how many vaccum inches of mercury are you supposed to have on a tbi 350. I took a reading off of the tbi in the front vaccum line and got about 21. is this right?

 

Your just idling in the driveway in neutral eh? That's ballpark.

 

yea just sitting there. i was using the vaccum to set my timing. I was always under the impression that it was supposed to be around 15 or so.

 

Idle Vacuum really depends on the setup. A Stock TBI 305/350 will pull 15 in/hg of vacuum sitting in gear with the A/C on. My 224/224 @ .050 cammed 305 pulled 15 in/hg unloaded @ 750 RPM.

20-23 in/hg is about right. Higher compression ratio typically gives higher vacuum readings, smaller cam has higher readings, roller cam increases reading, and reducing internal friction helps, as does gapless piston rings and good valve seal.

My old 312 with the tiny roller cam in it would run at 22-23 in/hg @ 650 RPM. At a steady 2,500 rpm it was pulling down about 24 in/hg unloaded.

My ZZ4 cammed 350 pulls about 18 in/hg unloaded.

 

ok so thats good. so now i have some more probs. thanks for the info.