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by Mike Holler
WARNING. As with all engine work, proceed at your own risk. See the Terms of Service.
In our modern world we are making great strides to improve upon the century and a half old Otto cycle internal combustion engine. Cylinder head and manifold design, along with camshaft profiles are improving continually. Another area of dramatic improvement is in piston design. If you are fortunate enough to have a popular engine, many of these new design pistons are available off the shelf. If, however, you are like me and favor the out-dated Mopars of yesteryear, you are left out in the dark.
While I have owned small block terrors, big block behemoths, Slanted A-bodies, and a few of the truck varieties, I’ve been tinkering mostly with the ’80s Turbo Dodge selections lately. While companies are releasing stroker packages to create engines big enough to drive a train, the four cylinder nephews are left from whence they came; in the past.
Back in the early ’80s, when Direct Connection was attempting to make the newly introduced 2.2 engine into a contender, TRW was commissioned to crank out a flat topped forging for a bit higher compression and improved durability. These pistons are listed on eBay from time to time. I happen to have a set or two and decided to look into the possibility of turning these dinosaur droppings into something of a modern marvel.
Figure 1: Careful mapping of the cylinder head's features.
A plan of action is requisite for a quality product. The concept was worked out, then mapped out, and then implemented. Measurements were taken of the cylinder head to place key points of the head on the piston top. The piston was then strapped to the rotary table on the mill to receive a Cellini work-over. Markings were scribed in red ink from a felt tip.
Figure 3 Piston strapped to rotary table on mill.
When the spark plug fires, it requires about a 13:1 or richer mixture to generate a good solid flame kernel (even though the overall mixture is happy being leaner). With this in mind, a dish was bored into the piston right under the spark plug. This will be the thinnest part of the piston at a depth of 0.18” deep, leaving 0.24” of material. The bit is then raised slightly to a depth of 0.15” and moved laterally toward the exhaust valve side. A trench is machined from the spark plug hole the whole way to the piston deck border, then back to the perpendicular dish border. Another trench is run to mark this border. At the other end, the trench is then run to the deck line again. Basically I just carved along the lines.
From here the bit is moved to piston center, then toward the exhaust side to the deck line again. Centering the bit is crucial to maintaining a concentric margin for the deck surface. If the bit isn’t centered, then the margin will either get wider or narrower as the rotary table is turned. The table is then rotated in one direction until it meets one of the previous trenches, and then the direction is reversed.
Now the outline of the dish has been established. The bit is shifted back and forth to remove the bulk of the material, leaving a coarse dish. The round nose bit is then replaced with a flat nosed bit to smooth out the floor.
The piston is then placed on an angle to create a ramp from the exhaust side dish to the higher intake side deck. Initially the round nose bit is used and the majority of the material is removed. Clean-up is done with the flat nosed bit.
Once the piston has been machined, a sanding roll is used in the die grinder to smooth out all sharp edges and blend the transitions. Medium grit emory cloth takes out the rough texture left by the sanding roll. A bit of Scotchbrite gives the piston a surface texture that will be quite resistant to carbon build-up.
As with any blueprinted engine build, the dish on all 4 pistons is measured with a CCing burette. Material is removed to equalize the largest volume on all of the pistons.
So here’s how the piston should work in the engine. The thinner the deck material the cooler it will be. The bottom of the piston is exposed to squirting oil from the connecting rods. Cooler piston areas will collect heavier liquid fuel droplets. The thinnest part of the piston deck is the cup under the spark plug. Therefore, the richest mixture in the entire cylinder at TDC should be right under the spark plug. How convenient!
Today’s engineers agree that combustion is more efficient on the exhaust valve side of the cylinder. Putting a dish on that side encourages the early stages of combustion to happen there. Venting the cup under the spark plug to this area further promotes that action. Since the exhaust side is of thinner material than the intake side, the dish will attract more of the dense liquid droplets than the less efficient intake side.
After combustion is underway, the ramp gives transition to the intake side. By this time, the piston is just beginning to move down the bore. Hopefully, we can have at least 60% of the charge combusted by 15 degrees ATDC and 90% + by 30 degrees ATDC.
Some of these tricks can be applied to conventional pistons. I’m using a set of Wiseco Forgings for a high output 2.2 Turbo rebuild. The pistons are shipped from the factory with sharp edges and machine marks. A sanding roll will give a small radius to these sharp edges to reduce hot spots and promote flame propagation. The same process applies where medium grit emory cloth will reduce the marks from the sanding roll, and Scotchbrite will give the piston a preferred surface texture.
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