Back in Part 1 of Boneyard Boost, we yanked a Gen VI 454 from the local wrecking yard and then liberally applied boost from a Vortech YSi supercharger. Despite over 100,000 miles, the Gen VI truck motor did more than just survive, it thrived. Originally fuel injected, we replaced the injection with a carbureted intake system that included an Edelbrock RPM Air gap intake and Holley carburetor. Additional upgrades included a mild COMP XM284 cam and beehive springs. Equipped with these changes, the normally aspirated 454 produced peak numbers of 427 hp and 476 lb-ft of torque. After adding boost to the boneyard big block with a Vortech YSi supercharger blowing through an 850 Holley modified by Carb Solutions Unlimited (CSU), the supercharged combination produced 791 hp and 713 lb-ft of torque. For our final test, we cooled things off with an air-to-water intercooler. Dropping the charge temps by over 130 degrees had a positive effect on power, upping the peak numbers to 848 hp and 761 lb-ft of torque.

Obviously most big-block owners, especially those who purchased their 454 for the paltry sum of $500, would be happy with 848 horsepower, but we couldn’t help but wonder if there was even more power to be had. With very little piston-valve clearance to work with, more cam timing was out of the equation so we turned our attention to the cylinder heads. The stock L29 heads were already equipped with two very desirable features, namely a full-size oval intake port (no peanut-port here) and small 100cc combustion chambers that help produce a 9.0:1 static compression ratio. Thanks to these changes, the 1996-99 Vortec 7.4Ls offered more power than the previous Gen V big blocks equipped with the open-chamber, peanut-port heads. Despite these beneficial features, we couldn’t help but wonder if additional head flow would improve power under boost? After all, isn’t a supercharger forcing air past all the flow restrictions? Well, there was only one way to find out, swap those heads, baby!

Choosing from the million or so available big block heads was difficult but in the end we went with a set of Brodix Race Rite heads for a number of reasons. First and foremost was the fact that the Race Rite heads offered aluminum construction. Iron heads should be outlawed based on weight alone, and as we all know in the power-to-weight ratio game, less weight is the same as more power. Now we might not suggest a head swap based purely on weight savings, but since they were designed as a direct bolt-on replacement, the Brodix heads also offered ease of installation. With factory valve angles and port locations, the Race Rite heads accepted all the stock intake and exhaust components. Though both weight and ease of installation were important, the real reason behind our choice was the power potential. Thanks to efficient port and chamber designs, the Brodix heads offered significantly more flow, and therefore power potential than the stock heads. Where the stock L29 head struggled to reach 245 cfm, the Brodix heads offered an additional 100 cfm, with a peak of 346 cfm.

More flow usually equates to more power but that’s what this test was all about. The one downside to the head swap was that the out-of-the-box Race Rite heads featured conventional (open-chamber) 121cc combustion chambers. When compared to the 100cc chambers on the L29 heads, the head swap dropped the static compression ratio from a decent 9.0:1 to a pathetic 7.78:1. Since each point of compression is worth roughly 3-4% in power, the loss of compression from the head swap hurt power by 5-6% (roughly 25 hp on the mild normally aspirated motor). The question now was would the substantial increase in airflow offered by the Race Rite heads offset the loss caused by the drop in compression? To illustrate the change in power offered by the head swap, we ran the motor with both sets of heads in normally aspirated and supercharged configuration. For each combination, the air/fuel was kept constant, but the two heads responded to decidedly different timing values (both NA and under boost). The L29 head worked best with 27 degrees NA, but was insensitive to timing from 25-30 degrees. The Brodix head ran best with 36 degrees NA and this difference carried over once we added boost.

Having run the 454 in both normally aspirated and supercharged trim with the stock heads, we quickly swapped them out for the Brodix Race Rite heads. The head swap necessitated a few other changes, as the rec-port heads required a different intake configuration. The oval-port, Edelbrock RPM Air gap was replaced by a dual-plane, Weiand Stealth designed to fit the rectangular intake ports in the Race Rite heads. The factory L29 heads also featured a bolt-down (non-adjustable) rocker assembly which could not be carried over to the Brodix heads. We substituted an adjustable set of COMP roller rockers using the factory rocker ratio. Naturally we made no changes to the Vortech Ysi supercharger, drive pulley combinations or use of the CSU blow-though carburetor. Also in place the air-to-water intercooler running dyno water though the core.

After adding the Brodix heads, we first ran the motor in normally aspirated trim and were rewarded with 455 hp and 459 lb-ft of torque. The extra flow offset the change in compression, at least higher in the rev range. From 4,700 rpm up, the Brodix head produced more power but the loss in compression hurt torque production below that point. Would this trend continue under boost? As it turned out, it did not. Hooking up the Vortech supercharger resulted in some serious power, as the Brodix-headed, boneyard big block thundered out 904 hp and 802 lb-ft of torque. Unlike the normally aspirated test, the head swap improved the power output through the entire rev range. The extra flow and chamber volume dropped the peak boost pressure by over 1 full pound (from 17.7 psi to 16.4 psi). Less boost and more power is a sure sign that these Brodix aluminum heads are the Rite Stuff.

Before adding the supercharger to the mix, we took the liberty of comparing the Brodix Race Rite heads to the stock L29 heads. Though the Brodix heads offered considerably more airflow (over 100 cfm at .700 lift), the stock heads featured 100cc combustion chambers that improved the static compression ratio by 1.22 points. The head swap dropped the compression ratio from 9.0:1 to 7.78:1. The change in compression ratio alone hurt power production by 5-6% (roughly 25 hp), but the extra flow offered by the Brodix heads offset the drop in compression. The head swap increased the peak power output from 427 hp to 455 hp, but the drop in compression made itself known lower in the rev range where the peak torque dropped slightly from 476 lb-ft to 459 lb-ft.

Recognizing that what a low-compression big block needed most was boost, we added the Vortech YSI supercharger to the equation. Run on the stock heads with a 32/80-tooth cog-pulley combination, the stock L29-headed combo produced 848 hp and 761 lb-ft of torque. After replacing the L29 heads (and Edelbrock RPM intake) with the Brodix Race Rite heads (and Weiand Stealth intake), the peak numbers jumped to 904 hp and 802 lb-ft of torque. Every bit as important was the fact that the head swap also dropped the peak boost pressure from 17.7 psi to 16.4 psi. Credit the addition flow and larger chamber volume for the welcome drop in boost. This supercharged 7.4L was inching ever closer to the 1,000-hp mark, pretty impressive for something we found in the bone yard!

https://youtu.be/3e60EGoJlyI

Sources

Brodix
brodix.com

COMP Cams
compcams.com

CSU
csucarbs.com

CXRacing
cxracing.com

Edelbrock
edelbrock.com

Holley/Hooker/Weiand
holley.com

Turbo Smart
turbosmartusa.com

Vortech Superchargers
vortechsuperchargers.com