header

^ Nice piece, but full disclosure, they will make no power on a stock engine. Money is better spent on a head port.

True, but they sure look cool.

A company called OBX makes a similar header. Slightly cheaper, looks to be of the import variety, so quality may be an issue.

https://www.ebay.com/i/172344638870?chn=ps

Here's a flange for those wanting to DIY or make a turbo manifold.

https://www.racemufflers.com/Chevy-4-2L-Inline-6-DOHC-Mild-Steel-Header-Flange-p/rm2096-mhf.htm

This is true - the stock exhaust ports were designed with emissions in mind. Even though there are 2 exhaust valve (4 valve head) the sub runners below the exhaust valve, short turn and merge into the common port need work. The stock exhaust port stalls @160 CFM @0.300 lift with 28" water depression on the flow bench. Porting the exhaust will gain 60+ cfm.(37%) increase.

The intake side need a minor clean up on the short turn - some heads exhibit a sharp edge - a quick blend brings the intake flow over 300 CFM @0.450 lift with 28" water depression.

I had flow test results from the Black Opel and my head .. need to find the link again..

With a ported head and custom cams we started to pull vacuum in the intake at WOT above 6000 rpm.. this was with a stock intake & exhaust manifold with an open 6' exhaust pipe. The throttle body is off a 6L LS and it was the restriction in the system.

With a ported head, custom cams and header a street worthy 4200 will crank out close to 380HP flywheel.