Mk7 GTI Downpipes
So you’re looking to add more power to your Mk7 GTI and you keep hearing the term “Stage 2” thrown around on the internet. Well, if you’re here, you’ve probably gathered that it has something to do with a downpipe. What is a downpipe? How does it add performance? Do I need a tune? Let’s get this cleared up.
What is a downpipe?
On an Mk7 GTI, the downpipe routes exhaust gases down from the turbocharger to the rest of the exhaust system. The factory downpipe also contains two catalytic converters, which is why the remainder of the exhaust section after the downpipe is referred to as a “Cat-back”. The factory downpipe sports a 3” V-band connection to the turbo and then reduces to 2.5” inch piping at the first bend.
How does an aftermarket downpipe add performance?
To understand why downpipes add performance, we first need to understand how the engine itself operates. In the simplest of terms, we can think of an engine as an air pump. The easier it is to suck air in and push air out, the more air can actually flow through the engine. More air plus more fuel equals more power, simple. In technical terms, we measure how easy it is to push air out of the engine as the amount of back pressure in the exhaust.
Downpipes significantly reduce the amount of back pressure in the exhaust system by upgrading the pipe diameter from 2.5” to 3” or even 3.5” in some cases. While upgrading just ½” in diameter may seem inconsequential, the actual flow rate is effectively increased by 44%*! In addition, after market downpipes replace OEM catalytic converters with high-flow catalytic converters, or, scrap them entirely.
High flow catalytic converters typically vary from 100-400 cell. This refers to the number of cells per square inch. As you can imagine, the fewer cells per square inch, the less back pressure. The trade-off is, the fewer cells, the less efficient the cat is at doing its job – reducing emissions. While there’s no single best answer, for what is right for you, all the downpipes we stock will work with Unitronic’s Stage 2 and Stage 2+ software (our recommendation).
*Based on cross-sectional area
Why not just upgrade the cat-back instead?
Back in the 70s and 80s, you could actually gain a sizeable amount of power by changing out the cat-back portion of the exhaust system on cars. This was because back then, mufflers and connecting pipes were under provisioned and often caused excess backpressure. However, the Mk7 GTI is equipped with a factory 2.5” exhaust system, which is more than adequate for 200 or even 300hp cars.
The reason is that by the time exhaust gases reach the second half of the exhaust system, they have cooled down by roughly half of the 1000F+ temperatures at the turbocharger. Throwing back to high school chemistry, you might remember the ideal gas law:
Essentially, if the exhaust gas has cooled by a factor of two by the time it reaches the cat-back (e.g. 1000F to 500F), then the amount of pressure also has decreased by a factor of two. While catback’s sound awesome, the exhaust portion isn’t nearly as critical as the downpipe in terms of performance.
Do I “have” to tune my Mk7 GTI if I buy a downpipe?
Let’s play devil’s advocate first. You can equip this part on a bone stock Mk7 GTI and notice instant improvements in throttle response from reduced back-pressure. We’ve also seen performance benefits of anywhere from 5 to 25whp from a downpipe only. However, we can’t really recommend this route to anyone.
Adding a downpipe without a tune will push the ECU out of its comfort zone. For instance, because boost is so much “easier” to make, it’s possible that the car can temporarily over-boost. This can throw an EPC light, sending your car into limp mode. For some this can happen the first pull, others might make it 10,000 miles safely. Either way, the proper way to keep your GTI running happy and healthy is with software that matches the hardware.
Consider that a tune alone nets 50-70whp (depending on fuel) at a cost of only $700. That’s about $10 per HP! A downpipe by itself might add 20whp for $500 ($25/hp), but pair that downpipe with a proper tune and you can net nearly 100hp over stock, and 40hp over a tuned car without a downpipe.
What about these eBay downpipes? Why should I spend twice as much?
Great question, and this one comes up a lot. You’ll often hear people suggest that “it’s just a metal pipe”, and it’s kind of hard to argue that. So why invest $500 or more into a brand-name downpipe?
The first major issue with nearly all “eBay” downpipes is a fitment issue near the ABS module. If you notice, the downpipe sits only inches away from this ABS control unit. Over time and shifting, this can melt away the plastic heat shield and fry the internal components entirely.
Nearly every manufacturer seems to separate their downpipes into two or more pieces. This helps ease the installation process, but when done cheaply can lead to leaks. These eBay downpipes most commonly have a pressure-tight fitment between the two pieces. This often results in a leaky connection and fumes in the cabin.
If that’s not enough to convince you, factor in fitment in as a whole. Often times these cheap parts don’t line up perfectly with turbo and exhaust flanges, which means “forcing it” and putting extra forces on head studs, mounting brackets and exhaust connections. No thanks!
Adding a downpipe to your Mk7 GTI is a great way to pick up some more power and change the exhaust note! We highly recommend investing in a quality part for maximum long-term enjoyment without hiccups. Lastly, make sure you pair your downpipe with a proper tune for maximum power and reliability!