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As a data point I had 68mm (exducer) VTGs and thought they felt lazy while driving. I think the 63.5 is the way to go. All bets are off if you decide to go non VTG, in that case I recommend a 5757 Xona or something in that range.
Thanks for all the input and real life feedback! This reinforces my conviction to keep the stock GT2 turbos / injectors and get the car tuned for 93 with basically every other mainstream bolt-on. Even if we cant quite hit 600 at the wheels this way, we are keeping the car (easily reversible back to) stock with low lag and good area under the curve and reliable for track use. Im hoping still for close to 600 on 93 with bigpack, inlets, and exhaust. Maybe have Todd tune it.
^^^ I put the IPD plenum and GT3 tb on my car only after I put on the Xonas. There was a thread by PTF showing the gains (with alpha turbos, not vtgs).
OP, 63s are a good upgrade for vtgs if that's how you choose to go. I have the datapoints showing how much better the 68s were over the 63 on pumpgas and since you would already be paying to upgrade your turbos I'm not sure there would be a big cost upgrade. Rix gave his opinion above and I guess it would depend on your main use for the car. If you were consistently doing autocross or something where you needed the lowest lag and earliest boost threshold then perhaps the 63s would be best but for highway pulls and general zooming around the 68s are measurably better....
……OP, 63s are a good upgrade for vtgs if that's how you choose to go…..
Im starting with a GT2, so my stock turbos (so im told) are not too different from the 63s, certainly not enough different to do the swap. In addition, there is that nagging concern that since it’s a GT2, we ought to keep things relatively original, or at least keep all the stock parts and be able to put everything back to original stock. Once those original stock GT2 turbos are gone, probably pretty much impossible to get original ones back.
Originally Posted by faiz
What’s the lag difference in hitting full boost 63 vs 68? 500rpm?
I don’t have experience with either, but from how its been explained to me, the 68’s are “lazier” compared to the 63’s, which from other turbo cars I completely understand. It sounds to me that if you are into the ¼ mile or half mile, 68’s would be a good way to go, but for road course track driving, not so much, so between the 63's and 68's my preference would be the 63's, but as explained above, stock GT2 is pretty similar to 63's, so that's where we plan to stay.
In another thread it lists the compressor side inducer size for stock, GT2, 63s and 68s. I can't remember what the difference in size is between the stock and GT2 but I'm pretty sure the "advantage" to the GT2 vs 997 vtg is the clipped turbine and the slightly different angle of the exhaust stream entering the turbine side (like a port match). Personally, I wouldn't touch the original GT2 turbos if I owned them. If the 63s are really better than the GT2 turbos then I'd try to find a way to get a used pair of stock 997 vtgs and upgrade them to 63s while leaving the GT2 turbos untouched. All depends on what your longterm plans are for the car...
Wanted to give an update on the stock GT2 turbos. Running a Todd OTS tune with about 1.4 bar, 85% duty cycle on stock injectors, and full bolt-ons: Do88 big pack, inlets, 2,5" exhaust, calculated dyno based on fueling data from Durameteric:
flat power curve from 4500->up due to the compressor wheel limitation. Also, some knock showing up above 5,000 after one or two hard runs, so flashing a new tune targeting a max boost of 1.25 bar. Still want to keep stock turbos. What do y'all think?
I think you should consider methanol.. methanol will eliminate any knock also u will get better air intake temperature.. I think methanol will totally change ur car for better.
Thanks for all the input and real life feedback! This reinforces my conviction to keep the stock GT2 turbos / injectors and get the car tuned for 93 with basically every other mainstream bolt-on. Even if we cant quite hit 600 at the wheels this way, we are keeping the car (easily reversible back to) stock with low lag and good area under the curve and reliable for track use. Im hoping still for close to 600 on 93 with bigpack, inlets, and exhaust. Maybe have Todd tune it.
I have a similar setup as what you intend to do, except for the GT2 turbos. I will be hitting the dyno in the spring! The setup I'm running is a monster in RWD, I don't think larger turbos would suit the RWD and I believe the chassis is almost at its limit with my stock-ish suspension. My car currently hits 10PSI @2500RPM and 22PSI @3000RPM this alone provides an experience like no other. I really recommend doing exactly as you have mentioned and go with the upgraded turbos if you still need more power, however I really do not think that will be the case. I have attached a data log, but please keep in mind we are currently working on a tune with my tuner.
Engine
RWD conversion
LWFW
D088 Big Pack & Y pipe
GT3 Throttle body 82MM
D088 Plenum 82MM
3.5 inch xpipe exhaust
Don’t want to mess with E85 or Methanol. Want to be able to take it to the track (road course) and not worry about hurting anything on 93, even with questionable quality 93.
So far no clutch slippage (knock wood) so our suspicions that the prior owner already upgraded the clutch may be correct.
So, we are trying to figure out what is really safely achievable on stock GT2/GT2RS engine, turbos, and injectors. The thing to remember here is that the 997.1 GT2 came with only 530 crank, while the 997.2 GT2RS came with 620 crank, same exact engine and turbos (people have gone so far as to compare part numbers in the PET catalog), the difference is tuning and slightly better intercoolers.
Looks like we are close to what we could have expected with the 63.5mm VTG’s, so happy we did not do the swap, even though we are running out of compressor wheel above 4,500 RPM. Do the 63.5mm’s have a similar dyno plot?
First off beautiful GT2 MezgerGT I read through the whole thread and figured I would give my opinion not that anyone cares
OK yes there are many plots showing what the 63.5mm would do, there is a ton of info I have dumped in my build thread but I'll put some on here. In regards to keeping your GT2 turbos stock, I wouldn't hesitate to use "standard" Turbo VTGs as your core to make the 63.5s. I realize and understand all the differences between the 2 however everyone is thinking about this in fixed geometry terms. You guys ever think about where all the possible throats are in the VTG? Turbine inlet throat area and wheel is not all of it. In other words I wouldn't lose sleep over it my friend. The 63.5 will have their turbine wheel clipped also when you upgrade, it doesn't need it anyways but they do it because of their experience on fixed geometry turbos which is fine. It will increase the natural frequency of the wheel and give you better HCF (high cycle fatigue) which will increase the shaft speed limit. But many don't realize that the turbine wheel will actually be spinner slower now when coupled with a larger compressor thus not needing the clip lol. This is why we speak in tip speed not in shaft speed when breaking down where the limits are in a particular turbo assembly. The turbo assembly, from an ultimate speed perspective, will have different limits for the compressor stage, turbine stage and bearing system, we pick the lowest one and call it the speed limit in shaft speed terms. This is extremely simplified and high level but wanted to give some perspective. Anyways probably more info than needed on a forum but any other turbo engineers lurking out there will appreciate this.
All that rambling to say, yes I think you have some gains if you go 63.5 VTGs . I recommend you check what vane positions you are running, reported as turbo duty cycle I believe. I run much more open vane positions compared to the stock turbos which means less pumping work (lower exhaust gas in cyl and less fuel for same power). In the dyno plots, my 93oct tune probably has more in it we didnt spend much time on it since I use e85 most of the time.
Thanks M3 Maestro for your reply and data! My understanding of my stock GT2 turbos is that they are much closer to the 63.5mm VTGs than normal stock 997TT turbos are. They are already clipped per my understanding, but they just arent quite up to the level of the 63.5’s. If you look at my calculated dyno chart on post # 25 above, I look pretty close to your Stage_3 63.5mm_93_Oct (dark green) plot on your dyno. Both of us are just under 600 at the wheels at that point, but im flatter 4,500-up as mine starts to run out of compressor wheel. Normally, id be all about upgrading to the 63.5’s to correct that, but if I were to do the swap, the car would no longer be able to be put back to stock, which is an important consideration for a GT2. The market for these cars is largely collector driven vs people like us who want to actually drive and enjoy them.
To combat the possibility of knock for track driving on hot days, im reflashing for about 18 PSI down from about 20. Hopefully, that will not make a huge dent in peak power and will reward us with a more linear curve. Hope to have that data in the next few days.
It looks from your chart that stock boost is around 16? Is that correct? You have some great data there. Thanks for sharing!
I read through the whole thread and figured I would give my opinion not that anyone cares 
. I recommend you check what vane positions you are running, reported as turbo duty cycle I believe. I run much more open vane positions compared to the stock turbos which means less pumping work (lower exhaust gas in cyl and less fuel for same power). In the dyno plots, my 93oct tune probably has more in it we didnt spend much time on it since I use e85 most of the time.
