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Honda CRX GT-4 Engine History Lesson - What You Never Knew

Dissecting Honda's race-winning, 1980's CRX SOHC powerplant

Aaron Bonk
Sep 9, 2014
Photographers: Doug Peterson, Nate Haines

As early as the first half of the 1980s, Honda sought to capture the attention of American performance enthusiasts. Before that, the company's racing efforts remained exclusively overseas as did the availability of high-performance wares from its affiliate, Mugen. The solution, according to Honda's executives, was to test American waters with the company's first-ever North American racing program, based upon its most performance-minded vehicle of its day—the CRX Si. Success would mean an abundance of interest in the company's grassroots racing efforts and nationwide availability of Mugen goods through authorized Honda dealerships.

1980s honda CRX GT4 engine honda GT4 team 01 Photo 2/8   |   Honda CRX GT-4 Engine History Lesson - What You Never Knew

Built at the hands of Honda Special Projects team members Charlie Curnutt and Dix Erickson—the automaker's in-house North American quasi-race preparation arm (among many other things)—and in collaboration with Mugen, the CRX was designed to compete in the SCCA's then GT-4 class. More specifically, driver and Comptech cofounder Doug Peterson says, the whole idea was to win the SCCA Runoffs race held at Road Atlanta in October 1985—a big deal at the time if you ask him.

Honda campaigned its CRX from 1985 to 1990, securing a number of wins and pole positions at every GT-4 Runoffs race throughout its six-year career. Its chassis, draped in Mugen's colors, and its suspension, designed at the hands of Honda Special Projects, shouldn't go unnoticed, but it's the CRX's powertrain, initially constructed by Mugen and later refined by Comptech, that you really care about.

1980s GT4 honda CRX 03 Photo 3/8   |   Honda CRX GT-4 Engine History Lesson - What You Never Knew

The Block

The Mugen-prepared single-cam engine was based upon the CRX Si's original engine block. Inside, the block remarkably retained many of its factory-issued pieces, like its crankshaft, 1mm-oversized cylinder liners, and connecting rods that underwent a simple polishing process before being balanced. Peterson says, "The [initial] block was amazingly stock other than [its] high-compression, cast-aluminum pistons." Peterson recalls the compression ratio measuring in at roughly 12.0:1 once the bore increase and higher-compression Mugen pistons had been fitted into place.

Mugen didn't just deliver the fully prepared long-block to its American affiliates with nothing more than well wishes, though; it sent along engineer Takashi Uno, who became responsible for maintaining the engine as well as to protect Mugen's U.S. interests. Peterson says, the engine's success was "life or death" to Uno, who moved on from the CRX project but remained in the United States throughout 1988 as American Honda campaigned a pair of Mugen-outfitted Integras in the IMSA series. Although Uno remained a fixture among the team for nearly four years, in 1986 engine development and maintenance was delegated to Northern California racing firm Comptech. Prior to 1986, Uno had rebuilt the engine three times, but any development beyond what Mugen had initially done wouldn't happen until Comptech was called upon.

1980s honda CRX GT4 engine 04 Photo 4/8   |   Honda CRX GT-4 Engine History Lesson - What You Never Knew

In 1986, Comptech reevaluated the engine, including its cast pistons and factory connecting rods. "Beautiful parts," Peterson says, "but not very strong. When I drove the car, I didn't rev the engine beyond 7,500 rpm if I didn't have to." As such, Comptech reassembled the short-block, this time with one-off Carrillo connecting rods and custom forged pistons that resulted in a 13.0:1 compression ratio—the maximum that could be achieved, Peterson says, without severely disrupting flame travel. The original steel oil pan, which had previously cracked upon high-rpm abuse, was also updated with the Integra's more rigid cast-aluminum piece. "After we developed and added stronger components, the engine would run a whole season," Peterson says.

The Head

The initial engine as prepared by Mugen was based off of the Japanese-spec cylinder head that'd been ported for increased airflow. Inside, it featured its original valves, a reground factory camshaft, and stiffer, dual valvesprings, also from Mugen. Peterson says, the cam wasn't anything particularly aggressive, which resulted in moderately increased lift and duration.

1980s honda CRX GT4 engine valve cover 06 Photo 5/8   |   Honda CRX GT-4 Engine History Lesson - What You Never Knew

A number of changes implemented within the top end by Comptech once it began its development process resulted in a 33hp gain. Here, intake valve diameter was increased, but doing so wasn't as easy as you'd imagine. "The three-valve layout limited valve size since the intake and exhaust valves would hit each other on overlap if their sizes were increased," Peterson explains. As such, only one of each chamber's intake valves were made larger, which allowed valve lift to be increased, pushing peak power from 165 hp to 198 hp as measured on Comptech's in-house engine dyno.

1980s honda CRX GT4 engine intake manifold 05 Photo 6/8   |   Honda CRX GT-4 Engine History Lesson - What You Never Knew

The Induction System

Fuel injection wasn't permitted within the SCCA's GT-4 class, which led to the CRX's 44mm Mikuni carburetor layout. Mugen fabricated a one-off, billet aluminum manifold that allowed the carburetors to bolt up seamlessly onto the cylinder head. A pair of low-pressure fuel pumps fed by a fuel cell delivered 110-octane gas into the combustion chambers. Nearly 30 years later, the air induction system remains virtually unchanged.

The Oiling System

The CRX's original wet sump oiling system was discarded in favor of a custom dry-sump system designed and built by Mugen. The three-stage oil pump featured two sections dedicated to scavenging oil from the pan and a third that supplied a high-pressure stream into the engine. The system was also made up of a custom oil pan, an oil reservoir located within the passenger-side foot well, a custom cast-aluminum oil filter housing, and -12 steel-braided hoses that circulated fluid throughout.

1980s honda CRX GT4 engine dry sump oil pan 07 Photo 7/8   |   Honda CRX GT-4 Engine History Lesson - What You Never Knew
1980s honda CRX GT4 engine dry sump oil system 08 Photo 8/8   |   Honda CRX GT-4 Engine History Lesson - What You Never Knew

The Exhaust Manifold

The CRX's exhaust system was based off of a 4-into-1 design that featured a relatively flat exhaust gas collector underneath the oil pan. Peterson says, going off course was highly discouraged and would undoubtedly result in all sorts of expensive exhaust system damage. Mugen, who commissioned the header, had it fabricated out of equal-length tubing that was formed and bent using a primitive albeit effective method. Here, the series of tubes were packed with sand, heated, and bent into shape by hand before being trimmed and welded to their corresponding flange and collector. "A work of art," they were, he says. The race-winning header, says Peterson who visited its production facility, was handbuilt in an 800-square-foot shop in Japan by three workers who barely had room to function.

The Ignition System

Spark was delivered using the factory distributor and ignition control module. Peterson says tuning was simple. Among the only changes made was a crank-trigger ignition implemented by Comptech that increased timing accuracy. "Tuning was pretty basic," he says of its extent, which didn't go far beyond jetting changes, "I loved it."

Peterson also says that prior to Comptech's involvement in 1986, the engine had yet to be dyno tested in the United States, although it'd undoubtedly been initially tuned at Mugen's headquarters the previous year. Baseline figures measured in at 165 hp at the end of 1985, after Peterson had secured the Runoffs victory that he and Honda had been working toward but before any changes had been implemented, which resulted in 198 hp by 1989.

Peterson, who along with Erb assumed care of the CRX program following Uno's placement among the company's IMSA efforts, says that, despite the Runoffs victory, the program later lost some of its importance. "But it was a step [toward] the next project," he says, "which was the 1,600cc four-valve [engines] we ran in the [IMSA] Integras."

Despite any perceived lack of interest, though, Honda's GT-4 racing efforts did its part in accelerating the Honda performance movement, not to mention the notoriety of Peterson, Erb, Comptech and Parker Johnstone who'd later take over driving duties. Honda's GT-4 CRX was America's first real taste of what's become a three-decade-long love affair with Mugen, has resulted in a number of victories for Honda, and, most importantly, what will always be the company's first automotive racing venture in the United States.

By Aaron Bonk
383 Articles

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