Cadillac Muffler/Exhaust selection based on Target HP

For a zero back-pressure exhaust, it can be shown that one should target 2.2 cfm (cubic feet per minute) of flow through the exhaust per engine horsepower of output.

The Cadillac STS-V has a dual-2.5″ exhaust with dual mufflers.  The stock mufflers probably flow 160-200 CFM each, or 320-400 CFM together.

The stock LC3 engine makes 469 hp through the stock mufflers.  However, this is a supercharged engine so at redline it likely makes 469 +~60 hp = 530 hp or so, and the exhaust needs to be suited to this figure.  Moreover, I would like to tune my STS-V to make 500+ hp not considering the supercharger load, so 560 hp with the charger.  Therefore I am targeting 600 hp with the supercharger load (rounding up).  Therefore the exhaust needs to flow 2.2 cfm/hp x 600 hp = 1,320 CFM.  Each muffler therefore should flow 1320/2 = 660 CFM.

What are some example flow rates for commonly used mufflers on the STS-V?   For 2.5″ mufflers:

Stock:  160-200 CFM x 2 = 320-400 CFM

Borla: 373 CFM x 2 = 746 CFM (at 15 inches of Water)

Corsa: 226 CFM x2 = 452 CFM

Gibson: 267 CFM x 2 = 534 CFM (at 15 inches of water)

Magnaflow 11226 (14326 polished) : 588 CFM x 2 = 1176 CFM (shown on a Dynomax video), at 20 inches of water.

and one that is relatively new but I have not heard tested on the STS-V is the Dynomax VT

Dynomax VT 17956: 841 CFM x 2 = 1684 CFM (shown on a Dynomax video), at 20 inches of water.

The 17956 is not recommended for dual 2.5″ installations.   The difference between the 17956 for single installations or the 17156 for dual appears to be the resistance of the spring for the diverter valve.  This Dynomax muffler has a diverter valve built-in, so at cruise it diverts flow for less noise.  The dual-muffler p/n is 17156.  This muffler is 4.5 in. x 9.75 in. 2.5 in. ID 14 in. Shell Length 19 in. Overall Length.  The Magnaflow 11226 is 4 x 9 x 20 inches.

Dynomax VT video:

I am thinking of either the Magnaflow mufflers, or the Dynomax VT 17156s.  Either should be a better match for the potential of the LC3 S/C V8 than the stock setup.  I had a Corsa cat-back system on my CTS 3.6L and loved it, but they don’t offer a system for the STS-V.

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