01. Last month we built this conspicuously stock 383 Magnum, and it was time to haul it over to Westech Performance Group to run it on the dyno. We planned to add some power to the bottom line before we were through.
Last month we bolted together a 383, built as true to original 383 Magnum/Commando specs as practical. Inside, our 383 benefited from Jim Grubbs' blueprint-quality machining, moly rings for increased bore life and seal, and modern KB hypereutectic pistons. The block was milled for "zero deck," which will eventually give us an ideal quench clearance if aftermarket closed-chamber heads are employed.
The stock #906 heads were milled .020-inches to compensate for the thicker replacement head gasket, thereby reducing chamber volume from the stock 87 cc to 83 cc. The deep valve notches in the KB pistons (6 cc) required to run long-duration high-lift cams, coupled with the 84cc open-chamber heads and the 383's short stroke, gave us a compression ratio of 9.2:1. This compares to 9.7:1, which would have been achieved with stock-style flat-top pistons with no valve reliefs.
02. For our baseline runs, the engine was fitted with the stock 383 AVS carb, iron intake, and iron exhaust manifolds with 2.5-inch extension pipes at the flange. Rick Stoner of Westech filled the crankcase with 10W-30 Unocal oil, and it was ready to fire. Stock, the 383 just nudged over its rating, delivering a respectable 338 hp.
Such minor modifications aside, this was a 383 just like Chrysler used to build. We gathered up an original 383 Magnum AVS carb, the correct PN 666 iron intake manifold, a real set of factory high-performance exhaust manifolds, and slid in a reproduction 383 "Road Runner" cam. A PAW SuperStock crank kit supplied the usual rebuild parts, as well as a crank and rods ordered as a balanced assembly to replace our too-far-gone originals. Overlooking the Mopar electronic ignition distributor--installed more for reliability than performance--the 383 on the engine stand was like an artifact from the past. We had only one question: How much power would it make?
From 1968-1970, the 383 Magnum was rated at 335 hp, although Mopar fans who remember these free-revving engines would compare seat-of-the-pants pull to the 440. Sporting the same heads and cam as the 440, but with a smaller displacement, the 383 certainly could be wound tighter. Was it good for its advertised 335 horsepower? A visit to the friendly dyno operators at Westech would give us the answer.
It was no secret that a 383 was responsive to traditional hot rod mods. Some may even argue that modified 383 cars played the key role in establishing Mopars' legendary reputation for performance. The 383 B-Bodies in particular were everywhere, while the Hemis were feared but seldom seen. The 383 cars were performance machines for the regular guy, and were used, abused, modified, torn-up, and sadly, often thrown away. The 383s fought it out in the trenches, simply because they worked. People weren't afraid to modify their 383s, and we weren't going to be shy about souping-up ours. We had a program of bolt-on parts waiting to prove their worth, with the dyno being the final yardstick.
03. With our baseline numbers in, we stripped the factory Hi-Po manifolds from the heads. Some will argue that on a mild engine combo, the factory iron exhaust doesn't give anything up to headers. We felt otherwise. A well-used set of Hooker 17/8-inch SuperComp dyno headers were bolted on. Power climbed to 355 hp, with a near constant torque gain across the entire curve. That's a meaningful improvement.
Truth or Lies
Much has been said of the horsepower numbers game in the musclecar era. Sometimes the claim is made that the gross ratings were wildly optimistic. In some cases the assertion is that certain engines were "seriously underrated." We've been here before, testing a stock 340 and a stock 440, and generally found that Chrysler was fairly accurate in its ratings compared to dyno numbers we've seen on other stock engines. We dutifully loaded our stock 383 onto the SuperFlow dyno at Westech to see how many of the 335 horses were real, and how many imagined. The 383 sparked instantly to life, and we were struck by how mellow it sounded through the factory manifolds compared to the open-header, big-cammed beasts we had been testing lately.
After the customary break-in cycle, we let the 383 fly. The readout at the end of the pull showed 335.2 hp at 5,000 rpm, and 392 lb-ft at 3,600 rpm. We adjusted the timing from 35 degrees total to 38, and ran a couple of backup pulls. The 383 responded with 338 hp at 5,000 rpm and 394.6 lb-ft at 3,600 rpm. No, Ma Mopar wasn't lying when she hung the numbers on the high performance 383.
Power Mods
While 338 hp for any stock engine is respectable, we wanted to explore the power-producing potential of the 383. This time around we would keep the basic 383 long-block package stock, but shake out some of the hidden power with basic bolt-ons. Our first target was the same chosen by countless thousands of 383 owners over the years--adding a set of tube headers. We pulled down the Magnum manifolds and replaced them with a set of Hooker PN 5101-1 HKR metallic-ceramic coated 17/8-inch four-into-one headers. Was there much to be gained over the sleek high-performance manifolds? The 383 definitely thought so, posting 358.3 hp at 5,400 rpm, and 413.7 lb-ft at 3,500 rpm. The headers added a solid 15-25 lb-ft of torque across the full rpm range. That's added power you'd definitely feel.
04. The old AVS perched atop our 383 was rebuilt and working well, but it was nonetheless next on our hit list.
Next, we set our sights on the original AVS carb. With only a guesstimated 600 cfm at hand, and the dyno recording over 1.8 inches Hg. of vacuum at wide open throttle, it was clearly short on airflow and costing some power. We went straight to the kill here with an 850 Demon carb. The 850 may be considered too much at this mild state of tune, but with our subsequent modification plan, it would be just enough.
Even though at this stage of the game we would be over-carbureting the 383 with the big Demon, we were confident it wouldn't be a detriment on the dyno. On the street, however, it may be a bit much for a stock 383 with headers. The added airflow was a definite improvement, with the 383 now edging up to 367.2 at 5,100 rpm, and 434 lb-ft at 3,600 rpm. Contrary to conventional wisdom, we found the big carb actually added substantially to torque in the low- to mid-range--as much as 25 lb-ft. Although the top-end numbers also climbed, the gains weren't as dramatic as lower in the rpm range.
05. We went with an 850 Speed Demon mechanical secondary carb--kind of big at this stage, but probably about right for the mods to come. It certainly looks more glamorous than the aged AVS, although slightly out of place on the stock iron intake. | 
06. The big Demon actually boosted torque right from the bottom of our test range at 3,000 rpm, and upped peak power to 367 hp. At the lower rpm levels it wasn't a matter of the increased airflow adding power, but rather, better fuel mixing. Up top the gain narrowed, likely because the intake was negating the airflow potential of the big carb. |
07. Steve Brule, Westech dyno operator and tuner, heaved off the heavy stock iron to make way for a more performance-minded piece. | |