JHM Limited Slip Differential (LSD) Review
I installed my JHM LSD about one year ago, and after a year of testing I’m finally ready to share my review here. I was eagerly awaiting this modification and was one of the first people to buy and install it, as I’d heard my race car friends talk at length about how great LSDs were and what a travesty it was that no one really sold a LSD kit for the B6/B7 S4 before the JHM kit came along. Not knowing much about LSDs, I had very, very high expectations that I could simply install the part and it would turn my car into a RWD drift machine, which in hindsight was not only silly but probably undesirable. After having the kit installed, at first I was fairly underwhelmed; with traction control on and driving around town in a normal fashion (how I do 98% of my driving), it is almost completely unnoticeable…after reading up about LSDs, this makes total sense, but to a LSD noobie I was disappointed I couldn’t feel the impact of the part in daily driving. So first, I’ll share a better understanding of what a differential is, and why you’d want a limited slip differential versus an open one, and thus when a LSD is noticeable or not…if you already know this, feel free to skip ahead to the bottom section and spoiler alert: I love mine and think its an awesome product!
How a LSD Works
All modern cars have a differential, as it’s necessary for a car to be able to turn, as the differential essentially allows to wheels on the same axle to rotate at different speeds. With an open differential, the power in the driveshaft can travel freely to either wheel on the axle, and that power tends to go to the path of least resistance. If a wheel loses grip, then an open differential will generally send all of the power to the wheel spinning freely, and therefore you’re unable to accelerate or throttle steer through the turn since all of your power is just going to a wheel that has no grip – big bummer. With a LSD, it does the opposite – if a wheel loses traction and starts spinning freely, it transfers power to the other wheel that still has traction. The result is more power and more traction to the ground whenever your tires lose grip, relative to an open diff setup. This video does a pretty good job at explaining it on a basic level:
Daily Driving with a LSD
So of course, a LSD is really only noticeable when you are experiencing a loss in traction, otherwise either an open differential or LSD are going to be splitting the power close to evenly since both wheels have grip and are rotating at about the same speed. To experience traction loss on a ~4,000 pound, AWD sports sedan like my 2006 Audi S4 is pretty rare, and requires a few conditions to be met…most importantly, ESP must be off, as the computer will electronically limit the power with ESP on if it senses wheel slip and thus it is a moot point what wheel the differential sends the power to if the differential isn’t getting any power in the first place. You’ll also need at least one of the following:
- You’re turning very sharply, thus the outside wheel is spinning much slower than the wheel on the inside part of the turn
- Accelerating very aggressively, which causes a tire to break loose
- Other factors are limiting grip (icy or wet roads, bald tires, etc.)
In daily driving, the above scenarios are pretty rare…so this mod is really best suited for those who do performance driving like road racing, auto-cross, or drag strips (helps with the launch), or I suppose those who like to showboat with some oversteer, drift turns, or donuts in a controlled environment. Armed with this knowledge, I tested this part how it was meant to be tested…
Road Racing & Auto-Cross with a LSD
The JHM Helical LSD is super predictable and easy to drive on the track. This is where it really shines. The car sometimes understeers, sometimes oversteers, but overall feels pretty neutral. You can do some throttle steering and power out of turns in ways I couldn’t before. Several times I would hear the rear tires squeal instead of just the fronts like I was used to. The LSD is definitely noticeable and helped a ton of the handling of the car on the track, a total game changer. You learn that each lap you can take turns a little quicker, and get on the gas a little sooner, than you thought was possible before. After an entire day of hitting the same course over and over, I was still finding was to improve my lap times and carry more speed through each turn.
Donuts & Show-boating with a LSD
While I’m not one to drive recklessly, and would never showboat out of a car show or around pedestrians, I have to admit I wanted to know if my underpowered, overweight 4 door sports sedan was capable of a donut now. While JHM has posted videos of their supercharged S4 lighting up tires, that car had nearly +100 horsepower on my car that was still naturally aspirated and only had a few bolt-ons, and the car with my old differential could barely break loose a tire. On dry roads, you can get the tail out pretty good and even do donuts, although it requires flooring the gas and taking the steering wheel close to full lock (warning, don’t go to full lock when doing this, it’s a good way to blow your power steering up). In wetter conditions, the effect is even more noticeable. The video above was with Hankook RS3 tires which are very grippy, so just imagine how much more traction loss you can get with cheaper/crappier tires, whereas the video below was with some cheap Nankang tires.
My #JHMLSD Review: Should You Get One?
Ok – so now we know what an LSD is, and what type of driving conditions and scenarios it helps improve…so now on to my review of the product itself. The JHM LSD is available for the B6-B7 A4 and S4, although not the RS4 at this time. It’s a Helical style LSD, which means it won’t need servicing like a mechanical/clutch plate LSD would, which is very nice. It also carries a limited lifetime warranty, is fairly affordable at $800, and is a direct fit for your car…as far as LSDs go, this one is well built and checks all of the boxes, JHM knocked it out of the park and gave it plenty of R&D so you can buy confidentially if you want an LSD for your S4.
Ultimately, this is a mod I’d recommend to anyone who does any kind of track events, or anyone who has added a lot of power to their car (forced induction, nitrous, etc.) and is looking to improve their handling and their ability to put that extra power to the ground. For the average A4 or S4 owner that hasn’t added a lot of power to their car, and only occasionally does spirited driving, this mod might not have a huge impact as our cars are fairly underpowered and maintain grip quite well from the factory (thanks, quattro!), so it wouldn’t be the first mod I’d recommend...but to those who have done a ton of modifications (as I have) and are looking to take their handling and power to the next level, this is a must-have. I have no regrets about getting mine, and it definitely ups the fun factor quite a bit whenever you’re going for a spirited drive. It’s also amazing to see fellow Audiphiles amazed to see my car get sideways…between that and the V8 sound note it’s hard to beat the experience of owning this 10-year-old luxury sedan 🙂
great post and educational from a technical standpoint. A little off topic, those side skirts you have I can find online easily. I see they are carbon? But the front one I have never been able to find? Do you have links for these and also how is the fitment?
That’s for our lovely silver b7 s4 down in NZ
The front lip is made by Kerscher, more info here: https://www.nickscarblog.com/reviews/kerscher-front-lip
Enjoy your S4, Jade!
But you can also replace front differ and even the middle one with LSD (for example Quaife LSD differentials). What do you think about that? Is it reasonable?
I’d contact JHM, they’ve done a ton of testing in this regard…