I think you are referring to the 350Evo gears. They were the first to come out with them. 350Evo actually developed the gears for their 350Z racing program.

 

Can anyone tell me what the rpm is at 75/90 mph with the 3.9 installed? I would consider the 3.9, but don't want it to be annoying when cruising on a long trip due to the higher rpm.

 

its not going to be annoying, with the gearing on the z, you would have to go pretty drastic in order to get annoying.

 

80 mph is approximately 3000 rpm in sixth gear with the 3.9.

 

That's impossible... 80mph is 3000rpm in 6th gear with the stock gears

 

http://www.f-body.org/gears/


depending on your tire size - specifically how tall the tires are, you can be at 3133-3350 rpms in 6th gear

the gears are actually 3.917, not 3.9 on the dot

it's perfectly fine. 6th gear is a LOT more powerful

 

+1 80mph is about @ 3400rpm with the 3.9 FD.

 

I just came back from a little ride to see. 80mph is 3250 rpm in my car. Like I said it is approximately 3000 rpm. Obviously it is more than 3000 rpm but close.

Based on my observations the 3.9 gears increase your rpm by approximately 300 rpm over the stock gears at any given speed.

 

IC thanks

 

I know 20's kill some of your acceleration do to the increase in diameter.Do you think the 3.9 will make up what you loss from using 20's?

 

Could you elaborate on how this effects the shift points? I don't see how a gearswap would affect shiftpoints. The individual gear ratios are the same. I can see how the FD can affect total torque multiplication, but if anything, that would just extend the shift points upward.

And on the revups, the 3.9 FD shouldn't force a shift on NA cars. On the stock tire size, with a fuel cut at 7200, the top speed of 4th with the 3.917 FD is 113mph. If you're doing 113.3 NA, then I think a shift to 5th is more than acceptable.

On my shorter tires that I presently run(25.7" vs 26.3"), the top speed of 4th is still 110.6mph. Again, I'm not too worried about having to shift to 5th.

 

The 3.9 gears will cause you to have to shift sooner than with the stock gears because your engine will be turning about 300 rpm more than stock at any given speed. Therefore your shifts will be made at slightly slower speeds than with the stock gears. This may make the car faster or may make it about the same as stock depending on, for example, your terminal speed and how many shifts you have to make to get to that particular speed if you are running 0-60 or 1/4 mile times with the 3.9 gears versus the stock gears.

The advantage of the 3.9 gears is that you get approximately 10% faster acceleration in each gear because the motor is turning more rpm at any gvien speed (therefore more in the power band) and the gears are shorter.

Personally I think the gears have more of an advantage in road racing or accelerating while in motion than accelerating from a dead stop, such as 1/4 mile times. That is based on a seat of the pants observation. I don't have any quarter mile times before and after my gears to compare.

I also felt like my car ran out of steam up top with the gears until I got cams. Now it pulls hard all the way to redline. Having a reflashed ECU raising the rev limiter also helps with the 3.9 gears.

 

The actual shift points, as they relate to your RPM speed are unchanged. When you upshift from 1st to 2nd, 2nd to 3rd, and so on, there is no difference between the 3.917 and stock as to where the tach drops on a shift. For example, if a particular upshift at 6500 rpm dropped you to 4300 rpm stock, you will still drop to 4300 rpm with the shorter final drive.

Where a lot of the confusion and misconceptions about the final drive come in is related to the actual speed of the wheels. Wheelspeed is lower to any given engine RPM for any gear compared to stock. So while you might've been able to hit 60 in 2nd gear stock, you would have to shift to 3rd. This is the kind of shift point changing most people quote since most people look at 0-60 and 1/4 mile times as the yardsticks to measure performance by.

Having the 3.917 on my 5AT, its had no ill effects on the TCM and how the transmission performs. The shift logic is unchanged as the internal gear spacing is unchanged. So the TCM's logic/timing for up and dowshifts doesn't need changing and reacts perfectly fine. The same concept applies to any driver with a 6MT who uses their tachometer as their main guide.

If you drive by your tach, your habits will be unchanged, they'll just happen more quickly. If you drive by your speedo, it will cause you to re-evaluate how you drive.




You can actually get the same effects by changing your tire diameter. Its another "gear" in the system of applying power from the crankshaft to the ground. Difference is, your speedometer is calibrated on wheel rotation so it will be affected, regearing doesn't affect the accuracy of any of those systems.

There's an old hot rod formula for understanding the relationship between RPM, final drive, speed and tire size. The 336 value is simplification for the constants of time, pi and distance. Take the numbers as rough approximations to understand the relationship and the same gearbox gear ratios for all.

rpm = (mph x final gear ratio x 336) / tire diameter

So start with a stock 6MT
2671 rpm = 60 mph x 3.538 x 336 / 26.7"

Take it up to the 3.917 FD
2958 rpm = 60 mph x 3.917 x 336 / 26.7"

You could get identical performance with a smaller tire on the stock FD
2960 rpm = 60 mph x 3.537 x 336 / 24.1"

Other than the fact that a 24.1" tire is near impossible to find, you see how changing tire size has an effect. Why most track prepped cars run a ~25.5" tire.

If you wanted the same effects of the 3.917 for track use, but without the same type of change for street you could get the 3.700 FD and short R compounds on 18's.

2960 rpm = 60 mph x 3.700 x 336 / 25.2"

That 25.2" tire is the measured height on a 275/35/18 Hoosier A6. So the guys with the larger turbo setups running track days with the shorter R's, they're effectively shortening their final drive for better acceleration.

 

That was my point; the gear spacing is unchanged.

When DaveB was talking shift points, he meant from a drag racing perspective, in that a shift should occur at the moment where more power will be made in the next gear than in the current gear. And that takes into account the torque multiplication of the current gear versus the next gear.

My point there was that by having the shorter FD, if anything, that would extends ideal shift points further up in the range.



I have looked into this option myself. The stock tires for my car(sedan) are 26.3" tall. BFG sells 17" DR's in sizes 205/40 and 225/45. The 205/40 tires are 24.0", versus a height of 25.1" for the 225/45.

I've considered mounting some of those tires on some stocker 17's I have in the garage; but as DaveB has pointed out, DR's and slicks need a good amount of sidewall compression to be effective; and 40 or 45 series tires just won't give you much compression.

 

I'm not arguing the general concept as it is true. If your engine loses enough power on the top end, upshifting and dropping the engine rpm's back so its in its peak power area can apply more power to the ground if the power loss is greater than the difference in gear spacing and torque multiplication. But that's based on the power curve the engine produces and your transmission gear ratios, not the FD.

 

http://www.*********.com/zwiki/Part:...rive_Gear_Sets

 

three five zero - tech

 

I agree. At least, I did before I thought about it more. The gear multiplication, to the ground, will be the product of the gear ratio and the final drive ratio.

To keep things simple, let's say you have two final drives: 3, and 2.
And each gear has the following ratios:

1st: 5
2nd: 4
3rd: 3
4th: 2
5th: 1

With the stock FD, 1st gear's final ratio is 5 x 2 = 10. Second gear's ratio is 4 x 2 = 8.

With the shorter FD of 3, 1st gears final ratio is 5 x 3 = 15, versus 2nd's of 4 x 3 = 12.

With the stock FD, the final ratio is smaller between 1st and 2nd gears, versus the FD value of 3.

In this simple exercise, the gap of final ratios between 1st and 2nd is 50% greater with the shorter FD. In the real world, the gap will be 10%.

Example:
With the stock FD, the gap between 1st and 2nd is 5.204. With the 3.9's, the gap is 5.758. The gap between the two gears has increased by roughly 10%.

This gap will play a factor when calculating ideal shift points for power.

 

Right, because the FD has changed by that same amount. But you're leaving engine power, transmission gear ratio and tire diameter as a constant.

Here's are 2 charts I created based off my dyno numbers for torque applied to the wheels. As you can see, the torque applied increases by ~10% in every gear by going from the 3.538 to 3.917. But the torque applied to the wheels is based off the engine torque and gear ratio. That part of the formula hasn't changed.





So lets assume you shift from 2nd to 3rd at 6600 rpms and it drops you to 4500 rpm. That won't change, regardless of the FD. Using the numbers above; at 6600 rpm in 2nd with the 3.538, you'd be appying 768.8 lb-ft and with the upshift, you'd be making 586.1 lb-ft. With the 3.917, you'd be putting down 851.1 and 648.8, respectively. But the gear shift still makes the same 24% torque loss, regardless of the FD.

Yes, the final numbers change base off the final drive, but if you look at the torque charts in the upper right hand corner, their relative % spacing doesn't.

 

nah that's wrong

15/12 = 1.25
and
10/8 = 1.25

the difference ratio is the same

you don't calculate the difference by addition and subtraction...


so
14.861 / 13.431 = 1.10647...
9.103 / 8.227 = 1.10647...


and btw it's technically 3.538, not 3.54 exactly (if we're talking about using the 3rd decimal place as in 3.917)


you'd have to change the individual transmission gears non-proportinally to actually create a rpm change from gear to gear compared to the stock tranny gearing. The final drive only multiplies everything one final time as a whole

the change in gap you're talking about doesn't change when you change the final drive. the only thing that changes is the accelleration, theoretical top speed in each gear, and the frequency you will shift during a given distance