: 1.00
5th: .74
6th: .50

Z51
Tire Size: 285/35/19
Rear: 3.42
1st: 2.97
2nd: 2.07
3rd: 1.4
4th: 1.00
5th: .71
6th: .57

I was able to obtain theoretical (Assume no drag limitation in 5th and 6th gear) top speed specs(mph) in each gear at 6500rpm (factory redline) using the following top speed calculator: http://www.reverendsgarage.net/index...calculator.php

F55/Base
Tire Size: 285/35/19
Rear: 3.42 Rear: 3.90 Rear: 4.10
1st: 57.1 1st: 50.1 1st: 47.6
2nd: 85.3 2nd: 74.8 2nd: 71.1
3rd: 116.8 3rd: 102.4 3rd: 97.4
4th: 151.8 4th: 133.1 4th: 126.6
5th: 205.2 5th: 179.9 5th: 171.2
6th: 303.7 6th: 266.3 6th: 253.3

Z51
Tire Size: 285/35/19
Rear: 3.42 Rear: 3.90 Rear: 4.10
1st: 51.1 1st: 44.8 1st: 42.6
2nd: 73.4 2nd: 64.3 2nd: 61.2
3rd: 108.5 3rd: 95.1 3rd: 90.5
4th: 151.8 4th: 133.1 4th: 126.6
5th: 213.9 5th: 187.5 5th: 178.4
6th: 266.3 6th: 233.6 6th: 222.2

It's a good idea to have your car geared so that you'll be crossing the 1/4mi mark right at the top of a given gear. This way, you save yourself a time costly shift into the next gear, while most effectively keeping the motor in the best power band. Note the top speed of the stock F55/Base gearing vs top speed of the Z51 gearing in 3rd gear. It's well documented that a stock Z51 can run 1/4mi speeds of 112-ish. In order to do this, either you'd have to shift into 4th, or simply rev the motor past 6500 rpm. Since another shift is very time costly, and apparently 6500 is not the absolute engine maximum, I would assume most magazines simply run the motor until it hits the fuel shut off. At 6600rpm you'd get 118.8mph, and at 6700 you'd get 113.5mph. I'm guessing C&D simply ran the car until the fuel shut off.

What if you swap out your 3.42s for 3.9s? Now the top speed in 3rd gear is 102.4mph and 95.1mph for F55/Base and Z51, respectively. This would force another shift before you get to the 1/4 mi mark. I'm not sure how fast people out there can shift, but I would imagine most drivers can't shift faster than .3 to .4 seconds. That means you'd be adding at least .3 seconds to your 1/4mi time. This also means, that for whatever new gearing you choose, if it doesn't give you enough additional torque at the wheels through the first 3 gears to overcome the time needed for an additional shift, even though the car pulls harder in each gear, you'll actually be hurting your 1/4 time.

The next consideration is how much are you going to be modding out your car? If you're going to be pumping it up enough to get trap speeds of 120-130ish (another 150-200hp at the crank I'm guessing), then any car with the stock gearing would be forced into a 3rd-4th shift anyways. In fourth gear both Z51 and non Z51 trannys have the same gearing, and you get 133.1mph with 3.9s and 126.6mph with 4.1s. Most heavily modded vette's I've seen sit right around here for trap speeds so it's rather perfect. Now if you're planning to reach for trap speeds of 140ish or more, then perhaps staying with the stock 3.42s in the rear would be your best bet.

These are just some thoughts I had this morning while I should have been working

 

I have been contemplating back and forth trying to figure out if I would want 3.90's or 4.10's in my non-Z56 6 speed that is street driven 99.9% of the time (I may make it to the track once per year...if I am lucky)

I really appreciate the analysis, it helps a lot and I think I could handle the 4.10 swap....do you think it would be faster than a stock Z51 car if all else is equal? What gearing are you leaning towards and how often will you track it?

Thanks again, Derek

 

virt
i put 4.10's in my non z51 6speed and absolutely love it.the seat of the pants feel is great.i will probably never see the drag strip so i really dont care about 1/4 times.

 

A couple of points:

1. The non-Z51 cars can hit the trap speeds you expect
without going over redline. The Z51 cars all need a shift to 4th.

2. Your calculation for the top speed in 3rd on a Z51 car
has some error I believe. The actual value is closer to 104
mph.

3. Adding a shift certainly isn't great for your et but it
isn't correct to assume a 0.4 s shift adds 0.4 s to the et.

If you were timing 0-60, this would be true but for
elapsed time measurements it is not. Why? Imagine the
extreme case: you never shift. If you need a shift to get to
some speed, like 60mph, and you never shift, you'll never
get there. And while you aren't accelerating, the clock is
running. So the shift time is directly added to the 0-60
time. But in a 1/4 mile run, if you're talking about a 3-4
shift towards the end of the track, you're already travelling
100+ mph, and even if you never shifted, you'd still
complete the 1/4 mile pretty quickly.

Pat

 

Very good points. And I have now had both a Z51 and non-Z51 C6. Pat is correct. A Z51 runs about 104 in third, and a non Z51 runs about 114 in third.

A better piece of information would have been what is the overall gearing in each gear between a Z51 car with the stock 3.42 rear end, and a non-Z51 with a 3.9 rear end.

Thanks for hard work you put into the post

 

Interesting write up. I'd like to change gears eventually, but I don't to 1/4 mile runs so I'm not concerned about an extra shift. I wish I could test drive a car with 3.9 and one with 4.1's first. When I changed gears on my bike I rode it for 6 months and then changed it again. Of course the second change only cost me $40 for a rear sprocket and was an easy install. I wish it was that simple in a car. The good thing is 6th gear is so tall that 4.1 might not be too much of a sacrifice for everyday driving. Whatever you do, I'll be interested in the outcome.

 

Scissors and I went at it on this very issue in Feb. Here's
the thread:
http://forums.corvetteforum.com/show...post1549811205

I still think I'm right but we've essentially agreed to
disagree. I still wish the Z51 had the MN6 transmission.

Pat

 

BTW, if you want to model different scenarios like this,
without having to buy all the cars and do all the work, I
highly recommend getting a drag simulator product. There
are many available and they are surprisingly accurate.

Using a simulator, I compared the non-Z51 3.42 to the
3.90 and the simulator suggests no appreciable
difference: less than a 1/10 in et and less than 1mph.

Pat

 

I agree with you. When the weather cools I'll find out for sure - I'm historically a pretty consistent drag racer and will take my new car to the track. The bad news is that it will be at least 3 months before the weather here get's cool enough to make it worthwhile

 

You guys are most likely right about this. If you follow the link to the online calculator, there is some checkbox that says "Check this box to factor in for tire deformation/flex". Clicking this effectively reduces the top speed in each gear by a bit to compensate for tire flex, though more so at higher speeds. If what you guys are saying about the Z51 is correct then that certainly is interesting. This probably means that even though at the 1/4mi the non-Z51 cars are only roughly .1s behind, a good part of that catch-up with due to that last minute 3-4 shift that the Z51 guys have to do. If you were doing some standing race just between friends, the Z51 probably looks much faster from 0-100. In fact, if we can find a 0-100 time for non-Z51 cars somewhere, it would be interesting to see how it compares to 0-100 time for Z51.

Given that all Z51 guys need the 3-4 shift even with the stock rear, I would recommend all Z51 guys interested in their 1/4 mile time to immediately swap out their rears

Pat,

You're right. Since during the shift time the car doesn't stop, it's simply not accelerating. I would suppose the actual time lost due to acceleration lost during the shift is much less, though I can't begin to guess what it would be. Realizing this, I also think it would always be better to make the shift than to coast through that last bit of the 1/4mi at redline. Keeping a constant coast at redline means your car is not moving much faster than a car coasting at neutral (which is what your car is doing in between shifts). At least if you attempt to make the shift, you can still get a bit more acceleration from the next gear, as opposed to being topped out at redline.

 

I thought about this quite a bit and I think I'lll go with the 3.90s. For the street they'll offer a tad better mileage (quite small I would bet, but better than nothing ), and for the track, the 3.90s will allow for more play room up top without having to worry about a 4-5 shift. I do eventually want a car that will run in the 10s (I figure eventually cam+heads+headers+intake+super/turbo/nitrous+tune). And eventually I will want to track the car quite a bit, but it will always be my daily driver. So I think the 3.90s will work better.

 

Hmmm that would be a very very close call. You'd have to actually use that drag strip simulator Catpat8000 was talking about to have a close approximation of which is better I think. Though I'm sure we can give it a try...

Looking at the mph measurements at redline, I would have to guess the Non-Z51 with 3.90s would be quicker in the quarter. Both cars would require 3 shifts to reach the 1/4mi (1-2, 2-3, 3-4), and the speeds attained in both 1st and 2nd gear are near identical, which means you're getting roughly the same amount of acceleration. In 3rd gear, the picture is starting to change however, since a non-z51 car with 3.90s start to top out earlier (which denotes more acceleration). In 4th gear, the non-Z51 should be pulling a fair amount harder than the Z51 stock. However, this is to minimal advantage since the 112-113mph trap speed is quickly reached once you're in the 4th gear. I'd wager they're roughly the same, but given you have good traction, a non-Z51 with 3.90s may pull as much as .1s over a stock Z51, a non-Z51 with 4.10s should give you even better gains.

 

Related to that article a few facts: With 4.10's:
You will have better traction in all gears due to slower spinning tires that regain traction faster than 3.42's even though it breaks loose easier. Myth number two is that your top speed is in fifth but everyone with 4.10's can tell you with over 400+ rwhp and 4.10's the top speed is in 6th gear. I did 180 in 6th at 4750 rpm and it was still climbing. Myth number 3 of poor mileage: I still get 26mpg with 4.10's
I don't find myself excessively shifting and the rpm at 80 in 6th for my C5 is 2200 (2350for the Z51 C6). I have the Z51 C6 and I am getting 4.10's. My motor will rev to 7100 when it is complete like my C5 does now. This allows for roughly 10% more speed in each gear over a 6500 rpm max.....135+ in fourth.

Cartek has 3.90's in their Z51 C6 that is currently running high 10.'s. Their application not being theory but rather fact, should put the issue to rest.

 

A lot of good information, but I'm still on the fence. 3.90's or 410's...decisions, decisions.

 

Heh, logging in on the cannery computer. Dont expect a response because I seldom get online while Im working and it may be a month or more before I check in again. Got the 3.90s. Love the 3.90s. Outside of any arguement about the performance merits they just plain feel right with the base car. SOTP is felt nicely in 4th and up. 6th becomes much more useable. You have to be a little more gentle with the throttle in first and second but as said it does drive out of wheelspin better. The first three gears match up fairly closely with the standard Z51 gearing so that will give you an idea of what to expect there if you have driven Z51. When I first installed them I hit the rev limiter a couple times in the lower gears because it pulls through the band faster and my timing was a little off. At 70 on the freeway I pull about 1750 rpm. I never did figure out any milage on it but I didnt notice pulling into the pump more often although obviously I had to lose some. Take care guys, cya next time.

 

Hey Crabman, I was driving your car the other day, your wife commented that I "hit" the gears better than you do

Good hearing from you!

 

Few other negatives with using a lower gear with stock engine which causes more wheel spin which increases 60 foot and shift times which is even more of a loss at the traps.
Also running lower gears means more heat buildup which causes horsepower loss.

Forcing the 4th gear shift happens late in the 1/4 mile, wrong powerband, another 0.4 to make shift and can lower topend speed.

 

Great hearing from you...I'm gonna wait til you get back so I can check your's out anyway. Didn't expect to hear from you, so didn't mention it in my earlier threads. Be safe, see ya in a couple of months....by the way, the headers scream!!!!! Your gonna love em!

 

Not true sir. The 60' times will fall from 3.90's or 4.10's since the tires are spinning slower than 3.42's and this is a traction aid. This is a universally proven fact: All the fast cars on this forum run these gears and get deep into the 10's with stock displacement and 1.4x 60' times. There is no additional shift if you run any of these gear set-ups. 4th gear even with 4.10's on a Z06 or Z51 will still reach 130mph which is a lot faster than anyone is going with a stock engine. You smply gear for the maximun rpm in your power band at the traps. I also have no idea where you get the heat issue information from or how it is supposed to affect the car's speed.

Like I always say, the only people that have anything bad to say about regearing are those that didnt try them. I never heard of someone with them thinking it hurt performance....not one person.

 

Everything said here about the 3:90s, just keeps gettin' better with the 4:10s. I've said it before, I'll say it again...4:10s ROCK. Best single mod I've made.



You won't be sorry. Just do it!



-Chris