Tech Talk #8 – Compression Session

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“Under extreme pressure and temperature, the cylinder charge virtually explodes in the chamber.”

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 vestpocket 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 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 pressure spike is almost vertical. Under extreme pressure and temperature, the cylinder charge virtually explodes in the chamber.

Detonation produces a distinctive “pinging” sound that can be heard in a muffled engine as the flame fronts collide and send shock waves through the engine. In a race engine with open headers, usually the only way to detect detonation is with an electronic knock sensor. One of the early warning signs of detonation is the appearance of “pepper” on the spark plug insulators – small black specks that are actually pieces of the piston transferring themselves to the plug. In more serious cases, the spark plug’s ground strap may be eroded or simply melted away by the extreme heat.

It’s important to make a distinction between static compression ratio and dynamic compression ratio. The static compression is what you measure on an engine stand with a burette and graduated cylinder. It’s a simple mathematical comparison between the volume above the piston at the bottom and top of its travel. Dynamic compression is much more difficult to measure because it depends on how efficiently the cylinders are filled when the engine is running. Factors such as camshaft timing (specifically the intake valve closing point) and the tuning of the induction system have a major impact on dynamic compression.

Dynamic compression ratio is what really counts in a racing engine because it determines the actual cylinder pressure. Think of two engines that are identical except for their camshafts. The engine with the shorter camshaft duration will typically have higher dynamic compression at low rpm because the intake valve closes earlier on the compression stroke. The engine with the longer duration camshaft will have less dynamic compression because its intake valve closes later after the piston has traveled farther up the cylinder. On the other hand, if the long-duration cam does a better job of filling the cylinder at high rpm than the short-duration cam, more air and fuel will be trapped in the cylinder and the resulting dynamic compression ratio will be higher.

So what the rewards of high compression that justify the risk of detonation? Raising the compression ratio generally increases power output across the engine’s entire operating range. Compression increases the efficiency of an engine by extracting more energy from the fuel that is burned. When we dyno test restricted oval-track engines with 9.5:1 compression, we usually see a Brake Specific Fuel Curve (BSFC) that’s significantly higher than the BSFC for a high-compression drag racing motor. This is an indication that the fuel is not being used as efficiently. Low-compression motors are usually not as consistent or as sharp on throttle response as high-compression motors – two major considerations in bracket racing.

High compression tends to make the cylinder more “active” by increasing the turbulence in the combustion chamber. This turbulence in the chamber can actually forestall detonation by creating a finely atomized, homogeneous fuel/air mixture that burns quickly. It is a serious mistake to cut compression ratio by installing a thicker head gasket or running more piston-to-head clearance because this destroys the “squish” between the head and piston that is essential to producing turbulence.

The best defense against detonation is to use good gasoline. We recommend VP C-14 for our bracket engines; other suppliers offer gasoline with comparable octane rating. Most racing engines are fairly sensitive to spark timing. Often retarding the spark advance just one degree will pull an engine safely out of detonation. A change of four or five degrees is way too much!

Every engine has its own optimum spark timing. Our 522ci big-blocks run well with 39 degrees of advance, while our 555’s prefer 36 degrees. In contrast, one of our 500ci Pro Stock engines runs best with only 30 degrees of lead. The difference is that the Pro Stock engine has smaller combustion chambers, shallower valve angles, sparks plugs located closer to the centers of the cylinders, more squish area, and lower piston domes that don’t disrupt the flame front. All of these features tend to reduce an engine’s spark advance requirement.

I think that 14:1 compression is about ideal for a bracket racing big-block. That ratio is high enough to produce good power without running on the ragged edge of detonation. It allows some leeway for tuning adjustments and changing track conditions – as long as you use good gas.

The combustion chamber volume in a typical bracket racing big-block with conventional cylinder heads is approximately 118cc’s. The chambers in these heads are relatively deep, and most aftermarket pistons have a steep dome. You can mill .200-inch of material off the dome and only lose 2 or 3 cc’s of volume. Unmasking the spark plug by trimming the dome can more than offset the loss of compression by improving combustion efficiency.

Like ice cream, red meat, and fried eggs, too much compression can be too much of a good thing!