Fe 427 hp?

[Excaliber]

When my original side oiler was dynoed (out of the car) adding the side pipes resulted in a 97 horse power loss. These are 4" round pipes with the typical restrictive stuff inside.

Of course the more power and the higher the rpm at which that power is made will have a big impact on how much you loose on any given engine. If the cam and intake are the "bottle neck" then side pipes will be less of an issue. With lower rpm your not flowing as much air so there again, side pipes will be less of an issue.

My engine was making power in the 7,000 rpm range, the "bottle neck" WAS the side pipes. So in my case it was a rather dramatic loss of power. It would be perhaps half of that if the motor couldn't breathe well from an intake bottle neck.

Of course on top of the side pipe loss you also have to add drive line and accessories power loss when your dyno the rear wheels with a car, not a bare engine. Add another 20% or so loss on top of the side pipes. End result can be a very discouraging horse power number, but it's all relative.