I've been driving around for a few weeks since completing the 700R4 to T5 mod on a 5.7 liter. It's much more fun than the auto. However I've noticed it doesn't seem to pull quite as hard as the 700 did. I have a 9 bolt 3.23 rear end. I'm not sure if it's that, or the TPI factory intake. I've just never really noticed it behind the auto. It doesn't pull to red line. Kind of peters out around 4200 RPM. I know there are limitations to the factory plenum. I seem to remember it being stronger through 5000 though. I'm waiting on a custom PROM, and I've since added a Racetronix fuel pump. I have yet to bump up the fuel pressure. Just not sure if the T5 has shown the limitation of the intake, or if I'm just off in gearing.

 

Gearing shouldn't have anything to do with where the engine runs out of power.

 

Limitation of the intake without question.

In my experience with the iron headed third gen engines and the T5,

1: A cammed 4bbl L69 is a LOT more fun than even the best year stock LB9. Revs to 6000 easy, and mates very well with 3.73 rear gears. The LB9 is going to flat choke around 4800 to 5000. And it's not going to respond to gears as well as a better flowing intake manifold. Regarding the 4200, maybe your tach is off. I don't know.

2: The TPI manifold is very old tech. Attempts at mileage and torque used long runners. It's a better stock manifold than a cross-fire manifold, and it looks cool. But as far as being a great manifold for the various sizes of SBC across various rpm ranges, it's really not a good intake at all.

3: Gearing matters heavily, but like Freddie said, it's not going to change the rpm where the engine powerband lies. The V8 T5 and 3.73 gears are a fun match that doesn't kill highway speed and works great for a lot of V8 combos. It's baby bear's porridge. Except for the rpm-choked TPI and cross-fire.

 

Peak HP on the stock 5.7L was made at 4400 rpm, so it petering out at around 4200 is about right given the tach may be off, and other little things.

You are not going to pull hard above the RPM that made peak HP.

Your 9 bolt rear should be 3.27 gear and no that has nothing to do with it really.

The T5 allows you to hold gear longer, shift higher, etc so you are reaching your engine's peak power point sooner.

The "redline" on the tach means very little in the case of how it or where it should pull too. In fact it means do not go above this RPM or engine failure can happen.