Back
in drag racing's Dark Ages when Reher-Morrison Racing
Engines was building 287ci small-blocks by the dozens
for Modified Eliminator, I used to wish that we could
build engines without fighting for every point of
compression. That desire now falls under the heading,
"Be careful what you wish for." Now that we
are building scores of big-block Chevrolets with nearly
twice the displacement of those vest-pocket
small-blocks, it's very easy to end up with too much
compression!
Big-inch
engines are in fashion, driven by the growing popularity
of Quick 8 and Quick 16 eliminators at many tracks. The
old adage "There's no replacement for
displacement" certainly applies in these fast
brackets. It's generally more cost-effective to increase
an engine's output by adding cubic inches than running
higher engine speeds and installing more exotic
components. Our 522ci and 555ci Super Series bracket
big-blocks are becoming very popular with racers who
want to run fast without the hassles of maintaining a
high-strung, high-rpm engine.
The larger
swept volume of a big-inch engine makes it all too easy
to exceed the prudent limits on compression ratio. With
nearly 70 cubic inches of piston displacement per |
cylinder in a 555ci Rat motor, it's possible to squeeze
out an 18:1 compression ratio with conventional
big-block cylinder heads and pistons. But if the
compression ratio is excessive, an engine is vulnerable
to detonation - the uncontrolled, spontaneous combustion
of the air/fuel mixture in the cylinders. Detonation is
extremely harmful to an engine. I compare it to hitting
your funny bone with a hammer - it sends a shock wave
through the entire system.
The dire
results of denotation are obvious: pinched and broken
ring lands, blown head gaskets, and caved-in piston tops
are unmistakable evidence of detonation. It's possible,
however, for an engine to be running in trace detonation
without any obvious problems. We usually don't see this
more subtle damage until we tear down an engine that's
been running over the edge. Detonation beats up the
wrist pins, distorts the piston pin bores, and hammers
the pin bushings in the connecting rods.
Several years
ago we worked on a project with GM engineers to measure
the cylinder pressure inside a running engine using
special transducers. Under normal combustion, the
pressure curve rises quickly but smoothly as the flame
front radiates from the spark plug across the cylinder.
In detonation, however, the |
