I am pretty intrigued by the dramatic difference in acceleration that this mod would make (the quarter mile difference between the 6MT and the 5AT is primarily due to gearing ) . However, the loss in mileage would be pretty ridiculous for a daily driver. Are there any 5AT guys out there who are planning to (or have already) made the switch to 3.9?

 

I have considered this FD, but am leaning more towards the 6MT FD. Mainly because of gas mileage and the overall increase in rpms the engine would see driving on the highway. It would add a fair amount of wear and tear to normal daily driving. On the track it would be very nice.

 

We have some 3.5 in stock for sale.
BJ@350EVo.com

 

Would a 3.9 FD (axle ratio) really be necessary on the 5AT? A torque converter has the ability to multiply torque on it's own without changing the final drive and the 5ATs come with 17" wheels as standard which would help in acceleration also. I would rather have the torque converter do it's job through torque multiplication and have it couple or lock at the taller ratio. You will still get the added torque at the low end and you won't sacrifice velocity at the high end. Look into modifying your torque converter before you start messing with your final drive. Raising the stall speed would have more benefit to your 1/4 mile times and you would still be able to have a decent daily driver. A slight increase in the stall will give you noticeable gains with very little affect on mileage. Changing to a 3.9 FD will increase your acceleration but you might also need to make tire and suspension mods to compensate for the 5AT multiplying to the new 3.9 FD. Traction might become a problem without mods because the torque converter will flash to the stall speed when you step on the throttle and multiplication will be more pronouced at the wheels.

Here's a basic explanation on how a torque converter multiplies torque.
Torque Converter Multiplication Explained

If you were looking to increase torque at the expense of hp on the 6MT then a 3.9 final drive would make sense. Since the 6MT can't create torque on its own similar to a torque converter. You would have to either modify one or more of the following: transmission ratio; axle ratio and or tire ratio. Changing the stock FD to a 3.9 final drive will increase your torque (acceleration) but you will decrease your max hp (velocity).

Here is a link on gearing ratios.
http://www.datsuns.com/Tech/tech_gearing.htm

You can see what effect final drive ratio has on our stock 5ATs and 6MTs. If you put both on a dyno, on average you will see that the 6MT will put down 3%-5% more hp mostly because of it's taller final drive. The 5AT gave up some of its velocity for more torque.

 

Thanks for the informative reply, but I am a bit confused as to what you mean by "the 5AT gave up some of its velocity for more torque." I attribute the discrepancy between 5AT and 6MT dynos to different levels of drivetrain loss, not gearing. After all, the dynos are being performed in the 1:1 gear regardless of the axle ratio. Am I incorrect in believing that gearing does not affect that actual horsepower/torque produced by the engine? An appropriate FD ratio makes it easier to stay within an engine's power band, but it doesn't actually *change* that power band, correct?

On a related note, since launching is performed at a fixed RPM, why would torque increase or decrease? I can vaguely understand that the taller gears (with a larger diameter) would yield more torque, but my understanding is a bit limited here. Is a rear end swap similar to, say, a lightweight flywheel, in that it reduces the inertia in parts of the drivetrain and makes launching harder?

As far as launching is concerned: yes, I was certainly planning to get a high-stall torque converter. I was just wondering if the quarter mile gains of a new FD - which still need to be verified in practice - are worth the loss in mileage.

Damn I wish i had a 6th gear

 

I have a torque converter and I just got thte 3.5 ring and pinion . Not installed yet . From what I understand if they made a 3.7...that would equal what the 3.9 is doing for the MT6 . Hope to have the gears in soon and will do a review

 

The transmission gear ratio is just part of the overall power transfer that goes to the ground. Neglecting driveline loss there are three basic areas that affect how much torque gets transmitted to the ground once pass the engine.

1) Transmission Ratio (the gearbox)
2) Axle Ratio (final drive power transfer)
3) Tire Ratio (tire size & weight)

The gearbox in the 6MT and the 5AT are both mechanical despite the popular misconception that the 5ATs gearbox is hydraulic. The 6MT uses a manually controlled gear cluster. The 5AT uses an electronically managed planetary gearset. Both gearboxes still require clutches to change gears. The different gearings in the gearbox is what allows the engine to operate in a given range and still produce different affects at the wheel. Mechanical driveline loss between the 6MT and the 5AT are virtually identical at the gearbox. Yes, you would want to dyno both transmissions when the engine to transmission gear ratio is as close to 1:1 as possible because it gives a more standardized reading. On the 5AT the 1:1 is 4th gear and on the 6MT the 1:1 is 5th. At the gearbox, the 5AT in 4th and the 6MT in 5th are virtually the same because power is coming from the same engine and the gearing ratio at the gearbox is the same. Dynoing in a gear ratio other than 1:1 would make it hard to compare against a standard. Unless everybody agreed to test at a different ratio my measurements compared to your measurements would be irrelavent.

Once you get pass the gearbox power has to be transferred to the driveshaft. This is where the hydraulic confusion comes into play. On the 6MT power is transferred through a simple mechancial connection using pressure and friction plates. A good example would be pressing two sanding disks together and trying to rotate them. With enough pressure and friction they will rotate in the same direction as if they were one. As a result one advantage is almost 100% power transfer. This mechanical connection also has it's disadvantages. One is that if one shaft is turning the other shaft also has to turn. You have to have the clutch engaged to allow both to operate at different speeds. On the 5AT power is transferred through a torque converter. A torque converter uses hydraulic coupling to transfer power. Because it is hydraulic it allows smoother power transition, torque multiplication and it allows both shafts to spin at different speeds. The torque converter can also act as a clutch below its stall speed. The downside to the torque converter was hydraulic sheer, weight and heat affected some of the controllers. Back in the day, torque converters were two to three times heavier than manuals. Today the 5AT is ~5kg (latptop computer weight) than the 6MT. Back in the day, hydraulic loss from free reving was as high as 20% at high RPMs. Today torque converters have at least one lock-up clutch which lock the fans together mechanically. 100% power transfer like an MT when locked. The 5AT has full lock-ups in 4th and 5th and a partial lock-up in 3rd. Back in the day, torque converters used a variety of mechanical govenors, modulators and cables to control operations. It was inaccurate and sloppy. Today most torque converters use computers and solenoids for control. In the Z, a locked 5AT will have virtually the same driveline loss as the 6MT.

Now we get to the axle. This is where most of the performance difference in the 5AT and the 6MT occurs. On the 6MT, the final drive ratio is 3.538. On the 5AT, the final drive ratio is 3.357. As you can see, the 6MT has a shorter gear than the 5AT. At the axle the 6MT is producing more torque at the same engine RPM than the 5AT but the 5AT would have a higher velocity at the same engine RPM than the 6MT.

If you dyno a 5AT Enthusiast and a 6MT Enthusiast you will find that most of the difference between the two would be at the axle because of the final drive ratio. Loss at the transmission would almost be identical and both of them have the same engine and wheels so there is no difference there.

Just to help out, the terms taller and shorter when comparing gear ratios came from the bicycle industry. Before they started using gears, bicycles had taller or shorter wheels. Taller wheels were able to reach faster velocities but they didn't accelerate well. Shorter wheels had faster acceleration but lower velocity. When they started adding gears to bikes they standardized the wheel to 27". A gear that was 1:1 went the circumference of a 27" with each complete pedal rotation. A taller gear like 1:0.5 meant that the bike did the 27" circumference in half a pedal stroke. The gear created the affect of a "taller" wheel. A shorter gear like 1:2 meant that the bike did a 27" circumference in two pedal strokes. The gear created the affect of a "shorter" wheel. In a car a 1:1 ratio meant that the gearbox made one rotation for every one engine rotation. Going back to the bike analogy, a ratio with a higher number is a shorter gear and a ratio with a smaller number is a taller gear. Sticking with our bike, if you wanted to see the effect that gearing has, get on a bike with multiple gears. If you put the bike in its shortest gear, when you pedal you can spin like mad and get very little velocity. What you do get is enough torque for faster acceleration or the ability to climb that steep hill more efficiently. If you put the bike in its tallest gear, you have sluggish acceleration but you can reach a higher velocity and you are more efficient at higher speeds. The average human is most efficient on a bike between 70-120 RPMS for performance riding. We use the gears to try and keep us in that range, outside of that range we are less efficient for the purpose of performance riding. Car engines are the same way. They operate most efficiently in a given range for certain types of activity and they use gears to stay in that range.

For a detailed answer as to how a torque converter can create more torque than the engine is outputting click on the link above about torque multiplication. Basically, if there is a difference between the impeller and turbine speeds, the torque converter will convert the difference into more torque by accelerating the fluid multiple times until the speeds match then they will couple or lock together.

It was long but I hope it helped.

 

A torque converter will definately help off the line, but will not be able to put you in a better rpm range for quick acceleration on a track. The main objective of a different final drive is to match the engine's output to overcome the drivetrain factors, wind resistance and weight. To aggressive and you shift more frequenly resulting in inefficient use of power, too weak and you never fully utilize the engines power. It is a balancing act, but I personally feel the 5AT would have been a perfect match for the 6MTs stock FD and the 6MT a perfect match for the 5ATs final drive. I think Nissan switched the two on the line by mistake

 

Yes!, this is what I am planning, the TC with the 3.5 and I assume the Nismo Diff?

Please let us know how it goes ASAP!

 

Are they used?

 

Ive got the G35 AT5 in the shop . Come to find out I have an open rear end . So the ring and pinion from the 6MT needs a ring gear spacer to get it to work...or I may try to go to a VLSD and the 6MT 3.5 ring and pinion . Trying to find out if the VLSD will go in the open rear end...ANY ONE KNOW ?????

 

If your thinking about putting an oem lsd in there you will also need new stub axles and now it becomes cost prohibative and you may as well get the NISMO Diff.

Sell me your 3.5!

 

How hard is it to put this in.

Thanks Glen

 

The offset of the ring gear on a automatic car differential is differnet than on a manual car. The best thng would be to install a LSD from a manual car of just get a Nismo Diff.

 

I think I talk to you yesterday on the phone [if you are John ] From what Im hearing I would need LSD axel stubs in addition to the VLSD unit .Im going to call Tony at HP MotorSports agian and talk to him first . Thanks for the good offer on the VLSD though....Bill
The offset of the ring gear on a automatic car differential is differnet than on a manual car. The best thng would be to install a LSD from a manual car of just get a Nismo Diff. [/QUOTE]

 

This sounds like a good idea for 5AT's. I'm a little concerned because I already spin the tires from a stop or a 5mph roll. I wonder if this would make my car less streetible?

I wonder if it changes the speedo? Also what other parts are needed? I think I may be up for a swap if any turbo 6mt's want to trade.

 

It doesnt change the speedo . Some one said sensors are after the gears . If you have a lsd already...just the 3.5 ring and pinion and some shims I think . I got to swap the whole pumpkin... mines an open rear end......funny thing is...Ive never left a one tire burn out....always 2....any time Ive check after wards

 

Hey Booger...My only other concern is shifting. This would be for those of us with F/I. My car needed the TS flash to help it shift. I wonder if changing the FD would make it harder to shift. I would not consider going over 3.5 by the way.

My car is an enthusiast so I already have the vlsd.

DWNSHIFT, are you saying it is better to swap the entire LSD with 3.5? (if I was using one from a 6MT) Why can't I just swap the 3.5 ring and pinion and add some shims?

I was speaking with Doug today at Crawford and he said the other concern he had was traction. I already get very little of that

 

whosdady, you would not want to go with the 3.5 FD or anything more aggressive than the 5AT stock FD. You have more than enough power for the FD you have in the car. You would have more shifting issues with the 3.5 FD as well.

 

3.5 ring and pinion for sale . PM me