Last month we built a mild street engine using a factory Chrysler 383 block, crankshaft, rods, and cylinder heads combined with some aftermarket goodies from Comp Cams and Summit Racing Equipment, and put it on the dyno to see what kind of horsepower and torque it could make. And while our numbers were significantly better than the factory rating for the 383 Magnum or Super Commando, we knew there was more to be had through efficient dyno tuning of the big-block. This month we'll show you how we gained 58 horsepower and 35 lb-ft of torque by spending a day on the dyno.
The factory horsepower rating for Mopar's 383 four-barrel varies by car model and year of production, but the highest rating the 383 ever had was 335 horsepower as the base engine of first generation Road Runners. Using a factory '68 block, crankshaft, connecting rods, and .030-inch over cast pistons combined with stock 906 cylinder heads for our engine, we're basically duplicating the '68 Road Runner engine. The only advantages our 10:1 compression 383 has over the factory version are the intake manifold, mild overbore, Comp camshaft, and Comp rocker gear. This is a very economical combination to build, costing less than $2,500 in parts and machine work, and is likely similar to the engine in many of your cars.
One of the questions we're often asked is whether or not dyno time is worth the money when it comes to an engine build. And while the answer to this is somewhat dependent on your budget, and the application, we generally favor dyno testing for a number of reasons. First, it's a lot of work to put an engine in a car, and if you have a leak or mechanical problem with the engine while on the dyno, it makes repairs a lot easier. Next, tuning an engine for peak power and testing multiple bolt-on parts requires accurate measurement, and an engine dyno is the only way to precisely measure changes to horsepower and torque. Sure you can do this at the dragstrip, but most street cars are limited by traction and the results of testing can be somewhat ambiguous. Because an engine dyno is a precise tool and removes the variable of the car from the equation, the results are accurate and replicable.
Another nice feature of dyno time is being able to break in the engine. Even better, a slight load can be placed on the dyno as the rpm is varied, which is a proven way to seat piston rings quickly. We performed the break-in procedure for our 383 on the Superflow engine dynamometer at Auto Performance Engines, checking for leaks or unusual noises, drained the oil and checked the valve lash. With new oil and a new oil filter installed, we were ready to make some dyno pulls on our big-block.
Knowing the parts inside an engine aren't really put under much of a strain until a load is put on them, the first real dyno pull on an engine always makes us the most nervous. It is at this time that any weak link, defect, or substandard part can be exposed, so we're always cautious during the first pull. For our testing we filled the fuel cell with BP 93 octane unleaded fuel, set the acceleration rate of the dyno to 300 rpm/second, and made our first pull from 3,500 to 5,500 rpm. On what was admittedly a conservative tune-up with a 750 vacuum secondary Holley carburetor and only 32 degrees total ignition timing, peak torque was 380 lb-ft at 4,600 rpm with a peak horsepower number of 360 horsepower at 5,500 rpm. Having already beaten the numbers of the factory 383, we were ready to make some tuning changes in the search for more power.
1 We built this 383 from a '68 block, 906 heads, forged crankshaft, and factory connectin
2 Our plan was to build this engine on a budget, so the 906 heads were simply treated to
3 To accommodate our solid lifter camshaft, we chose a set of Pro Magnum adjustable rocke
4 Assembled with rings, bearings, gaskets, seals, and an Edelbrock Victor 383 intake mani
5 Topped with one of Auto Performance Engine's Holley 750 break-in carburetors, we fired
6 After break-in we changed the oil, set the ignition timing at 35 degrees total advance,