GT45 will spool faster on a bigger motor. Like a 5.7 or 6.0. On a 4.8 or 5.3 they are laggy compared to a 76/65 or 78/75. The GT45 are hard to beat for the price and are solid up to 600rwhp, the 78/75 will push that a bit further like 700rwhp and a Billet wheel will get you more. Spool should be faster with a 78/75 than a GT45 but the 78/75 is more of a choke point on the exhaust side. You can get a cast wheel VS 78/75 for $399 and eBay has 15% right now on there eBay dealer.

yes and no. At 7psi aka low boost your turbo isn’t working hard so the exhaust isn’t backing up as much. But the housing can still only flow so much exhaust at a given psi. So a larger exhaust housing could provide more power due to less restriction even at 7psi at the same boost.

Its all about engine size, spool rpm and expected HP and boost.

A 76/65 would spool faster than a 78/75 and a GT45 but t won’t like rpm depending on Engine size. The Chinese turbos are less efficient (old tech) so a precision 76/68 will perform much better than a Chinese 76/65. The 76/65 will max out around 600rwhp like a GT45 but spool much faster but won’t carry higher Rpms like the GT45. Overall in the 5.3-6.0 range it’s hard to go wrong with a 78/75 given you keep Rpms within limits on bigger engines. With a manual a smaller turbo will help with spool on shifts etc. So the 76/68 or equivalent would prob be a better choice where I would jump to the 78/75 with a auto.

Fwiw on my 78/75 I run a full 3” highflow catback and it makes 6.5psi with a 12psi spring (cracking pressure) with a cutout I make 8.5psi and pick up a ton or spool time with no changes anywhere.

wastegate bench cracking pressure is different than installed pressure. Manufactures factor in exhaust backpressure pre turbo into the spring psi. So 12psi opens on a bench but when you factor in the exhaust pushing back on the valve when building boost you get it opening before hand. The amount of backpressure is based on exhaust turbine sizing and exhaust flow after the turbo (DP). Back pressure if high on mine bc I run a full 3” exhaust which is like 20ft of tubing, the cutout bypasses 1/2 that and this boost goes up and spool shortens. The same thing applies to turbo exhaust sizing, the difference is that if you go too small on the turbo exhaust your **** out of luck.

Since I know it takes 12 psi to open the valve and I make 8.5psi with the cutout open that means 8.5psi boost = 12-15psi back pressure to force the valve open. That’s almost a 2:1 ratio.

What’s the max power and boost you want to run and on what Engine and max rpm?

Looked back on ur first post, 350 lt1, 500-550rwhp and manual. I would be looking in the 70-76 / 65-68 range. So a cheap 76/65 will get you there if your budget is low. You could do a 76/68 and cost 4x as much. Or look for a 70/65 70/68 something of that exists. A smaller impeller will spool faster as well as the turbine side.