Hey man, I have done a bunch of R&D some of which I haven't put in the forum for obvious reasons, and some because it's so complex or just Nitty gritty and irrelevant for 90% of the people out there.
First I want to start this reply off by also tagging
D041987 I would like to know your peak boost targets and what rpm that peak boost comes on at as I believe you have set the correct way to tune large turbo's on stock motors, i.e pushing peak TQ higher in the rpm band and not following "stock turbo Boost curves" for bigger turbos, my maths suggest 550nm at the wheels is the max TQ we can safely run on these motors and if you target that TQ at the right RPM it means you could make 400whp on a stock motor. But this is maths and more testing is needed which is why I have acquired a stock motor to do some testing on whilst my build motor is getting some even extra love.
Now to reply to you
rjd_F30 until I can prove that they way N20s are currently tuned with big turbo's is incorrect because no one really cares about this platform and most tuners are "N55 or bust" I stand by forged Motor for 400whp as there is known evident supporting bent rods near this figure on stock motors. CP pistons and rods with CARR rod bolts (WMC bolts are trash and should be avoided, for an extra $150-200 it's worth it, for the gains in HP/TQ the carr bolts can take) rods are 1500whp capable, pistons are about 1000whp (this is just from reading around, I received an email from CP stated that they would survive 800whp just fine, but how much HP and TQ a rod and piston can take depends on its application a 1000whp piston used in a drag engine won't survive being used in a rally application that piston could quick become a 400whp piston so know that depending on how you plan to use your car will change how much power the piston and rods can take.
So that being said I recommend CP rods and piston with Carr bolts for the N20 for your power application.
Sleeving the block is necessary, not just from a power point of view and rigidity point of view but also because unless you can get the factory plasma arc coating reapplied after you bore the block out (you should always have the block honed/bored for a new set of pistons and piston rings). Sleeves are stronger anyways - leave it up to the engine builder to source although 99% sure the VAC Motorsports N55 sleeves fit.
Factory Mainstuds can be used if you don't want to spend money, ARP mainstuds/bedplate studs aren't necessary for 400whp but recommended. If you go with upgraded mainstuds get your engine builder to install these, they must be installed without washers, and the excess stud protruding from the nut must be removed so it won't interfere with the counterbalance module. Crankshaft tunnel tolerances must be rechecked incase you need a line hone(this is important and something your engine shop should do)
ARP headstuds are something I would say you should do, just due to the amount of boost you're going to run, the head will get very close to lifting on the factory studs causing you to blow a headgasket. N55 ARP head stud kit or EVO X ARP head stud kits will fit but read my first page about the 4 corner head stud threads.
Cut ring headgasket. Factory gasket is great but This gasket is better just get the install correct to avoid headaches.
ACL rod bearings due to the increased load on the rods get performance rod bearings, due to the sizes rod bearings come in, get your engine builder to work out what sizes you need before ordering the bearings you may need to contact ACL directly for purchasing, they offer 3 sizes for the N20.
Block needs to be notched for the rods, rotating assembly balanced.
Oil pan baffle stock pan is shit let's be honest, install a baffle as per my guide.
Stage 2 turbo, your choice of brand or you can diy a stage 2 or 2.2 turbo (I guess mines a 2.2 since it's a tad larger than the current stage 2 turbo's)
Replace your oil feel line whilst your at it - recent BMW recall made for them.
Replace your timing chain guides to the new set if you haven't already or your motor came with the newer set.
Install an oil cooler and oil thermostat (this is super important, not having oil thermostat makes the oil take forever to warm up, and these pistons expand up to 15% in the bore so the longer the oil is colder the longer your running gear takes to warm up or provide sufficient lubrication meaning excessive wear on a cold start, or piston slap occuring until your motor is warm, which wears your piston skirts out. - Mossleman make a great bolt on kit (designed for an N55 but it fits the n20 just the oil radiator might need abit of adjustment to fit)
Upgraded charge pipes, exhaust, downpipes, and the BIGGEST INTERCOOLER YOU CAN BUY FOR THE N20/N55 F SERIES this helps trust me
Stage 2 LPFP upgrade.
Custom ecu tune.
Optional extras.
Closed or semi closed deck, not really relevant for your power level but if you ever want to turn the power up later on I suggest getting it done helps eliminate cylinder wobble at high boost/cylinder pressures. ARP mainstuds (BMW M50 kit fits) since the motors open doing these now will help you if you want to go higher power later on.
On a stage 2 turbo even on a stock motor 20 psi and above on full E85 and E30 mixtures the hpfp starts to die out. E30 mixtures do last longer and can be run at the 22psi range okay but it is taxing the hpfp alot, especially if EWG car. That Brazilian n20 that made 380 whp on a PS2 turbo run E30 and 24psi from like.3500-5000 which is an extremely short time to run the boost makes me.wonder if they started feeling the stock hpfp crash on e30 at that boost I know it started flaking for me 24 psi at 4000rpm but my turbo is a touch larger. But you should be fine to make.400whp without the use of ethanol, 24psi from 3500r until 6500rpm yields some good results in logs and maths says it should be over 400whp.
You can drop the cost abit by running Maxspeedingrods connecting Rods for the N20 rated to 600hp with arp2000 rod bolts or 800ps with ARP LH19 Rod bolts, but the turbo and engine could handle 400whp no dramas and could with ethanol make even more but running ethanol would only be an option if you can't make 400whp on 93 (98 Ron ) and that's when I can tell you. You'd need to start looking at port injection.
PR ignition coils won't be necessary for your build unless you look at running high mixes of Ethanol.
Hopefully this answers your question, I'm sure you may have come across my posts in the N20 Facebook page, I have made some posts about creating a custom intake manifold and port injection set-up.
Any further questions just ask.
