hi guys , im not saying you should use this but just take a look :O) i recently fitting a new intercooler to my e4 that worked out to be to big " i didn't think you could go to big , my mistake dou !! so after some searching i came across this calculation for working out your inter cooler size . taken from a total vauxhall mag , the idea is to stay close to this number if you can , go smaller and loose power and thermal efficiency go to big and loose power and turbo efficiency . so if you car only makes 400bhp dont try and fit a 600bhp cooler dou !! as a rule of thumb you will need 3-litres of volume per 100 bhp a cross section of 50cm squared per 100bhp .. hight x depth = 50cm2 eg as a rough guide for 300 bhp you would be looking for a 9 L cooler with a cross section of 150cm2 .......... 3 x 3L and 3 x 50 cm2 for 400 bhp you woulds be looking for a 12 L cooler with a cross section of 200cm......... 4 x 3L and 4 x 50cm2 for 300 bhp , if say you where looking to get a cooler with a 2.5" core " 63.5 mm" to get the 150cm2 cross section you would be looking at a cooler with a height of around 24cm h x 6.35cm or 240mm x 63.5 mm = 152.4 you will also be looking at a length of 9L devided by 152.4 to get your volume = 9000/152.4= 59 cm giving us a cooler of 240mm h 590mm w 63.5mm d for our 300bhp i would treat this as a minimum size needed as a idea my cooler works out to be 300mm h 600mm w 100mm deep 18L 300cm2 cross section good for 600bhp dou !! making it 90% bigger in size and volume than my old cooler so to finish for my 420 bhp i have chosen a new cooler 300mm h 600mm w 76.2 mm d . 3 inch core 420 bhp x 3L per 100bhp = 420 x 3 = 12.6L needed cross section of 420 bhp x 50 cm2 per 100bhp = 210 cm2 needed the new cooler works out to have a 228cm2 cross section 300hx76.2w = 13.68L volume = 600w x 228cm2 = making the new cooler good for around 456 bhp .. this is just a guide that i wish i found earlier but i dont really do maths so forgive me if i got somthing wrong , as i stated i took the oe sum from another sorce thanks t vaux mag :O) .