By Ben Mozart
The race engine requires a precise mixture of air and fuel, approximately 13.0:1 by weight ratio.
But the power it makes depends upon how well the mixture is emulsified and atomized. How well it is delivered through the intake manifold runners and cylinder head ports. And it’s ability to negotiate the intake valves and to swirl in the combustion chambers, which are an extension of the ports, and to occupy the cylinders.
For most of us, arranging and controlling the movements of the gases in the cylinder head ports are beyond our imaginings. Is the air-fuel mixture moving efficiently in the intake tracts or clinging, vexingly, to its sides? If so, how could it be reintroduced into the air stream? And further downstream, how is it negotiating the short turn, the five valve-angles in the throat, and does it demonstrate swirl as it moves into the combustion chamber?
Over the past three decades, Dart Machinery has shown how it identifies and retains its engineering capital. Improvements in its power-making abilities are continually being added to its range of competition cylinder heads—first the aluminum models are upgraded then the cast iron. Today they use cylinder head flow data derived from their wet-flow bench, they then monitor the advances on the dynamometer and finally confirm the revisions on the race track.
Wet flow technology: Acquiring the unfair advantage.
At the turn of this millennium, Dart invested $80,000 in the largest and most elaborate wet-flow bench our aftermarket has known. Devised to better understand the flow characteristics in Pro Stock cylinder heads—chiefly wet flow behavior in the ports and combustion chambers—it took a further five years to fully understand the data it created. “Gaining empirical data from the wet-flow bench improves our minds,” says Dart’s founder Richard Maskin, so here is a sequence of images displaying air-fuel streams in the cylinder head ports and cylinder that might stimulate the ambitions of other minds.
To observe wet flow patterns in cylinder head ports Dart removes the metal port sides and replaces them with transparent Plexiglass.
Looking through the oblong Plexiglass window reveals the air-fuel movement. The roof of the port usually provokes higher air-fuel speeds, but to the trained eye a clockwise vortex is apparent which has greater significance. The brighter blue indicates more fuel in the air while the greener areas suggest heavier fuel.
The combustion chamber is an extension of the port and the chamber shape directs the curved fuel stream, arrowed and seen in the background, around the spark plug—not over it. The spark plug is obscured by the arrow head. To the right and in the foreground, the wider air-fuel stream is issued from the short turn.
Look closely and you can see the intake valve open by around 0.500 to 0.600in. The air-fuel steam on the left has traveled around the long turn and around the spark plug and is pictured running down the left side of the cylinder, the exhaust side. Slightly to the right the darker stream is also coming from the long turn, around the spark plug and down the bore. Moving slightly further to the right the lighter color indicates another stream of fuel but from the short turn. The goal is to make the fuel streams as wide as possible. Wider streams have smaller air-fuel droplets.
Note: The experienced eye can detect the air-fuel mixture shearing off the valve.
These three images depict wet flow patterns of a big-block Chevrolet at 0.300in, 0.500in and 0.700in valve lift. Notice how the wettest part of the stream becomes wider and better atomized as it spreads down the cylinder wall as valve lift increases.
For a small-block camshaft using a 0.440in lobe Tony McAfee tests up to 0.660in valve lift in 0.100in increments. But before he begins the wet-flow tests, he collects data, primarily lbs-per-hour of fuel and CFM information at each lift point at 55in of depression. This bench not only provides visual evaluation but also measures the fuel and records the findings at the desired air-fuel ratio.
Troy, Michigan, USA