[slowbird]

In a lot of threads here i see people mentioning idle vacuum or asking people about it. So why are we interested in it? Besides having enough vacuum for power brakes what else does vacuum do?

[Overkillphil]

The way it behaves can tell you a lot about the health and condition of your engine as well as its tune. There is plenty of established literature including paperwork which often comes with the better vacuum gauges. Look in repair manuals as well for all kinds of info. regarding vacuum readings and how to use them for diagnosing issues. There are also tons of things online. A simple search using "vacuum gauge to diagnose" revealed a plethora of results. Here is just one of them:
http://www.bing.com/images/search?q=...verlay&first=1

[george kujanski]

What Phil said.

George

[Tom Vaught]

X3 Maybe a race car guy, or a guy with hydro-boost brakes might not care much but the vacuum levels will help a carb guy, a vacuum diaphragm advance distributor guy, a street transmission guy (vacuum modular), and a sharp garage mechanic tuning guy with the engine tune-up or diagnostics if he has the info. I personally do not pay a lot of interest in idle vacuum/rpm when deciding if the idle air/fuel mixture is right, I use a wide band UEGO sensor for that deal these days.

Tom V.

[slowbird]

Im fully aware of the diagnostic part of the vacuum but why does everyone ask about vacuum at idle when the discuss of cams comes up? I am a racer and performance is what i look at so i thought maybe im missing something when it comes to street cars. Subject of vacuum never comes up in the race section.

[tpssonic]

What Tom said what George said that Phil said... So, you're pretty darn safe with those responses. Idle vacuum is one of the cheapest, easiest, and quickest diagnostic tools in the toolbox. Higher vacuum at idle makes carburetion tuning at idle much easier. Also, the additional advise that Tom provided with regard to the wide band would cover the rest of the bases.

[steve25]

What I want to know us why folks do not make the undeniable connection between low idle vacuum and Torque/ poor drivability below 2000 to 2500 rpm dispite getting a Carb set up right to deal with it!

[dmac]

Originally Posted by slowbird

In a lot of threads here i see people mentioning idle vacuum or asking people about it. So why are we interested in it? Besides having enough vacuum for power brakes what else does vacuum do?

Vacuum is what sucks gas into your engine. Poor vacuum doesn't allow the engine to fully atomize the fuel and distribute it evenly as well. Low vacuum also slows the airflow. Unless you plan to run only at higher rpm all the time, too low a vacuum isn't going to be as driveable for a street car.

[tpssonic]

Originally Posted by dmac

Vacuum is what sucks gas into your engine. Poor vacuum doesn't allow the engine to fully atomize the fuel and distribute it evenly as well. Low vacuum also slows the airflow. Unless you plan to run only at higher rpm all the time, too low a vacuum isn't going to be as driveable for a street car.

Yes.

[PAUL K]

Originally Posted by slowbird

In a lot of threads here i see people mentioning idle vacuum or asking people about it. So why are we interested in it? Besides having enough vacuum for power brakes what else does vacuum do?

Do you want to buy a can of vacuum? I have one for sale.

Would a camshaft with 214/224 duration at .200 require a vacuum canister?

[slowbird]

Originally Posted by dmac

Vacuum is what sucks gas into your engine. Poor vacuum doesn't allow the engine to fully atomize the fuel and distribute it evenly as well. Low vacuum also slows the airflow. Unless you plan to run only at higher rpm all the time, too low a vacuum isn't going to be as driveable for a street car.

What is considered low?
Take two 462ci engines one idles with 14" and the other 10", I know the idle will be slightly rougher (most likely anyway) on the engine with 10" of vacuum vs the 14" of vacuum but besides that what else? Once the engine gets off idle you'll never know there was a difference.

[blykins]

Originally Posted by PAUL K

Would a camshaft with 214/224 duration at .200 require a vacuum canister?

[slowbird]

Originally Posted by PAUL K

Do you want to buy a can of vacuum? I have one for sale.

Would a camshaft with 214/224 duration at .200 require a vacuum canister?

Im good on vacuum i think? But maybe i do need more?? I feel like im not understanding something when it comes to idle vacuum.

And as far as that cam goes, im guessing vacuum is the least of his worries, lol

[PAUL K]

Originally Posted by slowbird

What is considered low?
Take two 462ci engines one idles with 14" and the other 10", I know the idle will be slightly rougher (most likely anyway) on the engine with 10" of vacuum vs the 14" of vacuum but besides that what else? Once the engine gets off idle you'll never know there was a difference.

The engine with 10" of vacuum would drag the other one down the track because it would have the cam with the tighter LSA

[pontibeast]

As long as we are the subject of cans of vacuum, I put a vacuum can on my 350 Pontiac back when I ran a single pattern cam 244@.050 and .501 lift as the car was not happy providing enough vacuum for brakes. I later put in a Summit 2802 which I am sure didn't need the Vacuum can but it was hooked anyway. Now I am putting in a 455 with a stump puller and I am pretty sure it won't need the vacuum can but is there a down side to hooking it up or does it really matter one way or the other. Thanks and sorry for butting into your thread.

[Will]

Having plenty of vacuum for the power brakes is nice. AC cars also utilize vacuum for actuating the doors/diverters in the system that channel the airflow from one place to another, but I don't know how much vacuum you actually need for that. idle vac. might also be important to someone running manifold vacuum for distributor advance.

Those are specifics. In the larger scheme of things it's about low speed, off-idle driveability and response, how tractable the engine is in around-town driving. Many, maybe even most "street" car guys want an engine that starts easy, doesn't take forever and a day to warm up, and runs relatively efficiently and has good power right from the stoplight without having to run real loose converters or dealing with an engine that wants to buck or stall in a manual trans car when maneuvering in a parking lot or driving in stop & go traffic.

IME, Anything less than about 12-14" of vacuum at idle and the brakes start getting wonky.

[tpssonic]

Those with hide-away headlamps on their '68 and 69s like the extra vacuum. Those actuators are balky (IMO) at best, even with sufficient vacuum. They're a goofy engine-revving exercise/debacle to open/close the light-covers without good vacuum...

Also, with good vacuum it's easier to tune out the "burning eyes" syndrome.

That being said, without good headlights and eyesight due to low vacuum (and tight LSA-ouch, did i say that?), you won't be able to see that you're being skull-dragged down the track by a tight-LSA'd cam. Pick your poison.

[Tom Vaught]

I drove a 'Street Car' with a engine that had 5" of vacuum at idle, I had to play with the idle circuit and richen it up as less signal means less fuel going thru the idle circuit.

Outside of that deal the engine/car drove fine with the 4:33 gears in the back and the 4 speed. So sometimes GETTING THE ENGINE TO IDLE, PERIOD takes a bit of time if you are a novice with carbs but I tend to agree I never really worried about what the actual number was and tried to dial it in with a camshaft change.
Course I did not have a auto trans or a a/c system or vacuum doors on headlights to open or close, etc.

Tom V.

[TedRamAirII]

Originally Posted by pontibeast

As long as we are the subject of cans of vacuum, I put a vacuum can on my 350 Pontiac back when I ran a single pattern cam 244@.050 and .501 lift as the car was not happy providing enough vacuum for brakes. I later put in a Summit 2802 which I am sure didn't need the Vacuum can but it was hooked anyway. Now I am putting in a 455 with a stump puller and I am pretty sure it won't need the vacuum can but is there a down side to hooking it up or does it really matter one way or the other. Thanks and sorry for butting into your thread.

I think it would be to your benefit to keep the extra can for vacuum and have a check valve installed. That way the engine will pull higher vacuum on engine braking and higher engine speeds so it could store that highest vacuum in that can as a reserve. Can't hurt. ; )

[johnta1]

Idle vacuum readings is for sedate street cars.
Full throttle vacuum readings are for WFO race cars.