* Specific Caprice engine: 1988 Caprices came with a variety of engines, from small, fuel-efficient V6s to larger, more powerful V8s (like the 5.0L or 5.7L). The horsepower and torque will vary significantly between these options. A 5.0L will be considerably less powerful than a 5.7L.
* Engine modifications: Is the 1988 Caprice engine stock, or has it been modified? Modifications like upgraded camshafts, intake manifolds, exhaust systems, and carburetors (or fuel injection upgrades) will drastically alter the horsepower output.
* Transmission: The transmission (automatic or manual, and gear ratios) significantly impacts acceleration and quarter-mile times.
* El Camino's weight: Modifications to the El Camino itself (added weight from aftermarket parts, etc.) will affect performance.
* Driver skill: A skilled driver can achieve a better quarter-mile time than an unskilled driver.
* Environmental conditions: Track conditions (temperature, altitude, humidity) can affect performance.
In short: To get even a rough estimate, you'd need to specify the exact Caprice engine (displacement and any modifications), the transmission, and the condition/weight of the El Camino. Even then, it would still be an approximation, as dyno testing is the only truly accurate way to determine horsepower. Quarter-mile times are best determined via actual track testing.
Without this information, it's impossible to give you even a reasonable guess. You could search for performance data on stock 1988 Caprice engines of different sizes to get a *very* rough idea of the *potential*, but the actual result will likely vary considerably.
