That’s flywheel horsepower. As a general rule, tightening up the lobe centerline increases both the torque and horsepower numbers. Several things happen when the lobe centerline is reduced. First, the intake valve closes earlier thus providing an increase in the cranking and lowend cylinder pressures. Second, both the intake and exhaust valves are open together for a longer period of time which allows exhaust scavenging to be a bigger player assuming the exhaust system isn’t so restricted as to prevent this. Scavenging done properly helps to draw in the intake charge earlier by using the negative pressure that’s generated in the exhaust system at the tail end of the exhaust purge. Third, the exhaust valve closes later which results in a reduced manifold vacuum.
The drawback in reducing the lobe centerline angle is that the operating range does get narrower or more peakish while low rpm manifold vacuum also decreases. Because the exhaust valve does close at a later point with a reduced lobe centerline angle, this presents itself as a vacuum leak until the exhaust valve is fully closed; hence, the reduced vacuum seen at a gauge. And if running a single plane intake or even an open spacer under the carb, then that same valve overlap on an adjacent cylinder can come into play as an ‘internal’ vacuum leak. Said another way, one cylinder trying to pull in an intake charge can see an exhaust valve open on another cylinder and has to overcome this. Maintaining a true dual plane concept at the carburetor can minimize this particular effect and can be worth as much as an additional 1½”Hg to the vacuum gauge reading.
To a point, advancing or retarding the camshaft can move the operating rpm band around some while also varying the lowend vacuum attributes. Look for the idle to become increasingly more choppy as the lobe centerline is reduced while everything else about the camshaft remains the same. The reduced manifold vacuum simply makes carb tuning more ‘touchy’ in that lower rpm band.
A recent Y build (318 cubic inches) used a camshaft ground on 107° and that did produce a very choppy idle but peak horsepower and torque numbers were extremely good. And all this with this being a pump gas engine designed to be a daily driver. Because the overall intake and exhaust durations were kept on the lower end of the scale, drivability is still good even though the idle is ‘race car’ choppy. The particulars on the 318” Y build was in Y-Block Magazine issue #109.
Lorena, Texas (South of Waco)