Dropping this: https://shop.neatgearboxes.com.au/pr...-0-6061-ratio/
into both my cars and 2 other cars.

The maffs says it's at the real edge of "it's not going to work" and "might just work". If it's at the wrong side of the line then dropping a 5.1 diff will solve it AND give some torque bonus in the other gears.


Comparing it to the slushboxed 6spd with a much taller 6th than my... 7th gear, and a taller diff, I can't help but think it might just work. I regularly cruise at 110kmh on national roads and 140kmh on the highway and starting to lose some hearing from the engine doing cartwheels at stock gearing. Not hoping for any fuel economy benefits, at worst not a noticeable penalty.

So I went to town and started learning on how to tear apart the box to swap the needed pinions. I'd be doing it 4 times at least so no point in paying someone else to do this.
Got to the final stretch but got stuck on the output shaft pinion for 6th. Seems so hardly pressed I can't get it out without a hydraulic press, but for that I really need to take the whole thing apart down to the last bolt. Which is another full day of fun.



Only took me 3 hours to get here so if I solve this snag + how am I to get the new parts in if the old ones were so stuck in there, then it might just work. Taking the tail piece out is easy, taking the core out not so much.


After much faff I figured a way of removing the smaller 6th gear pinion out without taking apart the whole gearbox/gear set. I used 2 M10X1.5 50cm long threaded rods and some gear puller bits. It took some effort but it works. Unfortunately the threaded rods also get some damage but they are cheap so I can afford to use 2-3 sets of rods for 4 gearboxes.

 

After that the next obvious question is how to press in the new/smaller 6th gear pinion. I was worried that merely pressing the pinion against the gearbox case or the input shaft would be a bad idea so I took apart the gear train on this sacrificial gearbox to see what is what.



Bottom left is the input - engine side, bottom right is output - driveshaft side. 6th gear pinions are removed in this picture.


The input shaft is stubby and it is in the same axis as the output shaft, so I figured I can press against it. I had to validate this though.



I didn't want to crush anything on the good gearboxes so I had to see everything with this one.
I set the new pinion(well, still the old stock one for this gearbox as I was still learning) and pressed against the main shaft. The hydraulic press went up to 5 tons - quite a lot - and the pinion slid back into place.



I also wondered if to press against the transmission case(s), but then I feared that the output shaft bearing might get pushed out, or a snap ring would chip a groove somewhere so... not doing that. Simply press against the main shaft, it's perfectly safe.

 

Installed these longer 6th gear pinions on a good gearbox and here are the results:

old gearset(left) and new(right)

new gears in place


cruising rpms way down low

However an unexpected issue came about: ignition timing is sometimes pulled back 1-2-3 degrees by knock control. This is because this lower rpm/higher load combo sometimes pushes engine load above 50%, where the knock control is set to be enabled. This wasn't a thing with the stock gearset as you'd never get to such high loads merely cruising. I am still investigating this but I don't expect it to be a big deal. I will probably pull 3 degrees of timing in these areas and be done with it. Even 100% stock I've seen knock control be quite trigger happy and pulling timing a lot. Regardless of 95 or 100 RON E10 fuels...

highest recorded knock retard values in degrees BTDC


average knock retard


cruising AFRs all check out

I've also noticed fuel economy has improved by 1L/100km which is a noticeable 8-10% decrease over stock gearing at same speed and over a quite broad interval - 110 to 140kmh is now far more efficient. The downside is that in between 75 to 95kmh you're in between gears. 5th is too short, 6th is too tall. So... either I'm going or I'm not.

One more thing: cruise control strategy needed a tweak in order to keep car speed as sharply as it used to. Leaving this table as it was would cause the speed to dip or overshoot quite easily; a mere +50% everywhere has returned the crisp cruise control performance you'd expect by an OEM.


That's about it.

 

You don't have the proper gear puller for it, or you couldn’t get it off with that tool?

Thanks for the info/link, ordered the 3.63 R&P
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It's beyond generic gear pullers. The manual says I have to use a hydraulic press, but for that I have to take everything apart and I'd really avoid that if possible.

 

Yes, you remove the gears fron the shafts, you will need a hydraulic press and the correct adapters to do so. The hear set assembly will also have to be removed from the case. A few years back when I was a Toyota technician, I went to school to learn how to rebuild manual transmissions. Here are a couple of pictures that I took during the class and during the process that we rebuilt a manual transmission. This will give you an idea of what's involved in doing so and how far you will have to tear the transmission down. All the way is the correct answer:

 

I didn’t recall having to disassemble the main trans housing to get it off, but my memory is likely failing given how long ago it was since last reviewing the rebuild manual.
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Updated initial post on how to remove and reinstall the smaller 6th gear pinion without removing anything past the gearbox tail case. Excellent news for this mod.

 

Thank you and congrats, that was exactly the tool that I was referring to for the 6th gear removal from the mainshaft.

I’d probably recommend the 3.63 R&P over the 6th gear change for a turbo engine though, even at the higher cost. With the original 6th gear ratio a 70 mph cruise speed will be around 2850 rpm and it will also provide tire spin control in the lower gears. The 0.606 6th gear with the OE 4.44 R&P at 70 mph will be lower at 2550 rpm. That’s getting way down there in lugging range and there’s not much recourse for lower speeds, say 55 mph cruise, with so much gap from 5th gear.
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A taller diff affects all gears. Whatever little acceleration you get in gears 1-6 is reduced further. Modding only the 6th gear only affects that gear which I only use on the highway. Should it be too tall, I can always replace the diff R&P with a shorter one, and it doesn't have to be now, later, someday whenever I want. Bonus torque in the lower gears as well without affecting highway gearing.

 

Typically NA engine gearing isn’t needed or preferred for a turbo engine on street tires, but I’m probably overlooking that it’s not making any where near as much torque/hp as a strong 13B REW conversion.

 

nicer finish and appearance than the low cost 5.125 R&P being sold on ebay .
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Thought mistakenly that the ebay seller and the one above were the same, thanks for clarifying.

 

Updated initial post(s) with final results.

 

The update is specifically in post #3.
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