You'd think after two years this question would be answered, but quite honestly, the most common question I get about my WaveTrac diff is -- how does it compare to a performance pack (PP) diff? Likewise, Performance Pack members have questioned adding (or supplementing *cough* Justin *cough*) a WaveTrac. I've said before the WaveTrac is more responsive, but, I really had minimal time and minimal experience in a performance pack car, and of course, as a WaveTrac owner, I'm going to be biased. Thankfully, Bradd Kordenbrock was willing to swap cars with me for a canyon run with his PP-equiped GTI. Technical analysis, hard facts, and side-by-side opinions are all explored today. But fear not! If you don't want to grit through the technical details, feel free to skip to the last section for the direct comparison.
Differentials -- Why they matter
Before I dive into the comparison, I want to actually discuss WHY a differential matters so much. I think nearly every Mk7 owner has accepted the value of a diff, but most understand stand it simply as: 2 wheels turning is better than 1. And you're not wrong, if nothing else, that's a good place to start. But the thing is, differentials are so much more than that. A nice load up can inspire confidence and too much locking can slow you down. A friend of mine, Andy Hsiao, says it best:
"Many of us have heard about 1/1.5/2 way diffs, helicals, welded, etc, but it's hard to grasp how a diff impacts the car's performance as a whole. A diff can improve more than just your skids' scores. A well-tuned diff can give you more grip off the line and more grip through a corner while making the car easier to drive. The basics of it can be best explained versus an open diff. Open differentials don't have any locking mechanism to lock the two power tires together when one starts to get away from the other. Instead, power moves through the path of least resistance. The macro part that you will see with an open diff as a driver is in a one tire fire and profound understeer. ... Imagine instead, you're able to give the same amount of gas and have the car be perfectly composed through a corner. That's what a good limited slip will do. It's real and we can all have that."
While we don't have a plethora of differential options like BMW or Nissan owners, we do have a few options: Quaife, WaveTrac, Peloquin -- and of course the Performance Pack VAQ diff. And when selecting a differential, it's important to consider the different trade-offs.
Types of Limited Slip Differentials
Firstly, there's actually a fallacy in just the heading -- not all differential upgrades are even "limited-slips". In fact, the WaveTrac and other helical-type units are actually classified as "automatic torque-biasing differentials" (ATB's). Either way, the general effect here is the same -- transfer the torque into the wheel with the most traction. Generally speaking, the more effectively we can do this, the better a differential is doing at it's job.
THE "WAYS": Said earlier, you've probably heard the terms "1-way" and "2-way" and maybe "1.5 way" thrown around. What do they mean? Simple, actually. 1-way differentials only activate during acceleration. 2-way differentials activate equally during acceleration and deceleration. As you might have guessed, 1.5-way's are right in the middle. They produce maximum torque biasing when accelerating, but still provide some amount during decel. And in case you were wondering, whether the locking is 1:0.1 or 1:0.9, they are still called 1.5-ways.
HELICAL TYPE: These units are the most common in FWD applications. One reason for this is helical differentials do not require a friction modifier in the differential fluid. Since transmissions and differentials generally share the same gear oil in FWD applications, this makes them an attractive choice. However, the biggest trade-off in helical diffs is that under zero-traction conditions (e.g. lifting a tire under hard cornering), helicals fail -- they are no longer able to torque bias. Additionally, by default, all helical limited slips are 1-way differentials. WaveTrac claims to mitigate both of these issues, despite being a helical-type. For details, see the following link under the "How It's Different" tab. www.wavetrac.net/technical.htm
CLUTCH TYPE: Clutch type differentials overcome the two design "flaws" by design. In addition, clutch type differentials provide a huge amount of adjustability in the 1.5-way range. Additionally, clutch type designs range from basic single clutches on each axle, to crazy 32-deep stacks (check out OS Giken and Cusco if you're bored). There's really no downside to clutch types compared to other diffs other than service. They require more regular service intervals, and performance does deteriorate overtime.
THE VAQ DIFFERENTIAL: The best way to explain this is by simply thinking of this as a sideways haldex unit. I've included pictures from an Evo X's AYC (active yaw control) rear differential, because it actually functions nearly the same way. Essentially, power is transferred left and right, instead of front and back using a wet clutch actuated by an electrical pump. The biggest limitation is response time. The computer must first actuate the differential before any effect takes place. The other limitation is the locking torque -- 1600Nm or 1180 ft/lbs in English. Before you say wait, that's over kill, think about this: It's easy to hit 350 crank ft/lbs of torque in a tuned Mk7. When you factor in gear ratios, that 1180 number is easy to overcome. As you can see in the graph below, even in 4th gear, we can actually exceed the 1180ft/lb torque limit of the VAQ diff. Now, this doesn't mean it won't help when we make more power, it just means it slips. Keep in mind that this pressure is with the stock "diff tune". What I mean is, the clutch pressure is controlled digitally. For instance, the Clubsport and Clubsport S have more aggressive "tunes" which up the clamping force and sensitivity of the VAQ diff. Theoretically, we can turn it up even more than that. But for this article, we're consider the OEM setup of the PP differential vs the WaveTrac.
For more on the VAQ, check out this article: www.pistonheads.com/news/ph-eurocars/vws-vaq-diff-explained/30560
PP vs. WaveTrac -- Side by Side
Interview style from both myself and Bradd, we breakdown the WaveTrac vs. the Performance Pack. We discussed these questions after driving each other's cars through the canyons in Utah. He drove my WaveTrac car, I drove his PP-car.
Would you say the PP diff vs. the WaveTrac is something you immediately notice or did it take time?
[Bradd]: "I noticed the difference immediately when throwing a car into even a semi-sharp turn. In normal driving around town, it doesn't seem as noticeable but you can still tell it's there anytime you turn the wheel while on the throttle. The car just reacts instantly with the WaveTrac. The PP Diff is a bit more smoothed out (less reactive), which makes sense for a factory car of this caliber."
[Karl]: "The PP diff was so smooth, it wasn't immediately noticeable until I put it into a corner. Grip just continued, and that's when you could feel it working. VW has integrated the unit so seamlessly, that its operation has no better description than "smooth". The ramp up lets you know it's there, it's working, but it's not intruding."
Did you feel more confident in corners with the PP diff or the WaveTrac? Why?
[Bradd]: "I feel a bit more confident with the WaveTrac simply because it reacts slightly faster and is more aggressive."
[Karl]: "Bradd's answer here actually surprised me. I find the car easier to drive with the PP diff, which I guess could be described as confidence-inspiring. However, there's no doubt I can power out harder with the WaveTrac. I also feel the WaveTrac is more consistent in performance, so it's kind of a toss-up. I can get closer to the car's limits with the VAQ, but the WaveTrac is clearly capable of more. What I mean is, I can feel the WaveTrac load up in the steering wheel--I know when it's working."
In terms of straight line traction, what did you think of the WaveTrac vs. the Performance Pack?
[Bradd]: "WaveTrac has better straight line acceleration grip. However, tires have a lot to do with this aspect as well."
[Karl]: "I have to say, I think this is the most obvious weakness to me. The VAQ is kind of fighting back and forth with torque vectoring. The WaveTrac just HOOKS. This is actually really substantiated by the data in the graph presented in the previous section. Through 4th gear, the VAQ isn't providing 100% lock, assuming you're tuned."
If you didn't have a Performance Package car, do you think the ~$1000 price tag (not including installation) is worth it?
[Bradd]: "If I didn't have PP and the GTI was my only fun car (apart from a family SUV or something), a WaveTrac would be on my mod list."
[Karl]: "Obviously... I did it. No ragrets!"
How would you stack up the WaveTrac compared to the R? Essentially, aggressive limited-slip FWD vs. open-diff AWD?
[Bradd]: "As far as dry cornering, I think a GTI with the WaveTrac could match a well set up R turn for turn or possibly even out corner it on mountain roads. On a track, I think a GTI with the WaveTrac could probably pull the turns faster. In wet or snow, the R would take it every time."
[Karl]: "Haven't spent much time in an R, but based off what I've seen on track, I'd have to agree."
[Bradd]: "Let's see. My take away would be this: If you can get the Performance Pack, it's worth it. However if you already have the Performance Pack, I think it's good enough to make it not worth swapping out to a Mechanical diff unless you're building a track only racecar. If you know you want the most out of a diff to start with, you could try to get a non PP car knowing you'll upgrade stuff. However I think that's rarely worth it to do purposefully. Personally I think a car with the Performance Pack will give you 8 to 9 out of 10 that you'll get with a WaveTrac. Also I don't track my GTI. Just AutoX and curvy mountain/canyon roads. So for me, the PP is a better choice than doing a WaveTrac aftermarket."
[Karl]: "Couldn't have said it better. I think for my goals, I'm glad I have the WaveTrac, but for most it really doesn't make sense. The PP diff is great, the WaveTrac is just that last 20%."