Words And Photos: Richard Holdener
The induction system is a critical element in any performance motor and if this test is any indication, there is even more to the equation than the extra 40-50 horsepower. When choosing the induction system for your street or even street/strip small block, you will be forced to use a little (excuse the pun) inductive reasoning. To illustrate just what is at stake when choosing the right intake system, we decided to compare two vastly different combinations. In fact, not only did the induction systems offer decidedly different runner lengths (a critical intake design element), they differed in their manner of fuel supply.
The test involved comparing both a stock and modified Tune Port Injection (TPI) system to a conventional carburetor. The TPI system represented the ultra long-runner, EFI approach, while the dual-plane intake presented the more conventional carbureted system. Given the dramatic change in intake design, this was less about carburetion and fuel injection than it was about the runner length and its effect on the power and torque curves.
To illustrate the changes offered by the induction system upgrades, we first needed a test motor. The original L98 TPI induction system was designed to feed the airflow needs of a mild 350 small block (also used on the smaller 305). With a maximum power rating near 250 hp, the system was not designed with big power levels in mind. Despite this limitation, we wanted to tax the induction system(s) with something more powerful than a stock 350. To illustrate the differences in dramatic fashion, we built a healthy 383.
The 383 featured a 2-bolt block equipped with a forged stoker crank and 6.0-inch rods from Speedmaster. These components were teamed with a set of forged (dished) JE pistons and Total Seal Rings. The short block was further augmented with a COMP XR288HR cam and a set of AFR 195 Street heads. The AFR heads offered intake flow that exceeded 280 cfm (enough to support 550+ hp) and when combined with the healthy cam specs (.520/.540 lift split, a 236/242 duration split and 110-degree lsa), made for a stout little small block.
For dyno purposes, the stroker was first topped with the stock L98 TPI set up. We took the liberty of replacing the stock injectors with 50-pounders from Holley and the system was tuned to perfection using a FAST XFI management system. Run on the dyno with long-tub headers, an MSD distributor and Lucas synthetic oil, the tune-port injected small block thumped out some pretty serious torque numbers. As expected, the long-runner TPI system excelled at low-speed torque production and offered 501 lb-ft at 3,900 rpm. These same torque-inducing runners ultimately limited peak power production, as the 500+ lb-ft was combined with just 410 hp at 5,100 rpm. This differential between peak horsepower and torque was consistent with the stock L98 motor that offered near 250 hp and 350 lb-ft of torque. Anyone who has ever driven a TPI Vette, IROC or street rod will testify to the fact hat stomping on the gas results in a huge promise of big-block like power but this surge quickly fades as the torque curve falls off rapidly.
It should come as no surprise that an L98 TPI system original designed for a 250-hp 350 might be a tad restrictive on the larger, and considerably more powerful 383. Your first suggestion might be that the combination needs more airflow, but you’d be right, but only half right. For this first test we added a ton of extra airflow, but the results might surprise you. To increase flow to the 383, we first sent the lower TPI intake to Extrude Hone for porting. By forcing abrasive media through the ports, Extrude Hone was able to increase the flow rate of the lower intake by 50 cfm per runner!
The thin-wall TPI runners did not lend themselves to porting, so they were replaced by a set of big-tube runners from TPIS. The finishing touches on this modified TPI system included a ported upper plenum and dual 58-mm throttle body. Equipped with the modified TPI system, the peak numbers jumped to 451 hp and 533 lb-ft of torque. The extra flow improved power production through the entire rev range, but since the effective runner length remained the same, the overall shape of the curve did not change. Even in ported form, the long-runner TPI offered huge torque production but ultimately limited peak power production.
The final test was to replace the TPI system with a conventional carbureted intake. For this 383 stroker, we chose to combine a dual-plane, Eliminator intake from Speedmaster with a Holley 750 HP carburetor. Obviously we were swapping from fuel injection to carburetion, but the big change was the intake design itself. The long-runner TPI system was replaced by a much shorter (though longer than a single-plane) dual-plane intake. Compared to a single-plane intake, the dual-plane was designed to enhance low and mid-range torque production. If we view the single-plane as designed to promote top-end power and the TPI designed to promote Low-speed torque, the dual-plane can be thought of as an intake designed to fill the area between these two extremes.
Equipped with the dual-plane, Eliminator intake, the 383 produced an impressive 502 hp at 6,000 rpm but torque production dropped slightly to 493 lb-ft. The dual-plane intake increased peak power by 51 hp, lost 35 lb-ft of torque in the process. In fact, torque production was down by over 50 lb-ft lower in the rev range. A little inductive reasoning tells us that you will have to choose where you want your power production. Do you want the stump-pulling torque of the TIP or the top-end charge of the dual-plane? Is there any reason we can’t have both?
Since the stock TPI system was designed by GM for a mild 350-inch small block, it is not surprising that the modified version offered so much power on the larger 383. Equipped with the stock TPI system, the 383 produced 410 hp and 501 lb-ft of torque. After installation of the Extrude-Hone ported lower manifold, TPIS runners and ported plenum, the peak numbers jumped to 451 hp and 533 lb-ft of torque. The additional airflow offered by the ported TPI induction improved power through the entire curve, but note that the shape of the curve was unchanged. The fact that the two systems shared identical runner length ultimately limited the available power gains.
Swapping out the TPI system for conventional carburetion offered a dramatic change in the power curve. Measured peak to peak, the Speedmaster intake and Holley carb combo offered considerably more peak power (502 hp vs 451 hp), but torque production suffered below 4,800 rpm. The long runners employed on the TPI system were designed to enhance low-speed torque production and the extra 50 hp offered by the carburetion cost 50 lb-ft down low. As always, it is a question of where you want your power production. Would you utilize the 533 lb-ft of torque offered by the TPI or the 502 hp offered by the carbureted combo?