Throttle map
Your throttle may not be linear even if you set MHD to make it so. You can make some logs at different pedal positions to see what the pedal-to-load vs RPM map actually looks like. A lot of maps give you 95% torque already at 60% pedal position at higher RPMs. Here's an example of how two different throttle maps may behave:
Map A: 2psi/90 load at 57% accelerator pedal:
https://datazap.me/u/asbjoern/v43l-55?log=0&data=2-3-17-21
Map B: 16psi/172 load at 55% accelerator pedal:
https://datazap.me/u/asbjoern/50?log=0&data=2-3-17-21
For track driving I prefer map A, because it offers better pedal resolution.
Cats?
I would definitely recommend removing the mid-cats, or at least changing them to high-flow cats instead. I run removable 400cell mid-cats, and they do eat spool when installed. Same goes for the OEM mid-cats. There's no doubt about this.
What turbo? Well what track?
First of all, of course nothing N54 will feel like NA, so get that expectation out of your system first haha.
Personally I have only tried OEM and GC Lites (16T I guess, but with cast manifold). Smaller turbos and less target boost obviously helps. With the GC lites, I only feel boost lag at the smallest tracks, or if I try to come out of a corner from 3000rpm in 3rd gear because I missed a downshift.
There is one track in Hunan province where parts of the track is actually a go-kart track. Going through the tight go-kart section of the track, I do feel some lag with the GC lites. At larger tracks this is not an issue, and is not something I ever think about as holding me down on track. The oil pressure and cooling is a much larger issue with the N54 on track.
Anyway, I would say anything below GC lites should be fine at larger tracks. If you go larger, you can expect some turbo lag coming out of hairpins at any track. I know of one 1M that installed HP650s (17T I guess?) and got turbo lag coming out of hairpins even at the largest tracks here in China. But the sheer power of that setup means it is still more than competitive. So in conclusion:
- Small tracks or autocross: OEM or stage 1s are recommended in combination with a low boost map and a good throttle map.
- Medium sized tracks: Up-to smaller hybrids are recommended if you can afford a quality setup. Medium boost should be enough to be competitive. With DCT, GC lites and 16-18psi I dont have problems with lag on larger tracks, and I can keep up with the stock or slightly modded S55s powered cars.
- Large tracks: Larger hybrids or smaller singles are recommended. Here my 16-18psi map is not quite enough to keep up with the tuned S55s of the world. You can also max out a smaller hybrid, but that will cost some reliability. If single, you probably need ball bearing + twin scroll as a minimum to minimize lag in real world driving. However I can't imagine any single being right for you. I say this maybe not so much because of the spool, but more due to the transient response (weight of the wheels). The only experience I have here myself is the pure stage 2 single turbo for the N55. Although the peak power potential of that JB twin scroll turbo is arguably smaller than the GC lites, the actual boost lag feels much worse in daily driving.
- Drag racing: The larger the better lol
Cooling limp mode and safety
The car is FBO and I've ran flash only, but now run jb4+bef+meth, mostly for the additional safeties and cooling on track, I've been hitting water temp limp,
Some points to consider
- The reason you are hitting water temp limp is because your radiator system is undersized. This is a common problem with the N54. Non-M N54s feature an effective radiator cooling area of 2100cm2 (1800cm2 for DCT). In comparison S55 offers 3200cm2 just for engine water cooling. I am not sure how big the 1M aux cooler is, but I assume its less than 300cm2. So assuming you are running S55-ish power, and depending on which N54 car you have, the radiator is undersized by anywhere between 800-1400cm2 (124-217in2). The upsizing required is anywhere between 33%-77%. Most people install CSF radiators which amount to a total upsizing of 0%. Thats around 0 square inches in case you where wondering.
- Nothing you do in the flash or JB4 will change the cooling performance available from your cooling system. It will only delay the problem by a lap or so in the same way that hitting the track with a cold engine will. This is useless because you need time to heat up tires and brakes as well, assuming you are running proper track stuff.
- You can lower the threshold for when water temp limp mode starts with a custom MHD flash. Personally I lowered mine to 110C for added safety in one of my maps. When it is cold outside, I can go lap after lap without exceeding this limit. When it is super hot and humid outside I can barely do one full hot lap. My radiator system has a total area of 3100cm2 minus 300cm2 for the DCT core, so around 2800cm2 fyi. I even added external water spray but it doesn't help much.
- According to my track testing, the electric water pump is not the limiting factor of the N54. And you don't gain anything by adding a booster pump to the auxiliary system either, just fyi.
- Some people will tell you that selecting a single turbo will solve this problem. It won't. The N55 is equally prone to over-heating, and is equally difficult to keep happy on track.
- Some people will tell you that running 100% water and making an outlet in the bonnet solves the problem. It wont. Please read point number 1 again.
