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Rear end ratio recommendation


Cruzzar

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  1. I have recently purchased a stock 1975 280Z and would like to replace the 4 speed with a 5 speed from a 240sx with the .76 5th gear final drive ratio. With the overdrive it would seem to me that I should replace  the rear end gears with something lower such as 3.70 or 3.90. Any recommendations? I have done some of the tire diameter/gear ration calculations and it appears that I can get close to about 2600 rpm at 65 mph with the 3.90s. Is this about right ? If I go with the 3.70s the rpms would be down to about 2356 rpm at 65mph, would this be overdriving the stock l28 too much?

 

 

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Compare 1st gear ratios also, to see if 1st gear will even be usable.

 

Also, 1975 has an odd flange bolt size and pattern on the diff and propeller shaft.  So, if you get a different diff you'll need the propeller shaft flange also.

 

And the 240SX is not a direct swap, in case you didn't know.  http://zhome.com/ZCMnL/tech/240SX5spd/transmission.htm

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5 hours ago, Cruzzar said:
  1. I have recently purchased a stock 1975 280Z and would like to replace the 4 speed with a 5 speed from a 240sx with the .76 5th gear final drive ratio. With the overdrive it would seem to me that I should replace  the rear end gears with something lower such as 3.70 or 3.90. Any recommendations? I have done some of the tire diameter/gear ration calculations and it appears that I can get close to about 2600 rpm at 65 mph with the 3.90s. Is this about right ? If I go with the 3.70s the rpms would be down to about 2356 rpm at 65mph, would this be overdriving the stock l28 too much?

 

 

The automatics - some of them- had 3.54 gears and it was 3500rpm at 70. These engines like a cruise at 3k , it won’t hurt it . 
I have very close to your scenario with a 3.90 gears from a subbie. If you have a little higher rpm cam it would be nice . 
3.70 is a nice compromise , but not the easiest gear set to find . 
There’s a drone zone for some Z’s I’m that 2500 rpm range 

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I already have a 240sx trans and a front bell from a 4 speed and have read extensively on the machine work required to mate the early bell with the FS5W71c trans. I have a mill and don't anticipate any problems with the work required.

It appears that the later model Zs with the close ratio trans (80-83) came with the 3.90 ratio. Those trans came with either a 3.3 (first gear) and  .77 fifth gear or a 3.06 first and .745 fifth gear. If anybody has either or has experience with both of the ratio transmissions and have run the 3.9 or even the 3.7 ratios, I would like their impression regarding the engine's ability to pull the 5 th gear. Is the stock engine lumbering at 65 or is it buzzing and could use the 3.70 gears. 

I will keep the engine stock for now, if and when I do something with the engine I would probably do some port work, put the flat top pistons in it and match the ports. I will probably keep the stock injection but who knows may be switch to something more current with regards to fuel injection. I will not be putting on a turbo since that requires another whole layer of parts.

The car will be used of special occasion driving, short vacations, driving around town, etc. Won't be tracking it, no drag racing, not slamming it or putting flares on it.

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The later ZX's matched the 3.062 1st with the 3.9.  The 280Z 5 speed cars and early ZX's, used a 3.321 and a 3.54 rear.  Your 75 4 speed would have a 3.54.  79 ZX's have a wide range of diff ratios, based on the model and the transmission.  Everything from 3.36 to 3.9.  79 is an odd year for options.

 

Good luck.  Don't forget the funky diff pattern on your 75.

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It's not the R200 itself.  It's the 1975 model R200.  I think that 1975 was the first year for the R200, for the Z cars for sure, and for some reason Nissan changed the pinion flange pattern the very next year and kept it that way for many years.  So 1975 is an oddball.

 

I can't explain why, but here's a page that shows the variation and another that shows which ones are interchangeable.  The link you posted was for an R180.  I don't think that you have an R180 in your car.  R180's were used for automatic transmission cars.

 

https://www.thezstore.com/page/TZS/PROD/classic22e01/22-1003

 

https://www.thezstore.com/page/TZS/CTGY/classic22e01

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NewZed, Thanks for the info. I have the R200 not the R180, I just didn't pay attention to what rear end chart application I was looking at. Since I will have to shorten a driveshaft to accommodate the 200sx trans (about 2 inches) I will get another driveshaft that will fit the new rear end and keep the original driveshaft with the original 4 speed and original 3.54 rear end. Learn something new every day, by the time I get to 90 I should be pretty smart but probably won't be able to remember it. 

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It’s hard to figure your goals . The L motor can handle about any rpm range . The torque allows for lugging along just fine and cruise at 3500 also . 3.90 is nice for getting up to speed quickly . These cars are 50 years old and keeping up with fast highway speed there is other considerations besides just rpms. Noise, stability , aerodynamics. 
The 71C would be worth the effort because it’s a better trans and rebuilt parts are more plentiful . It’s a stronger transmission period. 
 

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My inclination is to go with the 3.90 gears (along with the 240sx transmission's .76 fifth gear). I will lower my rpms at 65 mph compared to the original 280s 4 speed and 3.54 ratio at 65 mph. The four speed's first gear was 3.59. The new 5 speed's first gear is 3.3, so going with the 3.90 rear gears will help a little to lengthen out the first gear starts. I just didn't want to replace the rear gears with 3.90s when a better "fit" would have been going with the 3.70s. Comments anyone?????

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11 minutes ago, Cruzzar said:

The four speed's first gear was 3.59. The new 5 speed's first gear is 3.3, so going with the 3.90 rear gears will help a little to lengthen out the first gear starts

 

3.5 is the early 240Z with a 3.36 R180 (I think).  Here's your 75 four speed.

image.png.57812e822e767c30be6bac75cce2a401.png

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I have a 3.9 rear with an '82 zx transmission. 3k rpm is right about 70 and I find that it's actually a very happy spot to cruise in. For an NA engine that hasn't had a ton of work done aside from megasquirt, I'd say the 1st-3rd gears are actually in great spots, but I can imagine they might feel too short in a turbo engine. 

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8 hours ago, Cruzzar said:

Madkaw. Does Megasquirt have the capabilities to run a knox sensor to back off the timing or does it need to with the aluminum head at 10.5 compression?

I have a knock sensor module installed , but have yet to use it . My max CR is 10.5:1 and we have 93 octane here . I have a N42 block so there is no knock sensor hole in the block , so I need to come up with an alternative . Currently running 35 timing , but have ran as much as 37 degrees . 

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