Hi guys. I thought I'd post this here as I know I just won't get the level of discussion in the 8P section that I will here I've been investigating the core size that I'll need to order to effectively cool the charge air on my 2.0 TDI. At present there is an aftermarket FMIC connected (by previous owner) but the stock FMIC was plumbed back in as he reported bad lag in the higher gears. I've not yet had a chance to measure the size of this core but I have been looking at what I will need. I have found a few interesting articles with mathematical equations to roughly work out the core size: How to Go Fast Faster: The Math Behind Turbocharging. Part 7: Intercooler Selection | Take me beyond the Horizon Calculating intercooler size - HomemadeTurbo - DIY Turbo Forum intercooler size calculator :O) - Evo Owners Forum The 3rd link was the easiest to understand so I went with the equations there and compared them to core sizes I'd seen on the net/eBay to see what would be best suited. Ok so here's the math: ----------------------------------------------------------------------------------------------------------------- 3-litres of volume per 100 bhp = 3L 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 cm2 "close enough" . 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 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 4.2x50 = 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 .. remember this is just the cooler not the pipe work ---------------------------------------------------------------------------------------------------------------- So if I apply the above to my situation I get the following: 220bhp x 3 = 660 (6.6L) 2.2(220bhp) x 50 = 110 cm2 These are the core sizes I've seen online and their stats: 1) 140mm x 550mm x 65mm - 5L - 91cm2 2) 180mm x 560mm x 65mm - 6.5L - 117cm2 3) 230mm x 560mm x 65mm - 8.3L - 149cm2 4) 415mm x 610mm x 57mm - 14.3L - 236cm2 5) 440mm x 610mm x 55mm - 14.7L - 242cm2 So as I need 6.6L - 110cm2 the closest core size is number 2 Obviously none of this takes into account the boost pressure the turbo is producing or the size of pipework so it really is just a rough guide. Can any of the engineering types out there add any info to this that might help me ensure I don't end up suffering with too much lag or a core that won't be up to the job.

When I read the first line of the post I was going to say 3L per 100bhp so yes option 2 would be about right. How big was the original intercooler that was fitted that caused bad lag in high gears ?. Was he expecting the car to pull in 5th gear like it did in second?. It was more likely a boost leak or a mapping issue . It's not just about the size of the intercooler but it's positioning to make sure airflow is unobstructed to it and the quality of the core construction and how much cooling / flow resistance it presents. There are many many threads regarding intercoolers on A3's already this is a well trodden path. Karl.

Thanks for your input Karl. I've been around on this forum a while now so I know full well that there's a lot of FMIC related info on here. I was trying to get some discussion going with regards to working out the best size cooler for people's needs. I know the likes of Bill, Tuffty, and Andrew normally love this kind of thing As I said in my first post, I've not had a chance to measure the core of the laggy FMIC yet so I don't know it's dimensions. My first thought when I was told about the lag was it was probably a boost leak/clamps not tight enough. Also a mapping tweak is not necessary when fitting a FMIC, there have been guys on here that have fitted the big Toyo core (aka the Welly cooler) with a generic stage 1 map and seen cooler AITs and no issues with lag. I myself had a Welly cooler on my old S3. BTW if anyone knows the dimensions of the Welly cooler I'd like to work out what BHP it works out for using the above formulars.

So by that logic the wellycooler core is way to big for say stage 2? Am i reading this right. Very interesting find!!

I don't know. If someone can show me the link to the Welly cooler I'll work it out because I can't remember what it's dimensions are.

Just checked and your right with the dimensions. That size core is also the one the guy uses on te Evo forum and it works out good enough for around 456bhp. So this means that the above math is pretty useless as there loads of us using the Welly coolers on cars running well under 300bhp! So why do these cars not suffer with horrible lag?! Hmmm need some input from the likes of Bill really.

i would take what the previous owner has said about lag with a pinch of salt. you know full well going from a stock 8L S3 cooler to something twice the size has little to no adverse effect. you would have to go HUGE or something really poorly flowing to feel it hurting performance.

Should take into consideration material grade or in other words quality of the intercooler. I have tested similar size intercoolers on same car and came out different results. Some of those chinese coolers arent that good

To add another doubt into the above calculations coolers 4 and 5 from above are the direct replacement coolers from THS and Darkside: 4) 415mm x 610mm x 57mm - 14.3L - 236cm2 5) 440mm x 610mm x 55mm - 14.7L - 242cm2 These two cores are a direct swap for the OE FMIC on the 140hp and 170hp engines yet according to the formulas they should be good for nearly 500hp! I don't think i'm getting anywhere with these formulas lol.

Yeh i agree. Never read a negative post about wellycooler. Bare in mind a k04 is a fairly small turbo?

Exactly. The boost pressure that my hybrid puts out on the TDI is between 23-25psi (going on VCDS logs) but the flow rate will be different to that of a K04 on 21psi. I think the key is in the pipework sizing. Need to work out the turbo flow to calculate velocity for sizing. Anyway, before my brain melts I think I'll just plumb in what I've got and see what happens

I would do that. Its exact to your set up then. And if u need to change the core size its no biggy when the pipe works in place.