When you look into tuning engines a little bit more, you might have come across the phrase 'blueprinting'. Most people might think of the construction drawings of a building or something like that, but no. When it's about engines, that's not what they mean.

Blueprinting a engine is a very expensive and time consuming way of finetuning a engine. Just because of the costs, it is often only done to all out performance engines and to racing engines.

When a engine is blueprinted, a lot of the factory impurities of the casting process are removed and the internals (like pistons, conrods, crankshaft, flywheel, etc.) are balanced while they are attached to each other.

Balancing everything like that is more accurate than when you balance every individual part.

When casting the engine components, slight imperfections occur. When these imperfections are within certain specs, the parts are fine for the engine manufacturer. The engine will run as intended.

But when you want your engine to run at its best and be an all out performance engine, these imperfections are not wanted. It is all extra mass that rotates in the engine with very high speeds. Moving this extra mass around costs some energy aswell.

Also, the mutual differences between the parts can cause stress on other parts (Like the main bearings that hold the crankshaft) and the imperfactions on the surfaces are more likely to break under stress than when the surface would be smooth.

When for instance the pistons and the conrods are grinded down to a (lower) weight by removing all the imperfections from casting and other redundent material, this not only saves weight, but the different parts can also be weighed to the exact same weight.

The advantage of this is that the load is more evenly spread onto other components, reducing stress. 

We are generally talking about differences of only severals grams or smaller, but when everything is rotating at more than 7000 rpm, this can make quite a difference!

When you do this (A very time consuming procces, everything has to been done by hand and be weighed!) the rotating parts have to be balanced out aswell again to compensate for the weight differences.

Blueprinting doesn't neccesarely mean that you gain power. Don't think you will ad 10 extra hp's to the engine or something like that. The engine will primarily run better and smoother, which is the base for any good performance engine!