I was just wondering what gear end gear ratio you would recommend for a turbo fb? I will be running a series 5 turbo with a bnr stage 2 upgrade. It will be a weekend driver with occasional track. The reason I'm asking is I just purchased a Guru diff from my friend in Australia and he thinks it just has the 84-85 stock ratio in it. In Australia they didn't have the 13b in any of their first gens so I'm thinking it is a 3.07 or 3.09 (weren't the 12a ratio's something like that?) I have a spare gslse rear end so was wondering if I should swap in the se gear (I think it's 4.07 or 4.08 right?) Here's a few pics of my diff.


One of my friends was saying maybe I would like the lower number gear ratio since it will be a high horsepower turbo car. He has a turbo ae86 corolla and says he's boosting when he doesn't want to while crusing at freeway speed because the higher gears keep his revs up where the turbo wants to kick in under cruisin.

Also, I'm thinking since the diff is already set up, maybe I should just leave it be for now so I don't have to mess with getting everthing put back in and lined up right.

 

the non-se rear ends were 3.90 not 3.09 biiiiiiiiig difference i still say use the -se chunk!

 

ah ok, that is a big difference

 

I put in a 3.73 gear in mine, And I am loving the difference from the old 4.076 rear gears I previously had.

 

Hey that's good news! I was hoping to to hear something like that from a turbo guy! I know with that turbo on there I'm gonna be going through the gears much faster with the gslse gear in there.

 

The 12A turbo cars came with the same 3.909:1as NA cars, however 5th gear was changed to .791:1 so you could get a higher top speed [140mph]. The 13B engined se came with 4.075:1. The FC 13B came NA 3.909:1, turbo 4.100:1

Several people I know who have gone 13B T have changed to 4.44:1. It depends on whether you want in effect a close ratio gear box as against better top speed/fuel consumption. The 1/4 and circuit times will change but not perhaps for oval tracks.

If in doubt keep it stock, change it latter if you are not happy.

 

honestly, i would like to get more mph per gear. with the turbo the gears fly by and it kinda sux cause traction is a big issue. i would really like to see what 3.73's would be like. what are your mph now in each gear directfreak??


edit: and how the **** did you get 3.73's???? hmmm did you already put that ford rear in there?? lol

 

It will depend greatly on your choice of tires/wheels.Overall diameter will affect acceleration and RPM greatly,regardless of the rearend ratio you select.
My SE pumpkin with 4.07:1 ratio was OK with my old 15's and 205/50-15 tires.A little on the low side,my freeway RPMS were a kinda high and traction was an issue because of the torque advantage from the low gears and smaller tires.

Once I stepped up to my current 17's with 245/35-17 out back,my cruise RPMs dropped a bit and traction/boost in lower gears was much improved.The higher gearing gave the motor more to push against and gained me more MPH per gear,thus more time to build higher boost in the gears that usually went by in an instant.
Im still toying with the idea of dropping down to 3.90 GSL gears just to see if it gets doggy or better.....

 

wow, i'm running 13's as of now. whenever robert sends me my 15's i'll see how that goes, but i didnt think stepping up to 17's would make that big a difference. SWEEEET. now maybe 1st gear might be worth something LOL

 

Ford Rear End? What are you talking about?! Who have you been talking to?






The gears are real nice - alot more mph per gear, less wheelspin, and faster boost response - just like Steve Noted.

I can only imagine when I go to 17's!

 

that's really odd. i usually though GSL-SE rear ends had more top end MPH to squeeze from them. when i had a GSL rear end, my cruise rate was exactly 3,000 RPM's @ 60mph. but when i switched to the GSL-SE rear end, my cruise rate dropped to approximately 2600-2700 RPM's @ 60mph. i run 15" rims w/ 205/50 tires, FYI.

anyone have a clue why...?

 

gsl-se's do have a longer 5th gear. but that's the only change. i'm sorry directfreak, i thought i read somewheres you were putting a 9" in there. either way, if these 15's don't cut it, i'm just gonna get a lower rear ratio like you. i'm glad you've tried it. WOOT!

 

gotcha. so if i switch back to my GSL rear end, the cruise rate RPM's will be much lower in conjunction with the GSL-SE tranny?

 

yes, when in 5th

 

It's a Ford 8.8 with 9" ends, and other...stuff.

 

Well thanks for the input, I feel much better trying out the 3.9 first, I think it will be fine now. Direct Freak.........damn that's a phat *** rear you got there and nice brakes too!

 

It does not matter what diameter rims you have. The critical measurement is the rolling circumference of the tire. The tire manufacturer will give this in his list of specifications.

Changing from 205 t0 215 changes your rpm per 10mph for a given rim size. The impact of changing from say 14" to 17" rim diameter will depend on the tire profile and width, thus your rpm per 10mph can go up or down depending on the tire selection.

 

I'll be using a 3.23 LSD out of a EB Falcon shortly, as the diff I got was a 2.77 open-wheeler when I pulled it out of the donor Ford. Ideally I want to run 3.5:1 with a 4-spider LSD as came factory in the ED-EF Ford Falcon XR6, but they're a little hard to come by right now.

3.23 should be good enough to run the engine in and do a few sub 12sec 1/4mile passes until I get bored with it.