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