I have such a car - vf37 on 2l, wrx tmic, my99 STi TypeR.
Well, not sure what makes more difference - twin scroll turbo, intercooler or short gearbox, but the car feels more lively than my other Impreza with vf34 and fmic, but standard, longer gearbox.
I think all those things contribute here.
So even despite figures on the graph (about 270HP for vf37, due to catted downpipe - vs about 330HP on vf34, decat) I can feel a huge difference, especially at lower revs.
But it might happen also, that the vf34 with tmic and short gearbox would be as great (I drove my TypeR with vf28, but quite honestly it was so many years ago, that I don't really remember any impressions, hard to compare feelings now - especially that when I bought my TypeR, it was my first Subaru, much faster than previous car, so it basically felt fast - even if there are now faster cars
)
Conversion, as you already know - takes turbo with proper frames, exhaust manifold, oil pan, oil dip, downpipe. A bit of work, but nothing really strange - just some pieces to put together. Worth maybe to remember about turbo oil feed - I replaced it with braided one.
What is definitely worth to do is to source a good, probably standard Subaru, downpipe with cat for NCT and keep it! - borrowing one becomes a problem, as most of people around will have only single-scroll downpipes
However you can always talk to reputable Subaru garage, they may have something to loan for NCT - for example Davelegacy/RSR, who offered me a help.
That's my problem - I was stupid enough to get rid of original twin scroll downpipe I had purchased with conversion kit - believing that sports cats from Ebay, even those 200cpi, quite reputable American brand, will be enough for my car... it wasn't...
I wanted to use sports cat downpipe to avoid pipes swaps every year at a cost of couple HPs.
I hope the Prodrive cat, which I'm waiting for to weld in my ts downpipe, will help - if not, I will probably rather do things as many other people - swapping pipes for NCT, decat to standard cat.
Is twin-scroll conversion on classic worth doing?
I'd say it depends on the price of parts and your goals. I was lucky to get the whole set for about 500GBP shipped to Dublin those couple of years ago.
Usually those kits are more expensive.
If not that price, I would probably have stayed with single-scroll anyway.
When I was buying TS kit I expected probably a bit more from it (especially in terms of power, but here we have limiting factor of 95RON petrol in Ireland etc. and as a matter of fact vf37 isn't much bigger turbo than any other from vf-series - on decat in the past I've seen 300HP and 400Nm), but lack of avcs makes figures a bit lower than on new age impreza, and spool a bit slower.
Especially when you look at graphs and compare it with some other cars, other turbos.
In the real life - as I mentioned at the beginning - even if the car doesn't make big numbers, it still feels quite quick, but again - it may be worth more to get short geared gearbox rather, instead (that's what I will try one day on my vf34/fmic subaru one day - depending on budget
)
It would be probably better to get vf36 on ball bearing - but these are harder to find. vf42? Well.. haven't seen too many for sale over last 5 years
Anyway - there are probably better turbos these days, but also very expensive.
vf36/37 according to some opionions, seem to have sometimes some wastegate issues - having problems with holding it closed due to the size of the wastegate flap (boost creep) - but I didn't notice anything particularly significant on my car.
So, well, if you're not afraid of some little inconveniences and you have conversion kit cheap enough to buy - you may try to go this way.
If not cheap or you don't like troubles - I'd stay with single scroll and a choice of proper turbo, depending on your power goals.
The fact, that you're rebuilding your engine and your turbo is gone, as far as I see in adverts - can make the decision a bit easier - but still anyway, I would look first at a price of the conversion.
(as you can see, I have a bit mixed feelings - I like to drive my car as it is, lack of "subaru burble" doesn't bother me, it feels quite quick, but probably problems with NCT are affecting my point of view a bit these days
- but that's rather a matter of car's age and some other, not turbo related, issues, so may be better to ignore my "moaning"
- it's more about how much money i spent on the car, for better or worse ideas and where I am with my car now, but learning is pricey
) and now, knowing what I know - I would probably do a lot of things different, especially reducing number of not so needed "modifications" and increasing number of kilometers on the road, happily driven
)
