CP-Carrillo’s New Line of Pistons and Connecting Rods
The trick to building a durable small-block for dirt track racing is to find the best parts that will do the job for the best price. Off-shore pieces are often attractively priced, but durability and reliability are often expensive lessons in what not to do. So at the most recent PRI show, we learned how CP-Carrillo has figured out a way to deliver made-in-America quality at a price budget-based builders can appreciate.
Besides brake pads and rotors, the most abused components in a dirt track car have to be the pistons and connecting rods. Short track racing is all about launching the car off the corner with a deep gear and a screaming small-block through a wide low-to-high rpm curve. That’s the life of a short-track small-block and where CP-Carrillo decided to make some inroads. Their goal was to deliver a set of affordable yet high quality pistons and connecting rods, and it led to what they call their Bullet lineup.
“The best thing about the Bullet line,” says CP-Carrillo’s Bryce Mulvey, “is we took everything we learned in our custom pistons and transferred it into a shelf part. You’re going to get the best of both worlds – a piston with the same technology used in NASCAR parts, and we’ve taken many of those little secrets and put them into a shelf part.”
We thought we’d take a little closer look into exactly what all that means in terms of specific applications like a small-block Chevy. Right off the shelf, the Bullet pistons are not repackaged street engine parts. The Bullet line is made from the classic 2618 race alloy, which is a much more durable aluminum forging than the street forgings made out of a 4032 alloy containing 12 to 13 percent more silicon to contain expansion. The Bullet 2618 material is far more forgiving under hard use and exactly the type of piston that can survive the harsh life of a short-track engine.
The ring package has also been updated to follow the trend toward tin rings. Even today, many “race” engines still follow the 1/16-, 1/16-, 3/16-inch ring formula. But frankly, when production LS engines now use a thin ring, there’s little reason to stay with old technology.
All other variables aside, thinner rings can immediately deliver an immediate power advantage based on simple physics. Thicker rings present a larger surface area in contact with the cylinder bore and that higher unit loading pressure. Thinner rings can reduce this radial load because there’s a smaller contact area.
The entire Bullet piston line has been updated to a more contemporary 1.2mm, 1.2mm, 3.0mm ring package. How does that compare? A 1/16-inch ring is 0.0625 inches thick while the 1.2mm ring is thinner at 0.047-inch. But the real difference is in the oil ring package. The traditional 3/16-inch oil ring measures 0.1875 while the 3mm is much thinner at 0.117-inch. Race engine builders will tell you most of the ring drag comes from the oil ring, so gains here will pay off with a greater return because of the smaller expander. Of course, experimenting with lighter tension expanders also is one way to improve power, but we’ll save that story for a later time.
Other benefits that have come from the racing world include computer-driven Finite Element Analysis (FEA) of the piston design, during which possible failure points can be quickly identified and redesigned to minimize these stress concentrations. This greatly improves piston durability. Other improvements include forged side relief design and accumulator grooves in between the top and second ring, all intended to improve ring seal. According to Mulvey, this makes these Bullet pistons easily reliable up to 650 hp.
As much as we can appreciate the piston being abused, the single-most stressed component has to be the connecting rod. While most might consider the compressive forces imposed on a piston during the power stroke as the most abusive, in fact it is the combination of a heavy piston at high rpm that imposes the biggest threat to connecting rod durability.
This is old news to the CP-Carrillo people whose history dates back to Fred Carrillo in 1963 designing a high-performance race connecting rod that has certainly withstood the test of time. The Bullet rod is based on this long-established history using the company’s famous 4330 alloy net forging steel that is the same alloy used in all its forgings. This allows CP Carrillo to rate these rods to 8,500 rpm.
But with current net forging capability, CP-Carrillo is able to produce an affordable rod with 7/16-inch WMC H11 tool steel cap screws that are one key to their durability. You’ll also notice that these are what CP-Carrillo calls A-beam rods, similar in design to I-beams. The most common application for the small-block Chevy dirt track small-block Chevy would be the 0.927-inch floating pin 6.00-inch rod that is far from the least expensive connecting rod you’ll find. But, consider this as excellent insurance against the alternative. CP-Carrillo even serializes each individual connecting rod and guarantees 100 percent traceability if there’s ever any question.
We touched briefly on weight, and this is one advantage of the Bullet series. A typical Bullet flat-top, two-valve relief piston for a 3.50-inch stroke small-block Chevy weighs in at barely 404 grams (not counting pins, locks, and rings). Combine that with its matching 6.00-inch CP-Carrillo Bullet rod that weighs 545 grams and you have a very lightweight reciprocating system that means a little extra torque coming off the corners at your favorite bull ring.
|Bullet Special SBC||4.00||3.50||6.0||-6.00||404||BCR1320-STD|
|Bullet Special SBC||4.030||3.50||6.0||-6.0cc||408||BCR1320-030|
|Bullet Special SBC||4.030||3.50||5.7||-6.0cc||455||BCR-1350-030|
|Bullet Special SBC||4.040||3.50||5.7||-6.0cc||455||BCR-1350-040|
|Bullet SB Ford||4.030||3.00||5.4||-6.6cc||485||BF6001-030|
|Bullet SB Ford||4.030||3.40||5.4||-8.7cc||418||BF6010-030|
If all this sounds intriguing, the next thing to do is to find your way to the CP-Carrillo website and run through their electronic catalog for the piston application you need. While we focused this story on the popular small-block Chevy, the Bullet piston line also covers GM LS, big-block Chevy, the small-block Windsor Ford, the Chrysler late Hemi, as well as the LA series engines, several iterations of the big-block Ford lineup, and even a few Pontiac and Olds engines. Check it out – and then start building.