Interestingly, the undercut valves do two different things with the different heads. With the 291 heads, the straight-stem valves perform better. But when the carb and intake are added the flow numbers are virtually the same. This shows that the two-barrel carburetor becomes the choke point and the undercut valves no longer have any detriment to the flow. In this situation the undercut valves would definitely be a better option (if cost isn't a factor) because the reduced weight will allow better high-rpm valve control with no tradeoff on flow efficiency.

But, for some reason the undercut valves actually flow better on the bare 462 heads than the straight-stem valves. However, when the carburetor and intake are added to complete the intake tract, the straight-stem valves perform better. We aren't sure exactly why this happens, but it does prove that it's always better to look at entire engine systems as a whole versus several small components individually. If you tested only the cylinder head and the undercut valves on a flow bench without looking at how adding the carburetor and intake manifold affected the intake tract, you would mistakenly be led to believe that the undercut valves are the better option.

Wet Flow
The wet-flow setup works off of the flow bench and uses a liquid flowing through the head to show how fuel reacts as it moves past the face of the valve. This is only a visual test, there are no numbers or measurements involved. Also, although the tube is several inches long, during wet-flow testing you should really only look at how the liquid behaves right around the face of the valve and the combustion chamber. UNCC

A comparison of how fuel (in green) flows through the 462 cylinder head as the intake valve opens. The first shot is with the valve at 0.150 lift, the second shot is at 0.300 lift and the final shot is as 0.450 lift. Woroniecki points out that as lift increases, accumulation of the "fuel" on the surface of the combustion chamber around the valve decreases. This should result in a more efficient burn. UNCC

Head Specs
  462 291
  Intake Exhaust Intake Exhaust
Valve Dia. 1.94 1.5 1.94 1.5
Stem Dia. 3.43 3.43 3.43 3.43
Throat Dia. 1.59 1.305 1.59 1.305
Seat Angle 45 45 45 45
Port Volume (cc's) 171 75 171 76
Avg. Port Width 1.074 1.303 1.092 1.322
Avg. Port Height 1.927 1.405 1.950 1.373
Port Length 5.0445 2.506 4.899 2.546

Port Mapping
Port mapping is also done on the flow bench. This time Dallman placed a pitot tube in different areas of the port and measured the amount of flow. Then he entered those numbers into a program that created a color-coded map showing the rate of flow through the port. A map like this can show you the areas of the port that move air most efficiently.

As you can see with the maps, the intake ports of both heads flow air most efficiently across the floor. This is consistent with most heads of this type. But the 291 head is capable of flowing air across the floor at a faster rate than the 462 head both at medium and high lift.