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Rear Gearing and Turbo Lag

1597 Views 25 Replies 8 Participants Last post by  KB240SR
I started to think today on how to help against the turbo lag of a larger turbo such as a 2871R .86 or larger. I remembered the different rear gears that are available for the car. Since our stock rear is 4.08 and if we make the rear a 4.36 or 4.63 then that should help with the turbo lag since the rpm will be higher in each gear than stock and the engine would go faster through each gear. Would this concept work or not?

4.08 rear gives 174mph @ 7500rpm (5th gear)
4.36 rear gives 163mph @ 7500rpm (5th gear)
4.64 rear gives 154mph @ 7500rpm (5th gear)

Chart For Tranny Gears
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no. it may seem better but it would only be a few tenths of a sec over the 1/4 mile. a 2871 isnt really that bad anyway. think about this, your gonna be spinning rear tires faster and traction can be an issue with a lightly modded sr. now your talking about 350whp with a 4.63 rear end, you might not be getting traction until you hit the top of 5th gear. if your worried about spool time go with a 25-50hp shot of nos.
im not really talking anything about 1/4 times and traction. im saying that with the 4.63 rear will have the engine climb through the rpm quick enough to lessen the lag of a larger turbo. Am i wrong?
also 4.36 is a common rear gear used in drifting and in many S13 and S14's in Japan. The chart shows the CA18DET Silvia came with a 4.36 rear gear but no LSD. 4.63 isnt that much different.
Your 0-60 time would be longer because you have to shift to 3rd to get up to 60. Around a tight track, it would be a good setup (like a 6-speed without a 6th gear) Traction could be an issue because the engine would have an easier time revving under load. What would you be using this gearing for? (street, track, drag, drift, grip, autocross...)
2nd gear would go a little over 60mph. Put the type of racing aside and just go by my question of:

Will a 4.36 or 4.63 rear gear make for a quicker spooling large turbo? If not, why?
i would think it would spool up slower since there is less of a load on the engine. either way you would want a longer gear with the more horsepower you have.
it wont spool quicker, its not physically possible. you might accelerate faster but you will also have to change gears sooner plus cruising around at 4k rpm doesnt sound very fun to me. if you had an auto trans then yeah it would accelerate faster but with a 5spd i dont know...
so my theory is bust. thanks anyways.
anyone know what the 4.64 would top 2nd gear out at? or a 4.4
the sites down for the gears but i calculated it to be around 62 - 64 mph. I still say the stock 4.08 rear is a little conservative with our cars since we have alot of RPM's to work with. 174mph is a little crazy for a car like the 240. Ive done 160mph in mine and it felt very unstable and i also blew a intercooler coupler doing it.
ive heard that the GT2871R .86 is a little laggy. Maybe ive been told wrong. I was really refuring to the GT30xx series.
I just got a couple reviews from Fresh Alloy of people using the 4.36 with the KA and SR and they claim they absolutly love it. One also said that after using the 4.36 the 4.63 would just be too low of a gear. Im really leaning toward the 4.36 rear at this point.
Doppleganger77 said:
ive heard that the GT2871R .86 is a little laggy. Maybe ive been told wrong. I was really refuring to the GT30xx series.
there is a significant differance between the gt2871r .86 and any gt30xx especially because tyhe gt30's are in a t3/t4 housing where the 2871r has a t2 housing. my buddies car ran the gt2871r and the spool time was almost identical to stock. by the time stock t25 boost would hit his car was already spinning tires like mad
GT2871R is also made in a T3 flange. My T25 hits 15psi at 3000 rpm in 3rd gear. Does a 2871R really spool that quick?
not sure about the t3 version but the t2 version spools at most 300-400 slower then the stock t25 with the .86
a buddy of mine had a 30r and his was boosting nicely at 3200.
I have the gt2871r .64 turbo and it is not a slow spooling turbo. It might seem like it though becouse its highest boost is alot higher than the stock turbo. Even in the low rpm's you make more boost than stock. when the turbo fully spools it is very noticable tho.

Just curious but where can I get some lower? gears? I want a higher top speed... I run thru the gears way too fast with this setup.
JDM 180sx rear is a high 3.xx i think. Have you gone 170mph with your SR?
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