Engine Vacuum Psi

[79MaliCrazz]

I want to know what is a normal engine vacuum psi i was told mine is 9 or 10 psi at idle and drops to 5 or 6 in gear and that's not normal the only problem is i bought my car with the motor in it and the guy couldn't give me the specs of the cam,heads,pistons,etc cause he didn't know it he only new that it had stock block,lunati master cam,flat top pistons,ported heads,victor jr intake ported to match the heads,and roller rockers. So no one cant figure it out without taking things a part to investigate.So my main question is whats a normal psi and what might i do to change the psi.

 

[DRIVEN]

By psi I assume you mean inHg? High teens is pretty typical on a stockish engine. Once you start playing with cam timing and duration it goes down pretty quick. There is no "normal" and without changing parts you can't really change it much. Fuel mixture and ignition timing will have an effect but if your engine is properly tuned now it won't likely increase.

 

[megaladon6]

it all depends on the build. a stock engine can produce almost 32inHG.
mine produces about 20in which is rather high for a performance build.
best thing to do is simply make sure the vac lines are good, and tune for the highest vacuum.

 

[DRIVEN]

a stock engine can produce almost 32inHG.

Never seen anything close to that at idle. Maybe momentary high 20s after the throttle plates slam shut following a high rpm pull. I do agree with checking vacuum lines and tuning for highest vacuum readings. Also check for internal vacuum leaks by pulling the pcv out of the valve cover and put your finger over the grommet. There should be no suction in the engine. If there is you might have a leaking intake gasket.

 

[khan0165]

a stock engine should see around 20inHg at idle. Motor with a mild cam can be anywhere between 8 and 20ish... You would need a pretty rowdy cam to see 5 inHg... or you might be over carb'd

you don't necessarily want tune for the highest vacuum... tho higher vacuum usually yields nicer idle and driving characteristics. You want to tune for the most steady vacuum.

if you're playing with your metering screws. With the car off, turn them all the way in, bottomed out. Then give both a half or 3/4 turn out. This will insure they are both equal, and give you a base line to start from. Then start the car, and slowly work your way up 1/8th turn at a time. Make sure you keep both equal... rev the motor a little between each adjustment to cycle some fuel thru, and keep going until you see the vacuum drop a little. Then back it off where it was steady and happy... At this point, if you want a little pep coming off the line, give them an extra 1/8th turn. This will insure you have a little extra fuel if you punch the throttle.

here's a good video for tuning http://www.youtube.com/watch?v=UPGLUsW5ZeE
here's a good video to diagnose any problems using a vacuum gauge http://www.youtube.com/watch?v=NrnzD7PgoHI

 

[-83MONTESS-]

Khan you just made it 100x easier! The whole vacuum timing thing always confused me but now I "feel" like I know how to do it :lol:

 

[79MaliCrazz]

Thanks guys I will look at adjusting my timing and carb and where it goes from there.

 

[79MaliCrazz]

They said I have 14 to 15 inches of vacuum not 6 the shop I first took it too are dumb asses.

 

[khan0165]

that's a very good reading if you have a roudy cam. You should be able to run 600-700rpm idle very smoothly there, depending on what kinda timing you have set.

The thing I've learned tho is... the number of your vacuum reading is not important in anyway. Whether someone tells you, you have 6 or 20, it doesn't really matter. What you wanna look for is where the vacuum is steadiest (given everything else is well adjusted).

what you do is, set a target for where you want the motor to idle and how much timing you want. Then try to achieve the steadiest vacuum at those settings. If you're having trouble, change the idle and timing, and then play with vacuum.

... keep repeating until you are happy

... and try not to get too caught up, there's so many settings, sometimes it's impossible to get a perfect setting.

 

[79MaliCrazz]

Thanks will do.