Having tested this on a dyno.
No.
I'm making 138hp with the standard cast manifold, with catalytic converter and exhaust.
I had a 4-2-1 branch manifold from a popular company, made the same power (actually made 0.5hp less peak but that could have been a difference in the dyno) with or without cat.
This is how my engine has been for 10 years, it's been on 4 different dynos and made the same power each time.
The last 3 power runs were on the same dyno, as I need a new power certificate for competing every year.
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The rain kept a trollin'
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Performance exhaust manifold
Linear Mode
