measuring eng vavuum

[jocko]

Edit - sorry, meant VACUUM, not vavuum (how many people are gonna click on this link to see an engine "vavuum"!!!

Ok, dumb question, but chasing eng vac on my truck (just ordered a booster and master cyl and THEN measured vac... I know, wrong order) Assumed I had decent vacuum, truck idles and runs fine, under load or not under load, stock cam, etc.

Measured vac by disconnecting the vac line from the carb and putting the vac gauge on in it's place where the distrib vac advance was attached. At idle, read zero vacuum. Vac increased as rpm increased, as you'd expect though.

I've read in several places where 17+" Hg is required to operate a brake booster properly, but never assumed I didn't have the minimum because I have a stock cam. Is the vacuum measured at idle rpm for this 17" comparison or at a specific rpm? I "thought" it was supposed to be a measurement at idle... Does air cleaner being installed/not installed make a difference?

Odd situation, really surprised me. Maybe I won't need that booster after all...

So, either -
i'm somehow measuring wrong
have a bad gauge(?) - but it did increase vac as I increased rpm... (went from 0 to about 18" Hg when accelerating rpm from idle up to about 2500 rpm) (is that backwards - should vac decrease as rpm increases?? maybe the gauge IS bad!)
i actually have zero vac at idle(!)
i have a massive vac leak
or this vac port is restricted/clogged, yada yada

Ideas? - feels like I'm missing something simple. Thanks for your help.

Was gonna fiddle with it a bit, but hobbling around on crutches right now and figured I'd just ask before I bust my other ankle hobbling around the garage...

[geezer#99]

You are hooked to a ported vacuum source which is zero at idle. Hook up to a full time source to check your vacuum. Some older boosters only need 14" of vacuum to operate.
Vacuum drops to zero when you put the pedal wide open but should remain constant under cruise or normal driving. If your motor is tired or worn out it will have lower vacuum too.

[jocko]

thanks geezer, makes sense - i assume the one out the back of the carb to the manifold would be full time - but I'll need to install a T or find an adapter to measure there - that will be the signal I likely tap into for the brake booster. But couldn't reach it on crutches today!!! That's what I get. Thanks - I believe you nailed it.

Ordered a 75 imp booster and 67 master cyl from oreilly's today. pretty cheap, the mc anyway. capt fab's bracket is enroute - looking fwd to being able to mill about the garage again.
thanks again geezer

[geezer#99]

What kind of carb? Usually a ported source on the front. Booster line can be hooked to the big port on the back of the carb or to a big port on the intake behind the carb.

[jocko]

it has an old carter 4 bbl. only port i saw handy on the front of the carb was the one i hooked the vac gauge to - the one the distrib vac advance is hooked to. Sorry, couldn't get up in there and climb around - that's as far as I cen get in with the $%^& crutches... There is the large port on the back but no tees on it for me to tap into for the gauge. Guess I better wait til I can walk again. Yes, booster to the big line on the back is the plan.

I assume a Tee should be placed in the line that I plan to hook the booster into - not running the line directly to the booster from the carb - assuming the line off the back of the carb is part of the crankcase ventilation.

[lakeroadster]

Hey Jocko.. check this out: How to Use and Interpret a Vacuum Gauge

I installed a vacuum gage as one of the new gauges in my truck. There is much that can be learned about a motor just by using a vacuum gage!

The other thing to keep in mind is altitude. Up here in Denver our motors don't pull as much vacuum.

And here is another thread where I asked a similar question as you did in this one: Vaccum Source Location

Back when I was a kid my dad bought a brand spankin new '65 Impala SS, it had a big factory vacuum gage mounted right in the middle of the dash. I can remember watching that gauge... being 5 years old, mesmerized by it.

[jocko]

You're breakin my heart! Here's the 65 I just sold a little bit ago... (have had two 65 Imp SS's) had the purdy gage in it too. Thanks for the great info Lakeroadster!

[Rich 5150 69]

There should be a port...on the manifold just behind the carb, maybe plugged with a fitting, that is the one the factory used for power brakes..good vacuum is 15 and above you won`t have any problem....some how I missed the crutch thing, did you try to kick your tires or what.....

[jocko]

Ha, no - but I'm about ready to. It's a 5th metatarsal stress fracture. To me, that means "not as bad as a REAL fracture" but I'm learning that's not the case. Initial prognosis was 6-8 weeks on crutches - now I'm hearing up to 20 (yes, TWENTY). I'm getting very frustrated. I'm not at the 6-8 week point yet, so there is still hope it will heal on that timeline, but so far, not going real well. It's jacking up my job, which I just started, not to mention keeping me from workin on the truck - now that we've been reunited after the move out west. So, yep, trying to not be cranky and roll with the flow.

I recall the fittings you mention on the manifold - just literally couldn't get that far into the engine bay to check it out (it's crammed into the garage and limited access to the pax side). But that rings a bell Rich - that's where I'll look to hook it up when I get it. Dropping by Oreilly's tomorrow to pick up the parts - maybe they'll "deliver" them to my truck in the parking lot! I'm lucky to have an all-tile-floor house right now - I slide all over the place on my office chair which gives me a nice break from the crutches!!! Life's little victories.

Thanks for all the posts on the vac subj guys, much appreciated!
jocko

[Rich 5150 69]

By the way Orreily`s might have the vacuum fitting on thier self help isle, for that, or rob it off any old chevy 60`s to 90`s I believe....they all had them.

[jocko]

by the way Lakeroadster - wish I'd been on the forum back when I had the impalas! Bot of mine had factory vac gauges (although I swapped a factory tach into one eventually) and the conclusion you came to was the correct one - they were both hooked to a manifold vac source (exactly like the plug with outlets that Rich described aft of the carb) not to the vac advance. Very interesting topic - I would have assumed the vac advance would/should be a ported vac source (i.e. zero at idle) but Markeb01's discussion was interesting. Seems odd to me to go to a full time source on vac advance and then adjust ones mech curve such that when you go wot the vac advance input to the distrib tapers off to zero. But I guess that's kinda how it works anyway - the total timing curve is always a combo of initial, vac, and mech advance once you rev it up and they're "all in" - so I'm not certain of it makes any difference at higher rpms whether you used a ported or full vac source - wouldn't they both be washed out at wot anyway and the full curve just a combo of initial and mech advance? Hmmmmmmmmmmm

I dunno. I'm just happy my truck starts, and now I have a good idea where to go look to hook up my brake booster if I ever get around to putting it on!



Got it Rich - I will take a peek under the hood (now that I know exactly where to look, I can probably climb in to see what I need to see somewhat) before I head to the store and see if I need one. Thank you!

[Rich 5150 69]

True, all manifold vacuum is gone at wot, either manifold or ported is correct, all depends on how your ignition is setup. I`m old school, still use manifold..

[C10CORIE]

At what vacuum do you get your best fuel economy??

[jocko]

good quesiton - I "think" that when you drive such that you are minimizing the manifold vacuum, you are getting better fuel economy. In other words, if you are having less fun, you are getting better mileage. For any given engine, the vac signal will increase to a point - so don't think actual #'s when talking fuel economy - think "change" in vacuum. That's how I understand the subj anyway - and I could be completely wrong. How's that for a disclaimer...


Lakeroadster - do you have any pics of your vac gauge installed? I'd love to see it - I may still have one laying around from the tach conversion I did in the imp. Might actually find a use for tha told thing (or I may have swapped it back in before I sold the car and kept the tach, don't recall). Still gotta unpack to find all the goodies!

[Rich 5150 69]

Originally Posted by C10CORIE

At what vacuum do you get your best fuel economy??

Your highest vacuum reading, if you bring the truck up to speed feather the thottle back just a hair you`ll see the vacuum come up a little more, that is where your best mileage will occur.

[jocko]

See, I did have it exactly backwards!! Thanks Rich.

[lakeroadster]

Originally Posted by jocko

Lakeroadster - do you have any pics of your vac gauge installed? I'd love to see it

Just an Autometer vacuum gage installed in my home made '67 Camaro "heater delete" panel.

[geezer#99]

That's the old saying Jocko. Drive to the highest vacuum for best mileage.
Or drive like you have an egg under the pedal.
Thanks for the pic of the Impala.
Brings back memories of my 65 SS 409 4 speed convert in white with a black top.
Sigh!!!!!!
As I recall the vacuum gauge was always close to zero on that car. LOL!!!

[Corts60]

This is a vacuum gauge I just installed in my Suburban. It shows where your best mileage will be under driving range. It is funny trying to keep it there when you drive. You basically have to drive like grandma when you take off to keep it in the green but for crusing it normally (for me) hangs around 13 inches.

The picture was taken when my truck was idling.