TII Boost Guage at Idle
 

when my car is idling (or trying to at least), what should the boost guage read? 0 or -20? everywhere i drive it says -20 until i step on the gas, then it jumps to 0 then starts moving up... i keep wanting to think it's supposed to be at 0 and that i have a vacuum leak, but then again, i don't know, thanks

the lower the Neg level at idle the better.

-20 is prolly off a bit, stock gauge sucks, but ya thats normal, and good.

Robert

hmm, thanks, looks like i'll need to look in a new direction for my idling problems

Yup.. the more vac at idle, the better. The closer to 0 you get at idle, the lower your compression. -20 lbs is like a new engine if its a true reading. I get -12 to -14 lbs at a 750 idle on my guessometer guage

Think of your engine as if it was you sucking on a straw.. Without boost, you have vacuum because its sucking in air.

I get -0.5 bar :( Having a new motor sucks... I want -1 bar :(

Gauges read in different scales, look on your gauge and use math to compare. Here is the standard which I'm pretty sure about, almost positive, but off the top of my head.

29.92" Hg=80cm Hg=1 Bar=14.7psi

Wait a minute you get -.5 bar? I better check for vacuum leaks or something. That would be around 7psi and I'm getting around 11" Hg which would be.......5.5psi about. But of course its a Guessometer so I'll check it with the vac gauge and see how accurate it is at vacuum. I will get better Autometer is just for now. Will probably dump the lightshow gauge for a EGT also. Maybe not, I dunno.

well shit. my aftermarket gauge shows like -1 psi when idleing. and I tested my compression like a month ago and its about 105 all around???? WTF???

I get -17 to -19" of vacuum at idle. I'd say its running well :)

Net wisdom is that you should see at least 14" of vacuum at a 750rpm idle on FC stock ports. Ported motors will have lower idle readings and if your idle is lower than 750 it will also read lower. Watch the units as well - the stock gauge is in mm Hg * 10. -20 -> -200 mm Hg which is about 6" vacuum (IIRC). At idle the stock gauge should always be pegged at -20.

At a 750rpm idle I get about 15-16" vacuum. When idling at 600rpm it drops to about 12-13". This is on a stock TII with 150k. If your seeing less, look for vacuum leaks and test your compression.

Henrik
87TII

BTW. -20# is physically impossible.

Re: TII Boost Guage at Idle
 

This is perfectly normal. As mentioned above your idle vacuum should be ~16inHg, which is ~400mmHg. This is way below the bottom of the stock boost gauge's scale. Anything less than mild acceleration should see it sitting hard at the bottom. It should only sit at zero with the ignition on and the engine not running.

The first thing you should do if you plan to modify your car is buy a proper aftermarket boost gauge. That will give you a much more complete and accurate picture of what your engine's doing.

Will a free flow intake and exhaust cause less vacuum at idle? I may have some vac leaks somewhere too, ordering the good hose as soon as I pick a color, cheap stuff is just that.