By "conventional engine tuning" we mean internal engine work. Not Turbocharging, Supercharging, Nitrous Oxide Injection, etc. No "bolt on power" modifications. but tuning the internals to give better airflow.
Getting more power from a standard car engine by conventional tuning means is at best difficult. (we will ignore standard turbocharged cars for the moment). The only way is to increase the airflow drawn through the engine. There are lots of ways to do this and it all involves a lot of work and money! For example:
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Bigger side draft carbs or throttle bodies
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Bigger engine bores / pistons (machined to increase capacity)
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Bigger engine stroke (offset ground machined crankpins and shorter connecting rods)
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better tubular exhaust manifolds and system
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better port shape in the head (ported head)
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Bigger inlet and exhaust valves fitted with specially shaped valve seats
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Higher compression ratio
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longer duration and higher lift camshaft
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forged pistons (for strength)
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stronger connecting rods to cope with higher RPM's
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uprated oil pump/system to help engine reliability at higher RPM's
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Crank / Connecting rod lightening, balancing, strengthening etc to cope with higher RPM's than standard engine
Depending on what you are trying to achieve you may need to do all of the above or just some! A fully tuned and "built" race engine is seriously expensive, loud, and unsuitable for road car use. It has bad driving characteristics, uneven idle, little low rpm power etc. And it is usually short lived as it has to be revved hard to get to any useful power. Obviously then it is best to start with the biggest engine variant of your model of car to begin with! This way you may not need to tune it at all to get the same power! If you do tune it it will not need as extensive modification to achieve the desired power so will be more user friendly.
Conventional tuning has low limits on how much power you can expect too. Its easy to get a few horsepower more, but very difficult to get any big power increases. And the cost of power goes up and up as more power is wanted...
Unless you have a good reason to use "conventional tuning" rather than fitting a bigger engine, or a turbocharger or Nitrous injection then these are probably better options for road use.
