Why does cold ambient temp (outside air) make your vacuum reading lower at idle? (closer to 0, say 13 instead of 15-16). Also, what is considered good vacuum on a rotary?

 

Mine has actually gone up since it has gotten colder, I was at 16 and now I'm at 18.

 

when it gets cold outside, all the little air molecules want to get warm, so they crawl up inside the engine and throw a party, then the car moves



But seriously, my guess is that because the air is denser, the engine doesnt have to pull as much. I would think it would be more vaccume though.

 

While experimenting with my fuel map at idle (he same rpm) for AFRs, I noticed and previously recorded….lower AFRs (richer) corresponded to lower vacuum values.

Perhaps possibly with an engine that doesn’t alter fuel with respect to temp…then when temps decrease your vacuum readings would increase since there would be denser air and a leaner mixture.

 

Thanks for all the good answers. From my experience, a good rotary will pull at least 13-15.
The best I ever get is 16, and while I suppose 12 could be ok, 10 definitely indicates a problem. Sometimes it's a loose spark plug wire or a vacuum leak, but can be a blown motor too. Mine is normally at 14, but when it sticks at 12-13 it pisses me off, so I rev it a few times to get it to stay on 15-16. My Syclone pulls like 17 with the A/C on.