There's no law saying every engine featured in Car Craft has to smash the dyno with huge power numbers. You can get as much satisfaction extracting more power from a mild engine as a supercharged big block. And when you nearly double the factory power rating with basic bolt-ons and inexpensive internal parts, that's worth writing about. In the case of this exotic-looking Mopar 225 Slant Six, the factory rated 100 (net) horsepower jumps all the way to 177 thanks to a mild Hughes Engines cam, Doug Dutra 2-piece exhaust manifolds, a 390-cfm Holley 4-barrel, and an Aussie Speed ram-tuned intake manifold.

While this subject engine happens to have one of the approximately 50,000 die-cast aluminum blocks offered in 1961-'62, everything else shown here can be applied to the nearly 12.5-million iron block Slants produced between 1960 and 1991. Better yet, those common-as-dirt cast iron Slant Sixes are among the most durable Detroit engines of all time. There's a reason the aluminum version never took off (you'll see why at the end).

Truth be told, if you're looking for maximum bang per dollar spent, grab a Chevy 350 and join the crowd, but if you can appreciate the Slant Six's eye-catching 30-degree list to starboard, follow along as the crew from R.A.D. Auto Machine takes a step-by-step approach to improved power.

See all 25 photos Our test engine is block number 37,415 of the approximately 50,000 die cast aluminum engines made. Optional only in the compact Plymouth Valiant and Dodge Lancer (never in full size cars, trucks or vans), the $47.35 price was $105.50 less than a basic AM radio. At 64 pounds bare, it's less than half the weight of a 130 pound iron block. But while regular cast iron blocks can safely be bored 0.060 inch oversize, the integrally-cast iron liners limited us to 0.020-inch.

See all 25 photos R.A.D. Engine Machine honcho Donnie Wood balances our 74.4-pound forged crank. The cast iron cranks used after 1976 are decent but have non-interchangeable narrow journals and bearings. Don't mix them up.

See all 25 photos Naturally aspirated applications get by with economical 16.4 ounce cast pistons from Egge (PN P1515-020B) with chrome-moly Sealed Power rings (PN E-196K-20). Enlarged by 0.020-inch, the new displacement is 230 cubic inches.

See all 25 photos The aluminum block's open cylinder deck and free-standing cylinder barrels shed weight but don't support the cylinders as well as the iron block's conventional flat deck; boost and nitrous are strictly off limits. The stock 6.707-inch forged rods weigh 26.8 ounces and get ARP rod bolts (PN 142-6001). At TDC, the crowns are 0.175-inch below the deck, yielding plenty of valve clearance.

See all 25 photos The beefy 59.2 pound head can safely be milled a full 0.100-inch. This bumps compression from 8.4 to 9.2:1. When O.E. type (one piece) solid lifters are used, the stock rocker arms and push rods still have enough adjuster range to absorb the change. Aftermarket (multi-piece) lifters require custom length push rods (more in a moment).

See all 25 photos Instead of splurging on bigger 1.70/1.44 (I/E) valves and hand porting, we back-cut the stock 1.62/1.36 units (pen points). Then R.A.D.'s "bowl hog" port throat cutter relieved the areas behind the valve heads. Attention Edelbrock: Thank You for supplying aluminum heads for more and more obscure engine applications. With 12.5 million made and plenty still on the road, we say it's time for an aluminum Performer head for the Slant Six! It's a winner waiting to happen.

See all 25 photos All pre-1981 Slants have solid lifters and adjustable rocker arms. The stock cam's 0.375/0.365-inch lift (0.406/0.414 after 1970) is a restriction. We replaced it with a Hughes Engines stick with 0.474/0.483-inch lift and 206/210 degrees of duration (PN STL0610I6). It ran great, even with the mild factory 1 and 2 barrel induction setups.

See all 25 photos Modern cam and lifter kits have moved away from Chrysler style one-piece lifters (left) to 2-piece construction. The lifter socket is nearly inch higher and stock-length 9.975-inch push rods are too long for the job. The solution is shorter 9.730-inch push rods. This issue was not triggered by our 0.100 head shave. It affects all Slant Sixes using non-OE 2-piece lifters (identified by a snap ring inside the body).

See all 25 photos The conventional head gasket (in hand) will not seal an aluminum block. For that, a (discontinued) Fel-Pro multi-layer gasket (PN 8000) is required. Shown on the table, the all-copper side faces down against the block and the darker side (with copper fire rings) faces the head.

See all 25 photos The stock adjustable rocker arms were set for 0.010 / 0.020-inch of lash cold (Intake / Exhaust). They are safe to 6,000 rpm with street-level spring pressures. The tall deck 225's under-square (3.40 x 4.125) architecture and modest port flow prevents much more than 5,000 rpm. By contrast, the 1960-'69 low deck 170 cubic inch Slant Six's one-inch shorter block deck height is over-square (3.40 x 3.125) and can nudge 6,000 rpm.

See all 25 photos Davis Unified Ignition (D.U.I.) recently released the Tri-Power, breaker-less ignition for the Slant Six (PN 70620). The self-contained Dyna-Mod feature and digitally controlled advance curve eliminate the need for an external module and vacuum advance canister. The D.U.I. Live Wires (PN C9082) are color keyed to match the Mopar Performance red engine paint we applied to the Aussie Speed finned aluminum rocker cover (PN AS0170).

See all 25 photos With a stock 20.6 pound cast iron exhaust manifold and 140-cfm Carter one-barrel carburetor, we got 130.2 hp at 4,300 rpm and 192.3 lb-ft at 2,700 rpm. Chrysler rated the 225 at 100 hp / 170 lb-ft. (net) so it looks like the Hughes Engines cam and head work delivered an extra 30 horsepower and 22.3 lb-ft of torque. Not bad.

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See all 25 photos Next, we swapped on a 260-cfm Carter BBD and iron manifold from a 1977-1980 Super Six. Usually found in Volare and Aspen station wagons, this was the best bang-per-buck upgrade, registering 151 hp at 4,700 rpm and 204 lb-ft at 3,300 rpm. Typically priced under $100 on the swap scene, it was a cheap way to gain 20.8 hp and 11.7 lb-ft. The Super Six's Carter BBD 2-barrel was also factory issue on Mopar 273 and 318 small blocks with slightly richer calibrations.

See all 25 photos Slant Six tubing headers have become scarce lately. Slant Six guru Doug Dutra has a solution in his new cast iron Dutra Duals manifold set. They delivered 162.4 hp at 4,200 rpm and 215.1 lb-ft at 3,300 rpm, gains of 11.4 horsepower and 11.1 lb-ft. At 28.8 pounds, they're 8.2 pounds heftier than the stock iron log but should last forever.

See all 25 photos Looking like a mini-Max Wedge unit, the stock 8x3 inch air cleaner absorbed 4.9 hp and 1.5 lb-ft (157.5 / 213.6). The baseplate opening had to be trimmed to fit the BBD 2 barrel carburetor.

See all 25 photos Since the early sixties, Offenhauser, Weiand, Mopar Performance, and Clifford have produced short-ram 4BBL intake manifolds for the Slant Six. They mostly shared the stock Chrysler manifold's 7-1/2 inch carb-to-head distance. Rather than offer more of the same, Aussie Speed smartly focuses on torque-happy long ram architecture. We selected the 12-inch ram (PN AS0470) on left and the 15.5-inch Hurricane (PN AS0024), on right. Both are fed by the same 390-cfm 4160 series Holley 4BBL (PN 0-8007).

See all 25 photos Slant Sixes secure the intake and exhaust manifolds via studs, nuts and thick triangular washers to prevent cracking from normal thermal expansion. Before engine assembly, we mocked each induction tract to the head. A few high spots had to be equalized with a die grinder.

See all 25 photos With the Holley's quartet of 1.425-inch bores feeding the short ram's open plenum from one foot away, we saw 170 horsepower, but the 16:1 air/fuel ratio was dangerously lean. The modest vacuum signal generated by the 230 cubic inches couldn't trigger adequate fuel flow. The box-stock carburetor calibration needed help.

See all 25 photos The Holley 0-8007 carburetor comes with #51 primary jets and #54 secondary metering plates. We replaced the jets with #58's but were still a bit lean at 15:1 (a/f) under load. Replacing the #54 secondary metering plate with a #60 corrected the air/fuel ratio to a safe 13.5:1 ratio. The result was 177 horsepower at 4,200 rpm and 232.3 ft-lb. at 2,800 rpm. Over 200 ft-lb was present from 2700 to 4400 rpm, avoiding the peaky nature that sometimes flaws small displacement engines.

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See all 25 photos

See all 25 photos With the Holley 390 swapped over, we were ready to see what the Hurricane could do. Unfortunately, the aluminum block's nemesis head gasket failure - reared its ugly head. The violence confused the dyno sensors so we didn't get any Hurricane-equipped power numbers.

See all 25 photos The dark spots adjacent to the spark plugs show where cylinder pressure got past the gasket's fire rings. The root cause was your author's decision not to mill the deck of the aluminum block before assembly. I didn't want to risk losing the geeky stamped id. codes. Luckily, it can be fixed and I have one more head gasket!