Hi,
I have a 1992 Z28 with the GM Fastburn 385. The current set up is all stock except for the motor/SDPC baseplate/Headers. The body/suspension is all stock.

If you open my hood, the only comment anyone ususally makes is "Nice headers" Which is just what I want. I even have my L98 valve covers on it.

I am using the stock tune, stock injectors, plenum, throttle body, runners, etc. I get 25mpg.

With this set up I've run a 14 flat in the 1/4. I recently took the car to a dyno and it produced 255hp/350fttq at the real wheels through my rebuilt 700 r4.

I'm thinking of AS&M runners which look stock but are bigger; port, siamese and internally polish the plenum; change to bigger injectors; and change to a larger throttlebody.

Does anyone have any experience with this set up? Which change will increase my horsepower? I don't want to start breaking the rear end. I just want to make use of the horsepower the the engine has by adjusting the induction. Any suggestions?

 

Did they dyno give you an air/fuel reading? with stock tune and stock injectors I would think the extra cubes and headers would need more gas. So yeah, tune and injectors on the fuel side.

If you are looking to improve upon the intake itself, have the SD base ported. With the cam that summit says is in that engine you'd definitely see some improvements with larger runners and a ported plenum. If you're looking for a bit higher HP number at the cost of a few lb-ft of torque go with a set of ported SLP runners; with as much torque as you are already making you'll likely see a good improvement just from injectors/tune, and the SLPs will give you a couple hundred more RPM, which your extra cubes could use.

 

When I put AS&M runners on my engine with L98 aluminum heads and a ZZ4 camshaft, I gained 4 horsepower and 10 torque at the wheels. I dont think you need larger injectors. 22lbs injectors are just fine at that power level.

What you really need to do is remove that intake and port it. We recently flowed a Vortec TPI intake (I'm assuming you're using that with the Fastburn heads) and it flowed only 235 CFM. A regular Edelbrock TPI intake flowed 260 cfm after extrude-honing, and my current intake after welding the roof and porting flows 305 cfm.

Any 52mm throttle body will work. Make sure you get one with an airfoil built-in or included.

 

The intake is the Scoggin/Dickie Vortec intake. I don't know what it's designed to flow. Interesting about the AS&M runners.