Would you think it's possible to increase horsepower, torque, and fuel efficiency by about 10 percent with just a tune-up? Neither would we, but that's just what a trip to The Dyno Shop got us.
We wanted to see what kind of gains could be had by dyno-tuning a relatively stock, high-mileage engine typical of what the average four-wheeler drives. Our test mule was an '85 Ramcharger with an Edelbrock Performer manifold, a Carter AFB carb, an MSD coil, a Mopar Orange E.C.U., high-performance plug wires, a 180-degree thermostat, 4.10 gears, and a Flowmaster muffler. The remainder of this 195,000-mile 318 was completely stock.
Getting Started
The first step in dyno-tuning a vehicle is to find out how it's used, what sort of gas it's fed, and the mileage and condition of the engine. Engines in better condition that run higher octane fuel can handle more timing than engines in poor condition that run lower octane fuel. In our case, our Ramcharger sees duty ranging from highway driving, to towing, to slow-speed off-road use. We use 89-octane fuel and we knew that the valve seals were wasted, allowing oil into the cylinders, thereby increasing detonation-causing carbon deposits.
With our vehicle information in hand, Dyno Shop co-owner Mark MacNeil and technician Alex Huerta went to work. They hooked our Dodge up to their Dyno Dynamics 850 dyno, connected the sensors for rpm, vacuum, ignition pattern, and air/fuel ratio, and performed a constant-speed increasing load run. With the tires spinning at 55 mph road speed, increasing amounts of load are placed on the dyno rollers, thus requiring more throttle input to maintain speed. The resulting readout allows the technicians to check the air/fuel ratio throughout all circuits of the carburetor's operating range in simulated real-world driving conditions.
The next step was a full-throttle run to see how much power we were generating. Our little 318 made 143 hp at 4,000 rpm and 204 lb-ft of torque between 2,800 and 3,200 rpm.
Performing Surgery
MacNeil and Huerta first performed a cranking rhythm test, rather than a compression test, to check the condition of the engine. With a compression test, unless the vehicle is started after checking every other cylinder to retain oil pressure, the lifters can bleed down, causing inaccurate readings. The cranking rhythm test is quick enough that this doesn't happen. With the distributor cap removed, MacNeil cranked the engine over while Huerta watched the rotor. Any increase in cranking speed would indicate less resistance in the cylinder and, therefore, a possible problem. In our case an increase in cranking speed around cylinder No. 5 and No. 7 was noticed. Out came the compression gauge to confirm their suspicions. Cylinder No. 7 checked out at 170 psi, but No. 5 read 145 psi-weak compared to the rest of the engine, but not terminal.
Next, Huerta threaded in a new set of Autolite Platinum AP65 spark plugs. The Autolite Platinums come with a three-year unlimited mileage guarantee. They should provide better fuel economy for a longer period of time because the platinum firing tip reduces gap erosion, ensuring that they will keep the 0.040-inch gap way longer than the plugs that they replaced. He then pulled and cleaned the distributor and carburetor so they could be properly tuned in accordance with the air/fuel and ignition pattern readings provided by the dyno runs.
In an effort to cure a detonation problem, we had tuned our carb to run way too rich. Ideally, the air/fuel ratio should be in the 15:1 range during cruising for optimum fuel efficiency, but this may cause surging on a high-mileage, high-compression engine such as ours. Therefore, Huerta chose to shoot for a slightly richer 14.5:1 ratio. Using the air/fuel ratio printout as a guide, the 0.0705/0.0475-inch primary rods were left alone, but the primary jets were changed from 0.101s to 0.98s and the secondary jets were changed from 0.095s to 0.092s.