Q: How is a rebuilt engine better than a new engine?
I once heard that if an engine is well rebuilt it can perform even better than it originally did. How is that? Also what modifications are made to the engine while it is being rebuilt to make it perform better? I heard you can add a lot of horse power if you modify it somehow.
A1: A rebuilt motor can perform better than the factory mass produced one IF it is built by a professional race engine builder OR a gifted/knowledgeable enthusiast who will blueprint it and ensure that all tolerances are exactly the same. A pro built motor will cost considerably more than a regular rebuilt motor.
For instance when we had street stock racing here they would bore/hone the block to the largest factory tolerance then measure heaps of pistons to find the lightest and best size along with testing boxes of factory valve springs to get a matched set with the most pressure to be able to rev the engine higher before valve bounce.
A2: It depends on your definition of ‘rebuilt’.
Engines are built to a design which includes size and clearance tolerances. As long as the engine is built within these it is considered suitable for purpose and installed in the vehicle.
However, when the engine was designed the performance was calculated based on all components and clearances being shot on, ie zero tolerance.
If you take a new, production line, engine from a mass produced vehicle and rebuild it to the exact specification you will see an increase in power (this is called blueprinting because before computers designs were printed out in white on blue paper). In race series where production engines with no modifications are permitted this is done in order to extract as much power as possible.
Having said all this, the increase in power is not as significant these days as it was in the past as engines are built to closer tolerances now.
When an engine is designed it has to meet many exacting criteria – performance, emissions, sound, fuel efficiency, longevity, variable fuel quality, varying climatic conditions and not least, cost – so they are built to a compromise. All engines have the potential to produce more power if modified but this will effect one or more of the above design criteria.
Modifying an engine to produce more power is basically very simple. The power an engine produces is restricted by the amount of fuel it can burn and this is governed by the amount of oxygen that can be crammed info the combustion chambers. Modifications that permit more fuel to be burnt will produce more power.
How to get more oxygen into an engine is dependent on many factors and can be done in various ways – improve air flow through the head, change cam shafts, larger valves, supercharge / turbocharger, nitrous injection, ECU remapping, etc.