When the high-performance parts industry began to make serious growth strides in the 1960s, the design path taken by most designers of engine power-enhancing aftermarket parts was “bigger is better.” Intake and exhaust systems, largely responsible for improvements in an engine’s volumetric efficiency (torque potential) were two types of components that provided gains in net flow, largely because the stock counterparts were intended for lower rpm use. Camshafts, the other major contributor to increased cylinder pressure (aside from compression ratio boosts) rounded out the trio of then-common “bolt-on” parts.

Along came the federal Clean Air Act of 1970, tetraethyl lead was reduced and ultimately removed from pump gasoline, stock mechanical compression ratios decreased accordingly, air quality standards (based on grams/mile of vehicle travel) emerged and Detroit introduced us to the use of emissions controls and systems. Add to the fact gasoline prices began their progressive rise to current levels and the performance automotive community was set on a clear path of change, at least when compared to its roots.

Consider this: From their beginning, exhaust emissions standards have been based on total engine air flow; e.g., grams of emissions per mile of vehicle travel. In concert with emissions-reducing add-on equipment, the OEM dropped engine speeds dramatically, via rear gear ratio changes. Suddenly, we experienced engines operating at rpm well below their most efficient range. Reduced compression ratios made the effects of this more dramatic. Then engine size reductions came into play, creating further decreases in net engine airflow, in the interest of reduced emissions. In time, these and related factors have evolved into vehicle families of lighter weight and powered by comparatively smaller and smaller engines. As the OEM became able to work through lead times for component redesigns, engines have enjoyed slightly larger displacements.

Over all, we’ve seen a need for improving low rpm torque and volumetric efficiency, both in an effort to increase net cylinder pressure at the time of combustion. TBI and MPFI (electronic fuel injection systems) have contributed much to contemporary gains made by the OEM, in terms of power improvements. On-board electronics, although directed to improve emissions, have helped the OEM achieve performance and mandated fuel economy levels, but the aftermarket is still playing catch-up. Looking for ways to accomplish this has become the contemporary goal of the performance enthusiast when selecting aftermarket parts and systems. Whether by some method of supercharging, use of nitrous oxide or engine component choices directed to parts that increase low and mid-rpm torque, the objective has been to improve cylinder filling and combustion efficiency, optimizing both where possible.

So what does this suggest to the performance enthusiast of today? A couple of thoughts come to mind. For example, if you’re buying a late-model vehicle, chances are pretty good you’ll be getting the benefit of most contemporary engine design improvements that include increased torque at relatively low engine speeds. After all, CAFÉ standards have put a premium on good low rpm torque, at least as far as it affects fuel economy and emissions. The newer engine packages are pretty much that; functionally integrated combinations of parts that were designed to work together, not a collection of individually configured components that bolted together but failed to optimize a commonality of functions…as in OEM years past.

Can you improve upon these engines? Well, since the majority of OEM engine packages are functional “compromises” required to meet an assortment of environmental and performance standards, your choices are more limited than in prior times. For this reason, depending upon the level of improvement you’d like to see, moderate changes to inlet and exhaust systems are quite popular (and effective), as are refinements in engine and power train tuning (the latter including automatic transmission functions). Consequently, there is growing popularity in the use of tuning products (chips, modules and hand-held programmers). You see the latter becoming of increasingly widespread use in the diesel truck community, especially among Duramax owners. Custom tuning for modern-day “muscle cars” has become a much sought-after commodity.

But for the builders of street rods and moderately modified street machines and daily drivers, particularly of the carbureted variety, the need for increased low rpm torque gains become a necessity, especially in the interest of improved (or good) fuel economy. And why not? Current and future pump fuel prices will emphasize the need for reduced, over-the-road engine speed, simply on the basis that less airflow translates into less fuel flow. It’s the concept behind contemporary engines that employ the DOD (displacement on demand) concept, along with CVTs (continually variable transmissions). Reduce airflow and at least the potential for mileage improvement will occur, the latter (CVT) approach intending to keep an engine operating in its most efficient range of rpm the majority of the time.

So make your bolt-on parts selection carefully. Include an honest appraisal about where in the engine speed range such parts are intended (and chosen) to be the most volumetrically efficient. You can do this through conversations with your specialty parts dealer or manufacturer of choice. Even if you’re buying off the Internet, it’s advisable to speak to knowledgeable information sources before purchases are made. Make a serious effort to realistically select the range of rpm in which the engine will operate a majority of the time, and choose parts designed to be the most efficient in that span. Don’t be led into the belief that power gains come only at high rpm, at least if you’re targeting performance and fuel economy. It’s all about improving combustion and volumetric efficiencies in the face of reduced engine speed, and whatever you can do to this end will be money not spent at the pump.  


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