Swapping rear gear analysis
Safety Car
Joined: Jun 2004
Posts: 4,430
Likes: 81
From: Whidbey Island WA
Originally Posted by Kreuzen
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!
Safety Car
Joined: Jun 2004
Posts: 4,430
Likes: 81
From: Whidbey Island WA
Originally Posted by jschindler
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!
Good hearing from you!
8th Gear
Joined: Mar 2005
Posts: 8
Likes: 0
Was wondering if the rear gear was changed to a 3.9 or 4.1, would that effect my top speed??
I know people that installed 3.90's and 4.10'
s on their C5 and their top speed was reduced after the instal, but YES their cars pulled much harder than the stock gear.
I know people that installed 3.90's and 4.10'
s on their C5 and their top speed was reduced after the instal, but YES their cars pulled much harder than the stock gear.
Team Owner
Joined: Jun 2001
Posts: 26,714
Likes: 344
From: Houston, TX
Originally Posted by crabman
What? Hit the gears better than me? GRRRR Lets backtrack though. I know that at the time you wrote that I had been gone long enough that there is no way my wife would have given you the keys to my car until after you had serviced her. You know what this means dont you? NOW YOU OWE ME TWO BEERS!!!!!
I hope to be up that way sometime in October - when do you get back? The beer is definitely on me
Safety Car
Joined: Jun 2004
Posts: 4,430
Likes: 81
From: Whidbey Island WA
Originally Posted by jschindler
Dude, your wife is worth WAY more than two beers I hope to be up that way sometime in October - when do you get back? The beer is definitely on me
I hope to be up that way sometime in October - when do you get back? The beer is definitely on me Hard to say. The season ends on 31 Oct, caught or not. I suspect it will be in that ballpark anyways. To stay somewhat on topic I think that Spinmonster is quite correct and is in an oblique way touching on my point. If the only criteria is quarter times then there may something to debate. If driveability out on the real world road is the question then going to lower gears is flat better. You give up top end that most of us will seldom or never see. You give up a wee bit of milage. The car drives better in every situation. The lower gearing is much better suited to real world conditions. The only thing I regret about the gear change was not doing it sooner.
Burning Brakes
Joined: Aug 2004
Posts: 893
Likes: 0
From: Reno NV
Originally Posted by cselz
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.
3.9:1; 4.1:1.
And dont automatically assume everything true for 3.9s is true for 4.1s, "too many cooks spoil the broth" and all that.
Burning Brakes
Joined: Jul 2005
Posts: 988
Likes: 26
From: Seattle WA
A6 conversion to not offered 3.15 rear issues
Originally Posted by Virt
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'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
Thanks
Scott
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Verdad Gallardo
Safety Car
Joined: Dec 1999
Posts: 4,148
Likes: 5
From: Roxbury NJ
When I regeared my C5, I had the same debate, 4.10 or 3.90. I like my car to feel like a refined sports car, not a drag car, so I didn't want to over do it. My car put down 390 rwhp, so DTE advised the 4.10 would be a better choice at that power level. They were right. I never wished that I had the 3.90, the 4.10 was just perfect on the street. If I had to do it again, I wouldn't even consider the 3.90 unless I had way over 400 rwhp.
The 4.10 gearing was also better than the stock Z51 stuff I have now. The 4.10 is harder to hook up at slow speed, but it's much better than the Z51 on the highway in 5th/6th.
The 4.10 gearing was also better than the stock Z51 stuff I have now. The 4.10 is harder to hook up at slow speed, but it's much better than the Z51 on the highway in 5th/6th.
