Datsun 4speeds

[Jesse OBrien]

It seems like everyone treats the 4speed like a red-headed step-child because it doesn't have overdrive, but I'm trying to figure out if there's more to it. Are the 4speeds especially weak, or do they have especially epic failures?

 

I'm just finishing up an l28e to carb conversion with a LIGHT refresh (it doesn't count for much, honestly ... I hope to end up with ~150hp if I'm lucky), and am looking at transmission options. The goal is to daily drive the car and participate in hillclimb races, and I can source 4speeds very cheaply (or free). If they're reasonably robust (I don't have any plans for big power adders, if things go VERY well I'll end up at 175hp and ~175ft/lbs).

 

I can sort of compensate for the lack of overdrive with a wider differential/final (I prefer the wider differential, since it keeps the stock transmission isolated and eases quick swaps). I could also go to a taller tire to get a little more gearing, but I have reservations so I'm asking for some feedback from you fine folks.

 

With this low-output engine, it won't be a blistering-fast car off the line, but I'd like to keep it responsive and fun to drive. I don't want to have to downshift to 3rd to pass someone on the highway, and don't want to wait 5 minutes to get out of 1st/2nd.

 

The way I'm looking at it is that my friends who race Porsche 911's re-gear their 4speeds rather than replacing them outright. They're making a bit more power (200-250hp) but I'm not sure that's a vast enough difference to make a re-geared Datsun un-driveable.

 

I'm just looking for feedback and others' experience. Is the 4speed worth working with, and if so, WHICH 4speeds are worth working with? At what power level do they become unstable hand-grenades? Are they really only worthwhile in vintage racing classes or other applications where the rules dictate that people need to deal with them?

[NewZed]

Up to somewhere in 1980 the 5 speed has the same gear set in 1 - 4 as the 75 - 79 4 speed.  5th is an engineered add on.  So it comes down to your desired freeway RPM or top speed, if you're considering the early 5 speed.

 

If you want the more evenly spaced 1 - 4 ratios, they only come in the 5 speed anyway.  That's the other reason people swap the later 5 speed in.

 

First gear is different between the early and late 4 and 5 speeds also, so the rear ratio can be important.  The very early 4 speeds, 72-74, have a much lower first gear to match the 3.36 rear ratio.  All of the options have a 1:1 4th gear ratio.

 

Talking about the transmissions available on the factory Z's and ZX's, not racing options.  Ratios are listed around the internet and in the FSM's.

 

 

Coincidentally, read Post #73 in this thread - http://www.classiczcars.com/forums/open-zcar-discussion/40249-hls30-01222-project-3.html

Edited April 8, 2013 by NewZed

[Jesse OBrien]

Thanks, that's right in line with what I was looking for. Running at 4k rpm @70mph is pushing some kind of limit. If I could stretch that down to 3k I'd be happy. I'm going to have to do some math to lay out what my gearing currently is, and what I really need to make a 4speed feasible. 

 

If I go with 5speed, it'd probably be out of a 240sx, since they're really plentiful around here, and I already know they're 'strong enough' but I wanted to explore the 4speed option first. Since my car isn't even running yet, I'm really just worried about highway speeds and 1:1. As I said, the only solutions I can see for that are final drive ratio or a wider differential, but I'm not convinced I'm increasing power enough to still have a driveable/raceable car.

 

As far as racing goes, I suppose you could take that term with a grain of salt. Relatively low power and short runs (a typical hillclimb run around here is less than 2 minutes long) mean that there won't be excessive load or heat, but I'd rather not have to replace a transmission more than once each season.

[cpark_12]

I just finished up on a 240sx 5 speed swap into my 83.  I had the original 5 speed in it first.  Now, I have a 3.90 rear end, but at 70mph, I'd run about 3500rpm.  That tranny went out and I had a 4 speed out of a 76 laying in the shop, so I swapped it in just to get it back on the road.  At 70mph, I'd run about 4000rpm.  Now I've swapped the 240sx tranny in and I'm running about 2500-2600rpm at 70mph.  Much better imo and I'm getting about 27mpg too.