Adding a performance camshaft to the stock 5.7 TBI 350 in your 1988–1995 GMT400 is one of the best bang-for-the-buck upgrades. Here’s what cams work best and what to expect.
Why Cam Swap on a TBI 350?
The factory 5.7 TBI 350 comes with a very mild cam designed for fuel economy and low-end torque. A performance cam with more lift and duration wakes up the engine significantly – especially with some head porting.
TBI Cam Requirements:
The TBI system uses a MAP sensor for fuel management. Cams with:
- Up to .480" lift: Works fine with stock TBI
- .480–.520" lift: May require tuning adjustments
- Duration: Keep under 240 degrees advertised (210 degrees @.050")
Top Cam Pick:
COMP Cams Big Mutha Thumpr – SBC 283/327/350/400 Camshaft Kit – Popular choice that works well in TBI trucks. Includes cam, lifters, pushrods, and timing set for a complete valve train upgrade.
What to Expect:
- Power increase: 30–50 HP over stock with cam only
- Sound: Distinctly lopey idle – classic truck muscle sound
- Fuel economy: May drop 1–2 MPG at light throttle
- Driveability: May require idle relearn with TBI
Supporting Mods:
For best results with a cam swap, also do:
- Timing advance to 36–38 degrees total
- HEI ignition (see other thread)
- Cold air intake
- Performance exhaust (header + cat-back)
Cam Installation:
Changing the cam requires engine disassembly to the point of removing the timing chain. Plan for:
- Drain coolant and oil
- Remove intake manifold
- Remove valve covers and pushrods
- Remove timing cover
- Replace timing chain and cam simultaneously
Tuning Note:
The TBI ECM can sometimes be jumpy at idle with a big cam. Use a scan tool to monitor and adjust idle air control (IAC) counts and timing for best idle quality.
What cam did you put in your TBI GMT400? How did it change the truck?