[Christ]

engine less header == garbage engine.

Two things happen here... the pulses of air allow the air to be pushed out at pressure, then lower pressure occurs when the valve closes, so air rushes back in around it, cooler than the valve is.

The other thing is that when exhaust is vented into freeair, it can still be on fire, since there is no method of arresting flame (another job that the manifold does.) This basically intensifies the effect of the first problem, according to the tech at BOCES.

If you think that having a wider open pipe will actually cause a problem with your car, watch a drag race on TV one day... see the top fuel dragsters? They don't technically even have a header. THey just have individual pipes that vent the exhaust (and fire).

If this were a problem, this could be a prime example of how to not do it.