engineermike
Well-Known Member
- Thread starter
- #1
I recently did some exhaust pressure testing and thought I would share my results for anyone interested. I've done exhaust pressure testing before and found power by relieving it. For reference, Lingenfelter IIRC once wrote that exhaust pressure should be kept under about 5 psi, but turbo setups can run over 50 psi.
The setup is a Stage 2 Whipple 2018 Mustang A10, running 93 octane, the XDI HPFP, and the 3.625" pulley for about 13 psi boost. The exhaust pressure rises with rpm, so the below numbers were recorded at peak pressure at the shift point, or about 7400 rpm.
Full stock exhaust measured 15 psi ahead of both cats. One cat is fairly new with about 5k miles, while the other is original with about 33k miles, but they both measured the same. My assumption was that most of the pressure was being imposed by the cats so I moved downstream of the cats and, to my surprise, measured 13 psi.
Next, I swapped to a Magnaflow Street series cat-back (model 19370). I measured 11 psi ahead of the cats, for a reduction of about 4 psi. Downstream of the cats measured about 6 psi, so the cat-back successfully knocked 7 psi off of the after-cat number.
On one hand, the Magnaflow reduced the after-cat pressure quite a bit from 13 psi to 6. On the other hand, I honestly expected it to be lower with the dual 3" system with straight-through mufflers.
Another oddity you might notice is that the cat dP went from 2 psi to 5 psi. The cats, of course, didn't suddenly become more restrictive. What likely happened was that the cat-back reduced the pressure at the cats, which in turn increased the volumetric flow rate of the exhaust and, thus, the velocity through the cats. With higher velocity comes higher pressure drop.
Next, I am wondering how much the 2.25" factory-connection is hurting it. It's 2.5" upstream and downstream of this connection and it's obviously the smallest diameter in the system.
On a final note, with a knock-limited supercharged Coyote, spark timing is the key to making power. The reduction in exhaust pressure of 4 psi should allow about 1 deg of additional spark timing according to the link below and professional contacts, which in turn would make about 20 more hp due to timing alone.
https://etd.ohiolink.edu/apexprod/r...ssion=dayton1375262182&disposition=attachment
The setup is a Stage 2 Whipple 2018 Mustang A10, running 93 octane, the XDI HPFP, and the 3.625" pulley for about 13 psi boost. The exhaust pressure rises with rpm, so the below numbers were recorded at peak pressure at the shift point, or about 7400 rpm.
Full stock exhaust measured 15 psi ahead of both cats. One cat is fairly new with about 5k miles, while the other is original with about 33k miles, but they both measured the same. My assumption was that most of the pressure was being imposed by the cats so I moved downstream of the cats and, to my surprise, measured 13 psi.
Next, I swapped to a Magnaflow Street series cat-back (model 19370). I measured 11 psi ahead of the cats, for a reduction of about 4 psi. Downstream of the cats measured about 6 psi, so the cat-back successfully knocked 7 psi off of the after-cat number.
On one hand, the Magnaflow reduced the after-cat pressure quite a bit from 13 psi to 6. On the other hand, I honestly expected it to be lower with the dual 3" system with straight-through mufflers.
Another oddity you might notice is that the cat dP went from 2 psi to 5 psi. The cats, of course, didn't suddenly become more restrictive. What likely happened was that the cat-back reduced the pressure at the cats, which in turn increased the volumetric flow rate of the exhaust and, thus, the velocity through the cats. With higher velocity comes higher pressure drop.
Next, I am wondering how much the 2.25" factory-connection is hurting it. It's 2.5" upstream and downstream of this connection and it's obviously the smallest diameter in the system.
On a final note, with a knock-limited supercharged Coyote, spark timing is the key to making power. The reduction in exhaust pressure of 4 psi should allow about 1 deg of additional spark timing according to the link below and professional contacts, which in turn would make about 20 more hp due to timing alone.
https://etd.ohiolink.edu/apexprod/r...ssion=dayton1375262182&disposition=attachment
Sponsored
.