my vacuum reading on my boost gauge sits at 18in hg without A/C with A/C it's about 15-16in hg. i also rely on the pressure reading of my VAFC to be a little accurate(i think) and i got about 457mm hg without A/C and 420+ mm hg with A/C.

i have no problem with the reading until yesterday when i saw an SiR like mine at an auto shop i go to and his vacuum reads 598mm hg and that's with A/C. the car has aftermarket air filter, DC sports headers, Tanabe exhaust and a VAFC while my car has turbo.

i was so surprised that i contacted my mechanic right away since my car was only rebuilt last 3 months ago and he told me the reason for low vacuum could be Cam timing and valve adjustment. i then asked if he put my cam timing to stock and he said he did. so that means it may be my valve adjustment since he did mine 1000km after the rebuilt.

now, is it true that cam timing and valve adjustment could be the result of it? by the way, he adjusted my valves with the engine hot since he said there's a hot and cold setting BUT even after the rebuilt without the turbo i don't see my vacuum go as high as 598mm hg on the vafc.

is there anything wrong with my car? what does it mean if a car has less vacuum compare to one with high vacuum especially in terms of performance?

i did compression test and i got: #1:185 #2:195 #3:190 #4:195. the engine is B16(2000 SiR).

 

what's your idle speed

 

idle speed is at 750rpm but on the vafc it jumps around which i believe is normal.

when the a/c kicks in the idling will go up to like 850rpm or 900rpm and my vacuum reading will decrease which is normal but low. on this other car i saw yesterday, he had his a/c turn and his idling is at 850rpm to 900rpm but his vacuum reading jumps around to 598mm hg so imagine his vacuum reading would be if the a/c was turned off.

why is my vacuum low? is there something wrong with my engine? i know no two engines are the same but could the vacuum reading differ by that much?

 

the a/c puts more of a load on the engine, hence the lesser vacuum (the idle air valve bypasses more air due the increased load, and consequently the vac reading goes down)

nothing to worry about :-)

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by lazerus &raquo;</TD></TR><TR><TD CLASS="quote">the a/c puts more of a load on the engine, hence the lesser vacuum (the idle air valve bypasses more air due the increased load, and consequently the vac reading goes down)

nothing to worry about :-)</TD></TR></TABLE>

but the problem is, his reading of 598mm hg is with his a/c ON while i got a reading of 420+ with a/c ON also. i believe that's a big gap there.

would adjusting the valve a bit tight results in that huge gap in vacuum reading, lets say if my mechanic adjusted it a little on the tight side?

 

750 seems really low to me..mine idles between 840 and 900...always did even when completely stock

 

Tight valve adjustment will give you low vacuum reading as well as richer idle. The richer idle is simply a side effect of the lower vacuum because the fpr increase fuel pressure as vacuum decreases. Try setting the valve adjustment to middle of the upper and lower factory limit.

 

I uninstalled my MSD box a few weeks back and afterwards the car was drawing significantly less vac. than before...so, after checking all my vac. lines (it was a 6btm, so I had to take out a line) I just leaned out the idle a bunch and the vac. went WAY up. now I'm drawing right around 21 in Hg at idle. Cold idle is still higher (less vac.) but that is to be expected.

I'd try leaning it out.

 

i guess what i'm experiencing is normal(i hope) coz i suspect my valve adjustment is adjusted tight and yes i'm rich at idle coz i'm using the vafc hack.

by the way, aside from running rich is there any negative effect of lower vacuum reading?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ion_four &raquo;</TD></TR><TR><TD CLASS="quote">I uninstalled my MSD box a few weeks back and afterwards the car was drawing significantly less vac. than before...so, after checking all my vac. lines (it was a 6btm, so I had to take out a line) I just leaned out the idle a bunch and the vac. went WAY up. now I'm drawing right around 21 in Hg at idle. Cold idle is still higher (less vac.) but that is to be expected.

I'd try leaning it out.</TD></TR></TABLE>

same here for me... i pull about 21-22 inches vacuum totally warmed up on my car. i dont know the conversion to mm

 

anywhere from 18-22 on the vaccum is normal but when your not running your a/c and it is about 15 then your rings could be going because your losing compression on your motor, well that was what i was told....