-Joel Gat  

Impreza tuner/ ProRally driver

LSD vs. no LSD: "...What's it mean in real life? ..."

-Shiv Pathak

Writer/ Tuner for SSC magazine   'Project Impreza 2.5RS'

"To answer the question whether an LSD is worthwhile once the suspension is max'd out, I would say ...."

- Robert O.

Impreza Enthusiast

"...There are 3 basic types of LSD, all of which use a different mechanism to ..."

"An LSD ensures that both axles coming out of it turn at close to the same speed. If you're not turning hard enough to reduce the traction on the inside wheel, you're not turning near the limit. I've seen plenty of Imprezas lift their inside wheel off the ground. No LSD means probably close to 99% of the power is going to the wheel in the air - no more acceleration for you. LSD means probably something closer to 40% or so of the power is still at the wheel with traction.

What's it mean in real life? You can power-on earlier in a turn. You can exit more quickly. You can drive faster."

-Joel Gat

Impreza tuner/ ProRally driver

"To answer the question whether an LSD is worthwhile once the suspension is max'd out, I would say yes. In fact, there's some merit to installing the LSD before suspension tuning begins. FWIW, we held off on SCC's (Sport Compact Car magazine) Impreza suspension comparison test until we got a rear diff in the car. Without the diff, the car could be coaxed into fits of inside wheelspin and terrible power-on understeer. Under power, the car could barely even hold a line through a turn. And this is on the dry. So, we felt it would be unfair to fault a suspension when the underlying cause is the diff. With the rear diff installed, the car drives much like a WRX. Finally.

A LSD effects a cars handling in ways that any amount of suspension tuning cannot. This is especially true on low friction surfaces and/or with high hp. I recently installed a Cusco clutch type rear LSD in my turbo RS. While it's a little chattery and overkill for NA use, it's the best upgrade I have ever made to the car's handling. Kicks butt in the dirt as well.

Without it, the car simply refused to power out of turns. Instead, it would dodge side-to-side, wiggle uncomfortably, and spin its inside wheels. With the LSD installed, the car holds real, throttle-induced lines-- almost as well as one would expect from a good rwd car.

Here's test: Do a quick wide-open throttle 1st gear U-turn in an MY00 (even in the dry). Then do it in any MY98 or MY99, modified suspension or not. In the MY00, the car will rotate nicely with the rear of the car doing much of the work. In a pre-MY00, the steering wheel will dodge side-to-side and the front tires will do almost all the work-- just like a FWD car. When I did that for the first time, it convinced me that these cars need a rear LSD. Heck, even the bean-counters at Subaru thought so."

-Shiv Pathak

Writer/ Tuner for SSC magazine    'Project Impreza 2.5RS'

"A differential gets its name because it allows the 2 wheels to travel at "different" speeds when you are going around a corner. Because the outside rear wheel travels further than the inside rear wheel, there needs to be a mechanism to allow this - the differential. If you had a one piece axle connecting the left and right wheels, the tires would scrub like crazy because they would be forced to travel at the same speed even though they couldn't and thus the scrubbing. Your rear tires would wear out in a few miles, your car would make ugly noises when cornering and it would handle very poorly. This is why you need a differential.

So far, so good. Normal "open" differentials solve the differential speed problem wonderfully, but they have one little problem. The design of an "open" differential has one strange side effect - power is always sent to the rear wheel that has the *least* amount of traction (most people need a good diagram and a few beers to understand this, but trust me). That is why on low traction surfaces, or when trying to accelerate on dry surfaces, only 1 wheel will spin (see My Cousin Vinny for a whole movie built around this fact). This is a problem if you are interested in performance, because when you floor it in the dry (unless you are driving a low powered car) or in low traction situations, you won't get very far because power is transferred to the wheel with the *least* traction.

LSD = "limited slip differential" seeks to solve this problem by having a mechanism that forces power to the wheel with the *most* traction while still allowing the wheels to turn at different speeds when going around a turn. (By the way, that solid rear axle mentioned above is great for traction in a straight line, but aside from the other bad effects mentioned above, it will tend to force the car to go in a straight line all of the time).

There are 3 basic types of LSD, all of which use a different mechanism to send power to the wheel with most traction:

1. viscous - this is what the 00 RS has (It actually has 2 - one in the center diff and one in the rear. This system uses plates and a viscous fluid with the interesting property of getting "sticker" as it heats up. Pros - it's cheap, it will last almost forever, no maintenance (the unit is sealed for life) and it works good. Cons - due to the nature of the fluid, I t does not "react: as quickly as the other types.

2. Mechanical - This system uses an unbelievably complex set of gears to function, the Audi's and "torsen" differentials are a gear type. Pros - works great, less "lag" time. Cons - expensive, some issues of durability (especially if a wheel gets in the air - but let's not start another debate).

3. Clutch type - This system uses clutches to operate. Pros - it works great, relatively inexpensive. Cons - the clutches wear out and need to be rebuilt. "

- Robert O.

Impreza enthusiast