3.9 or 4.1 that is the question?

chrisearl88 · Postby **chrisearl88** » Fri Dec 21, 2012 7:01 pm

Got an nb8b, cai, zorst. Its got a 3.6 open diff and looking to upgrade. Anybody driving both and could provide any insight into their recommendation.

Ive calculated that at 100kph a 3.9 is an extra 300, revs, while a 4.1 is 500. Obviously just lazy around at 2500rpm the revs will only jump by 200 and 333 respectively.

Any ideas on likely 0-100 improvements from someone who has done the swap?

lightyear · Postby **lightyear** » Fri Dec 21, 2012 7:34 pm

4.1 is the go if you want better performance and don't do too many country k's. 4.3 is what the track guys use.
3.9 would still be better than the 3.6.

Apu · Postby **Apu** » Fri Dec 21, 2012 9:17 pm

I have a 4.1 with LSD on my NB8C and it is the best bang for buck mid you can do. I can actually accelerate in 6th now, and it's probably only revving about 300-400 rpm more.

You will need to change your speed sensor, and funnily enough, a 3.9 one will give you 100% reading if you run 195/50 15 tyres. GPS tested!

chrisearl88 · Postby **chrisearl88** » Fri Dec 21, 2012 10:32 pm

thanks mate thats what i was looking for.

any idea on $$ for new speed sensor?

Did you notice a change in the fuel economy?

Apu · Postby **Apu** » Sat Dec 22, 2012 6:37 am

The sensor didn't cost much - I got it from MX5 Plus and from memory it was less than $100.

Fuel economy...I don't monitor it. It's not something you should think about it you want to modify your car, haha!

Mr Morlock · Postby **Mr Morlock** » Sat Dec 22, 2012 9:46 am

so what is the best bang for bucks in $$ terms - all up cost of replacement ( new or 2nd hand) and the cost of removal and refit?

pcmx5 · Postby **pcmx5** » Sat Dec 22, 2012 10:18 am

I ran a 6 speed with 4.1 in my LE and thought it was a bit short especially first; have a 3.9 and love it and it is the way i would go for a fast road car.

3.9 6 speed = accurate speedo as stated.

Best bang for $$ in all MX's after Nb8a is a diff ratio change,did the 4.1 in my NC and loved that too.

Peter.

Postby **Boyracer** » Mon Dec 24, 2012 6:52 am

Best all round diff with the 6 speed is the 3.9, I also think a 4.1 makes 1st gear too short.

Will be easier to find a 3.9 Torsen because that's what 99% of the NB imports come with.

chrisearl88 · Postby **chrisearl88** » Mon Dec 24, 2012 4:07 pm

Yea i think you've all convinced me. I really hate driving utes with short 1st so sounds like a 3.9 is the plan.

thanks for everyones input.

just downloaded a gps app for my galaxy that does 0-100 and 1/4 mile times. so i will do a few before and afters and post my results for future reference.

deviant · Postby **deviant** » Mon Dec 24, 2012 10:44 pm

I have a 3.9 in my NB8B, perfect for a compromise between fast road and track work. The in gear acceleration is improved but not to the point it drones and makes a racket just cruising. To be honest you won't notice a huge difference the instant you first drive it with the 3.9 but spend a bit of time cruising around, accelerating through the gears and going for a hard run and you will start to feel that the gearing is just that bit more 'right' for the car.

Mr Morlock wrote:so what is the best bang for bucks in $$ terms - all up cost of replacement ( new or 2nd hand) and the cost of removal and refit?

Depends how much you can do yourself. I had access to a torsen with no crownwheel on it. Finding a crownwheel is easy but the local transmission places wanted silly money and a long lead time to install the diff centre and set it up. If I could not have removed the diff myself it would have been hundreds more in labour for them to do the rest of the job. Reading around this forum it seems you chaps in the Eastern states can get the job done much cheaper.

I opted to get on to MX5+ who probably have a container load of the things. They sent me a used 3.9 complete in the housing for $800. Add another $30 for the oil seals and $15 or so for fluid. The oil seals are easy to do, the olds ones need levering out and the well lubricated new ones pressed in...Pro tip here, the old seals fit perfectly over the new ones and you can gently mallet them in place. The old seals will protect the new ones.

I am an okay mechanic with plenty of tools and I had a few read throughs of peoples threads and online guides. To be honest it is not a difficult job to do, I spread it out over about 8 hours over two days....there is a lot to do but it all comes out in an order and it is all fairly logical. It is heavy and awkward so you will find a second pair of hands useful...one person on the jack to lower / raise the diff while one keeps it lined up or levers the PPF out of the way.

I am not sure if MX5+ inspect the diffs they send out or not but mine is absolutely fine. No bad noises or leaks. I suspect it has low KMs on it or was treated well as it will still scuff the inside wheel on a very tight and low speed turn.