Big question about exhaust/intake

[shinn497]

So I've been looking into modding subarus and I have a question about the exhaust/intake. IT seems that moderate horse power gains can be achieved with using aftermarket variants of these. In particular, I've heard that the downpipe (I think), is heavily inefficient. With that said, would FFR's adjustments possibly cause an increase or decrease in engine performance. IF the engine is mounted from the front to the rear wouldn't this increase exahaust efficiency? Similiarly speaking how would the changed engine position effect the intake? The tertiary question is then would the intercooler also be effected, since I know subby owners switch from a top mounted to front mounted intercooler.

How does this change effect the GTM? Would it be more significant since the WRX donor is a FI car? IT would make me very excited if the exhaust that comes with the kit is more efficient purely due to its decreased length. FFR is a performance oriented company so I am not surprised.

[apexanimal]

it's the uppipe thats generally considered on the "to do" list of most modders as high on the priority list...

packaging wise we'll really have to see how the body turns out... they will take a look at channeling air into the bay from high-pressure zones, and create an inherent low-pressure zone in the engine bay so i have a feeling the tmic has a chance of being much more efficient in the 818 than a scoob...

exhaust wise, we're going to be seeing a much much shorter exhaust routing... that alone will help performance...

intake, again, we're going to have to wait and see... intake plumbing is generally quite easy with many different ways to do it... i'm personally not worried about any of that...

[bbjones121]

The downpipe is the first to usually get switched out. That is where the most gains will be as it will eliminate two cats. The uppipe has a single catalytic converter on some models. Usually you get an aftermarket uppipe for this or you buy one off a year that didn't have it. An intercooler the size of the top-mount is capable of close to 500hp out of the 2.5l, I am almost there with my Legacy and I am not running the largest turbo I could be. If you want to reduce heat soak on a hot day, want to push more than 500hp, or if you have the flanged turbo of the Legacies and more recent WRX's and want to utilize standard turbos, you would go front mount.

First, I think that over 500hp would be overkill on the 818, if you are worried about heat soak, it would be easy to add an intercooler spray. The donors are most likely going to have the non-flanged turbo housing (i.e., does not need to bolt directly to the intercooler), so you can be more flexible about location.

One thing that is nice about the limited space is that it will most likely require that you keep an unequal length exhaust header. This is one thing that distinguishes the exhaust note coming from this engine and it would be nice to keep it or refine it slightly. I imagine that the exhaust system will most likely stay the same. I am guessing that a WRX exhaust components are very similar to my legacy besides the twin tips I have out back. The turbo sits directly behind the engine block and sticks out about 10 inches (I will actually measure this tomorrow if I remember), then the downpipe goes down at about a 60 degree angle (this could probably be made more vertical to compact even more for potential trunk). For the 818, you would have a muffler right off the mid section of the downpipe or right off the downpipe section (but most aftermarket downpipes are a solid piece comprising of the downpipe and the midpipe).

Here is a WRX diagram courtesy of scoobytuner(dot)com http://scoobytuner.com/graphics/ExhuastDiagram.jpg

[Draco-REX]

The intake is going to be trickier. To keep costs down, FFR will have to make it so that the stock airbox and intake path can be maintained. Otherwise a tune will be a requirement of an 818 as Subarus MAF design is sensitive about the intake it's placed in. Granted, if you're getting a high flow downpipe, you're going to want to get it tuned anyways. But to minimize extra costs, the 818 will have to be able to mount the stock DP and stock airbox so a $150-$300 tune won't be a necessary.

The only other thing I could see happening would be for FFR to come up with their own exhaust and intake and offer a mail-in ECU tune for their 818 kit. Which wouldn't be a really bad thing, as it would make even a base 818 more powerful and have better driveability.

[305mouse]

I think most people do a DP first because it's easier to do than an UP. For 02-05 That UP is restrictive. With FF providing the exhaust, I hope that comes with a new catted DP assembly. With how the body and chassis work with a shorter distance, I could see a DP that goes straight back instead of that fun curve, then a cat headed to a muffler with hopefully a twin exhaust or a centered exhaust. Seeing as how the tranny is right there, a twin would probably be easier.

I wouldn't expect them to offer any ECU tune. That would put them out of their realm, unless they partnered with some tuner who would be willing to do something like that for them. But that would be assuming that everything else would be staying stock. Some people might keep the cat in the UP of their 02-05 donor and other people might get rid of it. That would create a cell and for those without the technical knowledge, they would need a tune. You could fix that thought with a resistor as well.

[apexanimal]

a tune for the intake is only really needed if the path in front of the maf is changed, but the track behind it can change without detriment and usually for some throttle response gain...

[BipDBo]

The engine bay on the WRX recieves ram air at high speed, so it is under positive pressure. This, combined with the negative pressure at the hood will be under negative pressure on the hood explains the very high hood scoop. It's just an uphill battle getting air through the intercooler. With the large scoop , the top mounted intercooler works well on the WRX when it is moving fast, because that's what it was designed for. When idling, or reving your engine at the start line of a drag strip, though, the heat from the engine and the radiator rises with convection and engulfs the intercooler causing heat soak. All these reasons are why many people switch to front mount.

With the 818, these problems will mostly be minimized or eliminated. The main reason is that most of the engine heat is rejected through the radiator, but with the radiator in the front of the car and the intercooler in the back, it will be much harder to heat soak it than with the WRX. There is no advantage, therefore in switching from top mount intercooler to front mount. That would require a lot more ducting, which would cause more turbo lag. Also, the 818 engine bay will be under negative pressure, so it will be much easier to get air through the intercooler into the engine bay. If the intakes are very low as they are on Rodney's car, it may be best to flow air through the intercooler in reverse, upflow, and to reject upwards, through the "rear hood." This would also work better when stopped because convection would keep the air moving upward.

The engine bay should be well ventilated, though. Jim's design as well as Ferarris, etc. or most any other mid engine car, has a grill of some sort over the engine to allow heat to escape. Also, the combustion intake should be routed to accept cold air, but this is easy, Engine Performance 101 stuff.

I assume that part of the standard build will be to replace all of the exhaust from the turbo, rearward. This will be a shorter, less convoluted routing, so there will be a small power bump from that.
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