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Discussion Starter #1
I have a 5 speed and the stock 3.5 rearend. What are the benefits of putting the 3.9 diff in it? I do not understand how this would make my car faster 0-60...will it? It seems like I would acutally go slower and have to shift alot more often. Please explain this to me I am not an engineer.
Thanks-Jeff 280z
 

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Discussion Starter #2
> I have a 5 speed and the stock 3.5 rearend.
> What are the benefits of putting the 3.9
> diff in it? I do not understand how this
> would make my car faster 0-60...will it? It
> seems like I would acutally go slower and
> have to shift alot more often. Please
> explain this to me I am not an engineer.
> Thanks-Jeff 280z

Im no engineer but ill try to explain this the best i can.....Basically a 3.9 rearend means that the drive wheels will rotate 3.9 times for every 1 revolution of the driveshaft. If the rearend was a 3.3 or 3.5, the engine has to produce more torque than a 3.9 to make it accelerate the same. If the power of the engine is the same, it would take longer for the engine to get to redline or whatever with a 3.5 or 3.3. If you had a 3.9 3.7 or 4.11 then the engine will rev much more quickly. Also the larger the number like 3.9 or 4.11 the more torque is produced at the rear wheels at the expense of loosing top speed. So there is a benefit to having a 3.9 and thats quicker acceleration, but a loss of top speed. I hope this makes sence. Ive been in a mechanical technology class for 3 years now.

Coop
 

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Discussion Starter #3
> Im no engineer but ill try to explain this
> the best i can.....Basically a 3.9 rearend
> means that the drive wheels will rotate 3.9
> times for every 1 revolution of the
> driveshaft.

Actually, I think it is the driveshaft will rotate 3.9 times for every 1 revolution of the drive wheel. I'm not an expert or anything myself, but I think this is right.

You would get better acceleration but have to shift sooner. If your car barley gets to 60 in 2nd before loosing power, getting redline, then if you lowered the overall ratio, youd have to make one more shift.
 

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Discussion Starter #4
> Actually, I think it is the driveshaft will
> rotate 3.9 times for every 1 revolution of
> the drive wheel. I'm not an expert or
> anything myself, but I think this is right.

Oh yeah....thats right...my mind was just thinking backwards. didnt get much sleep.

> You would get better acceleration but have
> to shift sooner. If your car barley gets to
> 60 in 2nd before loosing power, getting
> redline, then if you lowered the overall
> ratio, youd have to make one more shift.
 

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Think about riding a ten speed bike. You line up next to your twin brother, both of you on your identical ten speed bikes. Each of you are just as strong as the other. You start the race in 5th gear and your twin starts the race in 10th gear. You race for 20 ft, who do ya think will win? You will, although eventually he will be able to pass you on the top end you will be able to out accelerate him. The gear in your diff is the same concept of why you will accelerate faster with the 3.9 as opposed to the 3.54. It should feel like an instant 30 hp gain!

> Actually, I think it is the driveshaft will
> rotate 3.9 times for every 1 revolution of
> the drive wheel. I'm not an expert or
> anything myself, but I think this is right.

> You would get better acceleration but have
> to shift sooner. If your car barley gets to
> 60 in 2nd before loosing power, getting
> redline, then if you lowered the overall
> ratio, youd have to make one more shift.
 

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Discussion Starter #6
30hp gain! Just say here I come 3.9!

> Think about riding a ten speed bike. You
> line up next to your twin brother, both of
> you on your identical ten speed bikes. Each
> of you are just as strong as the other. You
> start the race in 5th gear and your twin
> starts the race in 10th gear. You race for
> 20 ft, who do ya think will win? You will,
> although eventually he will be able to pass
> you on the top end you will be able to out
> accelerate him. The gear in your diff is the
> same concept of why you will accelerate
> faster with the 3.9 as opposed to the 3.54.
> It should feel like an instant 30 hp gain!

thanks alot the bicycle thing made alot of sence to me. I think I will go with the 3.9!
Z ya Jeff
 

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In what year did the 3.9 come in? Is it just limited slip? and are the rear diffs interchangable say into an 83zx?
web
> Think about riding a ten speed bike. You
> line up next to your twin brother, both of
> you on your identical ten speed bikes. Each
> of you are just as strong as the other. You
> start the race in 5th gear and your twin
> starts the race in 10th gear. You race for
> 20 ft, who do ya think will win? You will,
> although eventually he will be able to pass
> you on the top end you will be able to out
> accelerate him. The gear in your diff is the
> same concept of why you will accelerate
> faster with the 3.9 as opposed to the 3.54.
> It should feel like an instant 30 hp gain!
 

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Discussion Starter #9
> In what year did the 3.9 come in? Is it just
> limited slip? and are the rear diffs
> interchangable say into an 83zx?
> web
Web- they are not Limited Slip. The 3.9 came in the non turbo Zx Cars so if you have a na ZX you have the 3.9 diff. My 77 5 speed is geared for the 3.5 so if I switch it will be very noticable however, the ZX 5 speed is geard differently then my 77z tranny so you need something more like a 4.11 Diff.-Travis 77z
 

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thanks travis. I have been spending to much time around a friend that has a jeep, he is also talking about changing the gears. I hear to much stuff from friends everyday to keep everything straight.
web
> Web- they are not Limited Slip. The 3.9 came
> in the non turbo Zx Cars so if you have a na
> ZX you have the 3.9 diff. My 77 5 speed is
> geared for the 3.5 so if I switch it will be
> very noticable however, the ZX 5 speed is
> geard differently then my 77z tranny so you
> need something more like a 4.11 Diff.-Travis
> 77z
 

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Discussion Starter #11
what is the ratio in a '70 240Z?

> I have a 1970 240Z with the stock rear end. Up front, I have a 280 engine with the SU carbs and a nice 5 speed tranny. Would a gear change make a difference for me?
 

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Re: 30hp gain! Just say here I come 3.9!

> With a 5 speed and 3.9 gears, what are the
> revs at 60mph.
The actual number depends on the 5-speed gear ratios and the size of the wheels and tires you are using.

A [very] rough estimate: about 2850rpm
 

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Re: what is the ratio in a '70 240Z?

I belive the early Zs came with a 3.9 rear. Unfortunately, it's real easy to beat up those rears.

The gear ratios depend on what you want to do with your car. If you want top speed or less engine sound look for the lower ratio number. If you want quick acceleration off the line, the high numbers are what you want.

Datsun engineered the early Zs for quick acceleration. When Z-car owners wanted a quieter ride and the feds wanted less pollution the higher gear ratios (low numbers) were selected.
 
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