Over the past 15 years at least 150 horsepower has come from cylinder head improvements on 355 cu inch oval track engines. During the same time span 467 cu inch oval track big blocks have increased 200 hp. Intake manifolds had to improve along with cylinder heads so they did not become the restriction. In the same time span there have been no more than 3% power gains in each of the following: cams, headers, carburetors. In other words you can take a cam of a relative size available in 1973 and a similar size current day cam and you will not see more than 3 % power increase. Most of the time there is a power gain by working out the camshaft - rocker arm - header - intake manifold combination for a particular engine. Power improvement from each individual item is typically only 1/2% to 2%. All four will not total more than 8% unless one item is terribly wrong. For example, headers that vary 17" in primary length with an average length of 20" will lose 40 to 50 hp. (Jere Stahl) www.stahlheaders.com Surf around this guy's site for some informative and great reading. He shows the difference between the pros and the amatuers (I'm not even sure I make amatuer anymore).
My local machinist (roundy round chevy, some Buick) sounds like the news letter. Wealth of information. He's on of those guys who, you can't get away with building a motor, out knowing him.
