Category Archives: Exhaust

GrimmSpeed EFR7163 Twinscroll Turbo Kit Development

The time has finally come to let the cat out of the bag on GrimmSpeed EFR7163 Twinscroll Turbo Kit development! We’ve been developing this system alongside our top mount intercooler since late last summer. The initial purpose was to build a single car that would produce big turbo power, but with responsiveness that’s similar to factory. This car would be an excellent test bed to prove that our TMIC would support big power with killer responsiveness, without sacrifice. We selected the Borg Warner 7163 Twinscroll Turbo for this project and immediately got to work with the design of a truly equal length header.


We quickly realized why nobody else is doing this. The configuration of the twinscroll ports on an aftermarket turbo is rotated 90 degrees from the factory turbo, making the routing of an equal length manifold below the car extremely difficult. We had to leverage all of the engineering tools and technology that we have access to in order to design a manifold that would fit the turbo that we selected without losing usable ground clearance and veering from our equal length requirement. The firing order of the motor dictates the runners that need to be paired for the twinscroll turbo to function properly, but it’s not even that simple. It’s not our style to hack something together that fits and works – we optimize. Finally, in such tight quarters, it’s also critical to figure out if a given manifold design can be fully welded and if there is adequate room for bolts/studs/nuts and tools needed for installation.

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FARO arm 3D scans of the underside of the car and of the factory manifold and turbo gave us a very good idea of the space that we had to work with. We experimented with three different collector designs and five different runner concepts before settling in on the one below. We were able to design a collector and runners in Solidworks that offered truly equal length (variance between runners is only 2.3%). Because we were concerned with ground clearance, we removed the EFRs IWG (internal wastegate) and used a pair of external wastegates. Knowing what we know now, our plan will be to utilize the EFR’s IWG on production kits if/when the project reaches that stage.

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For this design, we knew that we needed a perfectly symmetrical collector to help us achieve the performance that we’re targeting and utilize the full potential of the twinscroll EFR turbo. The way that the runners pair up and enter the turbo is critically important, so despite the tight space constraints that we’re working with, we made no sacrifice in the general collector design.

The best that we could fit is still a bit steeper than we’d have preferred, but we’re working with hard limits here. This collector has a 45 degree merge angle. Being that the turbo that we’ve selected uses a divided T4 turbine housing, that’s the size flange that we’ll use. Of course, it’s helpful that GrimmSpeed also manufactures the highest quality T4 gaskets around!

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Our engineering team and fabrication team worked together to prototype the kit with great accuracy. We were able to use a very slightly modified GrimmSpeed StealthBox Intake and a very slightly modified GrimmSpeed J-Pipe. For charge air cooling, we used a GrimmSpeed prototype charge pipe and prototype TMIC. The charge pipe is quite simple, as the EFR7163 has a bypass valve built into its compressor housing. After welding was finished and the pipes became cool, what we ended up with was a thing of beauty:

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Getting coolant and oil through the turbo just took some clever routing of hoses, since the factory turbo already uses both in the same location. For the turbo oil pan, we were able to simply reuse the factory oil pan, mounted directly to the turbo. Production kits will very likely include a smaller, custom pan, but that’s yet to be determined.

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We need to start with a shout out to DB Performance in Rogers, MN. We really enjoy the opportunity to bring our cars to a third party shop for dyno testing and tuning. We think that it helps us maintain a level of transparency in a world where closed-door dyno testing is so common and we love having a second set of expert eyes on the work that we’re doing. Shane, Danny and the rest of the crew run a top-notch operation and if you’re in the area, we highly suggest that you check in with them regarding maintenance, modification and tuning. These guys build some serious cars.

With the kit finally on the car, we knew a few things. We knew that fueling would limit our ability to make power. Our intention is to run the car on E85, so we’ll need a supplementary fuel system to deliver the volume of fuel that we’ll need to push this kit. We also knew that the kit would produce power and torque in excess of the limits of the factory motor. A phone call to our friends at IAG Performance for their Stage 3 shortblock and 14mm head studs solves that problem with ease. At the same time, we’ll have the heads gone through before putting it all back into the car. Other supporting modifications will be made at the same time (clutch, mounts, etc).

Step one, though, was to get the car to the dyno for some shakedown pulls and to build confidence in the kit before pulling the car apart for motor/heads/fueling. We pumped the 93oct out of the tank and replaced it with MS109. The thought was that this would help us safely and repeatably push the kit a little bit farther on the dyno than 93oct and E85 would allow with the factory fuel system. Our goal here was to determine that the cooling and oiling of the turbo was working flawlessly and to prove that the complexity of the manifold was justified by the increased performance offered in terms of spool and power production. The car was tuned by Shane at DB Performance (Rogers, MN) with the COBB Accessport.

2015 Subaru WRX, 6MT, 9000mi

  • GrimmSpeed EFR 7163 Turbo Kit
    • Borg Warner EFR 7163 Twinscroll Turbo
    • GrimmSpeed EL Manifold
    • Turbosmart Compgate 40 EWGs (pair)
    • GrimmSpeed J-Pipe (modified)
    • GrimmSpeed Stealthbox Intake (modified)
    • GrimmSpeed Big Turbo EBCS
    • GrimmSpeed Charge Pipe
    • GrimmSpeed Top Mount Intercooler w/ splitter
  • STOCK Catback Exhaust
  • STOCK Fuel System

Dyno: Dyno Dynamics
Stock 2015 WRX Baseline: 166whp/181wtq
Fuel: MS109

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Our first dyno session with this kit went even better than we had hoped. We kept very conservative, as the goal wasn’t to make power, but to prove that the setup functions properly. The coolant and oil feed and return for the turbo worked flawlessly on the dyno for 7hrs, which was one of our primary concerns, given the compact space.

Our primary goal for this kit was to provide a driving experience that isn’t unlike a stock turbo car, but with a power curve that leaves nothing to be desired. To achieve that, we knew we needed a turbo that could spool extremely fast but with the potential to produce really healthy power. This preliminary test was to prove the first part, as we’re quite confident in our ability to achieve the second. As the chart below shows, the GrimmSpeed EFR7163 Twinscroll kit spools just 200rpm slower than a ‘stage 2’ stock turbo car, hitting 15.5psi@3000rpm and 19.5psi@3500rpm as it approaches its target boost. With E85 and higher boost targets, we expect even faster spool, but these results alone are more than enough to demonstrate the capability of this beautifully designed turbo manifold paired with a killer EFR twinscroll turbo.

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Although making power wasn’t part of the goal, we know it’s on everybody’s minds, so we’ll share anyway. As you can see this car, in 100% stock form, produced 166whp on the dyno at DB Performance (known for reading very low). With the EFR7163 kit at only 20-21psi and relatively conservative tuning, the car was making 320-330whp and 300-310wtq. On our local dynojet, that equates to a ballpark 400whp/390wtq. The strange behavior at the top of the chart can likely be attributed to the fact that the car isn’t setup to make power (stock catback, for example) and some fuel/spark issues that we’ll sort out when we come back ready to turn it up.

What’s next?

We’re waiting for the weather to get into the 40s next week so that we can run the car on the road and do some logging and 2015 TMIC testing before we pull it apart for the following:

  • IAG Performance Stage III Short Block w/ 14mm head studs
  • Headwork/valvetrain
  • Supplemental fueling so that we can run E85
  • Water-meth
  • Capable clutch
  • Catback exhaust
  • TGV delete
  • Data acquisition hardware/sensors

Then we’ll be back to the dyno to push the car and find its limits. That’s all we’ve got for now! As many of you know, this project is also a way for us to push our 2015+ WRX TMIC pre-production units to their limits (maybe?). We’ll keep you updated on that front as well, but all we’ll say for now is that they’re keeping up with this kit very nicely so far.

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.