I agree with tuffty. Lag is cobblers.
Think of things in terms or useable power band rather than lag, and where that power band will be in terma of revs.
Tfsi ko4 = quite a big power band for a small-ish turbo from quite low revs.
3071 = capable of big power band but further up the rev range, which in turn requires more mods (valvetrain/cams/inlet manis) to utilise it.
Its a tough choice. Perhaps the tfsi might be too eager and torquey in such a lightweight 2wd car. And the garrett would be more useable.
garrett will cost more no doubt, but if you already have the turbo, its not so much a big deal. Just have to choose hotside wisely and budget maybe even more dosh for valvetrain.
Every setup has its pros and cons.
The issue is that we're all really talking about boost threshold, rather than lag.
Lag is what you get when your at 5000rpm, cruising at light throttle, the mash the foot into the carpet. IE its the time taken for the turbo to spool, once its well inside its working range. This is most noticeable on gear changes when pressing on. A laggy turbo will take ages to spool back up after a shift, a small lag free turbo will hit full boost pretty much instantly.
Boost threshold is the point at which the turbo starts to make boost, or makes full boost, depending on how you want to define it. Theres also the ramping up rate which is different. A small turbo will start to make boost, then VERY quickly go from 1psi to 20 or whatever your peak is. A big turbo will have a much slower ramp up as the energy to drive its much larger wheels builds up.
For a normal road car, be it a daily or a weekend toy, you want both low lag, and a low boost threshold, because normal driving dictates your driving around at normal RPM's, and the car will feel much nicer to drive if its got a nice wide powerband extending down into the normal crusing RPM range.
If you fit a very large turbo to a daily car, then you'll find that say at 3000rpm, you might now only have 60hp available, where a stock engine has 100 and a tuned small turbo might have 120 or more. That means when driving "normally", as opposed to giving it death, the car will feel sluggish, it'll feel like your always having to work at it to get the best from it. Its this feeling of tractable power that makes modern TDI's so popular, as you have power, everywhere. You dont have to slam it down three gears and rev the nuts off it to perform an overtake, just a gentle squeeze of the throttle and your past.
When driving a car hard, the priorities change, because your never in that low RPM range. If the turbo doesnt do much until 4k that doesnt matter, if your spending all your time between 5k and 8k.
Theres also the point about picking the right sized turbo for the job.
If you use a very small turbo, strung out to its last legs to make 300hp, then it'll make a great tractable street turbo with immense midrange, but it'll suffer at the top end as the turbo runs out of puff.
If you use a very large turbo, barely ticking over, but still make 300hp, then you'll have the boost threshold and response of a 450hp engine, but only 300hp to show for it. The balance between these two extremes depends on the use case.
The OP asked for 325-350hp. For that power level, a GT30 is absolutely into the "too large" category, (perhaps unless its the compact one with the smaller wheels), because its a 400-450hp capable turbo.
To get the best from a GT30, you need to do a lot of other engine work, valves, cams, intake, etc etc. After all of that, you'll have a nice 400hp. But you didnt want 400hp. So either the goal posts are already shifting, which isnt a great start to a build, or the GT30 is the wrong turbo choice.
At the original requested 325-350hp power range, the K04-064 is a great choice. Its an OEM part and comes with a decent manifold already attached. Its small enough to give a wide powerband within the standard engines safe RPM range, so doesnt need cams and valves and other stuff to get the best from it, and yet big enough to hold strong right to the red line rather than tailing off. It also has a bit of scope to go a bit futher, as shown by prawns 380hp figure, should you decide you want a bit more than your initial target figures.
It seems to me you need to figure out what you actually want from this build before selecting parts. Starting a build with a random assortment of bits and simply shovelling random parts at it is not the way to go.
