Under the Hood: DodgeSRT-4
The shorter piston never reaches the top of the bore; this is actually top dead center. The "ski ramp" piston dome encourages tumble in the cylinder, which improves idle stability and light-load performance. The best wide-open throttle performance, though, usually comes from a piston that comes as close as possible to the flat edges of the combustion chamber (called quench pads). The gas trapped between the piston and the quench pads gets squished out, flying directly into the flame kernel where combustion is starting. This added turbulence speeds flame propagation and reduces the chance of knock. Of course, a taller piston would raise compression, so the ski ramp would have to be replaced with a dish, but that's do-able.
If you add up the crank throw (half the stroke, or 50.5mm) the rod length (151mm), and the pin height (28mm) you'll find the top of the piston 229.5mm above the crank centerline. That's 8.64mm short of the top of the block (238.14mm). If it were us, we'd make the rod 159.64mm, which would improve the rod ratio from 1.5:1 to 1.58:1. Then we'd use a dished piston with flat areas to match the quench pads in the head. Of course, we don't care about idle stability and emissions, and we don't have the simulation software, math skills, or test engines to prove that this is a good idea, but it's a hunch. If we ever manage to blow up a stock one...
View Related Article
|
|