I vaguely recall reading about this last year. Anyone done it? Is it a worthwhile upgrade (with a modded compressor side too of course)? Does the lag increase noticeably?

Just wondering how it would affect 60-130, 100-200km/hr etc...

Reading all these threads about super-build monster turbo cars is giving me a little turbo envy.

 

Big turbos generally take more time to spool up to achieve pressure. That usually means more down low lag and less throttle response when prodded but as the revs build the pressure can be made so the response recovers in the higher rev range hence more up top. I can't comment on how larger 65/68 vtgs would behave on a 997tt but logically they aren't likely to give the same throttle response when you put the boot in at low revs as smaller wheeled turbos. If the 65/68 vtgs have the same throttle response then terrific! That's be a deal maker! Maybe Tom can give some guarantees here. But then that may mean its time to quit out of your turbos into some 65/68's. On a 3.6 engine its a fine line. It really comes down to where you want your power and, just how fast you want to go .

 

As Speed mentioned there are potential downsides to larger turbos in terms of drivability/lag, although some of the dynos of the alphas and other newer turbos suggest they won't lag anymore than the stock VTG's. Best to ask the tuners directly and have an idea exactly what role you want for the car - x% daily driver, y% street/track hero, z% garage queen, etc.

 

Eclou love the bit on the "garage queen" . Hey boggy please don't tell me yours is becoming the garage queen now..

 

Are you kidding me, mine is a dd. Rain, shine whatever.

Paul, I think you may have misunderstood my post - I'm not just talking about a larger compressor side like the Champion 65/68, I'm wondering specifically about using the GT2 turbine side on a modded compressor side (63.5/65/68).

I now recall that GT-TT (George?) did this on his car.

Gene, your point is spot on - trouble is that it's hard these days to get a reply from Proto about relevant questions never mind my turbo daydream questions.

I do recall a post somewhere a while ago indicating that after a certain point there is no point in increasing the compressor side b/c the 997tt turbine can't do any more anyway and that's why I wonder about the GT2 hotside. But then again, Champion has managed to get some good results with 65 and 68mm compressor side upgrades with regular 997tt turbine sections so who really knows what is truth and what is dogma.

 

If you look at Dave's AUC thread it's amazing how good the graph of the A28 looks. Massive USABLE torque. That's what I'm looking for. If the car is asleep till 4k rpm then it doesn't matter to me how much acceleration there is after that b/c I want the power down low and in mid-range. Perhaps the GT2 hotside isn't optimum for my goals, but just wondering about any user experience feedback on this.

I'd consider getting the A28s but I want to stick with VTGs - no EBC needed, uses the same oil lines etc... Champion's option is top of my wishlist, just waiting for a bit more data on performance times rather than just awesome dyno plots.