This is my understanding:
All engines run to a 'fiche'. This is basically a book/manual etc about the engine that states all the clearances, tolerances, specifications etc of the engine, and every engine in that class must run to those particular tolerances/regs/clearances. They would be stated in such a fashion that for example the piston crown height must measure XXmm plus or minus XXmm. So if the crown height is between 20-30mm the tolerance is 25mm plus or minus 5mm.
Now when the engines are manufactured, i.e the pistons, cranks, barrels, crank cases etc etc, depending on how good the tooling is, not every single part will be produced the same, a lot of the differences come down the wear and tear of the tooling used to manufacture the engines. Now in certain parts of the engine a tighter tolerance is better than a looser tolerance. I cant name any components off the top of my head but lets use piston crown height for an example. In this case a loose tolerance would be better as it achieves a higher compression ratio, which in a n/a engine is a good way of gaining power.
Now, mr engine man who supplies these engines will buy in a bulk say 20 engines. What he'll do is then break them all down into there individual parts, and measure the tolerances of each part, and put them in order of best to worst, so pistons will be arranged with the piston closest to the "30mm" slowly moving down the pistons measuring less and less, and at the other end you have the worst piston out of the lot. So he does this with each of his 20 engines, measuring every single part of every one of his 20 engines. Once he's done that he'll start putting them back together. However what he'll do is put all of the best components, the ones with the tolerances he deems to be the best to make the engine faster than the rest (as one engine builder may think one thing, another engine builder may think another so fiched engines can still differ from builder to builder) He'll put all the best components into one engine, then he'll pick the next inline of all the parts and build another engine, and so and so forth until he puts the last engine together with the components that particular builder thought were the worst parts for a 'fast' engine.
So his 'fast' engine, is now essentially 'tuned' to the way the builder wants, using only standard unmodified parts (as you cant machine or change any of the parts yourself) and can now sell it on as a 'fiched' engine. The engine will of course be Dyno'd before sale to prove its power output. Now if you imagine how much time and labour goes into stripping, measuring and building 20 engines the price can sky rocket, plus the extra added on because its a powerful engine, that's why you can hear engines changing hands of upwards of £10,000 when you think to yourself "There all the same aren't they?!?" - One of the reasons i'm sticking with my Biland at club level, no ones doing fiched engines, i bet you couldn't find 20 in the country! haha
I hope this makes sense, im having trouble putting whats in my head in text haha its late, thats my excuse :P
Hope this helps,
Thomas.............
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Fisched ?? by 'minikanga' (13 Feb 2013 @ 20:53)
