For those debating 3.90s or 4.10s...
05-05-2007, 09:24 PM
#1
For those debating 3.90s or 4.10s... With no other changes other than a set of new drag radials, I went from a 12.77 to a 12.70 swapping from 4.10s to 3.90s. MPH stayed the same, pretty much. 3.90s are stronger than 4.10s due to their having more teeth on both the pinion and ring gear, so they're less likely to break. They also let you run slightly less RPM cruising so they'll give you better gas mileage. A win/win situation in my opinion.
05-06-2007, 07:31 AM
#3
These are actually 3.91s, not 3.90s, but they call them 3.90s. 3.90s are usually noisy because of the low tooth count on the pinion, but these have 43 teeth on the ring gear and 11 on the pinion, unlike a true 3.90 gear. As for 60' times, on my 12.77 pass I had a 1.91 and on the 12.70 pass I had a 1.89. But I backed it up with another 12.70 pass with a 1.92 60'. So the 60's are pretty much identical.
05-08-2007, 01:42 AM
#6
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Nice, If I went with a 9" I was gonna go with 3.90s. so I guess the 3.89s arent the same as the 3.91s.
But Too bad I went with the 12 bolt. I wanted abs and I thought the 3.90s were semi non hunting gears, so I didnt wanna get em. Too bad I wish I knew they were hunting gears, since they are 3.91s and if I knew thery werent noisy. I may have disregarded the ABS and just went 9". If I knew these earlier. Oh well
But Too bad I went with the 12 bolt. I wanted abs and I thought the 3.90s were semi non hunting gears, so I didnt wanna get em. Too bad I wish I knew they were hunting gears, since they are 3.91s and if I knew thery werent noisy. I may have disregarded the ABS and just went 9". If I knew these earlier. Oh well
05-08-2007, 02:18 AM
#7
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Originally Posted by LS1Formulation
With no other changes other than a set of new drag radials, I went from a 12.77 to a 12.70 swapping from 4.10s to 3.90s. MPH stayed the same, pretty much. 3.90s are stronger than 4.10s due to their having more teeth on both the pinion and ring gear, so they're less likely to break. They also let you run slightly less RPM cruising so they'll give you better gas mileage. A win/win situation in my opinion.
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05-08-2007, 07:19 PM
#8
I had 3.90's in my 12-bolt until it broke, and they were really really noisy. I'll be running 4.10's. I would argue that your performance gain would probably be due just as much to the tires as the gears.
As far as gas mileage, I doubt you will see any measurable difference. You're talking about 100 rpm at 75mph in 6th.
As far as gas mileage, I doubt you will see any measurable difference. You're talking about 100 rpm at 75mph in 6th.
05-08-2007, 07:47 PM
#9
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Congrats on the performance increase. Any chance there were other factors involved like DA or the new drag radials installed? I'd like to see some others gain what you did from just the gear swap before passing final judgement on it however. I'm getting ready to switch out my 3.90's to some 4.30's or even some 4.56's. I'd sure hate to think I'm going to slow down by .07 or more from this switch. On the contrary, I'm expecting to pick up at least a tenth from my swap. I'll post up my results in another thread and possibly even back here if I back up your findings.
05-10-2007, 07:24 PM
#10
I got the gears from Summit. $210. You could say that the performance increase was from the tires, but either way the difference is negligible, so why not go with a gear that's stronger in the first place? I drive this car 40 miles each way to the track and it's 99% a street car. I would never put 4.30 or 4.56 gears in my street car unless it was running in the 10's or quicker like yours is. I never plan on this car being any quicker than 11.50's anyway, so i'll stick with 3.90s. As for backing up the times, I ran a 12.704 and a 12.707 back to back. The DA was within a couple hundred feet on the night I ran my best time previously.
05-12-2007, 09:19 PM
#13
Originally Posted by LSGunZ28
Jimbo, how did you break your 12 bolt?
https://ls1tech.com/forums/gears-axles/706083-apparently-you-can-break-12-bolt-highway.html
05-13-2007, 03:08 PM
#16
Originally Posted by LS1Formulation
With no other changes other than a set of new drag radials, I went from a 12.77 to a 12.70 swapping from 4.10s to 3.90s. MPH stayed the same, pretty much. 3.90s are stronger than 4.10s due to their having more teeth on both the pinion and ring gear, so they're less likely to break. They also let you run slightly less RPM cruising so they'll give you better gas mileage. A win/win situation in my opinion.
