I've got a 92 z28 g92 car. It has the stock 3.23s in it right now but also has the factory posi. Hawks sells a kit along with the gears. I'm a 3 series carrier correct ? And will the gears work with my factory posi ?

 

Yes that rear will have a 3 series carrier. So yes, 3.73 gears will bolt right in.

Unless you have SUBSTANTIALLY modified the motor, including doing away ENTIRELY with the TPI, you will find that 3.73 is way too much gear for that combo. You already have arguably the best gear for it.

You'll find that a converter is what you REALLY need. Leave the gears you have; put in a converter with about a 2500-2800 RPM stall speed.

 

^ I agree. Also keep in mind that shops will charge $300 to $800 labor depending on where you go for a gear swap. Then you have things like speedo being off and needing to swap gears in for that or go to a programmable one. That can easily put you in a $500-$1000+ range for this swap. My 350 TPI loved my ACT 9" converter. It felt like a different car after that install.

 

Beside costing a lot why would 3.73s be bad behind a stock 350 ?

 

Tuned Port Injection (TPI) uses an extremely long runner design. Air enters through the throttle body and has to flow to the runners (tubes from the plenum to the lower intake). Those tubes are fairly small and probably 1/4" or more flashing around the entire diameter of the opening due to GM casting process. Then each tube is only routed to it's designated cylinder. They cross over in the lower intake. So runner on driverside actually goes to a head intake valve on the passenger side. This gives it the very long design. It will make good torque which is mainly used on the street applications due to these cars being 3200-3600 lbs.

That being said a Stock 350 TPI will fall on it's face in high RPM. Usually by the time it gets to 4800 RPM the HP will fall out. Why? There is a peanut camshaft in it, low compression, and not enough air flow.

Most people say 3.23s or 3.42s gears. There are some guys running big numbers out of a TPI setup (check out some of Dyno Don's threads), but it takes some work to get there. My stock motor made 211 RWHP and 313 RWTQ.

IMO I would look at dropping the money in different areas. New Converter, LCAs, Reallocation Brackets, and SFCs would be a good investment. Remember with that higher flash torque converter you will see more torque leaving the line, which tends to reveal more traction issues. Also remember a smaller torque converter will be less weight and a higher torque multiplier.

 

Right: it's not about "stock 350"; it's about stock TPI.

The "T" in TPI stands for "tuned". As in, ACOUSITCALLY tuned, like a guitar string or the like. Actually, more like a flute. But, the point is, it uses the compression / rarefaction wave, aka "sound", produced by the air rushing down the runner and slamming into the back of the intake valve just as it closes, to reinforce THE NEXT cyl fill event by travelling UP the runner, THROUGH the plenum, and DOWN that next runner, thereby giving just a little extra ooomph to packing that cyl full of air/fuel. Problem is, it's ENTIRELY dependent on the length of the runners; which if you measure them, they are 22" from plenum to int valve, and that length cause the RPM at which the reinforcement effect is at maximum, to therefore be 3600 RPM (speed of sound...) and then above that, the effect diminishes, until it interferes DEstructively instead of CONstructively at 5400 RPM. NOTHING you can do to TPI will change that, as long as those runners are still there.

So what you end up with, is the typical TPI "Mount Everest" torque curve, with this MASSIVE peak at 3600 RPM, and then a RADICAL fall-off above that.

Which means, if you put gears in the car that force the engine to run above 3600 RPM, you slow it down. You get one of those familiar tire-smoking burnout machines, that can't pass a semi on a 2-lane road at 55 mph even when wound out to the red line.

You have the right gear for YOUR combo. What you need to make it leave harder, is a converter that won't pin the motor to 1500 RPM when you mat it at a traffic light. You get your launch by a different means, WITHOUT destroying the engine's ability to operate in normal traffic.