What affects vacuum readings?

[Puller]

I just put my motor back together, i still have to adjust ign. timing as it's way off, i only have o2 housing with no exhaust...could either of these things on a mostly stock motor reduce vacuum measurements. mine measures around 8. Someone said the lack of backpressure will reduce vacuum measurements as well as lean the motor out a bit. shoot me some info.

 

[98spydert]

Did you do a compression check on your engine? Doesn't sound healthy to me.

 

[erc's_gsx]

Normal reading (usually 17-22" Hg. in stock engines) Race engines vary "a lot" and in most cases will be considerably less.

have u done a boost leak test? there seems to be a problem, put back ur exhaust and see if your vacuum readings change any.

 

[Puller]

no boost leaks...compression 165 accross the board...new pistons rings rods and bearings. yeah i don't have an exhuast now but ill bend a 2.5" with a flange just to see anyother suggestions?

 

[psychlow]

A plugged cat will kill your vacuum.

Pretty common on these turbo cars....

Unbolt your cat and then run it, see if your vac readings improve.

 

[Puller]

i mentioned that i have no exhaust on, only o2 housing and im gonna put a downpipe on (or something homeade for now) to see.

 

[Defiant]

Reading vacuum:

http://www.iwemalpg.com/Vacuum_gauge.htm

 

[Puller]

thanks but that only answer ign timing...can the presence of, or lack there of, back pressure effect idle...? that only describes clogged cat.

 

[psychlow]

Originally posted by Puller
i mentioned that i have no exhaust on, only o2 housing and im gonna put a downpipe on (or something homeade for now) to see.

Sorry, I read "no exhaust" meaning no aftermarket exhaust, or that you were running stock exhaust. My fault.

Usually, ignition timing won't affect vac readings too much. Valve timing, on the other hand, sure would....

 

[Puller]

hmm...im not sure..it's just i replaced pistons and rings, 155 accross the board (typo above not 165) valve timing is perfect...im not sure it idles fine...could the 2g mas mess with it at all do you think...running rich or lean? once i log it ill put my safcII on and see what i can do...thanks for the help

 

[NOSNUSE]

If you installed a 2g mas with out adjusting your afc you are running lean and this can effect your vacuum, does this only happen when the car is warn or all the time.

 

[Puller]

i dunno really i don't have a pattern to explain right now cause i jsut got the motor together...i got it to run after countless attempts and it ran like ass but none the less...i went to start it the next day in teh cold and it fired...ran, quit, started up idled high ran quit...then i got it to idle on the 3rd try, around 1000 or so...im sure it just needs to warm...but as far as the mas goes...i have rotten vacuum when it starts cold, as it warms it steadies out to around 10.

 

[Puller]

i got it goin today, i must be running really lean because its hard cold starting, and it sputters like it's ridiculously lean...like danger zone...and it get's close to the "h" on the stock temp gauge. i have a 2g manifold, 16g with no heat shields and i also have just an o2 housing for an exhuast..is that my heat problem? just driving it normal is a problem as it overheats and kicks and sputters....do you guys think im running lean...im gonna put my safc II on today and richen it up to see if it helps...once warm it idles fine but still stays hot with fans on...any help appreciated...if you need more info just ask. also backing off the gas only gets me like 12 in hg opposed to 20 or so on a stocker...will you lose vacuum if you are too lean or too rich? thanks