Diagnosing and Repairing a 351 Cleveland with Improperly Machined Cylinder Heads
We are documenting the diagnosis and repair of a poorly running Ford 351 Cleveland stroker engine in Don Hicks’ 1973 Mustang, and this is part two of our three-part series. In the first installment, Mark Sanchez of Advanced Engineering West inspected the 393ci Cleveland and originally diagnosed the rough-running engine’s major problem as a failed No. 1 intake cam lobe on the installed hydraulic flat-tappet cam. Upgrading to a Comp Cams Mutha’ Thumpr hydraulic-roller cam and associated valvetrain components proved to be only half the fix.
Everything seemed to be back on track, with Sanchez proceeding to mock up the new valvetrain to check the valvetrain geometry, order the right length custom pushrods, and figure out the exact guideplate modifications. That’s when we discovered the biggest problem of all. It turned out a previous machinist had botched the adjustable valvetrain conversion on the engine’s rare, Ford Australian 302C-2V Cleveland heads to the extent they were beyond economical repair, with the valves and rocker studs on the canted valve casting ending up misaligned in all three axes. It’s likely this was the root cause of the cam failure.
Sanchez explains: “The canted-valve Cleveland engines are tough because you are not machining perpendicular to the deck or even the existing pedestal seat, and the intake and exhaust valve angles are different from each other. A conversion of this complexity must be done in a mill, not by a drill press.”
Whoever converted the nonadjustable pedestal-type valvetrain to an adjustable stud-mount configuration missed on all three axes: the front of the head to the rear of the head, the intake to exhaust tilt, and the rocker stud relationship to the cylinder head deck and valve seat. The valve guides were toast due to excessive thrust-loads, and the valve seats also needed re-machining.
“Fixing this requires boring the rocker-stud holes oversize on the correct angles, then installing a Keensert with locking keys that’s designed for oversize holes. Again, this is expert millwork to make everything come out right. Only after straightening out the angles can you install bronze wall valveguides, then properly do a concentric valve job. We’re talking $1,800 or more to repair the heads. And then what do you have? A heavy, 45-year-old head. It’s not as if you have a numbers-matching Boss 351 or a legal Stock Eliminator car. I call this ‘beyond economical repair,'” Sanchez concluded.
At this point, we decided it install a set of modern aftermarket aluminum heads that really do this engine justice. Our choice was a pair of Trick Flow Specialties (TFS) PowerPort Cleveland heads. Not only does TFS have a sterling reputation for delivering great performance per dollar, it’s one of the few aftermarket outfits currently making modern aluminum Cleveland-style cylinder heads that still retain production-based runners and valve angles. Out of the box, the castings fit Ford 351C, 351M, and 400 engines; with minor machining, they also work on widely available Ford 302/351W blocks to build a “replica” Boss 302 or 351 “Clevor” engine.
TFS’s intake runners are based on the OE Ford 2V design and have redesigned fast-burn, detonation-resistant, closed chambers as small as 60cc, with 2.08/1.600-inch valves. Note the TFS chamber’s centrally positioned spark plug angled slightly toward the exhaust side as well as its heart-shaped far-side wall, features typical of modern fast-burn heads. There are about a dozen different fully assembled TFS PowerPort Cleveland head combos. There’s a choice of 195 or 225cc intake runners; 60, 62, or 72cc combustion chambers; and three different valvespring configurations. Hicks’ engine size and dished pistons plus the new cam narrowed down the choices: 393 cubes pointed to the larger 225cc runner option; the hydraulic-roller cam to a compatible 1.550-inch dual valvespring package; and—to get as much static compression as possible with Hicks’ existing reverse-dome pistons—the smallest available 60cc chamber.
The part that ticked all these boxes is TFS-51616204-C00 (sold individually at Summit Racing for just under $2,690 each as of December 2019). As tested on Westech Performance’s SuperFlow bench, the TFS juggernaut outflowed Hicks’ Australian 2V heads at every lift point. The TFS intake port was more than 100 cfm better at high lift! Engine-builders know that an engine’s ultimate performance potential correlates well with intake port flow, assuming a complimentary cam and maximum compression ratio. On a normally aspirated engine with 0.700-inch valve lift, the math says TFS heads can conservatively support 644 hp, compared to just 407 hp for Hicks’ original heads.
After assembling the heads on the block, Sanchez carefully mocked up the upper valvetrain. Unlike Hicks’ old heads, alignment on the compound-angle TFS Cleveland heads was perfect, even with our selected oversize -inch pushrod guideplate option. Normally the heads come set up for -inch pushrods, but larger (albeit heavier) pushrods are the preferred choice these days for greater valvetrain stability. Save weight if you can on the valves, springs, and retainers, but not on the pushrods. After checking valvetrain geometry, only then order the final pushrod length. Be sure to make these checks using the camshaft, head gasket, and rocker arms you’ll be using, and after any final block or head machining—all this does make a difference! On Hicks’ engine, the correct length for the custom-order TFS pushrod length came out to 8.100 inches, but your mileage can (and probably will) vary.
For a fuel mixer, we wanted a tuner-friendly, 750-cfm-class, Holley-flanged, 4150-style carb. A double-pumper, mechanical secondary configuration works well with stick-shift transmissions (Hicks runs a transplanted T5), but we also wanted ample tuner options available if we needed to tame the Mutha’ Thumpr cam’s raspy idle. Major components locked in, Sanchez proceeded to final assembly, preparing the engine for a full test and tune session on Westech Performance’s SuperFlow dynamometers. Yes, that’s plural with an “s”: Next time, we’ll wring the motor out on both an engine dyno and a chassis dyno. How much power do you really lose through the drivetrain, anyway? And will all this fit under the Mustang’s stock flat hood? We’ll be back with a full report shortly.
Australian Heads vs 351C 2V and 4V Heads vs Trick Flow
Installed only on Australian-market Fords, the Aussie heads feature moderately sized 2V oval intake ports but with the superior 4V-style quench-chamber design that both raises compression and better resists detonation compared to North American-market 2V Cleveland heads with their smog-era, open chambers that slow the burn and lower compression. On the other hand, standard 2V open-chamber heads are lazy, drop compression, and are more detonation-prone than 4V chambers. That’s why the Aussies combined 2V ports with 4V chambers, which in their day, delivered the best overall performance in most applications. Why not just go with 4V heads? Years of real-world experience shows 4V intake ports are too huge for most street use and even many racing applications.
The Australian heads and closed combustion chamber are a very good combination compared to other contemporary stock heads from the 1970s, flowing well on the top end with about a 68-percent exhaust/intake flow-ratio at 0.500-inch valve-lift.
TFS ups the game further, offering an improved closed, “heart-shaped,” fast-burn chamber with relocated spark plugs plus 45 years’ worth of evolution in port shape, intake, and exhaust port contours, and short-turn radii. A revised oil return system also improves oil drain-back.
Source: Read Full Article