Horsepower and torque are words that are much used and little understood.
4-1 systems are not inherently superior to 4-2-1 systems or else we wouldn't complain about the 4-1 manifold and seek out the 4-2 manifolds.
A poorly designed 4-1 header can lose power over a stock US-spec exhaust with cat in place.
4-1 headers are not necessarily designed for high RPM power. They are tuned--or NOT tuned--as is often the case--for power production at a certain RPM range. An equal length header will be at its peak in a very narrow range and it is important to match peaks among porting, cam timing, intake length and header design.
I like 4-2-1 headers for street use because they are more forgiving of the various things that are out of our control--or under limited control from us.
I REALLY like 4-2-1 cast iron-dual downpipe setups for street engines. Cast iron manifolds LAST and LAST and are made right from the factory. The seal forever and ever. I have every confidence that a '74 iron manifold or a Yugo carb manifold, port matched and with a properly designed downpipe, will outperform any header on any reasonable street engine.
EVERY header I have encountered on an X1/9 has required tweaking or overhauling to make it fit right. The flanges are 3/8" at best, which is too thin to match thicknesses with the intake manifold for proper sealing (and not bending the studs) at the outer intake manifold shared holes. The inner holes are usually little chunks of angle iron cut approximately to length and tenuously welded in place. Sometimes it's not angle iron but 3/16" round bar. Either way, the length is usually wrong, causing more sealing problems. On top of all that, the welds are usually poor. Headers crack at the flange, at the collector and at tube-to-tube welds or somewhere in the middle of any tube along the way. They're sexy, but they just don't work great for street cars with street duty and street heat cycles. I am not being a fussy old man. I am being a practical hot rodder.
Headers are great when properly scienced out. On the fabrication bench at Bayless, I have a batch of flanges I ordered up to design a proper street header. Headers are down the road a bit, but when we do offer something, it will work and fit as it should.
