looking for info f5a auto tranny

MightyBoy tech questions and answers.
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arklan
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ok so iv been googling to find this info and cant find
i wanna know what the gear ratios r in the f5a auto box or what rpm the engine revs at 80k-90 kph
its for a diesel conversion(legal this time)
thanks
Last edited by arklan on Sun Dec 14, 2014 1:37 pm, edited 2 times in total.
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Brayden
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I don't know the exact ratios, but the final drive is roughly equivalent to an F5A manual. So the answer is that at 80-90kms your RPM is about 4500.
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rf4burns
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According to the SS40 manual
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ss40 auto ratio.JPG
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arklan
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ok so lemme see if i have this correct

using 175/70 r13 wheels, the diameter will be 452mm
.452 x 3.14 = 1.419 metres circumference
1000 / 1.419 = 704rpm at 60kph at the wheel or 704 / .6 = 1174rpm at the wheel at 100kph

final gear ratio on r4burns chart is 1 to 1 and the other ratio underneath that is im guessing the diff ratios which i dont really understand that much, but if i use the 4.684 as a multiplier...
4.684 x 1174 = 5501 engine rpm at 100kph which is ouch

from my searching on the forum, 175/70 r13 are the biggest wheels that will fit on a stock mb without scrubbing on full lock
its not an issue for me to use slightly bigger and just not use full lock but i wanna second opinion on my figures before i get excited

at 80kph it would mean 4400 rpm so pretty close to what brayden said
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Brayden
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175/70 R13 tyres have a diameter of 575mm, and you'll be very lucky to fit them in the rear wheel arches. Assuming they would fit, you'd wind up with about 4300RPM@100kph.
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arklan
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thanks for that, i forgot to count for 2 sidewalls in my calculations thanks for putting me on the right track :)
so something like a 500 tall wheel is the max i can go

do u know of any auto boxes that would fit and has a taller final ratio?
i was thinking of using a cvt from a sherpa or justy but they r pretty rare
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Brayden
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My 17's are 545, and they just squeeze in. F8B auto has a taller diff, same kind of difference you get going from F5A to F8B with a manual box.
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arklan
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whats the turning circle like? do yours scrub? and what offset r your rims?

im guessing a f8b auto box is gonna be a real mission to find
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rf4burns
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I have 165/75 13 on the rear these are "just fits" as any wear in the rear shackle bush's will result in scuffing under heavy braking (spirted driving).On the front 155/75 13 is as big as I can go without adjusting the steering stops and limiting the turn.
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Brayden
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arklan wrote:whats the turning circle like? do yours scrub? and what offset r your rims?
Turning circle isn't affected by the wheels, and they don't scrub but the arch lips have been folded flat. Offset is +42.
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Matty
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Do you have an updated build thread anymore Brayden?
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Brayden
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Nope, I haven't really done anything to it over the past 2 years. Moved on to other projects.
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arklan
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last question
is it possible to 'break' the gearbox so it just changes gears at the same rpm every time?
a little 3 speed or something would be better but google isnt my friend at the moment
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Brayden
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If you disconnect the vacuum switch then it will shift to 2nd at 40kph every time, or 60kph if you have your foot to the floor.
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