3.90 vs 4.10
Full Member
Joined: Jan 2009
Posts: 73
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From: Utica, NY
This is all wrong. Gearing gives is a means of mehanical advantage, the higher the ratio the greater the mechanical advantage of the engine to the wheels.
With the SAME torque applied, the mechanical advantage of a 4.10 will be greater (meaning it's easier for the engine to turn the wheels) causing easier wheel spin. NOTHING will change this.
The fact that you've got enough torque to spin 3.9's or enough torque to not bog doesn't change the fact that if you put 4.1's on the same vehicle they'd spin easier (and that usually implies quicker). If you've got enough torque to roast 3.9's, you'll explode the 4.1's because they'll spin considerably easier. The only area that this Big power and torque or little power and toque doesn't change mechanical advantage, so that's the only reason it shouldn't have been mentioned, not because the FD lacks it as you seem to imply.
The only difference as far as 'roasting' would be the overall top speed that the tire attained while it was spinning, as it'd obviously spin at a higher speed once the 3.9 wound itself out, but that's pretty pointless as far as launching goes if you're spinning yourself all the way through 1st gear to redline.
With the SAME torque applied, the mechanical advantage of a 4.10 will be greater (meaning it's easier for the engine to turn the wheels) causing easier wheel spin. NOTHING will change this.
The fact that you've got enough torque to spin 3.9's or enough torque to not bog doesn't change the fact that if you put 4.1's on the same vehicle they'd spin easier (and that usually implies quicker). If you've got enough torque to roast 3.9's, you'll explode the 4.1's because they'll spin considerably easier. The only area that this Big power and torque or little power and toque doesn't change mechanical advantage, so that's the only reason it shouldn't have been mentioned, not because the FD lacks it as you seem to imply.
The only difference as far as 'roasting' would be the overall top speed that the tire attained while it was spinning, as it'd obviously spin at a higher speed once the 3.9 wound itself out, but that's pretty pointless as far as launching goes if you're spinning yourself all the way through 1st gear to redline.
Joined: Dec 2001
Posts: 10,630
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From: NY, MA, MI, OR, TX, and now LA or AZ!
That gets to be pretty complex. Removing as many variables as possible you can say it takes 200ft-lbs of force to cause the rear tires to slip (example). The additional rotation of the 3.9 isn't going to increase torque like the mechanical advantage of the 4.1 will, so while although the wheel may 'spin' at a faster rate given the same RPM/torque through a launch (And get you out of the hole quicker), it won't break free sooner than the 4.1 would.
Full Member
Joined: Jan 2009
Posts: 73
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From: Utica, NY
That gets to be pretty complex. Removing as many variables as possible you can say it takes 200ft-lbs of force to cause the rear tires to slip (example). The additional rotation of the 3.9 isn't going to increase torque like the mechanical advantage of the 4.1 will, so while although the wheel may 'spin' at a faster rate given the same RPM/torque through a launch (And get you out of the hole quicker), it won't break free sooner than the 4.1 would.
Joined: Dec 2001
Posts: 10,630
Likes: 3
From: NY, MA, MI, OR, TX, and now LA or AZ!
Yeah, when you think about it only in turns of number of rotations it seems logical, but that's overlooking that it requires force to turn the wheel, and force to exceed it's gripping capability.
Sure, the 3.9 tire will rotate faster, but it'll have less torque applied to it than the 4.1. Thus, the 4.1 will exceed the 'grip' threshold because theres more torque behind the turning, even though it's rotating less.
Sure, the 3.9 tire will rotate faster, but it'll have less torque applied to it than the 4.1. Thus, the 4.1 will exceed the 'grip' threshold because theres more torque behind the turning, even though it's rotating less.
Well, unfortunately I don't have any proof. I understand what you are all trying to say, but I've seen the problem happen with my friend, I even asked him about it the other day and he confirmed it.
forcefed 240
Joined: Jan 2004
Posts: 123
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From: morgantown, west virginia
3.9 will have less wheel spin out of the whole than a 4.10 no matter what it. 4.10 will put more initial power down than a 3.9. higher the tq multiplication of the gear the more force you're applying to the ground. the more force you apply the more traction becomes an issue. i've never heard of a bigger fd getting more traction.
running a 4.10 to get better traction? for a high tq powered car?
running a 4.10 to get better traction? for a high tq powered car?
hell for all i know there were some unknown variables where his 60 foot improved such as his rpm and torque curve shifted out of optimum power, but a stall converter should have kept that consistent. I'll take a look around. Nothing like some unknown variables to blow a hole in an argument.
edit: the only thing so far that might go in my favor was one guy in a forum that moved from 4.11s to 4.56 gears. His 60 foot went from 1.32 to a 1.30 with the 4.56, but no mention of wheel spin so he could have been hooking up solid, the more i dig in it seems to be that it depends on the vehicle itself in that realm. I need to go to bed, an interesting discussion none the less.
edit: the only thing so far that might go in my favor was one guy in a forum that moved from 4.11s to 4.56 gears. His 60 foot went from 1.32 to a 1.30 with the 4.56, but no mention of wheel spin so he could have been hooking up solid, the more i dig in it seems to be that it depends on the vehicle itself in that realm. I need to go to bed, an interesting discussion none the less.
i made an excel sheet today, put in your transmission gear ratios and final drive, then tire size it gives you road speed, drive shaft, and axle speed at a given engine RPM...
its not 100% accurate but its pretty close to give an idea of what ratios do to top speed, shift points etc.
its not 100% accurate but its pretty close to give an idea of what ratios do to top speed, shift points etc.
Did your overall tire size change at all in the process? Should have noticed something i would think.
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Regarding the comment that a 4.1 will be less likely to spin then a 3.9 in a high torque car or any car for that matter is just innacurate. That would be like saying launching in 2nd gear breaks the tires easier then in first gear. If your melting the tires with a 3.9 a 4.1 is going to make it even worse. That high torque car has even more torque with the lower gearing (higher number). Thats not to say his times didn't improve due to shift points and accelleration in other gears, but it sure wouldn't solve a traction issue.
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Regarding the comment that a 4.1 will be less likely to spin then a 3.9 in a high torque car or any car for that matter is just innacurate. That would be like saying launching in 2nd gear breaks the tires easier then in first gear. If your melting the tires with a 3.9 a 4.1 is going to make it even worse. That high torque car has even more torque with the lower gearing (higher number). Thats not to say his times didn't improve due to shift points and accelleration in other gears, but it sure wouldn't solve a traction issue.
Full Member
Joined: Jun 2006
Posts: 95
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From: Lancaster, PA
question, I have a automatic with a TH350 tranny and ratchet shifter. since it originally had a FD auto tranny in it I assume it has 3.9 ratio. But it also has Kazz 1.5 lsd in it, so does the aftermarket lsd change the 3.9 gearing at all?
In my experience, the difference between 3.9 and 4.1 is splitting hairs and inconsequential in most instances. There is really nothing to be gained from this swap, it's just too incremental. (such a change might benefit an Indy or F1 racer) Gearing is a compromise of which you are trying to achieve the best balance. There are many factors affecting the outcome of this equation as has been noted above.
I would use the tallest gear I could (lowest numerically) that will launch the car for my intended purpose. I would not change gears unless you have a definite goal you are trying to accomplish, i.e., extended top end, better shift points, more grunt for a hard launch, etc.. When you change gears arbitrarily, you will make the car different, but not necessarily better.
Full Member
Joined: Jun 2006
Posts: 95
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From: Lancaster, PA
opps double post sry
question... i have an auto FD with a TH350 tranny and ratchetshifter. Since it originally had the stock auto tranny in i assume that i have 3.9 gearing. Now, I also have Kazz 1.5 lsd installed in the diff. how does that affect my 3.9 gearing in the rear end?
question... i have an auto FD with a TH350 tranny and ratchetshifter. Since it originally had the stock auto tranny in i assume that i have 3.9 gearing. Now, I also have Kazz 1.5 lsd installed in the diff. how does that affect my 3.9 gearing in the rear end?
Junior Member
Joined: Sep 2007
Posts: 6
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From: S.E England
The 4.10 ratio will give you better launches over a 3.9 ratio, forget about wheel spin for a moment.
Take the throttle to purely controlling the the wheel speed, with a 4.1 ratio you can control the wheels to 1900 rpm and a 3.9 2000rpm meaning you can find the point where you loose traction easier. You can also open up the throttle slower because of the increased torque.
But the real advantage come on track.
I think the 3.9 ratio was put on for the American market to give mpg on european and japanese roads you want the 4.3 idealy.
Take the throttle to purely controlling the the wheel speed, with a 4.1 ratio you can control the wheels to 1900 rpm and a 3.9 2000rpm meaning you can find the point where you loose traction easier. You can also open up the throttle slower because of the increased torque.
But the real advantage come on track.
I think the 3.9 ratio was put on for the American market to give mpg on european and japanese roads you want the 4.3 idealy.
Was a 3.9 used in jap automatics also? Or did they have a different ratio? Remember the automatics have different tranny ratios and also a lower redline. This will play a role in what final gear the factory choses. Depending on your power curve, in some cases a lower ratio may improve drag times by staying in a lower gear for longer or reducing the number of shifts.
