You want to look for the tuned length (and diameter) calculators online.
Then you want to pick the rpm range at which you want to run the engine and work from there.
BEFORE you get to the silencer, you want to look up:
david vizard exhaust terminator box
that goes at the end of your ...into 1 pipe, so its also tuned...
NB that the camshaft profile should be optimized for the same rpm range.
Similarly; the intake can be optimized, but that's even more difficult than getting a properly designed exhaust to fit.
I have noticed that properly designed bell mouths:
- Into the air filter box (Ram air is worth a look here too)
- From the air filter box.
- Into the turbo mouth. (especially)
- etc; all the way back. (Velocity Stack)
Is not too difficult and makes a surprising difference.