Tag Archives: exhaust

GrimmSpeed Downpipe – Frequently Asked Questions

grimmspeed wrx downpipe

There’s are certain questions that we typically get from customers, regardless of the product being discussed. Instead of making you call or email for an answer, we thought we’d address a few of the heavy hitters right here so that the information is available to you 24/7.

Question: I’m planning on going Stage 2 with my car, do I really need an aftermarket downpipe to run higher boost and make more power?

Answer: Yes! The purpose of an aftermarket subaru downpipe is to remove restriction from your exhaust system. In the stock downpipe, that restriction comes in a three different forms. The first and most important is that the factory downpipe contains highly restrictive catalytic converters. Aftermarket downpipes generally either remove the catalytic converter all together or replace it with a much higher flowing unit. GrimmSpeed offers options for both. The second restriction is a result of the design of the turbine outlet flange. The OEM flange is a flat plate, obstructing exhaust flow from the IWG and forcing it to merge with exhaust from the turbine immediately. As soon as you’ve seen one, you’ll know what I mean. The third is in diameter. Stock downpipes use a smaller diameter tubing than most aftermarket downpipes. If the purpose is to remove restriction, the reason for larger tubing is clear. Even a small increase in internal diameter has a large impact on the total flow area of the tube.

grimmspeed subaru downpipe engineering

Question: Ok, so I know I need an aftermarket downpipe. There are a bunch of different styles. Do I want bellmouth or divorced?

Answer: This is where things get a little bit tricky, because you’ll get different answers depending on a person’s personal opinion regarding the proper design of a subaru downpipe. It’s important that you understand the pros and cons of each and make an educated decision for yourself. To be clear, both will do what you want, which is to unlock some serious power potential, but there are differences that can facilitate more efficient tuning and better boost control that are equally important. The reason that we’ve gone with the divorced design, despite the increased cost and complexity, is that we believe that it’s the best, plain and simple.

Imagine that your downpipe is a freeway, your turbo is a tunnel and your internal wastegate is traffic trying to enter the freeway. On a typical freeway, a car would use an on-ramp to get up to a proper speed and direction before merging with traffic in a controlled manner. That is exactly what a divorced subaru downpipe, like the GrimmSpeed unit, facilitates. Our divorced tube is the perfect length for the exhaust gases from your IWG to merge with the rest of your exhaust with minimal turbulence. Any shorter, and the exhaust from your turbine is still recovering from leaving the turbo, expanding into a larger tube and making a turn down towards the ground. Any longer and you’re simply adding unnecessary cost, material, weight and complexity to the assembly. Now, imagine that the traffic entering the freeway simply enters from a stop sign. We’ve all had to make that nasty right-hand turn during rush hour, don’t make your exhaust do the same. Good, predictable exhaust flow for your internal wastegate will ensure that you are also able to create good, predictable boost control.

grimmspeed sti downpipe faro arm

Question: I have X downpipe right now and i didn’t have any trouble installing it, but I just can’t get rid of my exhaust leaks now.

Answer: This is something that we hear very often. Persistent exhaust leaks of this kind often come from flanges that are severely warped or from parts that don’t fit properly. Creating exhaust components that fit perfectly is more complex than most people (and other manufacturers, it seems) realize. Designing a part that installs easily is one thing. Engineering a part that installs easily, has appropriate clearances from other components and has nice, relatively flat flanges that are are at exactly the right angle to create a perfect seal is a bit more difficult.

Here’s where GrimmSpeed’s engineering process emerges miles ahead. Other manufacturers create their welding fixtures from an OEM downpipe to match it exactly. Sounds great, right? Wrong. We’ve been engineering top of the line exhaust components for a while now and we’re going to let you in on a little secret – Subaru isn’t perfect. In the design of this downpipe, we used our FARO Arm to 3D scan a handful of OEM downpipes and saw that from pipe to pipe, there was a significant amount of variation. We bring all of this data in CAD, overlay the scans, and then find the average of each flange location. We then design our pipe and build our fixtures to that average, instead of a single downpipe. This ensures that our tolerance range is at least as tight as the OEM, with no GrimmSpeed downpipe ever fitting worse than an OEM part. On top of that, we also use our FARO Arm to 3D scan the areas of each test vehicle affected by the downpipe. This ensures that we have proper clearances for the firewall, O2 sensors, transmission bracket, subframe and so on.

Question: So what sets the GrimmSpeed Subaru Downpipe apart from other downpipes that get decent reviews but are a little bit less expensive?

Answer: This is where you’ll have to use the information that we’ve shared to make an educated decision for yourself! If you’ve made it this far, you already understand the care that we take in our design and manufacturing processes. Now, we’ll overlook fitment and design and consider just build quality. All of our materials are top-quality, Made in the USA stuff. Aside from the obvious benefits of supporting local and domestic business, this means that we know exactly what we’re using to build these downpipes. All “Stainless Steel” downpipes are not created equal. Our 3″, 16ga tubing is beefy and our 1/2″ 304 stainless flanges are the thickest and flattest around. Thick flanges help us avoid warping during the welding process and ensure a fantastic seal to your turbo.

We periodically check our fixtures for accuracy using our 3D scanning equipment, to ensure that over time, it hasn’t pushed or pulled from the heat cycling. Each downpipe is welded by hand just feet (literally) from where they were designed and developed. Robotic welding looks great, but in an application like this, nothing can replace the skilled hand of a true craftsman. This type of welding requires that changes be made on the fly to ensure not only a cosmetically excellent weld, but more importantly, a structurally flawless one.

grimmspeed exhaust welding tig

The bottom line is that we’re all Subaru enthusiasts here. We get it. We’ve all had the ‘budget’ downpipes at some point and while some of us have had fine experiences with them, others of us have not. We take pride in developing and manufacturing products that we know won’t let you down in 5 days, 5 months or 5 years from now. A GrimmSpeed downpipe will outlast your car and we stand behind each one that leaves the shop 100%.

Need help with which GrimmSpeed Subaru Downpipe will work best for you? Check out the links below for more information on each fitment.