[69z28boy]

does anyone know of a vaccuum can that pulls advance with about 8-10 in HG?

my 302 had the vaccuum line plugged, and is running 12 degrees intial all the time, which is fine for racing, but i put around in the car alot, and it runs hot and is herky jerky until it gets into the mechanical advance... then it calms down a bit, but i want to get it running right under part throttle.

it is connected to manifold vaccuum, but only pulls about 10 at idle which is accurate for a 30-30 cammed 302 according to my research.

thanks
aaron

 

[zdld17]

Sounds like you are answering our own question. Have you tired moving initial up a little more? Will it give you problems with pump gas? Are you confusing this with fuel load up? I am running about 8 but have initial at 16* and 36* total with mechanical advance but I run Msd electronic. Once you add some initial timing , you may have to idle down to keep the dieseling down. Just suggestions.

 

[OAKLYSS]

Yes, I know this one! go to NAPA and pick up a Echlin VC-1810. It starts at 5" hg and is full advance by 8" hg and adds 15 degrees (info from John-Z). I am running an Accell adjustable unit on my ZL-1, screw is all the way in, so it starts at 3 1/2" hg because my vacuum is 5-6" at idle. I made a limiter on the advance so it adds 12 degrees instead of 20 at this setting. You can use it at about 3 turns in, it starts at 6"hg and is full at 7"hg. I dont know the advance at this setting. My distributor came with no vacuum advance, but car runs way better with vacuum advance hooked up to full time vacuum, (not ported), and cooler too.

 

[Silver69Camaro]

How's your power brakes with that vacuum? Just curious.

 

[69z28boy]

OK, mission accomplished,

i put the 1810 on it from Napa (I knew about that , but hadn't thought about it until after reading the post) and the car runs perfectly now..

the old vaccuum can wasn't an OE GM can i'm sure, in fact it looks exactly as the NAPA one i put on, but it had a different part number.... it was also setup with the spark advance hose going to the ported side of the carb.. which is another no-no. at best, it needed about 20 in HG to move that sucker LOLOL>, and the tube was blocked off with a ball bearing, so it wasn't getting any vaccuum advance.

now,with the 1810 can, the motor runs cooler, has much more part throttle response and is perfectlty smooth in all RPM ranges... where before you could actually feel the motor coming alive as the ignition advance came on, and then the secondarys even later,

i'm guessing someone set the motor up for race only applications... which isn't to smart for a car that is never on the race track LOL. I also put a different Holley Spring in the vac-secondaries, and the motor is dead smooth and revs very freely... much more pleasurable to drive.

by the way, the Vaccuum brakes work perfectly....

thanks

Aaorn

 

[satz28]

it is connected to manifold vaccuum, but only pulls about 10 at idle which is accurate for a 30-30 cammed 302 according to my research.

thanks
aaron

I'm curious. 10" seems very low at idle. My 302 pulls if I remember, about 15" or so at idle.

 

[OAKLYSS]

My power brakes work fine too at 5"-6"hg. My RS light covers also work great.

 

[Steptoe]

My old cam pulled 7 to 8 " at idle, no problems with brakes...after all do u need to use brakes at idle, other than sitting obn a hill at the lights...if u took your foot off, a a couple times the brakes where hard, like when the engine is turned off...but even at these times, it was not as if the brakes dont work, and how many times in places like this would one pump the brakes a couple times with out increasing revs a couple 100?..

 

[hhott71]

vacuum advance is only for part throttle conditions.
It is to be connected to a ported / part time vacuum source.
It is not to connected to a full time manifold vacuum.

 

[69z28boy]

i disagree with the last statement,

the more spark advance you have at part throttle applications, the better fuel economy you'll have, and the cooler the motor will run.

when you floor the car, you loose vaccuum anyway so your distributor runs on full mechanical advance at this point... so ported/manifold vaccuum is useless argument under racing conditions.

this would mean that at idle, (with vaccuum advance plugged in) you can see about 30 degreed initial and at cruising speed, you'll see about 45-50 degrees advance... maybe even more depending on how much advance your distributor has both mechanically and via vaccuum advance.

for what its worth, this motor (all stock) runs much better with full manifold vaccuum advance than it ever did with ported vaccuum. no hesitation, no bogging, actually runs cooler too.

thanks

Aaron

 

[Gary L]

vacuum advance is only for part throttle conditions.
It is to be connected to a ported / part time vacuum source.
It is not to connected to a full time manifold vacuum.

The "non-smog best for the engine setup" is to have the vacuum advance conected to manifold vacuum for the reasons listed in the original post.:thumbsup:

 

[hhott71]

Engine builders, GM, MSD etc all disagree with the shadetree idea of manifold vacuum on the vacuum advance.

Ported, period.

 

[deerhunter]

Somewhere in here there is a long post debating ported and manifold vacuum for the distributer. I have a 355 with a fairly radical cam and I got it running the best I could with ported vacuum. After reading the mentioned long post I decided to try manifold vacuum. After setting everything up like was described, I DID notice cooler temps and other differences-all good, but I really noticed an increase in 'in town driveability'. I now run manifold vacuum in two different engines I own and will probably will not go back to ported.

 

[69z28boy]

I think, in theory, ported is the ideal way to go...
however, with every HI-PO engine I've ever built or tuned, connecting the spark advance to the full manifold vaccuum yields the best results... for a driver....

the other thing to consider is that not many people consider a 302 with a cam that has 80 plus degrees overlap a streetable motor LOL... hell it only makes 10-12 vaccuum at idle, which is a race motor by all means... and these things really only run well, when your foot is escorting the loud pedal through the floorboard. in this case, vaccuum advance isn't an issue because you are either idling or on the limiter LOL.

now, the other extreme is the rarely driven show car that is completly stock, (still same motor and cam) but this version rarely sees above 3500-4000 rpm, and is driven in parades, and car cruisins etc.... that little high revving 302 fills the same boat.. in which case, its going to run hot if the timing isn't advancing, and if it sits and idles at 4-6 degrees initial advance, (which I believe is what the factory calls for) its gonna run like crap, load up on you, run hot etc... running full manifold vaccuum will pull the intial timing up to about 15-18 which will smooth all of these things out... plus make it much easier to drive from stoplight to stoplight.

if you are running a race motor on the street, swtich to full manifold vaccuum, reset your carbs idle and such. and I bet you'll be pleasently surrprised at how much more enjoyable it is to idle around in the car...

 

[69z28boy]

another thing to consider is that with smaller cams, especially in bigger motors, when running ported vaccuum, the minute you crack the throttle, you are still seeing more vaccuum than you would in a race motor at full manifold vaccuum.

this might account for why the debates on ported/manifold.

Aaron

 

[onovakind67]

When does 'idle' become 'part throttle' and why would the timing requirements be different?

 

[deerhunter]

As soon as you push on the gas pedal the butterfly's open.

 

[onovakind67]

And then?

 

[deerhunter]

Sorry, and then air rushes in through the butterfly's and the inside of the engine intake system loses vacuum...like opening a vacuum sealed can of food. The timing changes because of the increase in the engine RPM's.

 

[onovakind67]

Sorry, and then air rushes in through the butterfly's and the inside of the engine intake system loses vacuum...like opening a vacuum sealed can of food. The timing changes because of the increase in the engine RPM's.

Like a can of food? What creates the vacuum in a can of food? If I close the can, does the vacuum come back?
When you read the vacuum gauge in your car, do you have more vacuum at 2000 rpm or at an idle?