brodpphish

I believe KahnBB6 recommended the 3.72 gears as being peferable over stock 4.08 for a turbo car? Why is that? And car I switch the gears in the 4.08 pumkin or is another route better?

There is a member here with a stock 4.08 for sale and I am trying to figure out if I should buy it.

KahnBB6

iTrader: (5)

brodpphish, it's actually the 3.76 (technically 3.769:1) ratio from the MKIV Supra TT Auto (and JDM Aristo's with 2JZGTE's). A close second is the 3.92 (3.916:1) SC400 ratio. Some people with very powerful single turbo builds like the GS400 and 98-00 SC400 3.26 ratio.

Why are lower gears that 4.08 recommended? It's a good question I've asked before myself. I got some very eye opening reasons as to why but the short of it is that when you get into a substantially powerful turbocharged engine you don't need such aggressive gearing. The engine does more of the work versus short and aggressive gearing having a more significant hand in the work in an NA car.

Two threads I started on this subject on Clublexus and on Supraforums. Both are a good read:

https://www.clublexus.com/forums/per...-bpu-only.html

http://www.supraforums.com/forum/sho...ch-well-or-not

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The thing about the 4.083:1 gearing that stock NA SC300 5-speeds came with is that it's also exactly the same ratio that 1991-2000 JDM Soarer 2.5 GT's with 1JZ-GTE's and R154 5-speeds came with. This goes for both the early parallel twin turbo 1JZ's and the later single turbo VVT-i 1JZ's.

The reasoning for the SC300 M/T cars getting a 4.08 instead of the 4.27 from the Supra 5-speeds was probably to do with giving the car a more comfortable and less "racy" feel with the short gearing. It might have also been about distancing the SC300 5-speed's performance a bit from the Supra NA 5-speed's performance. Probably both.

With the 1JZ manual Soarers getting 4.08's it likely had to do with off the line acceleration and the laggy characteristics of the early twin turbo setup which only put out "280 horsepower". In the VVT-i revision with low end torque coming in much sooner the same unchanged gear ratio probably better emphasized the around town acceleration the car was designed for, still with not too much more than the advertised "280 horsepower" but much snappier than before due to the VVT-i system and newer turbo setup. Also, both these engines are 2.5L vs 3.0L and rev higher than any version of the 2JZ does. The lower displacement and higher revving nature of a the 1JZ is significant in the case of gearing.

Once you get well into 350whp-400whp-450whp+ territory with any kind of boosted JZ the short gearing makes less sense. Most folks going about turbocharging or swapping their SC's these days aren't going for near stock 1JZGTE power levels. They're shooting much higher.

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I thought long and hard about keeping my original 4.083 ratio even though I'm building a car for only about 350whp. It would have worked fine and will especially with a 1JZ swap but I reasoned that with the engine's power delivery being completely changed from what it is in NA form it makes more sense to have a 3.76 or 3.92 ratio.

With the turbochargers on 3.0 liters doing more of the work I won't need such aggressive gearing in around town situations. Also, as most folks will agree, the less aggressive gear ratio translates into a higher top speed in each gear, higher maximum speed and lower revs for better fuel economy in your overdrive gear.

Since I've been so many years on my stock NA engine, however, I did the opposite for the time being. I swapped from a 4.083 to a 4.272 ratio. I did this because without a turbo the opposite principle applies: you lose a bit of top speed in each gear but you go through the power you do have in each gear faster.

A point of comparison would be to look at just how aggressively R32 and R33 Nissan Skylines were geared in all turbo manual trim levels. This goes from their 2.0L RB20 to the 2.5L RB25 to the RB26. I'm not up on how Skyline owners change their gear ratios but... stock for stock... you can see where the logic of the engineers went in regards to gearing with turbos and the benefits and tradeoffs. It's interesting that Toyota did something similar (in stock form) with a big 3500lb+ RWD turbo coupe.

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Also, just to note: V160 6-speed transmissions *require* a 3.26 ratio (or the 220mm 6-speed diff) with any turbo engine. I believe a CD009 transmission also works best with a 3.26.

There are many folks here with boosted JZ's who will tell you they'd rather have a lower final drive ratio than a shorter one.

And yes, you can have your original diff pumpkin rebuilt with any known Toyota ring and pinion set that will fit it. It's recommended you get a professional shop to do this unless you're experienced with driveline rebuilding and setting up the correct shim clearances and runout.

Usually if you just want a different gear ratio all you need to do is remove the differential you have and swap in a different compatible one from an SC, MKIV or GS. The companion flange is the only thing that needs to be changed from an MKIV or GS for that, although there is a simple but very, very precise procedure required to swap the new flange on correctly.

Rebuilding a diff usually is desired when you either want a different gear ratio or an LSD installed... or both.

brodpphish

Wow Kahn thanks for all the info. I like the way my cares drives, I just use it around town. SO I made an offer on the stock diff.

You've been a great help.

KahnBB6

iTrader: (5)

Idlefree

Is the LSD 3.7 rear end from a Toyota Supra direct fit to our lexus sc300...help

gerrb

iTrader: (34)

when you say LSD 3.76 rear end ... are you referring on the torsen lsd diff only or the whole differential which includes the pumpkin ?

the TT auto diff (lsd + pumpkin) which has the 3.76 gearing is plug and play into your SC300.

Idlefree

Thank you Gerrb for all your help as usual through PM
Below is pictures of the issue





Sc300 smaller




Toyota supra 3.7 LSD larger radius

gerrb

iTrader: (34)

ok... now I see what you mean. It is the Axle Stubs that are not of same size.

If you are buying used axles , the SC400 from the junkyard will be easier to find and cheaper and should work.

But if you are buying brand new axles , just get the auto TT MKIV axles since you got that MKIV auto diff.