L98 Engine Induction - Stealth Mode TPI

Building a 458-hp L98 street sleeper

Richard Holdener Mar 10, 2007 0 Comment(s)
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To the stoker short-block, we added a Comp XR264HR that offered a .487/.495 lift split, a 212/218 duration split, and a 110-degree lobe separation angle.

Run with no accessories (only an electric water pump), a pair of 3-inch Magnaflow mufflers, and no induction system (only the stock throttle body), the L98 produced peak numbers of 327 hp at 4400 rpm, and 397 lbs-ft of torque at 3800 rpm. In typical TPI fashion, torque production from the L98 exceeded 350 lbs-ft from 2800 rpm to 4900 rpm. Even down at 2500 rpm, the TPI motor thumped out 340 lbs-ft. It was only after 5000 rpm that the torque curve plummeted rapidly, eventually dropping down below 250 lbs-ft at 6000 rpm.

Keeping our TPI small-block in stealth mode meant minimizing external modifications. Since a stroker is not externally visible, we decided to increase the displacement of the L98 from 350 cubic inches to 383 cubic inches via a 3.75-inch stroker crank and .030-inch overbore. The stroker kit supplied by Coast High Performance featured forged pistons that combined with our stock (but ported) L98 heads to produce a static compression of 10.6:1. The CHP kit included a cast crank (more than adequate for our power and rpm needs), a set of forged rods, and the forged .030-over pistons. The block was precision machined to accept the new reciprocating assembly, including slight clearance of the bottom of the cylinder bore for the rod bolts.

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Extrude Hone also worked its magic on the factory L98 lower intake and upper plenum.

The short-block was further enhanced with a (still mild) XR264HR cam that offered a .487/.495 lift split, a 212/218 duration split, and a 110-degree lobe separation angle. While the aftermarket is chock full of performance headgear, we decided to retain the stock look by subjecting the L98 aluminum heads to porting. The heads were sent to Extrude Hone for porting and finished up with a competition valve job. After all the work, the flow rate of the stock-appearing L98 castings was up to nearly 260 cfm.

The factory TPI lower intake was also given the stealth treatment, as Extrude Hone worked over the lower manifold, dramatically increasing the flow potential without any externally visible cues. Since the factory runners featured paper-thin walls, we had no choice but to run a set of big tubes from TPI Specialties. While they were not specifically factory, they looked a lot like the stock stuff, only larger. The factory TPI plenum was retained, but was treated to mild port matching, and flow enhancement via Extrude Hone porting. We knew the TPI system would be the limiting factor in terms of maximum power production, so every effort was made to maximize the flow rate of the individual induction components.

While we hoped to rely on a stock TPI throttle body, in the end we went with another TPIS piece to ensure that our new 383 had a sufficient airflow. With the new throttle body and TPIS runners, our new stroker motor looked like a mildly modified L98-we could live with that. It was what was underneath that counted, and we just knew the new stealth mode 383 was not going to disappoint.

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Though the ultra smooth wall finish was impressive, we were most interested in the airflow improvements.

The stroker was run once again with a set of Hooker headers, FAST XFI management system, and an MSD distributor. Although we installed a set of aftermarket valve covers on the 383, it will go back in the car with the factory covers, though it will be necessary to clearance them for the 1.5-ratio roller rockers. The new motor was first subjected to a break-in procedure with conventional Lucas oil, but was then switched over to synthetic oil after the break-in.

Tuned with the FAST management system, the 383 eventually produced 458 hp, and an amazing 534 lbs-ft of torque. You read that right, 534 lbs-ft of torque from a small-block. Talk about the perfect street stroker. The combination of displacement, cam timing, and airflow improvements added up to a gain of 131 hp (measured peak to peak), but that gain exceeded 150 hp out at 6000 rpm.

The torque gains were equally impressive, especially given the already torquey nature of the stock L98. While most small-blocks struggle to produce 500 lbs-ft of torque, this 383 not only produced 534 lbs-ft of torque, but also exceeded 500 lbs-ft from 3200 rpm to 4600 rpm. It was like we added a blower or turbo kit without anyone being the wiser.

TPI strokers rule.

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The combination of displacement and improved efficiency via the ported heads, cam, and induction had a positive effect on the horsepower production. Where the L98 350 produced peak power at 4700 rpm and fell off significantly thereafter, the improved breathing offered by the ported heads, cam, and intake mods allowed the 383 to carry power production out to 6000 rpm. Had the original L98 350 been equipped with a stock cam, the drop in power would have been even more significant past 5000 rpm. Stepping up in displacement, compression, and efficiency improved the peak power output by 131 hp. Out near 6000 rpm, the gains exceeded 150 hp, while looking like nothing more than a stock L98 equipped with headers and a throttle body.

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Given the more aggressive cam timing, we half expected the low-speed torque production to be down, but credit the hike in displacement and slight bump in static compression for the additional 65 lbs-ft of torque at just 2500 rpm. Measured peak to peak, the 383 upped torque production by nearly 140 lbs-ft. With nearly 540 lbs-ft of torque available, this is one TPI small-block that will pull like a big-block (at least through the mid range). The torque advantage continued all the way through 6000 rpm. We've always been fond of 383 strokers, but combining the additional displacement with the modified TPI induction makes one serious torque monster.

Sources

Lucas Oil
Corona, CA 92880
951-270-0154
www.lucasoil.com
Coast High Performance
Torrance, CA 90505
310-784-1010
www.coasthigh.com
Extrude Hone
Irwin, PA 15642
800-613-1065
www.extrudehone.com

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