Traditional tuning. Nowadays, with some chip tuning, you can easily add thirty horsepower. Bam!
In the past, things were different. When tuning, known as ‘fine-tuning,’ we worked on the engine’s breathing with files and milling, and adjusting a set of dual Webers wasn’t done with a keyboard. Improving the engine’s breathing was referred to as ‘flowing the block.’ How did it work? We removed all irregularities in the intake channel (and, to a lesser extent, the exhaust channel) that caused sudden contractions or expansions. For the same reason, we shortened the guides of the intake valves a bit. This improved volumetric efficiency, but also accelerated wear on the valve guides, as seen with the Honda CB750 OHC FII.
Polishing intake and exhaust channels had a counterproductive effect. So, we didn’t do that.
Dynamically balancing the engine also provided gains. In stock engines, a lot of power was lost due to internal imbalance. An engine with a balanced crankshaft, flywheel, pistons, and connecting rods ran smoother and with fewer vibrations, rotated more easily, achieved higher RPMs, and therefore delivered more power.
Increasing compression by skimming the cylinder head or engine block provided more power but carried the risk of engine knock. Adjusting the ignition timing was crucial. We didn’t skim the head or block so much that the engine turned into a diesel engine :-).
The intake is crucial
Tuning is about optimization. Therefore, ensure an optimal intake of air. Use the largest possible air filter and ensure a smooth transition from the filter’s diameter to the carburetor. Any sound damping chambers and baffles in the air filter housing and intake pipes could be removed.
Provide cool air. More oxygen fits in cool air. Additionally, cool air could prevent engine knock. So, we drew air from a cool place, not above the exhaust, for example. That’s why you often found an air scoop above the carburetors in tuned engines.
Ensure the right mixture. A few dynojet runs, including re-tuning the carburetion, were a wise investment. We’ve experienced a Simca Rally 3 gaining a sudden 16 extra horsepower at the rear wheels on the dynamometer.
Taking it further
Then, it got into serious tuning territory:
We fitted larger valves.
We installed a different (larger or multiple) carburetor(s). Keep in mind that the purchase of those carburetor(s) was only part of the expense because tuning those carburetors on the dynamometer cost extra since not every carburetor performed equally well in every setup. Luckily, there were a few specialists who mastered this game completely.
We installed a ‘hot’ camshaft.
Or, as a cheaper option, we fitted an adjustable plate that allowed us to set the camshaft a few degrees later than the factory value. A camshaft could usually be set about 5 degrees later without problems, shifting the RPM range of maximum torque upwards.
We chose the right length intake manifolds: shorter ‘thick’ manifolds provided more power at higher RPMs. Longer intake runners resulted in smoother operation. Of course, everything could be calculated and made from scratch, but for engines that were often tuned or came in various configurations, there was usually a wide selection of intake and exhaust manifolds.
We installed heavier valve springs and tried to lighten the valve mechanism. But then, we had to be careful about floating valves.
We adjusted the ignition: we could often ignite a few degrees earlier, which provided more power at higher RPMs. A fully electronic ignition unit with an adjustable ignition curve was certainly an option.
The exhaust
A standard exhaust system was designed to be cheap and quiet. An exhaust manifold with tuned (identical) lengths of bends that ended in a freer-breathing muffler system provided gains. As long as the carburetor jetting was adjusted to it (Dynojet).
Hallo