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

[TylerCharger69]

not late ignition timing...valve timing...i.e.   perhaps  bad lifter,  worn cam ,  weak valve springs, or loose rocker arms.  It sounds like a leak to me...but that is another possibility.

[mikepmcs]

not late ignition timing...valve timing...i.e.   perhaps  bad lifter,  worn cam ,  weak valve springs, or loose rocker arms.  It sounds like a leak to me...but that is another possibility.

crap hope i got a leak
explain to me the heat riser thing.  #2 cylinder right.  there are two holes on that side in between the headers(one by# 2 and one by# 6) and i can feel gasket there only. am i way off. shouldn't the heat riser port be somewhere in there or abouts.
v/r
Mike

[TylerCharger69]

if you have headers.....never mind the heat riser thing...it's the least likely candidate anyway.   On stock exhaust manifolds....on the passenger side, just before the outlet,  the heat riser is sort of a butterfly valve that "somewhat closes" during a cold start up.   What it does is,  it pushes hot exhaust gases back up through the crossover tunnel on the intake to warm the fuel while the engine is cold.   It also travels past that into the LH exhaust manifold too.  The manual choke gets heated up too.   These things are a pain in the ass, as they corrode,  and have been known to get stuck open, which is no big deal, or stuck shut, which  will cause troubles, and possibly cause a vacuum leak at that crossover point on the intake.......But   you have headers...so...dispense with the heat risor theory.    That 484/484  Purple Shaft stick you have isn't a Hoover as far as vacuum goes.   I had the same cam, and trashed it for that, and failure reasons.    I went with the Engle K-56 and my vacuum is now steady as hell  at 20 Hg.   It  helps dramatically on fuel mileage too  during leisurely  driving.

[dodge freak]

To check for a vacuum leak first plug ALL hoses up, pcv if you have it, brake booster, vacuum advance, ect. after you are sure the hose have no leaks then start the motor and check the vacuum. Sometimes its a bad brake booster-its happen to me brakes work fine-the pedal did get firm after the motor was shut off but I did not notice it at first and mine was only 7 months old from auto zone. A bad pcv valve or vacuum advance also could do it. You can check to see if those items, not the pvc, hold vacuum with a $20.00 hand held pump. Mine booster was so bad it did not hold any. Just something else to look for.

[mikepmcs]

To check for a vacuum leak first plug ALL hoses up, pcv if you have it, brake booster, vacuum advance, ect. after you are sure the hose have no leaks then start the motor and check the vacuum. Sometimes its a bad brake booster-its happen to me brakes work fine-the pedal did get firm after the motor was shut off but I did not notice it at first and mine was only 7 months old from auto zone. A bad pcv valve or vacuum advance also could do it. You can check to see if those items, not the pvc, hold vacuum with a $20.00 hand held pump. Mine booster was so bad it did not hold any. Just something else to look for.

i'm on it, i'll plug the pcv tomorrow(isolate)  it's kind of funky anyways.

[firefighter3931]

Mike, erratic timing can also be caused by a worn bushing in the distributor shaft or slop in the oil pump drive gear. Low engine vacuum shouldn't cause erratic timing. 

Try advancing the base timing (at idle). Keep advancing the timing until the engine stops speeding up and then back it off a hair. Check the timing at idle and full advance to see what you've got. Bigger cams like more spark lead....especially if the engine has lower compression. If you have the 284 cam in there it will want more than 12* of advance at idle. It should probably be in the 16-18* range. After the timing is set ; simply adjust the idle speed down to the desired level. 850-900 rpm should be just about right (in gear).

Ron

[Mfr426]

Mike, I just had a very similar problem. I have a 69 383 w/ the 484 MP cam and a new Eddy AFB 600CFM (I think that is a performer but and not sure). I had horrible vacuum (around 7 or and the car ran poorly. I took it to an old mopar dealer and the guy switched vacuum ports on the carb base to the passenger side one and BOOM! the motor now has 15" at idle and runs great (less my recent poppig issue). Anyway, the instructions said that the passenger port was for emission controlled motors so I didnt use it I used the drivers port (plust it had the vacuum as they suggested in the directions).

Anyway, I was embarrassed but greatful that it was as simple as that. It's worth a try...

[mikepmcs]

Mike, erratic timing can also be caused by a worn bushing in the distributor shaft or slop in the oil pump drive gear. Low engine vacuum shouldn't cause erratic timing. 

Try advancing the base timing (at idle). Keep advancing the timing until the engine stops speeding up and then back it off a hair. Check the timing at idle and full advance to see what you've got. Bigger cams like more spark lead....especially if the engine has lower compression. If you have the 284 cam in there it will want more than 12* of advance at idle. It should probably be in the 16-18* range. After the timing is set ; simply adjust the idle speed down to the desired level. 850-900 rpm should be just about right (in gear).

Ron

Mike, erratic timing can also be caused by a worn bushing in the distributor shaft or slop in the oil pump drive gear. Low engine vacuum shouldn't cause erratic timing. 

Try advancing the base timing (at idle). Keep advancing the timing until the engine stops speeding up and then back it off a hair. Check the timing at idle and full advance to see what you've got. Bigger cams like more spark lead....especially if the engine has lower compression. If you have the 284 cam in there it will want more than 12* of advance at idle. It should probably be in the 16-18* range. After the timing is set ; simply adjust the idle speed down to the desired level. 850-900 rpm should be just about right (in gear).

Ron

Thank you both so much for that great info and everyonew else as well.  I will try these tomorrow.  as far as the vac ports on the carb if you are talking about the front of the base, the driver's is plugged already and the pass side has the vac advance line in it.  i don't know of any other vac ports that i should hook too.  i don't have a stock air cleaner(summit chrome) so no other vac ports that i know of. I really appreciate the response fellas and i will follow through on suggestions.
v/r
Mike

[mikepmcs]

ok i adjusted the timing to what i think is around 16*+, the car never wanted to stop revving up so i stopped.  the timing tab has 4 division the second one down(as i'm looking at it appears to be 10*btdc, so what si the next line down, i'm guessing 15* and i went well below that line.  i don't want to hurt something but i wasn't experiencing any knock or anything.  took it out for a very short run and still can't really see/feel any difference.  it has speed but not like i remember a 383 having in my past. should i keep racking the timing?? is there a danger zone i should be looking out for, is it possible the cam degree was put in wrong(does that make sense).

My vac is steady 10 inches now though i guess that's a plus. i put a different vac gauge on it and the needle doesn't jump.  getting there slowly.  if you don't have any more suggestions i'm gonna start tuning the 600 eddy perf.  i got the cal kist and i think my metering rods are stuck in the rich position.  i'm gonna try a weaker step up spring first then go to 1 stage lean and so forth.  the car does run rich and i've all but slammed the IMS screws shut.
thanks again
v/r
Mike

[dodge freak]

When you keep advancing the timing till the motor stops speeding up or slows down you need to have the mechanical advance all the way in. That means 3000-4000 rpms , take a timing light and look at something on the balancer to see when the advance is all the way in and then speed the motor up a little more so you now it does not back off. Then move the dist. till the motor stops reeving. You are not looking for 10,15 20 degrees you are setting total timing with out vacuum, abiout 34-38 degress. Make sure the vacuum advance is unhook if you have it. I wear some ear muffs make for gun shooting cause it is loud and I am weak I guess. After you are done you might what to see what the timing is when idling but it really does not matter. This way when you are driving it though the gears the timing is right on. You may find if you change the springs in the dist. the motor will have more low - mid range power if it does not ping, but for now just try to set the total timing.

[deputycrawford]

     A couple of things here. One: you really need a timing light with the advance knob on the back of it. You put it at 36 degrees and rev the engine to about 3000 rpms and set the timing mark on the engine at zero degrees. Thats because the knob on the back of the timing light has done the rest for you. You then idle the engine back down and rev it slowly again. You want the timing mark to go back to 36 degrees by 2500 to 3000rpms. That means you might have to go to lighter springs in the distributor. I had to because mine felt sluggish like yours does. I wouldn't start with the carb until the timing is set. You might go to a mechanic and tell him to set it for you and show you. He should have an adjustable light. I bought a good one from Craftsman for under $70. You will have it forever and won't hurt an engine from it. I think that is worth it.
     Two: Eddy carbs need at least 16 inches of vacum at idle. You have 10. That means the Eddy carb is not using the idle circuit and you will have a permenant hesitation at part throttle tip in. You will probably not be able to tune that carb that well. The cam has too little vacum. I had the same carb and was told by the Eddy tech line the very same thing. I now have a Mighty Demon carb and will never give it up.
     Get the timing right first. I feel that is crucial. Then go carb. You are almost there but remember, you don't have a stock engine. Throw away the stock rules.

[TylerCharger69]