Read this if you want to change your final drive!


_JT

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Choosing your final drive

Lately a lot of questions have been asked about swapping your final drive. It's an easy and cheap way of gaining virtual whp and wtq that transform into faster acceleration. Let's sum it up.

+ Gears shorter - much extra virtual whp/wtq = lots of gain in acceleration for the money
+ In the city you probably won't notice any increase in fuel consumption since lighter load of engine is needed for same acceleration

- Higher rpm on the motorway means more noise
- Higher rpm on the motorway means higher fuel consumption
- With a very short final drive your top speed is severly limited

Here you can see an example of how much is gained with different gears and final drives.

There are different options. Honda B-series boxes contain 4.4 and 4.785 final drives. The latter one is found in the 98-01 spec ITR S80 boxes, the rest has 4.4 (correct me if I'm wrong). The two most common options are the OEM 4.875 and after market 4.928 final drive.

The 4.928
Most people agree that if you drive on motorways regularly the 4.928 is too short. Your engine will rotate at 3850rpm at 60mph. That's a lot. Also, if you drive on tracks where you easily run over 120mph, this will be too short as the gear limits your top speed at 140mph. You could overcome these problems by also installing a 5th gear that is a bit longer. The disadvantage of that is that the gap between 4th and 5th gear will be bigger, but you do get 4.928 acceleration in 1st-4th gear and stock fuel economy and noise on the motorway.

The 4.785
This is the best compromise. It will yield you almost 9% extra virtual torque/power in all gears over the whole range, but compared to the 4.928 your top speed is higher (4mph, which isn't that big but just a bit extra) and most important for driveability, rpm at 60mph is 400rpm lower. This is the best easy option if you drive a lot on the motorway.

98-01 S80 box
Another option would be to just get a 98-01 S80 box if you don't already have that. Given the same wheel/tire size, 4th and 5th gear is a small bit shorter but 1st, 2nd and 3rd gear are more noticable shorter. You don't have to open up your box, just change the box. This option has the disadvantage that the gap between 3rd and 4th gear will become bigger but you have noticably faster acceleration in 1st - 3rd gear while maintaining driveability in 4th and 5th gear.

In short:

Of course you can build your own box with custom gears etc but that is way more costly and if you already know how to do that and how to choose the parts that suit your needs, you won't be reading this topic ;)

Check out my flowchart if you don't know what to choose!

If there is anything missing just shout and I can add it to the topicstart. What I have tried is to make it easy to choose a different final drive for people that do not know too much about it and/or have trouble choosing.
 

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Richard.Siddle

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Thanks, very helpful. But can you recommend anybody who make make or supply a 3.1 Final Drive? Many thanks
 




_JT

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Tnx peeps.

@Richard: I don't know, sorry.
 


troopa9

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I have a y21 box from an sir2 del sol I believe. ITS SHORT. Any idea what final drive that is running?
I have friends with y21 boxes from uk cars and they are not that short. Ive raced other b-series and when I punch it in 3rd they can still be in 2nd!
I believe this is zapping my power because i have a b20vtec with a fair bit more torque than the average b. and I dont get to use it!
What box you guys recommend? Mb6 maybe??
 


_JT

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Short??? MB6 is longer than other b-series boxes and not shorter. And a shorter box makes you faster, doesn't cost you power. What are the max speed in all your gears?
 



chrissyej9

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heads up on the diagram u need another arrow between the race track question and the 4.785fd answer!lol

are all the fds interchangeable?? i.e will a 4.785fd from a s80 fit the mb6 gearbox? the reason i ask this is the mb6 gearboxes are alot cheaper than the s80...

good info above there too, thanks!!
 


elchunk

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think you need to swap first gear and not sure if the casing will allow the teg fd to fit in without modification???
 


chrissyej9

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ahh whole handlin then? lookin at s4c an s80 boxes but ther still big money!!
 



DOSvtec

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great write up, the diagram with the arrows is very useful if you dont want to read alot about final drives :nice:
 



firebass

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i just drive my b18c and from 1st to 4th gear is really fast the limiter in 4th gear is 140 klm !!!! 5th drive is very slow and its only for trip driving !!!!! well how does this sound ??? i didnt change the transimission probably the previous owner !!!
 


_JT

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Sounds like all the gears have been replaced by shorter except for 5th gear.
 


SPEED-DEMON

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Does a shorter FD gurantee better acceleration tho. I have gone against really short geared cars and I am on the stock 4.4 and will pull on them in 3rd and 4th. I am normally coming from half a car behind and end up in front. I think while they upshift and I am still in the lower gear I gain mega ground and pass them. And plus I can stay in 4th alot longer than they can and really open it out then. So is this just down to ratios or maybe i have a strong engine. Any help here appreciated.
I am pondering the idea of going to a 4.92 from the 4.4....but dont want it to hamper accelration, based on what I said above
 


elchunk

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sounds like you've got a good fit car then as my old civic's acceleration was vastly improved with the 4.92 FD,
4.92 = mega acceleration at the cost to high revs at 70-80 mph. if you do alot of long journeys i would avoid the 4.92! the 4.78 is ideal for me, as it gives the improved acceleration whilst still maintaining a decent cruising rpm.
 


H1ONDA EK9

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With a 4.78 what kind of acceleration gains are we talking about in seconds say in terms of 0-60 or 1/4 mile dash for instance?? :)
 


eg4lawd

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i hav a y21 an am putin in da 4.7 final drive but when it all built up da gear cluster is jammin against the final drive anyone knw if ther is a difference in the gear cluster bearings.. maybe a taller bearin to give it more room
 


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