Isky 270 Mega Cam P/N 390171

The difference in HP between hi-po manifolds will essentially depend on three things:
- how good the headers are
- how efficient the hi po manifolds are
- the cam timing.
Expect less difference with a short cam timing. So I can believe the numbers quoted if cam timing was short or stock-ish.

Factory hi po manifolds ARE headers. They just have very short primary lengths but fit the definition of a header, ie, exhaust dumping into separate pipes or runners that merge into a larger common area [ the collector ].