Team Chevelle banner
1 - 20 of 84 Posts

·
Registered
Joined
·
4,844 Posts
I will tell you one thing I know for sure, Vortec heads do not like vacuum advance at all. Seems anything over about 36° timing total with those heads seems to make problems. I only added 10° with my 32° total mechanical timing and the Vortec headed 350 didn't like it all.
 

·
Registered
Joined
·
1,703 Posts
In a stock cam type situation I feel ported is just fine.

min a performance cam I prefer full is the way to go. Smooths put the rough idle quite a bit.

some will disagree with me but I feel the engine cools better at idle without and vacuum advance so plus side for ported.
 

·
Registered
Joined
·
804 Posts
Discussion Starter · #4 ·
I will tell you one thing I know for sure, Vortec heads do not like vacuum advance at all. Seems anything over about 36° timing total with those heads seems to make problems. I only added 10° with my 32° total mechanical timing and the Vortec headed 350 didn't like it all.
There's total timing , IE mechanical + initial = total timing. Like the magic 36 number .
Then there's total total timing which is
Mechanical + initial + vacuum advance = total total...IE 27 * mechanical, 10* initial, and 13* vacuum advance = 50* total total .
That's what they're referring to in the one video .
 

·
Premium Member
1970 SS454 LS6 11 second street car
Joined
·
32,405 Posts
All due respect, this topic has just been plain beaten to death over the years.
Everybody has an opinion and experience on the topic. Read ignition 101 and Dave Ray's advice. All you ever need to know.
 

·
Registered
Joined
·
1,593 Posts
Ported and full manifold vacuum work EXACTLY the same once your throttle blades are open to expose the port which feeds the ported vacuum advance.
As long as your engine meets the following points, full manifold vacuum is the way to go. Better mileage as more timing allows the throttle blades to be closed more to obtain the same idle speed, and cooler engine at idle because of more advance.
1. Engine makes enough vacuum at idle in gear to keep the particular vacuum advance cannister you have pulled in all the way. If it cannot, you will have varying timing and therefore varying idle, or an extreme rpm drop from park to in gear.
2. The cannister doesn't add an amount of timing which then makes the total at idle excessive. Total being initial plus the added vacuum advance.
 

·
Registered
Joined
·
4,844 Posts
There's total timing , IE mechanical + initial = total timing. Like the magic 36 number .
Then there's total total timing which is
Mechanical + initial + vacuum advance = total total...IE 27 * mechanical, 10* initial, and 13* vacuum advance = 50* total total .
That's what they're referring to in the one video .
I hate to tell you how bad my Vortec 350 ran on the road with 50° timing that inc vacuum advance, kicked back and ran like crap. Pulled off the vacuum hose and the car ran smooth as glass again down the road, never had that happen on most of my GM stuff. Every engine will want different amounts of vacuum advance due to chamber design and compression for sure.
 

·
Registered
Joined
·
51 Posts
Mr Ray always speaks to timing settings that work. It is not to often I've seen him speak to flame propagation...more than likely due to the complex nature of the subject. Peanut heads do not excel in that area, Vortecs shine in comparison.

Perhaps Mr Ray might chime in on such matters, personally I like manifold vacuum...constant stable repeatability always wins the day speaking for myself...along with the fact my low compression peanut engine thrives on big advance and today's highly modified fuel.
 

·
Registered
Joined
·
804 Posts
Discussion Starter · #11 ·
Ported and full manifold vacuum work EXACTLY the same once your throttle blades are open to expose the port which feeds the ported vacuum advance.
As long as your engine meets the following points, full manifold vacuum is the way to go. Better mileage as more timing allows the throttle blades to be closed more to obtain the same idle speed, and cooler engine at idle because of more advance.
1. Engine makes enough vacuum at idle in gear to keep the particular vacuum advance cannister you have pulled in all the way. If it cannot, you will have varying timing and therefore varying idle, or an extreme rpm drop from park to in gear.
2. The cannister doesn't add an amount of timing which then makes the total at idle excessive. Total being initial plus the added vacuum advance.
In reference to your #1 point/condition I'm getting exactly that which is somewhat of an erratic idle . Im currently hooked to full manifold vacuum. Vacuum gauge bouncing some and ofcourse rpm as well . Fresh on the mild side sb motor . Pulling average 12ish inches vacuum at idle . I say average because of the fluctuation in vacuum at idle .
If I hold it at as low at about 1000 rpm and vacuum gauge is locked in nice and steady.
Maybe the issue is what you've raised here which Is that the engine isn't pulling enough vacuum at idle in order to pull the VA in all the way . Its an oem style distributor that I've done nothing with yet . Maybe I need to limit it so that the 12 inches of vacuum does pull enough.
This thing right now before doing anything is showing me
29* in at 2300 rpm
14* VA
I set initial at 8* figurin I didn't want my mechanical and initial to sum up much more than 36*
Good info . Gave me somthin to shoot at oher than from the hip. Thanks
 

·
Registered
Ryan
Joined
·
1,759 Posts
Generally fresh out of the box distributors have about 22 degrees of mechanical advance. That puts you at 30 total. I’d increase your advance.

An eratic idle plugged into full manifold vacuum could be indicative of your vacuum advance can fluctuating the amount of advance it is adding as the diaphram and spring is withering in and out.

what are the specs on your engine and cam? Stock GM 350 vs solid roller 12:1 BBC will have vastly different ignition advance requirements.
 

·
Registered
Joined
·
804 Posts
Discussion Starter · #13 ·
Generally fresh out of the box distributors have about 22 degrees of mechanical advance. That puts you at 30 total. I’d increase your advance.

An eratic idle plugged into full manifold vacuum could be indicative of your vacuum advance can fluctuating the amount of advance it is adding as the diaphram and spring is withering in and out.

what are the specs on your engine and cam? Stock GM 350 vs solid roller 12:1 BBC will have vastly different ignition advance requirements.
Idk man this thing is showin me about 29* mechanical at about 2300 .
Course then again I'm usin a dial back light which I know Ray along with quite a few other very knowledgeable guys say no to .I probably should just eliminate that question and get timing tape on the balancer.

But as it stands I'm setting initial at 8 because mechanical is showing me 29.

On the erratic idle you're saying the can could be fluctuating the ammount of advance causing spring to whither in and out .
That would be a problem with the distributor itself or would it mean an adjustment needs to be made.

Stock bottom end 350 with some decent aluminum heads. Runners are too big for the engine but the heads were there so I put em on it . Jones flat tappet 218 dur. .468 lift
Air gap intake . Holley street demon 625 .
And yeah I know the distributor is gonna Need work . Whats the chances of throwing an oem style distributor on an engine that's anything but stock and be optimal? Right?
 

·
Registered
1966 Chevelle SS396
Joined
·
1,352 Posts
In reference to your #1 point/condition I'm getting exactly that which is somewhat of an erratic idle . Im currently hooked to full manifold vacuum. Vacuum gauge bouncing some and ofcourse rpm as well . Fresh on the mild side sb motor . Pulling average 12ish inches vacuum at idle . I say average because of the fluctuation in vacuum at idle .
Have you tried retorquing your intake? Might be sucking air.
 

·
Registered
Ryan
Joined
·
1,759 Posts
Idk man this thing is showin me about 29* mechanical at about 2300 .
Course then again I'm usin a dial back light which I know Ray along with quite a few other very knowledgeable guys say no to .I probably should just eliminate that question and get timing tape on the balancer.

But as it stands I'm setting initial at 8 because mechanical is showing me 29.

On the erratic idle you're saying the can could be fluctuating the ammount of advance causing spring to whither in and out .
That would be a problem with the distributor itself or would it mean an adjustment needs to be made.

Stock bottom end 350 with some decent aluminum heads. Runners are too big for the engine but the heads were there so I put em on it . Jones flat tappet 218 dur. .468 lift
Air gap intake . Holley street demon 625 .
And yeah I know the distributor is gonna Need work . Whats the chances of throwing an oem style distributor on an engine that's anything but stock and be optimal? Right?
29 seems unusually high. But I’m not doubting you. That very well could be. That cam is healthy but not huge. It could definitely use more than 8 degrees at idle. Full manifold vacuum for advance will be best. It definitely needs more than 8 at idle.

As far as withering in and out (that’s just the best word I could come up with) , it doesn’t mean there’s a problem with the vacuum can, it just means that the engine isn’t pulling enough vacuum to hold it steady. Timing goes gown, idle speed goes down, this causes timing to go down more, then idle rpm goes down even more. So forth and so on until it either does or you blip the throttle enough to get rpm back up and start the downward spiral all over again. Does this sound like that’s your problem?

is your vacuum advance can adjustable?

with the engine specs you laid out, starting out I’d shoot for something like 14 initial, 22 mechanical, 36 total, 12 degrees vacuum advance plugged into a full manifold vacuum source. You’d have to put a positive limiter stop on your vacuum can. Lots of literature about how to do it here.
 

·
Registered
Joined
·
1,593 Posts
If you have an HEI large body dist, you can use MSD vacuum advance stop plate 84281. I would start with adding that. Also look for a VC302 or VC1703 advance cannister. It starts to move at 3-6" and is all in by 7-9" vacuum. You need the cannister to be all in 1.5-2" below your in gear idle vacuum for it to work properly.
 

·
Registered
Joined
·
804 Posts
Discussion Starter · #17 ·
29 seems unusually high. But I’m not doubting you. That very well could be. That cam is healthy but not huge. It could definitely use more than 8 degrees at idle. Full manifold vacuum for advance will be best. It definitely needs more than 8 at idle.

As far as withering in and out (that’s just the best word I could come up with) , it doesn’t mean there’s a problem with the vacuum can, it just means that the engine isn’t pulling enough vacuum to hold it steady. Timing goes gown, idle speed goes down, this causes timing to go down more, then idle rpm goes down even more. So forth and so on until it either does or you blip the throttle enough to get rpm back up and start the downward spiral all over again. Does this sound like that’s your problem?

is your vacuum advance can adjustable?

with the engine specs you laid out, starting out I’d shoot for something like 14 initial, 22 mechanical, 36 total, 12 degrees vacuum advance plugged into a full manifold vacuum source. You’d have to put a positive limiter stop on your vacuum can. Lots of literature about how to do it here.
I agree . 29 seems high and that's why I need to 100 percent verify TDC and where that balancer is at Before trying to use numbers .
This morning I fired it up let it get to temp and it ofcourse was still giving me an erratic vacuum/rpm . So I Said screw it . I advanced the distributor Maybe 4 to 6 degrees by eye.
.. I'm sittin in the cab with it runnin right now . 700 rpm steady .15 inches of vacuum pretty steady enough /not varying much at all .
By advancing that distributor 4 to 6* I'm between 12 and 14 initial now because I was 8 with the light last night.
If the balancer and light are true then right now I'd be like 41 or 42 * total. Too much.
The other thing is I need to get a fuel regulator and gauge lol . This thing is just kind of roughed in right now .
Honestly I've never been one much with timing light vac gauges , none of that . I'd just drive em and feel it . Move the distributor and or tune the carb untill whatever I was drivin was gettin it the best it could . You can tell when an engine is on the verge of too much advance/detonation. If not then probably best leave the hood closed till it gets dropped at the local shop lol. Darn things are like women I tell ya . What works on one probably won't work on the next haha
 

·
Registered
Joined
·
804 Posts
Discussion Starter · #18 ·
If you have an HEI large body dist, you can use MSD vacuum advance stop plate 84281. I would start with adding that. Also look for a VC302 or VC1703 advance cannister. It starts to move at 3-6" and is all in by 7-9" vacuum. You need the cannister to be all in 1.5-2" below your in gear idle vacuum for it to work properly.
I think I've got that stop plate in my arsenal at the garage. I'll look for it later on .
If not ill get one. I'm pretty sure I got the canister too .
1.5 to 2" below in gear idle vacuum . Got it !
I'll give er a shot . Thanks pal
 

·
Premium Member
Joined
·
7,708 Posts
Is this a HEI or points distributor?
If you can get weights or limit the mech advance to 18-20 deg and limit your vacuum can to 10-12 deg I think you can get it run much better. 16-18 initial and 36-38 total and 10–12 from the vacuum can.
 

·
Registered
Joined
·
804 Posts
Discussion Starter · #20 ·
Is this a HEI or points distributor?
If you can get weights or limit the mech advance to 18-20 deg and limit your vacuum can to 10-12 deg I think you can get it run much better. 16-18 initial and 36-38 total and 10–12 from the vacuum can.
Yep its HEI . And I've got a bunch of that stuff hangin around. Weights stops this that . Never got to playing with them .
I thought I heard one time that limiting mechanical very low and having much more initial = go faster ?? Idk
 
1 - 20 of 84 Posts
Top