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#1 |
Senior Member
Join Date: Mar 2003
Location: St robert Mo
Posts: 2,001
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vacume advance question
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.
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1968 c10 lowered 3" 4" 355/Th400 built by Hatfield racing in joplin MO Last edited by 76bonanza; 05-15-2007 at 11:48 AM. |
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#2 | |
Account Suspended
Join Date: Sep 2005
Location: Monroe, WA
Posts: 3,815
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Re: vacumm advance question
Quote:
![]() 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 ![]() |
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#3 |
"Trucks with Class"
![]() Join Date: Jul 2000
Location: OLYMPIA, WA, USA
Posts: 8,132
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Re: vacumm advance question
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. |
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#4 |
Registered User
Join Date: Aug 2006
Location: Bakersfield,CA
Posts: 7,893
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Re: vacumm advance question
no it wont run better with the vacumm advance disconnected, SBC like 32-36* of total timing, and about 8-10* of initial timing
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-steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME |
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#5 | |
Account Suspended
Join Date: Sep 2005
Location: Monroe, WA
Posts: 3,815
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Re: vacumm advance question
Quote:
![]() 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 ![]() |
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#6 |
Registered User
Join Date: May 2003
Location: Shreveport LA
Posts: 3,170
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Re: vacumm advance question
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.
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#7 |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
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.
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#8 |
Registered User
Join Date: May 2005
Location: PA
Posts: 2,696
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Re: vacumm advance question
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#9 |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
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. |
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#10 |
Registered User
Join Date: May 2005
Location: PA
Posts: 2,696
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Re: vacumm advance question
Ahhh - I see. I thought you were supposed to use full manifold vacuum for vac. advance.
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#11 |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
It's possible that some people make it work the way you're talking about. But no carb manufacturer will tell you that.
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#12 | |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
Posts: 6,821
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Re: vacumm advance question
Quote:
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. |
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#13 | |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
Quote:
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#14 |
Account Suspended
Join Date: Sep 2005
Location: Monroe, WA
Posts: 3,815
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Re: vacumm advance question
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.
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#15 |
Account Suspended
Join Date: Sep 2005
Location: Monroe, WA
Posts: 3,815
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Re: vacumm advance question
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#16 |
"Trucks with Class"
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Location: OLYMPIA, WA, USA
Posts: 8,132
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Re: vacumm advance question
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. |
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#17 | |
Account Suspended
Join Date: Sep 2005
Location: Monroe, WA
Posts: 3,815
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Re: vacumm advance question
Quote:
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 ![]() |
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#18 |
67-72 Addict!
Join Date: Feb 2002
Location: The TAX State!
Posts: 7,857
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Re: vacumm advance question
I run 19 degrees initial and 42 total on my truck.
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Jim 1970 C/10 Fleetside w/Ghost Flames Lowered 4.5" front and 4" rear (Raked) 355/350 Turbo w/shift kit 10" Redneck Performance Verter w/2500 stall Hooker Super Comps part#2808-1 Performer RPM Air-Gap 12 Bolt w/3:73 gears- Eaton Posi Comp Cams XE262 with 1.6 Crane Energizers, Road Demon 625 and Brodix IK 180 heads |
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#19 | |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
Quote:
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. |
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#20 |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
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 |
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#21 |
Senior Member
Join Date: Sep 2006
Location: Vancouver, WA
Posts: 1,015
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Re: vacumm advance question
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. |
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#22 |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
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#23 |
Senior Member
Join Date: Sep 2006
Location: Vancouver, WA
Posts: 1,015
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Re: vacumm advance question
?????
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#24 |
Senior Member
Join Date: Sep 2006
Location: Vancouver, WA
Posts: 1,015
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Re: vacumm advance question
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. Last edited by 71chevylowrider; 05-17-2007 at 02:45 PM. |
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#25 | |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: vacumm advance question
Posted by PICKMUP;
Quote:
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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