Im After some advice recomendations on fmic piping ive already bought a 690x300x100 core with 3" in/out but am unsure on size of tube,ie;is it best to try and stick with the larger diameter where possible or just fit the whole lot out in 2.5"also is it good practice to fit cowling as well ????

think most people us 2.5" it is what i have use and if your intersted i have about 4metes of 2.5" ally tube in my shed for sale and a load os samco bends to

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As a general principle it is good practice to size the intercooler piping to the same ID as the end tank inlet/outlet ID. The main restriction to flow/bhp will then be the throttle plate diameter and inlet plenum capacity. The table below gives a guide to optimal sizing. You can always push these figures of course, they are not a hard limit. The selection of IC pipe diameter depends on what your ultimate goal is. 2.5" pipework may be perfectly adequate if you are aiming for no more than 400bhp

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1994 Toyota Celica GT-Four ST205WRC JDM 269bhp @ 0.9bar
1994 Toyota Celica GT-Four Special GT 590bhp @ 1.8bar
1989 Van Diemen RF88/89 Formula Ford 1600
2008 Nissan Patrol GU 3.0L ZD30DDTi 154bhp