4 speed stick shift diesel

[240ZD]

Hi all, I thought I should share with you the results of my 4-speed manual transmission swap.

When I lost my mind and bolted an LD28 into my 240 Z five years ago, I was under the impression that a really low rpm range would necessitate having a 5th gear. So I also stuck a 280b 5-speed transmission in there as well, since the stock 4-speed was having some bad issues.

Because the stock differential was a 3.53:1, the "close-ratio" gears of the 280b ran REALLY tall. This made driving the car around the city a rough go, so I installed the R200 3.90:1 differential that the 5-speed was designed to match.

http://datsunzgarage.com/trans/index.htm

This was a great setup until a turbocharger was added about 2 years back. The new boost dynamic made the gear ratios seem way too short, and it took some unpleasantly aggressive acceleration to get the boost up before the next shift until 4th gear, or just a burst in 3rd. I'd had it with this nonsense.

SO, I got lucky and picked up a 1973 4-speed, and it's accompanying 3.53:1 R180 differential for only $60. These were installed and filled with Lucas full-synthetic gear oil.

The new tall legs of this setup are perfect. First gear is no longer just for pulling stumps. Second gear is actually useful now, and is reasonably capable of getting the turbo excited. Third gear is good for any city speed, from burbling along at 20 mph to forcibly bending one's neck from there all the way up through 60, and with no let-up in power....actually, it just rapidly pulls harder all the way to the rev limiter, boost still rising. Shifting in to 4th gear at that point just rockets the speedometer up relentlessly through 100 mph. 4th gear is also fine all the way down to very low speeds, such an 35 with a deep exhaust lope fit for a king.

This setup is much more fun and easy to drive than the 5 speed, resulting in fewer yet more effective shifts and really takes advantage of the flat, turbo-boosted power curve and conspicuous torque of the LD28T.

Highway performance is improved, as before 4th gear was too short above 70 mph, but 5th gear constantly compelled me to do like 90 or it felt too strained. It was actually a bit difficult to make the car go as slow as 80 in 5th on the highway without annoyingly light pedal pressure. Now, 4th gear provides comfortable highway cruising at any reasonable speed and rpm, excellent gas mileage, and the joy of having the shifter back (so you can rest your hand on it and feel the transmission working)..... all with acceleration that a Viking would find insensitive.

There you have it. I would love to see someone turbocharge their diesel maxima, perhaps lighten it up a bit, and then put a 4 speed in it, to confirm these findings.

[rlaggren]

Thanks for the report.

I think the weight of the Max would almost certainly change the optimum ratios. I'd guess the rear end should be shorter (higher ratio). And five gears might actually be useful then - or not. But it's a heavier car and less aerodynamic.

I can't recall. Did you make fuel mods to tie fueling to boost when you installed the turbo or are you running with the stock IP ?

Rufus

[dieseldorf]

You could have kept the 5-speed in because all the gears from 1-4 are the same on all gas tranies, 4th gear is 1:1 anyway. The only different are the diesel and pickups trannies.

[240ZD]

@ Rufus,

Yes, I plumbed a vacuum line from the intake into a barb I installed into the altitude-diaphragm "bolt" in the back of the IP, a mod lifted from Teknomage, another turbo diesel Z owner in this site, haha. As the boost goes up, it pushes on a metal diaphragm designed to raise or lower the volume of delivered fuel to compensate for high altitudes. This tricks the IP into thinking that it is traveling below sea level, which increases the fuel flow in a nice curve that follows the increased pressure and air density.

Here is teknomage's diaphragm mod:

http://nissandiesel.dyndns.org/viewtopic.php?t=2214

What I wonder is if a vacuum under no-boost conditions actually makes the IP think it's at a higher altitude, and reduce the overall fuel volume curve when you back off the throttle. If so, this effect could help account for the large increase in fuel mileage and reduced black smoke reported by those who turbocharge, in addition to the more complete combustion you get with turbo pressure.

Other than that, I did give the smoke screw just a tiny little tweak above stock, because it makes the car pull a lot harder during rapid acceleration. I once had the screw turned about 270 degrees clockwise from stock, and I'm sure I could have blown the "Home Depot Racing" turbo-to-intake pipe right off the engine under max boost. That being said, leaving the fuel screw at the stock position still delivers a really nice turbo effect.

[240ZD]

@ Dieseldorf,

Fourth gear being 1:1 is actually the only ratio the different Z transmissions share.

This chart shows the differences:

http://datsunzgarage.com/trans/index.htm

The small differences in gear ratios make a big difference by the time the rubber pushes on the asphalt. I read that the Mazdaspeed Miata's 6-speed gear ratios are tuned down to the thousandth for perfect shifts.

[rlaggren]

You reported such good things I figured you might have connected up the fueling. IIRC it needs an 83 IP to get the aneroid; perfect use for it! <g>

Rufus