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2005 FXT
353 Posts
Discussion Starter #1
I found this on the websight of a company called PDE,

"Why does the WRX / STi / Legacy Downpipe reduce to 2.5 inches at the exit? Back to Top

All PDE downpipes for the Subaru line reduce to 2.5 inches at the exit so the stock Subaru "donut" gasket can be reinstalled. There are two reasons for this:
1. The downpipe is bolted to the turbo, and also to the transmission in the same location as the stock Subaru downpipe. Because the downpipe is attached to both the turbo (engine) and transmission, when the engine moves on the engine mounts relative to crankshaft acceleration and deceleration the downpipe must move with the engine. The remainder of the exhaust system behind the downpipe however is attached to the chassis and not the engine, and so must be able to move independent of the engine and transmission.

For this reason Subaru installed a "donut" style gasket with compression springs between the downpipe and remainder of the exhaust system, enabling the donut gasket joint to flex when the engine and transmission move on their mounts. PDE engineers considered using a 3.0" bellows type flex joint to allow the required movement between the downpipe and the remainder of the exhaust, but after careful analysis on the Superflow flowbench determined that because of the very short (0.625") length of the 2.5" reduction exhaust flow was compromised in a very minimal manner and determined that using a quality aftermarket bellows type flex joint was not worth the additional manufacturing expense.

In fact, during development of this product exhaust flow characteristics between the current design and a bellows type flex joint showed zero measurable difference on the chassis dyno. It was found however, that when a donut gasket or flex bellows was not installed on the system, some vehicles created a harmonic vibration through the exhaust system at certain highway speeds, an unacceptable compromise by PDE standards.

2. The second reason for the continued use of the stock Subaru donut style gasket on PDE downpipes is simply because nearly all aftermarket cat-back systems are designed to attach to the stock Subaru downpipe. Because the stock downpipe uses the donut style flange, and testing during product development showed zero loss on the chassis dyno by using the stock donut gasket, PDE engineers thought it important that our customers have the widest choice possible in choosing which aftermarket cat-back to use on their vehicle when purchasing a PDE downpipe."
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