Modified Mustangs & Fords
Supercharged 351C Cleveland Build - Clevelands Rock
Supercharging a carbureted 351 Cleveland.
Ford's Modular engine program is in full swing and getting more than its fair share of press thanks to the recent introduction of the new four-valve (4V), 5.0L Coyote engine. The Coyote is an impressive piece, capable of exceeding 400 hp right off the showroom floor, and considerably more in modified trim. It is plenty powerful, but the modular engine isn't the first Ford to wear the 4V designation. Applied to the Cleveland family back in 1970, the famous Cleveland family lasted only to 1974, but continued on in spirit somewhat in the 351/400M engines through 1982. In terms of factory performance, the 351 Cleveland was the high-water mark for small-block performance. In fact, it wasn't until the recent introduction of the 4V Coyote engine that Ford eventually eclipsed the (normally aspirated) power output of the original 351 Cleveland (in '71 Boss22 351 guise). Rated at just 330 hp, the Cleveland-headed 351 was every bit as underrated as the previous Boss 302.
Considerable credit obviously goes to the impressive Cleveland cylinder heads. Thanks to massive port volumes (especially in the 4V heads), huge valves (2.19/1.71) and power producing quench combustion chambers, the 4V Cleveland heads handily out-flowed the "Fuelie" heads offered by the General. Where a production set of DZ302, or LT-1 heads flowed just over 200 cfm, peak flow from the 4V Cleveland heads checked in as high as 270 cfm. Even a professionally ported set of iron Chevy heads was no match for the off-the-shelf Cleveland heads. Sure, they were big, but airflow is what wins races and in that arena, the Cleveland was king. Having recently tested reproductions of all the famous Ford and Chevy small-blocks offered in the muscle car era, we can safely say the factory Fords were more powerful than the factory Chevys. The Boss 302 made more power than the DZ 302; ditto for the Boss 351 over the LT-1 (especially the lower compression 1971 version).
As cool as the factory stuff was, there is obviously more power to be had from a 351 Cleveland. With stock heads that will support well over 550 hp in normally aspirated trim, a Cleveland is just begging for more power. Rather than take the all-motor route, we decided to leave the high-rpm builds to the drag race guys and concentrate on power below 6,500 rpm.
When it comes to building power without resorting to elevated engine speeds, the two most popular routes are increased displacement and forced induction. For this test, we chose the latter, as nothing adds power like an efficient supercharger. In fact, our Paxton Novi 2000 supercharger offered so much power that we were forced to limit the boost pressure in deference to the somewhat weak Cleveland block castings. Not quite as fragile as the production pushrod 5.0L, the Cleveland is not the ideal choice for a maximum power adder effort. If four-digit power levels are in the cards, better check out a new block from Dart, but for our supercharged street-engine needs, the 4V Cleveland was a perfect choice.
This supercharged Cleveland combination was originally built as a reproduction of the famous Boss 351, using a two-bolt, 4V Cleveland block. The 351C was treated to some machine work courtesy of L&R Engines in Sante Fe Springs, California. The block was bored 0.030 over to make room for a set of forged pistons from Probe Racing that features valve reliefs for use with wilder cam timing. The factory connecting rods were polished, shot-peened, treated to new ARP rod bolts, and then hung on a cast-iron 351C crank. The std/std crank was polished and after balancing, the entire reciprocating assembly installed using new Clevite rod and main bearings.
For our supercharged combination, we selected a Comp XE284H hydraulic flat-tappet cam. The inexpensive (compared to a roller conversion) flat-tappet cam offered 0.584-inch valve lift, a 240/246-degree duration split at 0.050, and a 110-degree LSA. The solid flat-tappet cam was installed with plenty of moly lube on the bottom of the lifters (to protect during the all-important break-in procedure). We also took the liberty of adding high-zinc break-in additive and oil supplied by Lucas oil.
The Cleveland cylinder heads for our supercharged testbed originally came from a '71 4V. The 351C 4V heads featured the desirable (smaller) quench chambers, but lacked the adjustable valvetrain of the original Boss 351 heads. This was easily cured by the good folks at L&R, as the non-adjustable pedestals were machined to accept the adjustable rocker assembly. This allowed us to successfully run the hydraulic-lifter 351C cam and adjustable 1.73:1 roller rocker arms. The 351C heads were also treated to new stainless steel intake and exhaust valves courtesy of Pro Comp. We replaced the factory multi-groove valves with the more common single-groove variety, milled the heads slightly to achieve the desired 64cc combustion chambers, and treated the heads and new valves to a performance valve job. The heads were installed using Fel Pro head gaskets and ARP hardware. Fel Pro also supplied the intake gaskets as well.
The ideal intake manifold choice for the street-oriented 351 Cleveland is the Edelbrock Performer RPM Air Gap. This, despite a port mismatch between the large 4V heads and smaller intake ports on the Edelbrock intake (they were designed for the Edelbrock aluminum Cleveland heads). The Edelbrock head and intake combination would be a better choice, but we stuck with the stock stuff. Time constraints didn't allow us to secure the Air Gap intake, so we opted for the stock cast-iron 4V manifold. The factory did offer an aluminum version, but the factory Boss 351 intake is both expensive and all but impossible to find. In naturally aspirated trim, we ran a Holley 750 HP carburetor, but swapped that out for a modified Holley from Carb Solutions Unlimited (CSU) for our supercharged blow-through application. Additional components used in the test included a billet distributor, Meziere electric water pump and 17?8-inch Hooker Super Comp headers. Run in normally aspirated trim, the cammed-up 351C produced 447 hp at 6,400 rpm and 411 lb-ft of torque at 5,200 rpm-solid power given the number of stock components.
Now it was time for some boost. While blower kits for the pushrod 5.0L and 351W Fords abound, finding a bolt-on kit for the Cleveland is somewhat more difficult. Undaunted, we simply adapted a Novi 2000 Renegade kit that was designed for a 302. We machined an adapter plate that accepted the bolt pattern to secure the blower bracket and bolted to the 351C head. The trick was to make the plate the proper thickness to line up the blower and crank pulleys (also from a 5.0L application). Once we had the thickness correct, it was a simple matter of bolting the blower in place and securing the proper belt length. Since our 6.5-inch lower crank pulley was a six-rib, we purchased a K060585 belt and relied on the adjustable tensioner built into the Paxton Renegade kit. Our original intent was to run the new Novi 2500 supercharger capable of exceeding 1,000 hp, but the Novi 2500 was probably overkill for this application and we instead ran the Novi 2000. Having reached 1,000 hp with the Novi 2000 in another application, it too was more blower than we needed on this mild Cleveland, but it has proven itself efficient and powerful, even at more modest boost levels.
In addition to the Paxton Renegade kit, we secured a new Powerhat from Vortech Engineering. Since we had no intention of running EFI on the Cleveland, this was the perfect opportunity to run the new Powerhat. Blowing through the carburetor is a viable alternative to EFI, but care must be taken to properly dial in the fuel curve both at part and wide-open throttle. In the past, carb tuning on a blow-though application has been difficult, especially at elevated boost levels. Since fuel flow is dependant on the airflow, the problem is getting the air to flow uniformly through the carburetor. Swirl and turbulence in a typical carburetor bonnet can make tuning difficult, if not impossible. The orientation of the bonnet and inlet air relative to the carburetor is critical, as rotating the bonnet as little as 10-15 degrees can literally destroy a perfectly good air/fuel curve. To help eliminate this, Vortech designed its unique Powerhat with Diffusion Technology that allowed for near 360-degree orientation of the inlet air. Extensive research and development went into integrated air management that minimized restrictions while maximizing distribution.
On our 351 Cleveland, the custom Novi Renegade kit was equipped with a pulley combination to provide less than 10 psi at our self-imposed rev limit of 6,500 rpm. The motor could certainly rev higher, but limiting the engine speed limited the boost and power production experienced by our 351C block. After configuring the necessary aluminum tubing to connect the Novi 2000 to the new Powerhat, we finally got a chance to run the engine in anger. Equipped with 100-octane Rocket Brand race fuel, the supercharged Cleveland eventually produced peak numbers of 674 hp at 6,500 rpm (it was still climbing rapidly) and 579 lb-ft of torque at 5,100 rpm. Torque production exceeded 550 lb-ft from 4,400 rpm to 6,400 rpm, making for one sweet torque curve. The small crank and large blower pulley combination produced a peak boost pressure of 8.6 psi at 6,500 rpm. With a forged rotating assembly and a Dart block, this combination could reach 1,000 hp, and easily exceed that with the more powerful Novi 2500. For now, what early Mustang owner (or any Ford for that matter) wouldn't love to blow by the Chevy guys with a supercharged Cleveland under the hood? Clevelands Rock!
In addition to the Paxton Renegade kit, we secured a new Powerhat from Vortech Engineering. Since we had no intention of running EFI on the Cleveland, this was the perfect opportunity to run the new Powerhat.