ok guys, cannonbear is selling a 3.92 differential for the GS in the classifieds. i am definitely interested in it, but i have NO IDEA how it would affect my car. i have a STOCK 99 GS400 with 95000 miles, full maintenance completed. my questions are:

1. what are the positive effects of installing this in my car? i know i will be faster, BUT BY HOW MUCH? if our stock 0-60 time is 6 seconds, what do you think my new 0-60 time will be with this mod? i'm trying to justify if its worth spending over $400 with shipping. this is my last mod before grad school. i have no more money for TC or ANYTHING haha

2. what are the negative effects of installing this? will my MPG go down? will i be running at higher RPM's at 55MPH than normal? will this item wear out my engine and transmission by overworking it? if my car breaks, i'll have no money to fix it after this mod...

3. anybody done this before with these gears specificly?

4. last, how much would this cost me to install in my car? at a lexus dealership? at a toyota dealership? anywhere?

thanks for your time CL, any help would be greatly appreciated.

vince

 

1. Car will be faster through all gears. Higher RWTQ at the ground. Expect somewhere around .3-.5 secondes faster on 0-60. Your car will be in third gear when you are 60 mph, instead of the top of 2nd gear.

2. MPG will be less. Expect 1-3mpg less. RPM's will be higher. I'd estimate 80mph would be around 3200RPM instead of 2800 with stock gears.

3. Yes it has been done here, but I recall someone mentioning the SC400 differential (probably what the person is selling) doesn't have the same driveshaft/axel mounts so you might need to find other ones (don't know detials about this..)

4. I got a good deal, $150 installed. Estimate about 2hrs labor.

- the easiest "direct" swap is the Supra automatic Turbo differential which has 3.76 gears and a limited slip core (LSD). This item is "hard to find" because it's not a well stocked item, but there are a few vendors who can get it. (Carson Toyota & TM Eng.) I believe.

 

thanks for the input lvangundy...does anyone have more info on what he said?

vince

 

slowdiver did in fact swap in a sc400 diff and it bolted right in. thats why i choose it, and if anything did need to be altered, the shop will more then likely be able to take care of it.

 

With the 3.92 gears, the car will need to shift into third gear to get to 60 mph. Due to the extra shift, the 0-60 time will likely be a little slower than stock. Another CL member has experienced this by checking against a G-tech; this isn't 100% accurate, but they are pretty reliable.

I would be more concerned with getting a limited-slip. Many drag racing guys will say that an LSD alone is good for .3 in the quarter mile (when the final drives are the same, non-LSD versus LSD).

 

So far, I have not heard of a measured before / after improvement using the 3.92. This is a relatvely new mod, so there are not many who can talk about it.

I say relatively new, because the 3.76 has been tested and proven and used by members for over 5 years now.

If you are shifting to third gear before 60mph, then I seriously doubt you will be faster than a 3.76 geared car.

My best 0-60 with TC, LSD, 3.76, Intake, Exhaust mods is 5.2 sec. and I'm shifting to second at roughly around 50 mph.



Even with the 3.76 gearing I am dealing with managing the excessive wheelspin. This is with 295 size tires on 20" rims.

If you are running stock tire/rim sizes, etc. the traction will be worse. This is the most challenging aspect of getting a low 0-60 on the heavy GS.

Also take note that on a Supercharged or Turbo motored GS, even the 3.76 is marginally too much gear. Some of those with FI have run with the 3.76, but there are also many who believe the stock gearing with LSD is a better choice for higher powered set ups. I can think of atleast one member with a supercharger who tracked his car regularly and switched from the 3.76 to stock gearing in order to improve his times.

If you plan on keeping a very mild motor set up and / or are running oversize 20" rims the 3.92 gearing might work well, but if you are planning on FI I'd say forget it.


I'd really like to see before / after times of a 3.92 geared car. Butt-o-meter conclusions don't mean much, because we hear the same "WOW" comments from those who switch to the 3.76 gears, so it is impossible to compare the two ratios subjectively unless someone has tried both set ups.

 

hmmm interesting post.... i'm picking up a supra lsd this weekend...i heard that it'll improve the 0-60 time but i didn't know there were issues with excessive wheel spin - right now i'm running my supra rims on the gs til the supra project gets done (285/35/18 on the rear) - but i'll have to get some track times to get some verifiable data including same weight/fuel etc... to even include atmospheric data...i'm curious how much air pressure your running with the wheel spin..i normally drop down to 20-25psi on street tires and 15-20psi on slicks also, what tires are you running - i usually get bfg kd's or the perelli p-zeros...

this is dependent or not if my supra project doesn't bleed me dry.....lol

 

thanks for the input...i guess i'll shoot for the supra diff instead!

 

Well when I say excessive, I am correlating that to what my times are when I spin and grab versus when I spin a couple revolutions. The difference in 0-60 is between 0.4 - 0.6 depending how long you spin em. That's a big loss timewise, just because of that little extra wheel spin.

When the tires spin hard, it "feels" like you are hauling faster than when it spins just slightly, but the G-tech doesn't lie. Spinning more than a couple revs is always slower. This is the main reason why I put little value in butt-o-meter opinions. No two butts are exactly alike you know.

I've got the new Michelin PS2's, so the traction is pretty much as good or better than anything except a drag radial or slick.

At the strip I go down to around 30psi, but that's no where close to what you need to be running on the street. If we want traction just put on some slicks, but that's not my point. My point being, when using standard upgraded size street tires, under real world conditions, it says that traction will be the limiting factor of how much gear is really usable. Pro drag cars use gears up in the 5.xx range, but look at the ***** slicks they use to get grip.

The 3:76 is near ideal IMO. Using more gear and you have even more traction issues, less gear and you are not running up in the power band of the motor.

When I go FI on the car, for faster times, I may need to get rid of the 3.76 and switch over to the trd stock gear set up.