When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Does anyone here have any time slips that shows the gains you made when swapping in a 3.69 gear from the stock 3.35. My car on has a few bolt-ons but I and looking to do this swap!
To over simplify it. The % difference between FD ratios will be the same percentage increase in acceleration but only to the top of each gear. Then once you shift while you are now in a higher gear losing mechanical torque you will be slower while you otherwise could have continued in the previous gear, but then once you would have had to shift on the previous FD to the higher gear you will go back to being faster. FD change will also be the same percentage difference in the top speed of each gear and the increased rpm at any given gear. (With 10% FD higher ratio 2500 rpm at 60mph becomes 2750rpm at 60mph.
Another example say a car with a FD of 4.0 and hits rev limit in 2nd gear at 70mph in 7.5sec and does 63mph in 7sec and hits rev limiter in 3rd at 100mph in 14.5sec and does 90mph in 13.5sec. The same car with a 4.4 FD will rev limit in 2nd at 63mph and 3rd at 90mph (10% less) however it should theoretically get to 63mph 10% faster then the 4.0FD would get to 63mph. So 63mph in 6.3sec (0.7faster) and 90mph in 12.15seconds (1.35 sec faster).
However because everytime you shift you will lose mechanical torque compared to previously not needing to shift the 4.4 is slower from 63-70 and 90-100.
now from a dig the more aggressive numerically higher FD will almost always be faster given traction not being an issue and not factoring in launching. The higher RPM you can launch without traction being an issue the benefits of the numerically higher FD decreases.
Now from a roll it will be very dependent on what speed you are starting from and also running up to. Many situations the 4.0 will be faster.
Ideally you pick a FD to limit needing to have an extra shift while getting as close to peak WHP as you cross the line if you trap 100mph a FD with limiter at 103-105mph would be ideal, if traction isn't an issue then this would be an aggressive enough FD to have it be 4th. If you go FI and increase power you’d want a numerically lower FD if you now can trap 115 you’d want an FD that puts limiter most likely still 4th at 118-120mph.
for track purposes it gets very complicated and very dependent on the track, serious people have multiple diffs with different FD ratios for quick swaps depending on the specific track.
for a street car decide what speeds A-B you like to run, 40-90? 60-130? Some other given speed to given speed and then you’d choose the FD best suited to that specific speed range.
To over simplify it. The % difference between FD ratios will be the same percentage increase in acceleration but only to the top of each gear. Then once you shift while you are now in a higher gear losing mechanical torque you will be slower while you otherwise could have continued in the previous gear, but then once you would have had to shift on the previous FD to the higher gear you will go back to being faster. FD change will also be the same percentage difference in the top speed of each gear and the increased rpm at any given gear. (With 10% FD higher ratio 2500 rpm at 60mph becomes 2750rpm at 60mph.
Another example say a car with a FD of 4.0 and hits rev limit in 2nd gear at 70mph in 7.5sec and does 63mph in 7sec and hits rev limiter in 3rd at 100mph in 14.5sec and does 90mph in 13.5sec. The same car with a 4.4 FD will rev limit in 2nd at 63mph and 3rd at 90mph (10% less) however it should theoretically get to 63mph 10% faster then the 4.0FD would get to 63mph. So 63mph in 6.3sec (0.7faster) and 90mph in 12.15seconds (1.35 sec faster).
However because everytime you shift you will lose mechanical torque compared to previously not needing to shift the 4.4 is slower from 63-70 and 90-100.
now from a dig the more aggressive numerically higher FD will almost always be faster given traction not being an issue and not factoring in launching. The higher RPM you can launch without traction being an issue the benefits of the numerically higher FD decreases.
Now from a roll it will be very dependent on what speed you are starting from and also running up to. Many situations the 4.0 will be faster.
Ideally you pick a FD to limit needing to have an extra shift while getting as close to peak WHP as you cross the line if you trap 100mph a FD with limiter at 103-105mph would be ideal, if traction isn't an issue then this would be an aggressive enough FD to have it be 4th. If you go FI and increase power you’d want a numerically lower FD if you now can trap 115 you’d want an FD that puts limiter most likely still 4th at 118-120mph.
for track purposes it gets very complicated and very dependent on the track, serious people have multiple diffs with different FD ratios for quick swaps depending on the specific track.
for a street car decide what speeds A-B you like to run, 40-90? 60-130? Some other given speed to given speed and then you’d choose the FD best suited to that specific speed range.
Thanx for your reply. My main concern is just to run quicker 1/4 mile times with good traction. And coming from the Mustang world a rear gear swap like this would always achieve that goal. I was hoping to find some one who had some real world results I could see to help with my decision. And moving from a 3.35 to a 3.69 is not a bad move for the price I can get it done. (G35 diff swap) Also by running shorter rear tires(25" vs 26.7" stock) my effective gear ratio went from 3.35 to 3.60. I can tell it's quicker from that change.
You use OEM spare wheels for the front on the track? Wow. You're really making the most out of that G for the dragstrip.
IDK about the track, but in daily driving there was a serious difference going from 3.69 to 4.08 rear gears in my 6MT. Obviously it moved the shift points down, and highway RPMs went up a bit, but man was it a worthy project.
Thanx for your reply. My main concern is just to run quicker 1/4 mile times with good traction. And coming from the Mustang world a rear gear swap like this would always achieve that goal. I was hoping to find some one who had some real world results I could see to help with my decision. And moving from a 3.35 to a 3.69 is not a bad move for the price I can get it done. (G35 diff swap) Also by running shorter rear tires(25" vs 26.7" stock) my effective gear ratio went from 3.35 to 3.60. I can tell it's quicker from that change.
A smaller tire or shorter (numerically higher) FD will always "feel" quicker, because every gear will accelerate at the same percentage change as I described above but to a slower speed, just because it "feels" quicker does not always mean it is actually quicker depending on the circumstances.
a 3.35 to 3.69 is approx. 10% difference. it should accelerate 10% faster in every gear, but to a 10% slower speed this makes it feel to the driver 10% faster, but it isn't always the case.
with the auto trans gearing, stock tire diameter, and stock rev limiter going from a 3.35 to 3.69 will have the following results.
3.357 - 1st - 36 / 2nd - 55.6 / 3rd - 86.94 / 4th -125.82
3.692 - 1st - 32.79 / 2nd - 50.56 / 3rd - 79.05 / 4th - 114.4
From a dig because you have so much mph/rpm range in 1st gear and how much the 60ft impact the 1/4th a more aggressive gear will almost always be faster especially if the resulting gear doesn't require an extra shift.
if you are NA you probably wont be trapping over 114mph so the 3.69 doesn't result in needing an extra shift. In your case specific to the 4th a 3.69 will be very beneficial. You could probably go even more aggressive assuming you have the tire to hook up and as long as it doesn't require you to shift to 5th.
Last edited by 14Q60awdSPORT; Apr 19, 2022 at 09:28 AM.
You use OEM spare wheels for the front on the track? Wow. You're really making the most out of that G for the dragstrip.
IDK about the track, but in daily driving there was a serious difference going from 3.69 to 4.08 rear gears in my 6MT. Obviously it moved the shift points down, and highway RPMs went up a bit, but man was it a worthy project.
GL
The spare 17" (17x3.5 & 18x3.5) are just as good as aftermarket skinnys but much cheaper. I get a good deal on real street tires that last and handle well for street driving. Maxxis T145/80/17 or T145/70/18. Have driven 3k miles on a pair with little wear. The diff swap is the most affordable option for me that I can do myself. Lost out on a 3.91 diff from a G Vert. I am hoping to drop about 2 tenths off my times. I will be well worth it then!
145/70/18 Maxxis Tire
OEM 17" Maxxis 145/80/17 Spare painted Factory color
[QUOTE=JME377;4309947]The spare 17" (17x3.5 & 18x3.5) are just as good as aftermarket skinnys but much cheaper. I get a good deal on real street tires that last and handle well for street driving. Maxxis T145/80/17 or T145/70/18. Have driven 3k miles on a pair with little wear. The diff swap is the most affordable option for me that I can do myself. Lost out on a 3.91 diff from a G Vert. I am hoping to drop about 2 tenths off my times. I will be well worth it then!
The spare 17" (17x3.5 & 18x3.5) are just as good as aftermarket skinnys but much cheaper. I get a good deal on real street tires that last and handle well for street driving. Maxxis T145/80/17 or T145/70/18. Have driven 3k miles on a pair with little wear. The diff swap is the most affordable option for me that I can do myself. Lost out on a 3.91 diff from a G Vert. I am hoping to drop about 2 tenths off my times. I will be well worth it then!
145/70/18 Maxxis Tire
Are you looking for a 3.6 Diff?
No looking Have one already and just installed it. But no times as of yet
Wow. You're really making the most out of that G for the dragstrip.
