FIRE POWER FLIRTATIONS
How Does the 392 Hemi Whale Motor Respond to Bolt-on Goodies?
Making the most out of a 392 Hemi.
In a 1952 presentation to the Society of Automotive Engineers titled “New Horizons in Engine Design,” Chrysler Engine Design Chief James Zeder wrote, “The power of an engine should be based on physique, not on stimulants.” Had Zeder been a sports coach addressing a locker room full of athletes, his message would have been akin to urging the guys to hit the gym instead of pumping steroids.
As we’ll see in this buildup of a Chrysler 392 Fire Power V8, there’s a lot of “physique” in the legendary whale motor. But who exactly was Zeder accusing of juicing with “stimulants” in his presentation? That would have been General Motors’ groundbreaking ’49 Caddy and Oldsmobile “Kettering” V8s (named after GM engine design boss Charles Kettering). No, GM wasn’t stashing hidden nitrous oxide systems in early Olds Rocket 88s and Coupe Devilles; rather (in Zeder’s opinion), GM’S “Boss Kett’ was too reliant on high compression—and highoctane gasoline—to complete the picture.
In 1921, GM’S fuel research lab, led by Charles Kettering, discovered tetraethyl lead (TEL). The petroleum industry adopted it, and the problem of spark knock in car engines was greatly reduced. The addition of TEL to the nation’s fuel stocks allowed engine makers to nudge compression ratios upward from the Model T’s 3.98:1 into the 5s, 6s, and low 7s by 1940. The added compression was good
for overall efficiency, not just peak power, as each drop of gas was better utilized.
So when GM conjured its new, postwwii OHV V8s, it banked on everescalating compression ratios and publicly predicted the average passenger car engine would be as high as 12.5:1 by 1960. This was to be made possible by wartime advances in fuel quality, as the average octane jumped from 65 to 80 points after the war.
But Chrysler’s Zeder didn’t agree. He reasoned that relying on external factors like high-octane gas (the “stimulants”) would spell disaster if fuel shortages ever materialized and only low octane was available—which is what happened in the early 1970s with OPEC.
Elsewhere in Zeder’s 1952 SAE paper he wrote, “In recent years, so much attention has been paid to fuels that it is necessary to remind ourselves periodically that we work with a hot-air engine; and in order to get power out we must get air in…it is amazing how often the hot air finds its way into the advertising, and not into the engine.” Thus, instead of GM’S compression/octane-based strategy, Chrysler embraced the hemispherical combustion chamber for its “octane indifference,” i.e. ability to run on lousy gasoline.
Between July 1945 and December 1947, Chrysler built a group of experimental engines with many combustion chamber configurations, compression ratios, cylinder counts, and crankcase layouts. One of these engines would be chosen for mass production and play the key role of motivating Chrysler cars for the next decade. No pressure, right? Wedges, pentroofs, Heron-style, and hemispherical
combustion chambers were all evaluated, and through it all, the hemi-type proved its ability to make solid power without the need for high-quality fuel. “Physique” won out over “stimulants.”
In January 1948, the decision was made to bring the 331ci Hemi to reality for the ’51 model year. It was a humble start. With its single two-barrel carburetor, single exhaust system, and 7.5:1 compression, the inaugural ’51 Chrysler Fire Power delivered 180 hp at 4,000 rpm and 312 lb-ft at 2,000 rpm. But it was a full 20 hp ahead of Caddy’s top ’51 V8, which also utilized a 7.5:1 compression ratio, displaced the same 331 cubes, and inhaled through a single two barrel, but had wedge-type combustion chambers.
From there, the Hemi grew from strength to strength and spawned smaller Desoto (1952) and Dodge (1953) siblings. By 1958, the Fire Power maxed out at 392 ci; and with a solid cam, dual Carter WCFBS, and 10:1 compression aboard the mighty 300D, it delivered 380 hp at 5,200 rpm and 435 lb-ft at 3,600 rpm.
But as history has shown, the octane race never materialized, and octane stabilized at 90 to 104 points for the next two decades. To suit these conditions, Detroit compression ratios languished in the 8.5 to 10.5:1 range, with sky-high numbers reserved for factory Super Stock applications. (Chrysler’s 413 Max Wedge tickled the 13.5:1 mark in 1962.) None of this validated—or discredited—either Kettering or Zeder’s strategy.
Seventy years later, we now know that the massive Hemi’s octane indifference was a largely unappreciated/unnecessary marketing feature. Instead, its breathing potential became the key selling point. Unfortunately, performance potential didn’t sell new cars, and the substantial added cost associated with its dual rocker shafts and jewel-like rocker arms triggered termination after the ’58 model year (though some ’59 Dodge trucks were Hemi equipped).
As the cheaper-to-produce wedge-head 413 big-block took over as Chrysler’s prime mover in 1959, the early Hemi became the go-to engine for nitro drag racers like Don “Big Daddy” Garlits and growing legions of rodders who simply appreciated its “physique.” With its broad shouldered look, there are few engines that gather a crowd when the hood goes up like a Hemi.
Let’s watch as Donnie Wood and the crew at R.A.D. Auto Machine exploit the whale’s physique with some interesting intake and exhaust goodies.
1. A 0.040-inch overbore brings the cylinders to 4.040 inches. The firstgeneration Hemi block lacks the crankcase skirt extensions of the second- (’64-’71) and third-generation (2003-present) blocks. This rules out cross-bolted main caps, but with quality machine work and assembly, it’ll handle 600 hp.
2. Like the post-wwii Cadillac 331, Oldsmobile 303, Studebaker 232, Ford Y-block, Buick nailhead, Pontiac 287, and Packard 320, the Fire Power’s tappet chamber is sealed by a baffled, stamped-steel cover. The CE57 stamping on the right-hand end of the block identifies this block as being from the ’57 model year.
7 7. The nearly flat pistons with valve relief notches yield a pump-gas-compatible 9.5:1 compression ratio, just half a point lower than the ’58 300D. With its 0.040 overbore, this 392 now displaces 398 ci. Rings are plasma-moly faced Speed Pro (PN 24122) gapped at 0.018/0.020 (top/ second).
3. The 68-pound, forged-steel 392 crank’s output flange is drilled for eight flywheel fasteners, just like the later 426 Hemi. But obviously, they’re not interchangeable. Key swap meet giveaways are the Fire Power’s non-threaded flange holes and absence of the raised flywheel/flexplate register ring seen on 426 Hemi cranks.
5 5. The pen points to the polishing work performed on the forged rod beams. For contrast, the factory-issue grainy surface texture (right) can trigger stress cracks. Earlier 331 and 354 rods are shorter than 392 units, 6.625 inches versus 6.951, and have smaller big ends, 2.250 inches versus 2.6875. The 392 block’s 10.865-inch deck height is also taller than the 331/354 block’s 10.38/10.32-inch respective spans. As such, 331/354 and 392 intake manifolds do not interchange without effort.
6. Spiral locks secure the floating pins within the KB Silv-o-lite hypereutectic pistons. The 392’s “smallish” 103cc combustion chambers don’t require massive (and heavy) piston domes to achieve compression. By contrast, second-generation 426 Hemis have 172cc combustion chambers and require larger domes to achieve the same compression ratio.
4. To set the thrust bearing, constant forward pressure is applied to the crank while the 2.6875-inch-diameter main caps are torqued to 90 lb-ft. Earlier 331 (’51-’54) and 354 (’55-’56) Hemi mainand rod-bearing diameters are smaller at 2.500 inches.
12. Pop quiz: Which cylinder head is heavier, a 331 Hemi or a Chevy 454? Surprise! At 64.4 pounds bare, the Hemi is 3.8 pounds lighter than the fabled Rat! The free-flowing square intake (top) and ovoid exhaust ports (bottom) complemented the hemi chambers and have won countless races. Later 354 and 392 iron heads are also much lighter than Bow Tie goodies. 12
15 15. Any Mopar 273, 318, 340, or 360 distributor will fit the 392 block as long as a Hot Heads intermediate shaft (PN 21930) and alignment collar are used, as shown here in the lifter galley.
13. A set of Hot Heads dual valvesprings (PN 40062) works with the stock retainers and locks to deliver 135 pounds of pressure (closed) and 325 pounds at full lift. Installed height is 1.700 inches. P.c.-style seals control valve stem lubrication. 13
10 10. With the fresh ARP rod-cap fasteners torqued to 52 lb-ft, a stock 5-quart Chrysler center-sump oil pan and matched pickup tube work with the Hot Heads oil pump (PN 21920). Hot Heads offers a rear sump pan that’s ideal for most hot rod applications (PN 21811). Higher capacity oiling systems are also available from Hot Heads and Milodon, but the stock arrangement is adequate for this 6,000-rpm street machine.
14 14. R.A.D.’S Donnie Wood carefully guides the heads into position atop the Best composite head gaskets (PN 585). Compressed, the gaskets are 0.040 inch thick and help deliver the 9.5:1 compression ratio. First-gen Hemi heads are interchangeable side to side as long as bolt-on caps seal the unused water passages at the firewall end of things.
8 8. Lack of the rare and expensive adjustable rocker arms used in Chrysler 300D and certain marine/industrial applications forced us into a hydraulic cam. For this street-and-strip build, we opted for an Isky Mega 280 with 0.485-inch lift, 280/280 degrees advertised duration (232/232 @ 0.050-inch lift) on a 108-degree LSA. It’s hotter than the 300D cam’s 0.435/0.442inch lift and 276/276 degrees of duration. The hydraulic lifters impose no penalties whatsoever at this level of performance.
9. A nifty vintage finned aluminum FGT multi-piece timing cover contains the Cloyes True Roller timing set (PN 9-1103). R.A.D. painted the engine black rather than the stain-prone silver paint originally used by Chrysler.
11. Though 392 heads won’t spoil the recipe, earlier 331 heads are said to have better port floors. Bearing casting number 1556157-1, the stock 331’s 1.81/1.5-inch valves and seats are upsized to 2.00/1.75 inches, matching stock 392 dimensions. Metal was also removed from the throat area directly behind the valve heads.
16 16. Proven stable to more than 7,000 rpm (with sufficient valvesprings), the cast iron rocker-shaft stand hold-down bolts do double duty as head bolts. Though only four bolts pinch the head gasket atop each cylinder, the arrangement is bulletproof when naturally aspirated. Supercharged applications must employ studs and other tricks to increase clamping force.
17. For an extra measure of control, Hot Heads 3⁄8-inch diameter chrome-moly pushrods (PN 21086) with adjustable ends connect the lifters to the rocker arms. R.A.D. set them at half a turn beyond zero plunger lash.
19 19. Working with the Hot Heads driveshaft and alignment collar described earlier, a box-stock Pertronix Flame-thrower billet aluminum distributor (PN D141700) fires the Autolite No. 65 plugs at 32 degrees BTDC. Though first- and second-generation Hemi spark plug tubes are not interchangeable, the wires and rigid insulators are. A set of 8mm Taylor wires (PN 75089) need only final trimming and terminal crimping for use.
18. R.A.D.’S Steve Chmura installs the vintage Mickey Thompson finned, castaluminum rocker covers. At 6.100 inches, second-generation (426-style) Hemi spark plug tubes are 0.250 inch too long for use on the 331-392. Hot Heads offers replacements for lost items.
20. The stock cast-iron 392 thermostat housing (in hands) is too short when 331 heads are used on a 392 block. Hot Heads remedies the hassle with a hose kit and adapter fittings (PN 21404). Though this dyno setup uses an electric water pump, Hot Heads offers numerous street- and strip-friendly alternates.
21. Hot Heads offers these adapter spacers (PN 20209) when using shorter 331 heads on the tall-deck 392 block. Two sets of gaskets are needed.
26 26. With the headers still in place, the oldschool Weiand dual-quad intake manifold and tandem 500-cfm Edelbrock Performer carburetors (PN 1405) were installed. The twin Edelbrocks work just fine right out of the box, though the bottoms of the linkage arms interfere with the intake bolts. A simple trim solves the matter.
25 25. Switching from stock manifolds (in hand) to Hot Heads 17⁄8-inch block-hugger headers (PN 60010) unleashed an extra 23 hp and 25.6 lb-ft. Hot Heads also offers a full line of under-chassis and equal-length headers as well as reproductions of the “big mouth” iron manifolds used on Chrysler 300 letter cars.
23 23. For the single-carb test, R.A.D. used a 750-cfm Quick Fuel double-pumper (PN HR-750) to meter fuel and air.
22. To evaluate the classic and the new, let’s compare a vintage 7263 Weiand dual-quad manifold (left) with a modern Hot Heads 50000 high-rise, single-four-barrel unit. With so much distance between the M/T rocker covers, will the single carburetor look naked compared to the dual-quad setup?
24. Exhaling through stock center-dump Chrysler New Yorker exhaust manifolds, the Quick Fuel carb delivered 408 hp at 5,500 rpm and 463.6 lb-ft at 3,300 rpm. Box stock metering is ideal.
27. The dual carbs delivered 436.5 hp at 5,700 rpm and 440.4 lb-ft at 4,400 rpm. Sure, the extra 6.5 hp is nice, but losing 44.8 lb-ft of torque is a hard pill to swallow.