scoutcamper

Hey All,

I havent seen a combo of these questions here before, and I have used the search button.

Just bought a 1990 GMC Jimmy with a 350 TBI in it. It is painfully slow. Literally my Miata can out run it, which is killing me. I have a 305 TPI from a 1991 Camaro sitting on the engine stand at my parents house.

Thought 1: Can't do TBI->TPI intake swap that I *really* want to do, thank you California SMOG.(See my last TPI swap here: https://www.thirdgen.org/forums/tpi/...-65-chevy.html )

Thought 2: That 305 TPI engine likely has aluminum heads(need to verify this weekend)

Now to the question: Would it be worth it to swap the 305 TPI heads onto this 350 TBI, along with buying a new flat tappet cam(working on a super cheap budget here otherwise summit racing heads would be my weapon of choice)?

I am going to do Exhaust and I have already done a *good* tune up(had to to get it to pass smog). I will also preemptively be doing the fuel pump.


One last note, this rig is being used for Search and Rescue(Personal response vehicle), so I need bulletproof reliability(People need rescuing at the most inopportune times), which is one area that the TBI shines above a carburetor .

midias

If the 305 is a factory camaro then it will not have aluminum heads. The heads will swap but make sure the combustion chamber volume will not raise compression too high. Other than that Look up Ultimate TBI mods and do those after the cam.

Schurkey

Fairly sure the 305 heads will have smaller valves than the 350 heads. Perhaps 1.85 intake diameter for the 305, vs. 1.92 for the 350.

Are you prepared to re-program the computer? I'm not sure how much power you're going to gain with smaller intake valves and a less-mild camshaft. A tiny boost in compression is not likely to make a big difference. 'Course, the computer recalibration all depends on the camshaft you select.

When it was me, I stuffed a '92 Caprice LO5 into my pickup. The Caprice 350 added 1/4 point of compression, and a (mild) roller cam to the pickup-truck LO5, yet required ZERO computer re-calibration. Even so, not a huge power improvement over the truck engine.

Scrapping the swirl-port heads sounds like a wonderful idea.

I threaten myself with adding aftermarket heads--which I've already purchased, and an aftermarket intake manifold--which I've already purchased...but I just cannot get myself to disassemble such a sweet-running (if underpowered) OEM engine.

Unless you intend to replace the intake manifold, you'd do well to find heads that have the screwy tilted-center-bolts that the OEM intake manifold requires. You'll also want an exhaust-gas crossover provision on those heads, so the EGR continues to function.

I put Edelbrock "Tubular Exhaust System" (EPA/CARB compliant headers) and a 3" catalyst and cat-back on my pickup. I can't say that it made much difference. I was a bit underwhelmed. Which is why I torture myself about the aftermarket heads and intake manifold. Maybe even a mild cam.

Wise. Don't forget to verify voltage as close to the fuel pump as you can get, and that the in-tank wire harness is often corroded. Any loss of voltage results in poor pump performance.

You'll want to keep the mods fairly mild, then.

RBob

iTrader: (1)

My guess is that there are other issues with the truck making it slow. Does the fuel pressure hold under load? Is the base timing set correctly (can even bump it a few degrees)? Is the hot air flapper in the aircleaner working correctly (if not it will kill power)?

Cap/rotor/wires/plugs OK?

Does the TBI actually go wide open when the go-pedal is stuck to the floor?

Any chance the rear diff pinon seal has been replaced? The retaining nut may have been over-torqued causing a drag on power.

RBob.