At this point, it's super uber hard to guess cause no one has ever posted results with a true dual ball bearing, Precision made 3-bolt .85 A/R PT70, 72, 74, or 76 GT-Q turbo on a Buick. But....
If I had to guess, based on past results of going from non ball bearing to true dual ball bearing on the other turbos, there is about 400-500 rpm minimum, less stall speed required when going from non bb to dual bb.
Example:
If you run a 3500 stall converter on a non bb PT6776 on a 231 CI v6 now, you really only need a 3000-3200 to spool a dual ball bearing PT6776R. As time goes on, we might find people running an orange stripe 12" converter that can only foot brake to 2800, will spool the PT6776R just fine. But until someone does some real world testing, the general consensus right now is 3000-3200.
With that said, typically the non bb PT70 GT-Q likes a 3800-4000 stall converter on a 231-235 ci motor to be able to race it. (IE: Pro Tree and Trans Brake heads up racing) These new stroker motor combos will require slightly less, I'd say about 100-200 less. 270+ ci Stage motors are another story. Now, if we apply the 400-500 rpm rule, this would theoretically put the stall requirements for this new dual BB 70 GT-Q turbo at around 3400-3500 and still spool it up enough to race. The big WOW factor with this info is, it will allow you to run a tighter converter with less slippage, which equates to more mph. Not to mention that you can get a nice 9.5" lock up that can handle some serious power at the 3400-3500 rpm range. A real win win for everyone. So instead of running say 10.00's at 132-133 on moderate boost, you might see cars running 10.00s at 136-138 on the same moderate boost level.
Time will tell once more people get them on their cars and hit the track. But I have a feeling that this is what we will see. The other thing is, the larger you go with the compressor wheel, the more the stall requirements go up. This is due in part to more rotating mass for the larger compressor wheel. Exactly how much more, is also a grey area. On a 235ci motor with a 76 GT-Q, we would typically need a 4200 stall. On a Stage II 270+, a true 4000 would get the job done easily with the 76 GT-Q non bb. On my old car with a 274ci Stage II and a PT74 GT-Q, my Art Carr 19930 that stalled to 4000 would spool that turbo up so fast, that I had to really be careful on how much boost I left with or it would just smokem like a top fueler. I really could have lowered the stall to around 3800 and it still would have been raceable. Hope some of this helps, and I hope my predictions come true. If they do, it will make a lot of racers very happy I can assure you.
FWIW, there have been some Buick racers that have run the GT4276 in both non ball bearing and ball bearing who have reported favorable spool up results. But that turbo has only ever been available with a T4 inlet 4 1/4" discharge style Garrett turbine housing in 1.01, 1.15, 1.28 and 1.44 A/R. TSL guys love the GT4276 non bb with the 1.28 A/R Garrett housing.
HTH
Patrick
P.S. It would be sweet if the powers that be could develope a 3-bolt .95 to .96 A/R housing that would take a internally gated 3.5" downpipe and bolt up in the stock location. They could call it a PT4294R. (70mm comp wheel) Or how about a PT4202R (74mm comp wheel) And lastly would be the PT4276R (76mm comp wheel) But there I go again speculating.
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