[dchavezo]

Hey everyone. I've searched thru my fsm like you wouldn't believe and I still haven't found it.

Do any of you know the exact number the 4.9 engine vacuum should be at idle with no leaks?

 

[onecool]

I don't know that there is an exact number I would think the altitude you are at would be factored also I'm doing a quick guess here may be 12-15 in at sea level?

 

[dchavezo]

That seems a tad low. I read a post on here that someone said 21 is what the "manual" says but I'm not sure what manual they were referring to.

I'm getting a solid 18, that sometimes bumps to 19.

Read another post that said 15-20 is good but that to much could be a problem. If anyone has an exact number and a pic of a FSM or a credible source I'd appreciate it!

 

[Ranger]

I'd say 17-19 should be expected.

 

[Submariner409]

All engines differ on idle vacuum - injected, carbureted, cam selection, ignition advance, rpm, number and size of emissions and working vacuum bleeds - all makes a difference. A mildly radical, lopey cam absolutely destroys idle vacuum.

That said, idle vacuum in a normal road car V-8 engine at about a 650 idle speed should run from 18" - 22". There is practically NO airflow into the engine at idle speeds and very little at highway speeds, steady running. In my Northstar at 65 mph the throttle is only open about 9%.

On the road, the higher the vacuum for a given engine speed/load - the better the fuel mileage.