Vacuum is important if ...

[Headrope]

Why does vacuum matter?
Lately there have been many questions regarding whether Mopar cams are good or not. Eventually in each thread something negative attributed to the Mopar cams is that they don't generate much vacuum. Why is that important?

[RD]

it is important if you have power brakes or cruise control.  both of those items are able to function based upon the vacuum generated from your engine.  (there is a vacuum line that runs straight from the intake manifold to the brake booster).  Vacuum from an internal engine aspect is just as important because your engine, in all practical sense, is a vacuum pump.  vacuum causes the air/fuel mixture to enter your cylinder, vacuum pulls the fuel from your carburetor, vacuum allows for piston rings to seal better.

I am sure there are more reasons, but I am tired, sick and at work and really should be doing something else

[ck1]

I think also you need it to open your head light doors that operate on vacuum.........

[John_Kunkel]

Vacuum is important even if you don't have vacuum actuated components on the car.

Low vacuum caused by a leak or a narrow lobe separation angle contributes to poor idle quality and miserable low speed driveability.

[Headrope]

I drive an original Charger ('66) - no vacuum actuated components on the car, that I'm aware of.
How much vacuum is necessary for good idle quality and terrific low speed driveability on a 400?

[firefighter3931]

I drive an original Charger ('66) - no vacuum actuated components on the car, that I'm aware of.
How much vacuum is necessary for good idle quality and terrific low speed driveability on a 400?

I would say 10in hg minimum and preferably around 15 for a nice running street combo that's easy to tune and drive. The less vacuum you have, the harder it is to dial in the carburator. Strong idle vacuum makes for good off idle response/bottom end power.

Ron

[Chryco Psycho]

 
low vacuum can create the need for adjustable air bleeds on both idle & jet circuits as weel a drilling holes, in the throttle plates to keep the idle transfer slots doing there job , adjusting acclerator pump circuits to get enough fuel in as the throttle is opened

[Headrope]

Much thanks.

[mikepmcs]

ok i'm reviving this post cause i'm dealing with a low vac issue right now and am experiencing all of what has been said here, difficult to adjust carb and all.  i have a mopar cam  .484 and i'm reading in the 5-7in range on the vac needle at idle and it fluctuates wildly in between those 2.  am  i to believe after reading these posts that the low vac i have is normal because of the cam i'm using??

eddy performer carb and intake, i've disconnected all vac lines off the back of the manifold.  i am hooked straight up to the plug on the back of the manifold reading the vacuum.

thanks
v/r
Mike

[Chryco Psycho]

Yup
longer duration & more overlap in the cam grind causes low vacuum at idle 

[mikepmcs]

haven't fired it up yet cause it was real late when i finshed changing the plugs and stuff to prepare for a tune up(timing), but i'm hoping after last night's find it will be a good night.  I didn't like the way the previous owner used bolts instead of studs on the carb, so i got correct studs and when i was looking at the rear left bolt, i noticed it was bottomed out but not all the way contacting the carb base(like 2 mm gap).  hopefully that will produce a positive result tonight when i get home and give me a couple more inches of vac.
we shall see.
thanks chryco for the sanity check.
v/r
Mike

[TylerCharger69]

my car runs at 20 Hg    5-7 would indicate a manifold leak or a carb leak or a vacuum leak someplace.  Late valve timing will cause that too.   But you say it's erratic.....I believe it's a leak someplace.   Spray some good 'ol WD40 around the base of the intake as well as the carb while the engine is running and see if it makes the car idle up for that moment.   If it does....you've found your leak

[mikepmcs]

my car runs at 20 Hg    5-7 would indicate a manifold leak or a carb leak or a vacuum leak someplace.  Late valve timing will cause that too.   But you say it's erratic.....I believe it's a leak someplace.   Spray some good 'ol WD40 around the base of the intake as well as the carb while the engine is running and see if it makes the car idle up for that moment.   If it does....you've found your leak

so tough to tell with the cam i have in it, but i'll do it just the same. i'd have to spray it in each place for about 30 seconds just to get the idle to react.  blop blop blop.. the GL hits a mean lick if you know what i mean.
thanks
v/r
Mike

[TylerCharger69]

spraying  should cause it to idle up instantaneously...if there is a leak.  What cam are you running?   Gross lift?....duration?

[mikepmcs]

mopar purple 484 484

[TylerCharger69]

That could be an issue...considering the factory cam is a split duration and lift.  I had one of those exact cams...and...I had vacuum troubles too...that POS Purple Shaft also lost the exhaust lobe...it was ground down to nothing,  and no other engine component had damage.   I went with an Engle cam..and It's great!!!   I've heard a few horror stories about the purple shafts  (unfortunately AFTER  my problems and not before)   but oh, well....car is well tuned now!!!

[8WHEELER]

I have run the 484 284 mopar cam since 1980 never had any problems yet, I did run the mopar 509 for a short
time, but that thing is NOT for a street car, and I would not use it to race either. I think my next cam will be an Engle
cam, just have not decided witch one yet 

Dan

[Chryco Psycho]

Eagle or Engle

[Ghoste]

I have the MP 484 cam as well and it just isn't a vacuum monster and that's all there is to it.

[mikepmcs]

I have the MP 484 cam as well and it just isn't a vacuum monster and that's all there is to it.

ok good enough for now, thanks to all, i will still try the wd40 idea as it seems a lot safer than propane.
now i'm worried about this mopar cam, hopefully it was just a bad cam(reference to post above by tyler69)

ok if i want to change cams in the near future, what should i go with. i'm gonna start a poll.
thanks
v/r
Mike

[deputycrawford]

Just start the car and check the vacum now. I had the same thing and tightend down my carb and cleared it right up. Comp .545/ .545 lift cam on 383. 10 inches of vacum at idle.

[mikepmcs]

Just start the car and check the vacum now. I had the same thing and tightend down my carb and cleared it right up. Comp .545/ .545 lift cam on 383. 10 inches of vacum at idle.

i put new studs on the carb last night and started her up and gained an inch, i'm now fluctuating in the 7-9 range.  needle bounces like a madman, never steady.  someone said that indicates a leak right there. so i understand vac needle should be steady even with a heavy lope in it.
thanks
v/r
Mike

[TylerCharger69]

Either a leak  at the carb, intake, or heat riser...or late valve timing will cause fluctuations too...Check for other vacuum leaks first before tinkering with the timing

[deputycrawford]

You are correct, the reading should be constant. 7 to 9 means you're close. Check and tighten the carb, tighten intake slightly and check for leaks with the spray trick. Its gotta be a leak.

[mikepmcs]

Either a leak  at the carb, intake, or heat riser...or late valve timing will cause fluctuations too...Check for other vacuum leaks first before tinkering with the timing

you say late valve timing, if when i get to that stage are you saying I need to advance initial even more than it is right now(roughly 12btdc  at idle, 850-900 is about as best i can steady him up right now)  i have headers too.
i won't do it though til i've sprayed for a leak

thanks all so far. i love this place