Hi all,

 

I know its been done to death but I want to know what ratio to go for 3.89 or 4.11?

 

I'm having a custom diff made which is a combination of...wait for it! A commodore/r31 centre housing (due to diff gears), ke70 axle housings, ef/el falcon lsd centre, r31 crown and pinion ratio's, ef/el axles shortened and re-splined and ef/el brakes (5 stud).

 

I'm going to be running a 4agze, jdm big port rwd inlet, sc14, T50 (till it lets me know I shouldn't be) and some other goodies!

 

I want good highway cruise rpm (so not screaming its guts out) but also want good acceleration, in saying that I don't want 1st gear to be rendered useless cos its too short, so I used gear calculator and the difference in RPM between the 4.11 and 3.89 is about 150RPM at 100km/h (obviously 150rpm lower for 3.89).

Would this minimal difference be so noticeable if using it as a street car and for the odd hill climb? if not would it be 4.11 all the way!?

Its 150rpm... Not too much difference really. That's a weird setup you are running as well, what is it exactly? like a ke70 diff housing modded to suit the center you are putting in? How much is this costing you? Or you made it yourself? My hilux diff I used cost me $400 for the diff with 4.3 gears and used a jig to put the mounts on and just welded the center so until that brakes then ill just get a full spool or maybe a LSD.

yeah I know its not much but it can make a difference, I know I could have gone T series or hilux or something else but I wanted lsd with easily available ratio's thats not going to break and 4 wheel disc that matched front to rear of which I could get parts cheap and easily so that's why I went for this set up, its going to cost around 1500-2000 once I have finished buying all the parts and paid for the engineering (welding & machining) side of things. I'm getting Graham at Boss differentials to do the work as he does lots for drag, drift & street cars.

4.11 or 4.3 would be winner.

Thats pretty expensive, my hilux diff was $400 plus my time to weld on mounts and $250 for rear disk brake setup that was for caliper brackets, new rotors and calipers all ma70 supra brakes, huge things! LSD are redily available for hilux diff and and alot of different ratios as well.

If you're using it on the highway go the 3.9

 

You'll never use top in competition, so gearbox ratios will be more important anyway.

 

I found tyre size made a much bigger difference than diff ratio, fat 14" geared me up more than changing the 4.1 for a 3.9 for this trip up to Cairns.

3.9 is ideal, I ran that in my ae71 when I converted to a manual and the difference was considerably noticable as opposed to the 3 speed auto and 4.11 diff gears. Still good top end speed and response and fuel economy.

Cool thanks Ben-89, i went for the 3.89 as after talking with Graham he swayed me to this cos I'll be doing a few little trips in it I think.

 

Thanks for the input guys

Hey mate,

As stated above a 3.9 ratio would be more suited to your setup and would give you half decent highway cruising rpm. In all honestly with the torque throughout the rev range of a 4agze there is going to be stuff all difference in acceleration times between a 3.9 and 4.11. However this is just my 2 cents.

 

Cheers,

Justin

Thanks Justin, i thought that would be the case, so keen to get this finished and in the car along with the motor of course!

But not sure how long that's going to take...

Interesting. I found the ratios on a t50 sucked hard with anything less than a 4.77. When I had 4.11 the rev drop from second to third used to just annoy me. However as you are using it for highway as well, I'd suggest 4.1 over 3.9, as around town I think the 3.9 would chew.