Timing & Vacuum Advance 101 - Page 2 - Team Camaro Tech
Performance Our High Performance area

Reply
 
LinkBack Thread Tools Display Modes
post #16 of 80 (permalink) Old Jul 6th, 10, 08:22 PM
Senior Tech
Wes
 
Join Date: Dec 2003
Location: Leander, TX
Posts: 2,758
Re: Timing & Vacuum Advance 101

John Z,..beautifully written, especially the advice regarding vacuum advance on the street. I couldn't agree more. There are a half dozen reasons to run it,..no good reason not to (for street apps).

For clarification, I thought, vacuum advance was 'turned off' at WOT because of higher cylinder pressures and combustion chamber heat, both of which conspire to cause pre-ignition (pinging, knocking, etc).

Your experience and research says it's more to do with AF ratio and the speed with which different AF ratios will burn.

We may be in agreement,..cause and effect.

Can you expound?
Straight-line-69 is offline  
Sponsored Links
Advertisement
 
post #17 of 80 (permalink) Old Jul 7th, 10, 04:59 AM
Banned
Bethanne
 
Nashville Beth's Avatar
 
Join Date: Jan 2007
Location: Nashville TN
Posts: 3,192
Re: Timing & Vacuum Advance 101

[QUOTE=Straight-line-69;1449158]

For clarification, I thought, vacuum advance was 'turned off' at WOT because of higher cylinder pressures and combustion chamber heat, both of which conspire to cause pre-ignition (pinging, knocking, etc).

QUOTE]

Vacuum advance is "turned off" during wide open throttle because there is no vacuum in the manifold .
Nashville Beth is offline  
post #18 of 80 (permalink) Old Jul 7th, 10, 10:58 AM
Senior Tech
Wes
 
Join Date: Dec 2003
Location: Leander, TX
Posts: 2,758
Re: Timing & Vacuum Advance 101

Thanks NB,..yes, very low vacuum conditions remove vacuum advance functionality.

But the question above wasn't how VA was 'turned off', but why.

John Z believes it's necessary for differing burns speeds of differing AF ratios.

I thought it had more to do with combustion temps and pressure caused by WOT coupled with load, which produce pre-ignition.

I'm thinking we both may be right but wanted his angle.

John Z,..you still out there?
Straight-line-69 is offline  
 
post #19 of 80 (permalink) Old Jul 7th, 10, 02:16 PM
Senior Tech
Steps
 
Join Date: Dec 2005
Location: New Zealand
Posts: 13,316
Re: Timing & Vacuum Advance 101

Quote:
John Z believes it's necessary for differing burns speeds of differing AF ratios.

I thought it had more to do with combustion temps and pressure caused by WOT coupled with load, which produce pre-ignition.
John doesnt believe..he knows, this is not a document of opinion, its a doc of what actually is
a/f, compresion/ octane/chamber design/engine load/rpm all influence burn speed and the required advance needed.

My Spelling is not incorrect...it is creative

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Steptoe is offline  
post #20 of 80 (permalink) Old Jul 7th, 10, 05:05 PM
Banned
Bethanne
 
Nashville Beth's Avatar
 
Join Date: Jan 2007
Location: Nashville TN
Posts: 3,192
Re: Timing & Vacuum Advance 101

Quote:
Originally Posted by Straight-line-69 View Post
Thanks NB,..yes, very low vacuum conditions remove vacuum advance functionality.

But the question above wasn't how VA was 'turned off', but why.

John Z believes it's necessary for differing burns speeds of differing AF ratios.

I thought it had more to do with combustion temps and pressure caused by WOT coupled with load, which produce pre-ignition.

I'm thinking we both may be right but wanted his angle.
I think I know what you are saying. You are saying that that the amount of advance at any given time is dictated by combustion temps and pressure (true), but that is not looking at the big picture.

Engineers had to develop a mechanical system that would manipulate spark timing over a wide range of load and give the best possible gas mileage. If they were only concerned with power at WOT, there would be no need for VA.
It's cruising at very light load where you need a lot more advance because the mixture is lean (for gas mileage) and needs more time to burn.

And I'm just repeating what JohnZ has so eloquently written......
Nashville Beth is offline  
post #21 of 80 (permalink) Old Jul 9th, 10, 07:13 PM Thread Starter
Team Member
John
 
JohnZ's Avatar
 
Join Date: Sep 2000
Location: Washington, Michigan USA
Posts: 7,728
Re: Timing & Vacuum Advance 101

Quote:
Originally Posted by aaronjohn20 View Post
Okay let me get this straigt. I've got an Edelbrock 1407 in my 85 iroc. My vacum advanced needs to be hooked up on the left port
I'm not familiar with how the Edelbrocks are ported, but you want it connected to whichever port provides full manifold vacuum (sometimes called "full-time" vacuum) at idle, not the one that's a "ported" vacuum source, with little or no vacuum at idle.


JohnZ
CRG
JohnZ is offline  
post #22 of 80 (permalink) Old Jul 9th, 10, 07:35 PM Thread Starter
Team Member
John
 
JohnZ's Avatar
 
Join Date: Sep 2000
Location: Washington, Michigan USA
Posts: 7,728
Re: Timing & Vacuum Advance 101

Quote:
Originally Posted by Straight-line-69 View Post
Thanks NB,..yes, very low vacuum conditions remove vacuum advance functionality.

But the question above wasn't how VA was 'turned off', but why.

John Z believes it's necessary for differing burns speeds of differing AF ratios.

I thought it had more to do with combustion temps and pressure caused by WOT coupled with load, which produce pre-ignition.

I'm thinking we both may be right but wanted his angle.

John Z,..you still out there?
There are lots of factors influencing what the optimum timing should be under many diverse operating conditions, most of which have been mentioned in the last few posts above; the challenge in the 60's was to develop simple and reliable mechanical systems that although separate, would complement each other and manage ignition timing based on both engine rpm and load without resulting in engine damage. The result was the combined centrifugal and vacuum advance systems, which worked amazingly well for decades considering their simplicity. Those systems are also relatively simple to tune, IF you understand how they work and what they're supposed to do, and understand that the rules are different between street cars and race cars.

The onset of emissions requirements began to complicate the ignition equations, and subverted the centrifugal and vacuum advance systems into doing things they were never intended to do, most of which reduced driveability and efficiency.

Catalytic converter technology overcame many of those problems, and the ultimate solution that we enjoy in our daily drivers today came as a result of computer technology that senses, measures, and changes both A/F ratios and ignition timing maps hundreds of times per second and compensates instantly for temperature, altitude, load, onset of detonation at any level, and self-learns the driver's habits, and you can tune them with a laptop without getting your hands dirty. That's progress.


JohnZ
CRG
JohnZ is offline  
post #23 of 80 (permalink) Old Jul 10th, 10, 06:20 AM
Senior Tech
 
Join Date: Aug 2001
Location: north carlina
Posts: 230
Question Re: Timing & Vacuum Advance 101

Jonn Z what is manifold vacuun hooked to? What about the ajustable distributors like this? www.professional-produsts.com
dill is offline  
post #24 of 80 (permalink) Old Jul 10th, 10, 06:28 AM
Senior Tech
Scott
 
Join Date: Aug 2008
Location: CT
Posts: 7,277
Re: Timing & Vacuum Advance 101

Quote:
Originally Posted by dill View Post
Jonn Z what is manifold vacuun hooked to? What about the ajustable distributors like this? www.professional-produsts.com
Fix your link.
Sauron67MM is offline  
post #25 of 80 (permalink) Old Jul 10th, 10, 06:54 AM Thread Starter
Team Member
John
 
JohnZ's Avatar
 
Join Date: Sep 2000
Location: Washington, Michigan USA
Posts: 7,728
Re: Timing & Vacuum Advance 101

Quote:
Originally Posted by dill View Post
Jonn Z what is manifold vacuun hooked to? What about the ajustable distributors like this? www.professional-products.com
Manifold vacuum is connected to the vacuum advance diaphragm on the distributor. The distributor in the link allows you to change static timing without using a timing light; it has the same centrifugal and vacuum advance systems as any GM distributor. Fixed your link.


JohnZ
CRG
JohnZ is offline  
post #26 of 80 (permalink) Old Jul 10th, 10, 09:02 AM
Senior Tech
 
Join Date: Aug 2001
Location: north carlina
Posts: 230
Question Re: Timing & Vacuum Advance 101

Is it a good one to use in a bbc?
dill is offline  
post #27 of 80 (permalink) Old May 22nd, 11, 01:32 PM
Tech Team
 
Join Date: Aug 2005
Location: Colorado
Posts: 81
Re: Timing & Vacuum Advance 101

JohnZ;

I'm posting this just to give testament to what you have stated here. I implemented the change to full manifold vacuum and adjusted the timing and mixture on my GMPP 350 / 350 hp crate motor with FAST EzEFI and what a difference! Smoother idle, more responsive throttle, cooler and better mileage. I'm in Denver and the engines run lean to start with up here and my idle was always unstable. Now, it sits at 750 without much variation with 12 deg static / 25 deg with 13 " vacuum. 36 deg total timing w/o vacuum. Air Fuel ratios at idle = 13.8:1 AF, 14.8:1 at cruise and 13.8:1 WOT....all on regular gas.... lovin it!

Thanks John!!!

Bob

69 Camaro
Le Mans Blue Coupe, 350 GMPP/HO with FAST EZ EFI, TH350
boboftherockies is offline  
post #28 of 80 (permalink) Old May 22nd, 11, 01:59 PM
Senior Tech
Steps
 
Join Date: Dec 2005
Location: New Zealand
Posts: 13,316
Re: Timing & Vacuum Advance 101

Quote:
Air Fuel ratios at idle = 13.8:1 AF, 14.8:1 at cruise and 13.8:1 WOT....all on regular gas.... lovin it!
For petrol you are looking at around 14.6 /14.7
The idle can be leaned out a little... within reason, lean mixtures require more advance. At least 115.1 to about 15.3....big economy savings to be made while sitting at interections.
Cruise again can be leaned out further...that is the basic concept of the VA add a few degs of advance for the leaner cruise mixture....u are only looking at 10 to 15 hp requirement to cruise...so around 15.1 to 15.3

Under power WOT dont even risk leaning..hi temps /kill engines
And more power is made with a far more richer mixture...getting down into the 11.5 to 12.5 AFR range....the richer the mixture...withing reason gives more power.

If messing around with data loggers /AFRs get the timing in ball park, re jet/powervalves etc the carb.....fine tune the timing curves and VA....MAY have to re set the catb again.. but generally once at this stage mixtures are pretty spot on... or close enough for a good street machine....influences like ambiant temps altitude will have more influnence from this point.

My Spelling is not incorrect...it is creative

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Steptoe is offline  
post #29 of 80 (permalink) Old May 23rd, 11, 01:27 PM
Tech Team
Bob
 
Join Date: Jan 2010
Location: Texas
Posts: 40
Send a message via Yahoo to PRO1CAMARO
Re: Timing & Vacuum Advance 101

I understand the vacuum advance at idle and high vacuum part throtle helping with the cooling and performance. But just a thought/question, since my combination is basically a drag motor (800 hp-540) and likes the locked out set up at start up and idle. Would running it locked out w/ the vacuum advance functioning for the part throttle advance work the same? I changed it back to centrifigual advance 12 deg. inital and 32 total and made my heat problem worse plus it just didn't seem to like it, harder to start, more cold natured etc. I went back to lock out setup and set it at 36 total and the heat problem got better and the motor was more "snappy" which tells me it may need more during part throtle cruise. I just ordered the same dizzy that I have except it has a vacuum advance and it is possible to set it up that way. Figured I would try the vacuum advance before I go the bigger radiator way - it's cheaper this way too if it works.

100% home built '68 4-link back half, 540 solid roller Dart block, ported Dart 325's & a tunnel ram w/ (2) 750's
PRO1CAMARO is offline  
post #30 of 80 (permalink) Old May 26th, 11, 12:29 PM
Senior Tech
Wes
 
Join Date: Dec 2003
Location: Leander, TX
Posts: 2,758
Re: Timing & Vacuum Advance 101

I'd run the vacuum advance. Even at the drags, your engine spends a lot of time running at less than wide open throttle (returning, staging, tuning, etc).

At WOT, vacuum advance is out of play so there's no penalty for running it.
Straight-line-69 is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Team Camaro Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address.
NOTE we receive a lot of registrations with bad email addresses. IF you do not receive your confirmation email you will not be able to post. contact support and we will try and help.
Be sure you enter a valid email address and check your spam folder as well.



Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome