What VAC does ur car idle at? (that sounds really stupid, but i dont know what else to call it.. opposite of boost)

 

between 15-20" is normal

yes it's called vac.

 

Thanks

 

What would cause a car to run at like 10" of vacuum?

 

low compression or vac leak somewhere.

 

Or an intake leak. But yea, 15-20 should be good. If you are getting low vac definatly check for intake leaks.

-Andrew

 

All engines in good health, turbo, NA, sc, whatever, idle at 14-20". Ported engines (or cammed piston engines) with more overlap can have much less vacuum. Low vacuum on a rotary that is not wildly ported can be an indication of poor compression...vacuum is formed by the engine ingesting air...since compression is a measure of how efficiently an engine moves air, a lack of compression means it moves less air, thus pulls less vacuum.

Any leaks in the intake tract between the internals and the throttle plates will cause a loss of vacuum as well. The throttle plates being closed are the reason vacuum forms and is maintained between them, and the internals. The engine is spinning, pulling in air, the throttle plates are closed and not allowing much, thus you have vacuum in between. IF you have a leak in that area, however, atmosphere will be pulled in, and the level of vacuum will raise toward atmospheric.

 

mine reads about 20ish

 

mine's much closer to the 14" end of the scale.

I'm sure I've got a vacuum leak somewhere.

 

Yeah I think I have a leak in my tb (when it was ported it got nicked at the mating service.. amazing huh?)

 

Bad corner/side seal clearance.

 

I believe that would be covered under "poor compression" as mentioned above...