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Modifying an LT1 Corvette - To Top It All Off

An Up-Close Look At Trick Flow Specialties' New 430HP LT1 Top-End Kit

Jay Heath Nov 26, 2009
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MODIFYING AN LT1 Corvette for improved performance can be a trying business. Aftermarket parts are scarce, the work itself is often maddeningly difficult, and even the most intrepid Corvette tuners may be inclined to treat you like an end-stage Ebola victim. Given these hurdles, we were pleasantly surprised to learn of Trick Flow Specialties' new Fast as Cast GenX LT1 top-end kit, which should be available by the time you read this.

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Carrying PN TFS-K304-430-400, the package generates a claimed 430 horses and 400 lb-ft of torque when installed on a healthy, stock-displacement LT1 engine. If accurate, those figures would put a '92-'96 Vette's output on par with Bowling Green's latest, and restore much sorely needed performance credibility to the aging Gen II small-block. We'll be installing the full kit on our '96 C4 coupe in the weeks ahead, but for now, let's take a closer look at the package's specifics.

Heads TFS has offered GenX LT1 cylinder heads for a few years now, but the company's first effort employed largish 195cc intake ports and was intended for stroked or forced-induction engines. The new Fast as Cast version relies on a downsized, 185cc port that makes it ideally suited to naturally aspirated, stock-cube combinations. As the name implies, these heads are designed to offer superb flow right off the shelf, eliminating the need for costly and time-consuming port work. TFS' Al Noe explains the concept:

"Fast as Cast is a process where we optimize the port design and shape (just like a CNC-ported head) and then make port core tooling to be an exact duplicate. With current manufacturing processes, we can make a cylinder head with tight-tolerance cast ports. We also CNC the chamber, CNC bowl blend, and CNC the intake manifold port opening."

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Another characteristic that separates these heads from everything else on the market is the orientation of the valves. Rather than using the standard SBC valve angle of 23 degrees, the Fast as Cast heads employ valves that are rotated to 21 degrees.

"We did this to allow us to make a very tight, small [54cc], efficient chamber to keep the stock compression ratio, and also to keep the valve seats from protruding into the deck surface of the head," says Noe. "We could make a [23-degree] small-chamber LT1 head, but two problems would occur: first, the valve-seat ring would protrude into the deck, which makes milling the head a nightmare. The second problem is the small, tight chamber with a 23-degree valve angle is not a great design. Rolling the valve angle two degrees allows us to have a small, torquey intake port with an excellent-performing chamber design attached to it."

Other notable features include 67cc exhaust runners, 2.02/1.60-inch valves, and heavy-duty valvesprings suitable for up to 0.600-inch lift. In addition to being the prime mover in the Fast as Cast LT1 top-end package, the heads are available separately, as well as in a bare version.

Camshaft Realizing that few Corvette enthusiasts are willing to trade driveability for a few extra high-rpm horsepower, TFS engineers designed the package's camshaft to embody an acceptable compromise between enhanced output and everyday civility. The specs-0.530/0.530-inch lift (with 1.6-ratio rockers), 219/227-degree duration, and a 113-degree lobe-separation angle-are not dissimilar to those of GM Performance Parts' highly regarded LT4 "Hot" cam, which is perhaps not surprising given the latter's reputation for superb all-around performance.

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Like any internal engine modification, the cam does require post-install ECM tuning, preferably performed on a chassis dyno. Given the power increases involved, that seems like a small price to pay.

Supporting Hardware
To minimize last-minute parts-store shuttling, the kit comes with a complete set of gaskets and all required fasteners. You will need to supply your own timing chain, 1.6-ratio rocker arms, fuel injectors, and pushrods, all of which can be obtained from TFS or its parent company, Summit Racing.

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In our case, Summit supplied a GMPP Extreme Duty Timing Chain Set (PN 12370835), a single-roller setup that uses a steel water-pump gear as well as steel cam and crank sprockets. Note that while this unit is designed for '95-up LT1/4 engines, it can be used on earlier versions with some modifications.

TFS, meanwhile, sent us a full set of its own 1.6-ratio roller rockers (PN TFS-31400513), which feature CNC-machined aluminum bodies, needle-bearing fulcrums, roller tips, and a machined relief for improved valve-spring clearance. Best of all, they come in a natty black-anodized finish that's guaranteed to wow onlookers any time you pull the valve covers.

To meet the fueling requirements of our power-enhanced LT1, the company also kicked in an octet of 30-lb/hr TFX fuel injectors (PN TFS-89030). In addition to their increased flow rating (the stockers are rated at 24 lb/hr), the TFS squirters incorporate upgrades ranging from Viton fluoroelastomer O-rings and EV1-style wiring connectors to low-magnetic stainless-steel bodies and 100 percent duty-cycle testing.

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The final elements of our particular package included pushrods and a TFS pushrod-length checker. Noe recommends verifying this critical measurement prior to installation, as TFS engineers have observed considerable differences in length requirements among various years and versions of LT engines. To minimize downtime during our install, the company shipped us pushrods in two different lengths. After determining which one is right for our combo, we'll simply ship the unused set back to the company.

The Bottom Line
Offering top-quality engine upgrades is one thing, but doing it affordably is critical when dealing with an older vehicle such as the LT1 C4. Happily, TFS appears to have kept its target customer in mind when setting the package's list price, which comes in at $2,499.95. While that's not exactly couch-cushion money, it's a bargain compared with the cost of a full-on engine swap or trading up to a newer-model Vette. And besides, with 430 horses corralled under the hood, who needs a C6?

We left off last month's torque converter-installation story with a promise of follow-up testing, so when a cold front blew in recently and dropped temperatures into the low 50s, we headed north, deep into Gator Country, for a day of quarter-miling at Gainesville Raceway. Considering that our '96 coupe had already logged a best pass of 13.28 seconds with the low-stall factory converter, we were hopeful that our new TCI StreetFighter unit-with its approximately 2,800-rpm stall speed-would combine with the crisp, dry air to produce a legitimate 12-second pass.

Our initial run started out promisingly, with the fat (315mm) Nitto NT555R drag radials planting hard and hoisting the nose smartly skyward off the line. But just as the rear wheels were passing through the beams, acceleration fell off sharply, to the accompaniment of a woeful sputtering sound from the engine compartment. The car recovered quickly and completed the run, but the e.t. damage was done: the resulting 13.519 at 104.46 mph was more than a half-second off the desired pace.

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A glance at the glowing "Reserve" light beneath the fuel gauge confirmed the problem: the sheer violence of the launch had caused what little gas there was in the tank to slosh rearward, away from the pickup, temporarily starving the engine of fuel. After hying it to a nearby BP for a few gallons of 93-octane premium, we were ready to give it another go.

We used the same launch technique for Round Two, staging as shallowly as possible, bringing the revs up to around 2,500 rpm, and rolling smoothly into the throttle as the light went green. The force of the initial "hit" suggested a record-setting run, but even we were surprised by the timeslip: a 12.866 at 105.60, the pass was more than four tenths quicker than our previous best. A follow-up blast produced a 12.870 at 105.60, proving that the C4 was not only a legitimate 12-second performer, but a consistent one at that.

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Perhaps even more impressive than the e.t.'s were the car's 60-foot times, which came in at 1.706 and 1.704 seconds. That our stock-engine C4 can crack off such times using nothing more than a pair of drag radials to enhance traction speaks volumes about the fundamental soundness of the factory fourth-gen suspension. After all, attempts at similar antics in C5s and C6s have been known to end in violent bouts of wheelhop-and worse.

If you're interested in replicating our results with your own C4, it also bears mention that all of our quickest (and fastest) runs have come with the car's digital coolant-temp gauge reading between 170 and 180 degrees. Unlike the LS-series engines, whose all-aluminum construction does much to inure them to the power-sapping effects of heat, the iron-block LT1 always performs at its best when run cool. In the case of our C4, we've had the benefit of conducting our testing at private track rentals, making it possible to crank the car, drive directly into the water box, and start our pre-race routine before the engine builds much heat. While this procedure may not be practicable at a typical "test and tune" session, you can achieve similar results by simply leaving the engine off, putting the trans in Neutral, and pushing the car through the staging lanes.

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But regardless of how closely you choose to mimic our drag-racing regimen, it's clear that a good high-stall torque converter and a pair of sticky rear tires are by far the most effective modifications you can make to an automatic LT1 Vette. And now that our low-buck C4 is a real, live 12-second runner, we're confident that the new Trick Flow top-end hardware will enable it to hit or exceed our previously outlined e.t. bogey of 12.5 seconds. Stay tuned.

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Unlike bias-ply "street slicks" or full-on racing tires, the Nittos don't require a woolly, John Force-style burnout for optimum performance (unless, of course, you're staging a magazine photo). We dropped ours to 20 psi for testing, then refilled them to 32 psi for the drive home.



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