How to read a Vacuum Gauge?

[Mustang5L5]

Yes, i know how to read a Vac Gauge, but this question is more of theory as to what should go on when you test your vacuum.

I suspect my car has clogged cats (actually, been that way for a while but i am just now getting to it). Reason being the car gets lethargic up top and my brakes are worse when I brake under high RPM's. All signs of clogged exhaust and poor vacuum.

So I have a boost/vac gauge. (the boost part is for later ) and plan on hooking it up so i can monitor vac while driving and under load.

Question is, what should I see? Over 20 inHG at idle? What about cruise? I won't be able to start my car for a couple weeks to actually begin testing, but i'd like some basic ideas as to what to look for.

I might grab a 3-pod A-pillar and stick my Water temp, oil pressure and vac gauge there. I know i said I wanted to put them under the radio, but i kinda want to see what's going on and that is the best spot

Vac could come in handy for troubleshooting and fuel economy, so nice to have that gauge in a good spot. (I'll just have to hide the "boost" half until I get some)

 

[393strokervert]

your vac readings @ idle will depend on what cam you have in the motor. I have a Trick Flow stage1 cam in mine & I pull a very steady 15hg @ idle & I beleive the same 15hg while cruising( my needle is so still that it almost looks like it is stuck...until I touch the throttle.
With a stock cam I think you should pull 18-20hg @ idle. Where you have your timing set will also effect idle vac readings. on my old carbed stang I used a vac gauge to set the base timing by advancing it until I reached max vacuum (which was around 22-24 hg with a stock cam) & then backing it off apprx 4* (19-20hg)it's been a while but I'm sure those ##'s are really close. a vac gauge is a great diagnostic tool.
according to my old Chilton manual, a clogged exhaust w/excessive back pressure will cause a gradual drop in vacuum @ idle. at startup you will have close to normal idle readings & then it will back down slowly.....at least that's how me & Mr.Chilton see it

 

[stykthyn]

IIRC you still have the stock cam in your car so you should see between 17-20 inches at idle and between 15-18 inches at cruising speed.

 

[skunk21]

you don't have to check it at cruising speed. There are 2 methods to check for it. testing via vacuum gauge and measuring backpressure. Hook the vacuum gauge at get your vacuum reading at idle ( should be about 20 like stated) then raise rpms to 2500 ( I have seen it done at 1600rpms so either way ) the gauge will drop quick when you crack the throttle then stabilize, if there is restriction the vacuum will to drop off - example 20inches @ idle then drops off to 15 inches @ 2500rpms good indicator that pressure is backing up into the exhaust system. you can also measure back pressure with a low pressure gauge (get cheap gauge that reads 1-10 lbs). you can take out the check valve and use that port in AIR system or take out the 02 sensor and taking a reading. normally a reading of 1.25-1.5 psi @ idle or more than 3psi @ 2,000rpms means a restiction for sure in exhaust.

it's either clogged or not. the backpressure reading will really tell you what is going on but either test works.

 

[Mustang5L5]

Okay, well finally got around to testing. I had 20 mmhg at idle and at cruise had about the same...maybe a little more..23 or so.

So exhaust is not clogged