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

[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.

[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.

[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.

[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.

[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.

[PICKMUP]

All HEIs are not created equal. Most drop in, set up pretty close, and run good...but some of these have just way too much dist advance built in (vac & mech combined) and have to be "fixed" to make them compatible with our heavy trucks.
This is one of those. I don't have separate vac & mech figures on this one yet, but total is over 50.
My 68 GM serv manual (396) calls for mech: 28 @ 4200rpm (only 15 @ 2000) and vac: 15 @ 15"+ with 6 initial setting.
My 79 GM serv manual (454) call for even less with mech: 20 @ 4200 (only 14 @ 2800) and vac: 10 @ 13"+ with 4 initial setting.
Sorry, I don't have a '71 manual close, but it looks like GM set these heavy trucks with BB engines to run around 30 degrees total advance for running in mid-range cruising rpms at part throttle. That seems to agree with what has worked best in the past.
So, until I can get an adj vac adv pot or limit the mech adv to shorten up the curve...I will have to leave the vac adj off. Not best, but the pinging will burn a hole in the pistons over time...and it drives me crazy.

[GREASEMONKEY72]

i have a stock 350 with the points dizzy and i have the VA hose connected to the carb (qjet) and its set at 8* initial, i have no idea what the total is

what would you all say to that? right/wrong, what?

all the talk thats gone on so far is giving me a headache, so i thought id just ask about mine instead

[jimfulco]

Some race motors can benefit from vacuum advance. It won't do squat at full-throttle on the track, but it can help it run smoother and cooler in the pits.

I had a 292 with a 310* cam in a Nova for a short time, and hooking up the vacuum advance (manifold vacuum, please) made it run smoother at low rpm, such as in residential areas & school zones.

Ported vacuum was introduced for smog devices, and the HEI distributor was designed as a smog device. Recalibrate it like Crazy said and it should work much better. Just don't tell the smog police.

[76bonanza]

One other question, I am from the camp of full vacuum at idle also. But I have always wondered about tuning the carb. Do you tune the carb idle screws with advance plug in or not. Because if you tune it with advance disconnected then when you connect it to full manifold vac the idle tends to go up also?

[67chevemall]

Man I put a edel 1406 on my truck a few months ago and I can't get it to stop pinging and poping.
This brings more light no the subject 4 sure

I think I am going to try turning the dizzy to full advance and see what happens with out the VA.......

Good thread keep er going

[Billla]

Originally Posted by 67chevemall

Man I put a edel 1406 on my truck a few months ago and I can't get it to stop pinging and poping.
This brings more light no the subject 4 sure

I think I am going to try turning the dizzy to full advance and see what happens with out the VA.......

Good thread keep er going

If it's pinging you have TOO MUCH advance. Do you have an adjustable timing light?

[76bonanza]

What does pinging sound like exactly? I have what sounds like an exhaust leak louder under truck and very metalic sounding, but maybe it is pinging it does it idle.

[piecesparts]

In most cases the so called "Pinging", otherwise known as spark knock happens under load or acceleration. If you have a metallic noise at idle, that would be another thing. I would try driving with the window down and listen as you accelerate your truck to get out onto a street or highway. If you here something that sounds like a light rod knock, then you are experiencing the "pinging" we have referenced to.

[rage'nrat638]

sounds like a bag of marbles in a tin can