Your Big-Block Chevy Can Produce 530 Horsepower On Pump Gas, Just Perform a Traco-otomy

Originally published in the July 1982 issue of Hot Rod magazine

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By Marlan Davis

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Stretching all the way back to their domination of the Trans-Am series and 24 Hours of Daytona, the accomplishments of Traco Engineering on road courses, in endurance racing, and as the powerplant suppliers for the IROC series are legendary. One visit to their unassuming whitewashed building in sunny Culver City, California, reveals that Chevy power is definitely spoken there. All-aluminum big-blocks and small-blocks, turbocharged freaks (one with an aviation carburetor), and assorted NASCAR "bow tie" motors are scattered about in various stages of assembly.

With all this mega-muscle you've got to wonder which one of these brute powerhouses is manager Jim Jones' street engine. The answer is none of the above, for over in one corner is what appears to be a stock, cast-iron, low-compression (9.5:1) big-block being assembled.Thisis his street engine?! "Yes," says Jim, "but it'll run high 12s in a 3600-pound car on pump gas and street tires, and I'll be able to drive it to work every day."

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Traco considers a forged steel crank mandatory for high-performance use. They check the crank for straightness in this special V-block fixture by measuring runout on the center main bearing journal (of course the journal itself is first mic'ed for roundness). Chevy allows .001-inch tolerance.

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Here Jim uses a 3-point hole mic to check bearing clearance. Traco watches the block's bore diameter closely, "since bearing crush is what keeps the bearings from spinning." Bearing clearance is held at a tight .002-.0025, "the closer the better." Out-of-the-box, unturned cranks are preferred, since clearances can be precisely adjusted with available .001-over and undersize bearings that are only available for standard diameter cranks. Indexing the crank is not critical on a street engine, since the piston spends relatively little time at top dead center. If for any reason the crank has to be turned (up to .020 is okay), it should be re-Tufftrided or carbon-nitrided.

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Since they're good through 720 horsepower, the heavy-duty big-block rods were left basically unromanced, except for being drilled for pin oiling. The holes were drilled out through the bottom of the pin eye instead of from the top down, which can weaken the rod. Center-to-center length is not critical either, as long as the shortest and longest rods are within .005 inch. If you really want a fully-prepped rod, buy a Carillo; they have 200 percent greater big-end strength, come ready to run right out of the box, and the cost is about the same as a fully-prepped L88 rod. Both rod and main bearings are Speed-Pro-marketed Clevite 77s.

That Jim can produce such a potent combination is a tribute both to Chevy's basic engineering excellence (stout bottom-end, bulletproof oiling system, and free-flowing heads) and to Traco's years of research into cam profiles and induction combinations that make power and last (imagine what it takes to build an engine that can win the 24 Hours of Daytona). The foundation is a 4-bolt-main, 4.25-inch-bore block found in thousands of late '60s Corvettes, including Jim's '69. It easily accepts a .060-inch overbore with no overheating tendencies. Combined with the forged 3.76-inch stroke crank, that yields a total of 440 short-stroke cubic inches.

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Contrary to popular belief, it is possible to get a relatively inexpensive 427 forged piston. Use a TRW 11:1 427/435 horsepower replacement, but slice the dome and 9.5:1 compression is attainable with open chamber heads. After milling, the sharp edges on the dome were rounded off to enhance flow and reduce detonation. The pistons come set up for pressed pins; to convert them to a full-floating configuration with Spirolock retainers use a shorter-than-standard Traco 4340 steel heat-treated, hard-chromed, centerless-ground wrist pin. The rings are a .005-over Speed-Pro plasma-moly set that can be custom-fitted to each cylinder. For this application, the stock piston pin oiling groove is adequate, so no additional holes need be drilled. However, the pistons are deburred, especially near the wristpin hole. The skirt's scraper edge is left sharp as it's supposed to scrape oil off the cylinder walls.

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It's absolutely critical to check rod bolt stretch. Any bolt reaching .009 stretch before 67 ft.-lbs. must be discarded. Jim has even seen bolts come through with no threads on them! With boron bolts, three disassemblies are about the max; always change the nuts.

Jim prefers 427-based engines over the larger 454 since the smaller engines make more horsepower per cubic inch due to reduced internal friction brought about by slower piston speed. Forged 427 cranks are much more plentiful (and cheaper), and are all internally balanced. Besides, most of Traco's experience is with the 427, the most common road-racing big-block.

After the block is stripped, cleaned, and deburred, it is bored exactly to the piston size so the slug will just slide down the bore. The consequent elimination of piston rock facilitates accurate deck height checking. A torque plate and Sunnen CK10 hone running a JHU-820 stone is used to achieve the desired final "mirror" finish with the 5/64-5/64-3/16 Speed-Pro rings used. The rings are of conventional configuration, the top being plasma-moly, the second cast-iron, and the third a standard tension oil ring for good oil control. To allow custom-fitting to each bore, the top two rings come .005-inch oversize (although Jim checks the oil ring clearances, too, just to be on the safe side).

Once the Tufftrided crank is checked for straightness, chamferred, and deburred, it is placed in the block along with Speed-Pro Clevite 77 bearings, preferred on both mains and rods for their superior load-bearing ability. If the straight crank spins freely, the main saddles need no further work, but in the event that saddle alignment is required, Traco recommends a unique combination of align boringandalign honing: Hydro Head's machinists and balancers first use a Van Norman boring machine to eliminate any chance of bell-mouthing the bearing bores, then finish up the last .0005 inch with a Sunnen align-hone machine.

The Chevy 7/16-inch bolt, heavy-duty connecting rods are not reworked extensively. There's no need to polish the beams, but the mating surfaces are ground smooth and polished before the big ends are resized on a Sunnen rod machine, which according to Traco is best for avoiding bellmouthing. The rods are not dowel-pinned for bearing retention, since Traco relies on relatively tight bearing clearances to provide the proper crush. Jim says, "Dowel-pinning is a waste for steel rods 99 percent of the time. If the rod distorts enough, you'll have a problem regardless." Either 454 LS6 rods or L88/ZL1 rods can be used. The 454 rods are set up for pressed pins, come with an inferior knurled shank bolt, and lack the L88's final heat-treat, but are a lot cheaper.

Alternatively, the L88 rods are honed for a floating pin and have superior boron bolts. If the bolts on the 454 rod are upgraded to L88 or SPS bolts, you'll never know the difference on the street. You can also rehone the small ends to accept a floating pin, but there's nothing wrong with running a pressed pin, either. In fact, it'll probably last longer on the street, since there's no pin retainer to work loose.

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One aspect of Chevy motors that is nearly bulletproof is the oiling system. On a big-block you can't go wrong with heavy-duty factory-type components, like a ZL1 or TRW oil pump, Chevy windage tray, and Chevy heavy-duty oil pump driveshaft with steel (instead of plastic) sleeve.

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This engine's success secret is the Traco-designed cam. Developed from years of road-racing experience and ground by Engle, the flat-tappet, dual-pattern bumpstick features a broad power band with lots of lift versus duration, so it "fools" the engine into thinking the compression ratio is higher than it actually is. Engle also supplied the dual valvesprings, chrome-moly retainers, and keepers. Chevy "H"-type big-slot rockers and grooved balls provide an economical and satisfactory alternative to aftermarket roller rockers. Traco also likes Chevy 7/16-diameter pushrods, since "any time you increase tubing diameter (even if the wall is thinner) you gain structural strength." Use them with the appropriate guidelines. The rocker studs are Traco's own, and feature a better fillet radius that flares into the hex for added strength, plus they've hardened, heat-treated, and black-oxided. Speed-Pro made the chrome-plated stainless valves that measure 2.19 and 1.88 inches on the intakes and exhausts, respectively.

The retainer that has the least chance of jumping out if you choose fullfloating is a .072-inch Spirolock, according to Traco. They don't believe in bushing the small-ends to accept the floating pins, since that only removes meat from the critical small-end of the rod. Instead, running metal-on-metal is recommended, but that requires a pin with a low coefficient of friction made from material that's dissimilar to the rod's. Traco's own taperwall, chromed pin fits the bill. Its .001-inch-over-stock .991-inch diameter permits honing the piston and rod holes round, while its shorter-than-factory 2.640-inch length allows room for installing the Spirolock retainers in the reworked TRW pistons that, out of the box, come set up for pressed pins. These pistons were intended as an 11:1 427 closed-chamber replacement and offer 9.5:1 compression with a 0 deck-height clearance and .040-thick head gasket once .050 is whacked off the dome. Without the dome massage, compression would be 9.75:1, "too high for today." Why not just use a lower deck and avoid cutting the piston? Because in Traco's experience any combined deck/gasket clearance greater than .040-inch reduces the effectiveness of the combustion chamber's quench area, actually enhancing the chance of detonation.

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Good as the oil pump is, two things need to be checked: the rotor-to-pump-housing clearance and the oil pump spring. Traco also safety wires the pump bolts.

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Cast-iron open chamber heads are much cheaper than the exotic aluminum heads and get the job done about as well as on the street. Other than a good deburring and flash removal, the ports were left nearly stock, with only some minor valve pocket work. The sharp edges on the intake port behind the valve were blended without reducing the short-side radius. A three-angle valve job was performed following the recommendations in the factory'sChevrolet Powermanual, with no abrupt transitions between each angle. Naturally, the heads were cc'ed and the chambers polished, too. The valves were also back-cut at a 20-degree angle to the seat to eliminate excess material obstructing flow around the valve head and through the pocket. So that each cam lobe would have equal lift and to ensure correct valvetrain geometry, all valve stem heights were equalized. You can measure stem height by indexing off the valve cover gasket surface.

In Jim's engine, piston skirt clearance was held to a relatively tight (for forged pistons) .004 inch. He recommends opening it up to .006 inch "if you're going to take it right out and beat on it." Unlike some other engine builders, he does not consider rod side clearance critical as long as it remains within the stock tolerances. "If the side clearance is greater than the bearing clearance, it won't affect oil flow," Jim maintains.

Speaking of oil flow, the lubrication system is left pretty much as Chevy designed it—blueprinted ZL1 oil pump, stock windage tray, heavy-duty oil pump driveshaft, and Vette oil pan.

No internal oil passage restrictions or modifications are performed, "since a roller cam isn't being run." Jim does make sure that the pickup is positioned between ⅜ and ½ inch off the floor, and adjusts its length as necessary. If you're drag racing exclusively, the screen can be rotated 180 degrees to face the rear of the pan. If you plan to make extended top-end banzai runs, an oil cooler should be added; it will plumb right into the high-performance block above the oil filter mount. Likewise, if turning corners is your game, Accusump is the name; the pressurized oil reservoir provides an inexpensive alternative to a full dry sump. Valvoline 20W-50 racing oil is recommended for street use.

The real secret to this engine's power output and street tractability is the camshaft profile, developed as a result of Traco's long road-racing experience, where low-rpm power coming off the corners is required. It features a broad torque curve and lots of downstairs power, yet is still pulling at 6500. There is also the necessity to compensate for the lowered compression ratio, so, quick valvetrain opening and closing rates are utilized to maintain good cylinder fill characteristics. The dual-pattern, solid-lifter cam is ground by Engle exclusively for Traco, and all they're willing to reveal is that it has .560-inch intake/.580-inch exhaust lift and 284/290 degrees advertised duration.

A combination of Engle and stock Chevy components complete the valve actuating assembly, which is kept in time by a Cloyes chain. To prevent wear on the block thrust face, Traco machines the re&r of the Cam Sprocket to accept a Cloyes .030-inch-thick shim. No "anti-walk" button is needed, since the lobes on a flat-tappet cam have a .003-inch taper that causes the lifters to rotate and hold the cam in the block.

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A red Victor composition head gasket carried by Chevy ensures good sealing, especially when Traco coats it on both sides with aluminum paint. The side where the metal beads join faces "down." Note how the cylinder bore valve relief notches were ground out to match the gasket.

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Keeping the incoming fuel charge cool is worth horsepower. A Chevy L88 valley baffle and Traco intake gaskets with exhaust heat block offs and notches already trimmed for the baffle provide the solution.

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McLeod's clutches are both cheaper and stronger than the equivalent Chevy parts. The 30-pound flywheel is backed by an 11-inch disc and pressure plate.

Speed-Pro (Sealed Power) 2.19-inch intake and 1.88-inch exhaust valves are installed in factory cast-iron, open-chamber heads mildly massaged by Cylinder Heads West. After a pressure check and deburring, the heads were ported in the valve pocket area and treated to a good valve job. The circumference of the valve seats was increased to match the outer edge of the valve face; then the seat was narrowed by increasing the inner seat diameter with a 35-degree cutter for the exhausts and a 25-degree cutter for the inlet. This left a .060-inch-wide inlet seat and .090-inch-wide exhaust seat. The unused portion of the valve seat on the valve itself was backcut 20 degrees. K-Line bronze insert valve guides were installed to prevent excess wear with the high-lift cam and stiff springs; their tight guide clearance offers better oil control as well. The chambers were then cc'ed and polished and the heads once again pressure-checked. Stock rocker covers are used on top of the heads, since their built-in oil rails help keep the factory rocker arms adequately lubricated.

Two different intakes and a late 850 Holley double-pumper (dp) were tested on the dyno(see sidebar).Jim settled on an Edelbrock Victor Jr. 2R, and actually uses a smaller 750 c.f.m. dp carb for street driving. A ⅜-inch fuel line feeds gas from the tank to a Carter high-output mechanical pump, which is connected to the carb via A/N-6 line.

Since it's "the best distributor going," Jim relies on an old GM breakerless L88-type ignition system. For street driving, the vacuum advance is retained for better throttle response and gas mileage. With the vacuum hose plugged, the timing totals 38 degrees at 2800 rpm.

Doug Thorley supplied a newly-designed set of under-chassis-style headers that offers adequate ground clearance on the low-slung Corvette without the horsepower restrictions of side-mounts. With 36-inch-long, 2-inch-diameter primaries that dump into 3½-inch collectors that are 10 inches in length, the headers are nearly the same dimensions specified by Chevrolet Engineering for a 4-speed car like Jim's. On an automatic car, Jim would try a bolt-on collector extension (and also a 3500 rpm stall speed converter). The rest of the exhaust system consists of dual 2½-inch pipe feeding into Thrush Turbo mufflers. Jim highly recommends a crossover tube, although its use under the Vette is precluded due to limited clearance.

Not only is dyno-testing the proof of the pudding, it often turns up surprising and unexpected results. Traco fitted the short-block described in this article with both closed and open-chambered heads. The closed-chamber heads were fitted with the open-chamber 1.88 exhaust valve to make the comparison as valid as possible. With the same short-block, the closed chamber engine had one point higher compression (10.5:1). Traco theorized that the two combinations' horsepower outputs would be nearly identical, since the higher compression ratio (CR) would offset the superior combustion chamber design. However, this held true only through 3500 rpm; above that the open-chamber engine pulled away, putting out 50 more horses on the top end. Making the result even more surprising was the fact that the 10.5:1 ratio required a 50/50 pump gas/AvGas mixture to prevent detonation, while the open-chamber motor used 50/50 leaded/unleaded pump gas.

A comparison between the dual-plane C427X intake and single-plane Victor Jr. 2R also didn't validate "paper theories." Supposedly, the dual-plane would be the stronger low-end manifold, but it turned out that just the reverse was true, with the Victor showing slight gains downstairs while only being off 1-3 horsepower on top. Note the relatively broad, flat torque band in all tests that pulls from Day One. That's the legacy of road race development, and it's also the ticket for the street.

Dyno Tests Traco Engineering Street Big-Block Chevy

All tests performed with 850 c.f.m. Holley carb 38 degrees total timing

TEST1:10.5:1 CR, closed chamber heads, 50/50 pump/AvGas, C427X intake

TEST 2:9.5:1 CR, open chamber heads, 50/50 unleaded/leaded pump gas, C427X intake

TEST 3:9.5:1 CR, open chamber heads, 50/50 unleaded/leaded pump gas, Victor Jr. 2R intake

Horsepower/torque peaks in bold face

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All Traco engines are run one hour at 2500-2800 rpm to monitor vital functions at a light load similar to a 60-mph freeway cruise. After a hot retorque of the heads and a change of oil and filter, the power runs shown in the accompanying chart were performed. Then, after a 500-mile break-in period (short bursts of throttle are okay, but don't go overboard), the engine was taken to Orange County International Raceway for drag tests. Corked up, in Jim's 3600-pound, 3.55 geared Corvette, it ran a best of 12.92 @ 113.35 with the smaller 750 dp carb on street tires and pump gas. It seems a rat is one species that will not become extinct. Why not contact the Traco "Wild Animal Preservation Society," 11928 W. Jefferson Blvd., Culver City, CA 90230, (213) 398-3722?

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All other specs as given in Chevrolet shop manual.

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Chevy's 8-inch heavy-duty balancer is machined round before balancing. Naturally, it's also fully degreed, with Traco's trademark stamped on right at the desired 38-degree total timing mark. It's not necessary to pin the balancer on the street.

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GM's old Corvette breakerless ignition system, transistorized coil, and cast-iron, tach-drive Corvette distributor is still one of the best setups available, especially when the amplifier is rebuilt using modern heat-resistant components. Alternatively, you can replace the amplifier with an MSD6A box and Blaster coil. Kits are still available from GM to convert point distributors to breakerless operation. Thunder Volt supplied the 8mm resistance wire.

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Both the Edelbrock Victor Jr. 2R and C427X intake manifolds were tested, with surprising results. Note that the Victor has no vacuum tap.

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1. Must upgrade with 3969864 bolts and 3942410 nuts.

2. Requires four 3902885 attaching studs.

3. Out of production.

4. Part No. not yet assigned.

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