gearing???

[riot936]

I'm having trouble finding a bolt in 5 speed or auto in my neck of the woods for my 72' 620. But there are ample gm corporate 10 bolts in the boneyards. That rpute I can get one with a posi and unlimited gearing options on the cheap So I'm thinking about narrowing one down and putting it under the truck when I bag it. Which leads me to this question, what gear ratio should I go with that will give me lower highway rpm's without sacrificing all my power.L16 with weber and L20b cam and factory 4 speed ill be running 225/45r17.

[datzenmike]

I'm having trouble finding a bolt in 5 speed or auto in my neck of the woods for my 72' 620.

At first I was "Ah he needs a 5 speed"

 

But there are ample gm corporate 10 bolts in the boneyards. That rpute I can get one with a posi and unlimited gearing options on the cheap So I'm thinking about narrowing one down and putting it under the truck when I bag it. Which leads me to this question, what gear ratio should I go with that will give me lower highway rpm's without sacrificing all my power.L16 with weber and L20b cam and factory 4 speed ill be running 225/45r17.

Then I was like "He wants a rear axle. Did I read that wrong?

 

 

Shortening will be more expensive than finding another differential I think. The L16 doesn't make enough to need LSD (but I admit they are cool) Your '72 will have a 4.375 ratio. A 4.11 or 3.889.. maybe even a 3.70 (if not carrying loads and just a DD). 720 trucks have a variety of ratios.

 

How about a Mustang rear axle? LSD, rear disc brakes, a wide variety of ratios, already narrow. Drill the axle ends for 6 lug so you can keep your rims

[riot936]

You are correct I do need a 5 speed but can't find one around here. I'd much rather hsve one than axke swap. A ford 8.8? That's option too. Its just the gearing that has me thrown for a loop as I want lower rpms on the freeway. Running 3k only doing 60mph is kinda aggrivating. I wouldn't need to redrill as I'm getting custom cut billets that hide the lugs.....altgough it would give a reason to do a 5 lug swap. In the land of 4x4s and houstons race scene I can get damn near any commomn axle for dirt cheap. You happen to know the 4 speeds ratios??

[datzenmike]

Well all 4 speeds are 1 to 1 in 4th (even 5 speeds) but the 5th gear is an overdrive that drops the RPM about 12-16% This would be 2640 @60 with 12% and 2520 @60 with 16%. The 280zx non turbo 5 speed is 25% overdrive or 2250 rms @60.

 

Have a care, as a drop in RPM also means a drop in power output at 60 MPH. The L16 may not make enough power to comfortably cruise @60 without having the throttle too clode to the floor and the secondary open.

[riot936]

Ok I see what you are saying, what do you think would be about middle of the road? Little less rpm but not a huge loss in power? I really nees to find a 5 speed. From what I've read ia that I need one from a 77-79 200sx?? There are to 720s in the local boneyard. But they have z motors.

[datzenmike]

Yeah a 5 speed will (more or less0 not affect your acceleration through 1 to 4 but will reduce high speed cruise RPMs. It's having your cake and eating it too. A differential change will reduce rpms at all speeds and affect all acceleration in every gear.

 

 

The 620 came with two 4 speeds. A 26" shorty and a longer 31.5 71B model so there are two ways to do this...

 

1/ The '77-'89 200sx (first gen.) had a 26" dogleg 5 speed that will bolt to the L16 and is the same length so your driveshaft will fit. You will need to sort out the transmission mount below the shifter. The FS5W63A dogleg is adequate for an L20B so will be ok for an L16. It is not heavy duty so if hauling loads, towing or you have plans for a stronger modified L18 or L20B 'stroker' motor, I'd skip it, it won't stand up to that.

 

2/ The '74 and up 620 4 speeds and the '77 and on 5 speeds are heavy duty and were used in the 240Z and even the early 280zx cars with the six cylinder L motors. Very strong and reliable. Because they were used in a 620, all the parts needed to swap one into the early 620 already exist. The best thing is to find a scrapped 620 with a 5 speed and strip out all the parts needed. Basically the 5 speed, the front driveshaft (the one directly behind it) the cross member under it that holds the rear mount, the shifter and boot.

[riot936]

Sweet I appreciate it. One more quick question what model number are the 5 speeds?

[datzenmike]

The short '77-'79 200sx doglegs are FS5W63A

 

The longer 620 5 speeds are FS5W71B

 

They are also in the following but may have different 123 and overdrive ratios...

'80 720 2wd

'77-'78 280z

'79-'83 280zx (non turbo)

'78-'84 810/Maxima (gas engine)

[Z-train]

what gear ratio should I go with that will give me lower highway rpm's without sacrificing all my power.

No such ratio.Generally speaking,for every 10% increase in power,you can drop the gear ratio accordingly.