vacume advance question

[76bonanza]

I have had several people tell me my truck would run better with the vacuum advance disconected. I have a mild 355 (330hp) quick fuel mech secondary carb. Truck is not daily driver but not a drag racer either when drive it is street aplication. Does the vacuum advance soley help fuel milage?
what are the pros and cons of this. And one last question is it proper to just pull the hose or get the lockout kit?

Thanks for any insight.

[Billla]

Originally Posted by 76bonanza

I have had several people tell me my truck would run better with the vacuum advance disconected. I have a mild 355 (330hp) quick fuel mech secondary carb. Truck is not daily driver but not a drag racer either when drive it is street aplication. Does the vacuum advance soley help fuel milage?
what are the pros and cons of this. And one last question is it proper to just pull the hose or get the lockout kit?
Thanks for any insight.

Vacuum advance provides correct advance based on engine load, mechanical advance only on RPM. Without the vacuum advance based on engine load your engine will not "run better" (does it run badly now?) and yes, your milage would likely decrease measurably - especially in cruise. Disconnecting the vacuum advance only makes sense (IMHO ) when you have a specific advance curve (such as for drag racing), you have a static engine load (circle track) or if the engine is making > 1.4 HP/CID.

The quick and easy way to feel this is just to pull the hose and plug it - then go for a drive around town

The mechanical secondary carb isn't helping fuel milage, either

[PICKMUP]

It depends on how much total advance is in the dist. I find some HEIs from the 70s & 80s that have a total of 50 degrees or more...they were probably in real light vehicles.
If you set the initial up to 8-12, the bottom end is strong, but they ping like crazy at cruising speeds/loads. You need to get the total back down to around 32-36 by either blocking the vac advance or limiting it and still have good 8-12 initial.

[GREASEMONKEY72]

no it wont run better with the vacumm advance disconnected, SBC like 32-36* of total timing, and about 8-10* of initial timing

[Billla]

Originally Posted by GREASEMONKEY72

no it wont run better with the vacumm advance disconnected, SBC like 32-36* of total timing, and about 8-10* of initial timing

, with the slight caveat that SBCs like 32-36 total advance and whatever initial is required to achieve that total advance.

Vacuum advance is in addition to mechanical advance, adding advance to 50+ degrees at idle and cruise.

I usually get called BS on that one - but it's a fact

[jimfulco]

Vacuum advance can help it run smoother at idle and at low rpm. Too much vacuum advance can be a bad thing, however, causing an annoying miss at cruising speed.

[bryanw1968]

Everything everyone is saying is right on the $$. Another reason people run vacuum advance is it allows you to not have a ton of initial timing at idle. Therefore saving gas and not making your motor run hot at idle.

[Jim_PA]

Originally Posted by bryanw1968

Another reason people run vacuum advance is it allows you to not have a ton of initial timing at idle. Therefore saving gas and not making your motor run hot at idle.

[bryanw1968]

Say you're running 8 degrees of initial timing. That would typically not be enough timing at initial. You're truck would fall on its face when you tried to go. When using vacuum advance you should have a hose from your ported vacuum port to the vacuum diaphram on the distributor. That diaphram won't see vacuum until you crack the primary throttle blades thus allowing the diaphram to see vacuum and add timing.

In the distributor you still have to consider the weights and springs to control how much total advance and what RPM it comes in.

[Jim_PA]

Ahhh - I see. I thought you were supposed to use full manifold vacuum for vac. advance.

[bryanw1968]

It's possible that some people make it work the way you're talking about. But no carb manufacturer will tell you that.

[piecesparts]

Originally Posted by Billla

, with the slight caveat that SBCs like 32-36 total advance and whatever initial is required to achieve that total advance.

Vacuum advance is in addition to mechanical advance, adding advance to 50+ degrees at idle and cruise.

I usually get called BS on that one - but it's a fact

YEP, I agree and I won't wave any BS flags. My stroker is set to get 35 degrees tOTAL and the response is amazing. I keep my vacuum advance connected to get the distributor to control the rest of the load.

I know a gentleman tha has his 440 Charger set up with the vacuum disconnected, the mileag is bad on these cars from the GET GO, but his is downright nasty. He can't run over a 100 miles on a tank of gas and his gearing is not that low.

[bryanw1968]

Originally Posted by piecesparts

YEP, I agree and I won't wave any BS flags. My stroker is set to get 35 degrees tOTAL and the response is amazing. I keep my vacuum advance connected to get the distributor to control the rest of the load.

I know a gentleman tha has his 440 Charger set up with the vacuum disconnected, the mileag is bad on these cars from the GET GO, but his is downright nasty. He can't run over a 100 miles on a tank of gas and his gearing is not that low.

In my mind, the only reason to run w/o a vacuum advance is to run a distributor that doesn't have one on it. For example I run a full mechanical MSD setup w/o a vacuum advance. So there is initial and whatever else the mechanical distributor throws in(33 total). My heads won't let me run anymore than that.

[Billla]

Originally Posted by bryanw1968

Everything everyone is saying is right on the $$. Another reason people run vacuum advance is it allows you to not have a ton of initial timing at idle. Therefore saving gas and not making your motor run hot at idle.

Ummm...no. There is significant vacuum advance at idle; this improves idle quality. This is why vacuum advance must be disconnected to set initial timing.

[Billla]

Originally Posted by bryanw1968

It's possible that some people make it work the way you're talking about. But no carb manufacturer will tell you that.

Ummm...again The vacuum advance SHOULD be connected to manifold vacuum as ported vacuum will be zero at idle.

[PICKMUP]

I just went through this again today with a stock 71 3/4T w/402 & TH 400. I put in a used HEI...55degrees total from the mech & vac and 8 initial.
The thing pings BAD at crusing when you put a slight load on the engine...AGGH! It did not do this with the stock point dist & vac advance.
I pulled off the vac hose and took it for a drive this afternoon. The ping is GONE! and it runs just as strong as before. I did not get the adjustable timing light hooked up, so I don't know what the total is now with just the mechanical advance.
I would say if you are having this problem, try no vac and see if it works for you.

[Billla]

Originally Posted by PICKMUP

I just went through this again today with a stock 71 3/4T w/402 & TH 400. I put in a used HEI...55degrees total from the mech & vac and 8 initial.
The thing pings BAD at crusing when you put a slight load on the engine...AGGH! It did not do this with the stock point dist & vac advance.
I pulled off the vac hose and took it for a drive this afternoon. The ping is GONE! and it runs just as strong as before. I did not get the adjustable timing light hooked up, so I don't know what the total is now with just the mechanical advance.
I would say if you are having this problem, try no vac and see if it works for you.

I'd be real careful drawing cause and effect conclusions here, and I'll stand by my statement that although it may still "run strong" you're losing milage and driveability.

Mechanical advance should be limited to 34-36 degrees by 3000 RPM, using whatever initial is required to get there. If you're getting more mechanical than that, then you need to bush the pivots. What curve are you seeing with *just* mechanical advance?

This sounds a lot more like a stuck advance than anything else to me - have you verified the movement of the mechanical and vacuum advance?

I'm not just disagreeing to disagree My point is that unless you get in there and figure out what's happening I would be careful about generalizing a recommendation - especially when it's contrary to information that the top tuners provide for a street-driven engine.

[70c10]

I run 19 degrees initial and 42 total on my truck.

[bryanw1968]

Originally Posted by Billla

Ummm...again The vacuum advance SHOULD be connected to manifold vacuum as ported vacuum will be zero at idle.

Biila,

I have in fact heard of what you're talking about as far as disconnecting the vacuum advance to set the timing. And it does actually make sense that direct vacuum might make for a smoother idle. However, why do carb manufacturers(barry grand in particular) tell you NOT to connect things this way? If you have direct vacuum hooked up to the VA at idle won't that bring too much initial into the equation? Maybe unless you just set less initial with the VA unplugged and then plug it back in to find out what your total initial is?
Me confused.

[bryanw1968]

So I did a little more research and I remembered some stuff I forgot.
There were some distributors back in the '70's that actually worked the opposite of how most GM distributors work. They actually need to have the VA on the distributor connected to direct vacuum to keep it from pulling timing into the initial equation.

That being said, it still doesn't coincide with what you're saying(billa).
Definitely not arguing I'm just trying to get it all straight in my head.

Connecting a standard GM distributor to full vacuum at idle still doesn't make sense to me. Especially if the motor is not stock and has any kind of a camshaft which would fluxuate vacuum so much. If you had a stock motor and connected the VA to full vacuum at idle I guess it makes a little more sense but not much?

Help me understand this further.
Thanks,
B

[71chevylowrider]

Hey guys, can I add something here, I changed out my carb recently from a Demon that I couldn't get to run right, I have a nice 350 with aan edelbrock perforem rpm manifold, and it has a high lift Isky cam, (very notible lopey idle) I went back to a brand new edelbrock 1406 vac secondary carb. I hooked it up to the ported vacum tube on the carb, (the one on the left when your in front of the truck looking at the carb, & I couldn't get it to run all that great, not enough power, not a very good idle, so I hooked it to the other side of the carb (the one on the right front, not the big one in the middle, (I beleive it's constant vaccume) & after adjusting my idle mixture screws & getting the idle the way I like it, the truck idles, starts, & hauls A$$ better then ever before, I was concerned about having it hooked up wrong even though I loved how the truck was running, I also want to say I respect Billa's knowledge of motors & tuning them, so check this out, I cpied this right out of the owners manual for my 1406, apparantly it's okay with edelbrock to hook it directly to manifold vaccume.

Long Duration Camshaft

If the engine has a fairly radical camshaft it may require an excessive amount of throttle opening for idle and/or have low idle vacuum levels. Either condition can lead to poor levels of adjustability and erratic idles.

# Another fix for the above condition is to run as much spark advance as possible at idle. If the distributor is fitted with a vacuum advance unit, connect it directly to manifold vacuum. If you are not able to employ vacuum advance for some reason, then the mechanical curve should have a low limit, which will allow you to use plenty of initial spark advance.

[bryanw1968]

Originally Posted by Billla

Ummm...again The vacuum advance SHOULD be connected to manifold vacuum as ported vacuum will be zero at idle.

Please excuse my reading TOO fast. I just re-read that you said SHOULD be connected to PORTED vacuum. I agree

[71chevylowrider]

?????

[71chevylowrider]

Originally Posted by Billla View Post
Ummm...again The vacuum advance SHOULD be connected to manifold vacuum as ported vacuum will be zero at idle.


Bryan, when I read that it sounds like he is saying it "should be connected to manifold vaccume", & that is what I did, & that is what edelbrock says in the owners manual if you can't get enough ported vacuume, as in a long duration, high lift, radical camshaft.

[WorkinLonghorn]

Posted by PICKMUP;

I just went through this again today with a stock 71 3/4T w/402 & TH 400. I put in a used HEI...55degrees total from the mech & vac and 8 initial.
The thing pings BAD at crusing when you put a slight load on the engine...AGGH! It did not do this with the stock point dist & vac advance.
I pulled off the vac hose and took it for a drive this afternoon. The ping is GONE! and it runs just as strong as before. I did not get the adjustable timing light hooked up, so I don't know what the total is now with just the mechanical advance.
I would say if you are having this problem, try no vac and see if it works for you.

>>>I would say if you're having this problem,get AN ADJUSTABLE VACUUM ADVANCE and set it correctly and watch your fuel mileage and your part-throttle performance increase.