Ported Vacuum or Manifold Vacuum? - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 09-03-2008, 10:04 PM
Regal Beagle's Avatar
Registered User
 
Join Date: Apr 2008
Location: Calgary, Alberta
Posts: 199
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Ported Vacuum or Manifold Vacuum?

I was adjusting my idle mixture today and speaking to a mechanic friend for some assistance to be sure I got it right. My distributer was hooked up to the ported vacuum port but my friend says it needs to be hooked up to the manifold vacuum side of my Edelbrock Performer. I switched it over and adjusted the idle and got the car running and performing well much the same as when it was hooked up to the ported side.
My question is which side do I need to have my distributer hooked up to for maximum performance?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 09-03-2008, 10:18 PM
Rustydawg's Avatar
Cruzin w/Elvis in Bigfoots UFO
 
Last photo:
Join Date: Nov 2004
Location: Edmonton AB, Great White North
Age: 48
Posts: 248
Wiki Edits: 0

Thanks: 2
Thanked 0 Times in 0 Posts
Go with manifold vacuum

Here's a quote from a posting on this forum:

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively.

Read the full post "Timing and Vacuum Advance 101" here:

Distributor Tuning and Theory - Part 1

I was pondering this same question myself the other day and was searching the subject...
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 09-03-2008, 10:19 PM
C-10's Avatar
Revolution Calling
 

Last journal entry: Poverty caps live on.
Last photo:
Join Date: Apr 2008
Location: Nashport, Ohio
Posts: 920
Wiki Edits: 0

Thanks: 2
Thanked 6 Times in 6 Posts
At WOT it doesn't matter which way it's hooked up. The ported side just keeps the vacuum advance from moving the spark until you move the throttle. This will help part throttle drive-ability and economy and provide a more stable idle. When you are at WOT engine vacuum is at zero.

The vacuum adv. canister should only see vacuum at off idle cruise speeds.

This is my take...
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 09-04-2008, 07:02 AM
Greg T's Avatar
www.krusinklassics.net
 
Last photo:
Join Date: Jul 2007
Location: Escanaba, MI.
Age: 60
Posts: 1,148
Wiki Edits: 0

Thanks: 20
Thanked 41 Times in 32 Posts
Full manifold vac will aid in cooler city driving and better throttle response because your low speed and idle timing will be more advanced. Use manifold vacuum.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 09-04-2008, 07:23 AM
Irelands child's Avatar
Registered User
 
Last wiki edit: Ford engine specifications Last photo:
Join Date: Jan 2005
Location: Upstate New York
Posts: 4,907
Wiki Edits: 8

Thanks: 14
Thanked 203 Times in 184 Posts
In general, and my rule of thumb - a Ford=ported vacuum. A GM=manifold vacuum. But of course there are exceptions just to make life interesting

Dave W
__________________
Irelands child
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 09-04-2008, 10:34 AM
Regal Beagle's Avatar
Registered User
 
Join Date: Apr 2008
Location: Calgary, Alberta
Posts: 199
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by Rustydawg
Here's a quote from a posting on this forum:

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively.

Read the full post "Timing and Vacuum Advance 101" here:

Distributor Tuning and Theory - Part 1

I was pondering this same question myself the other day and was searching the subject...
That is a great article ad really seems to make sense. I guess the proof will be in the driving. I'm headin' out for a cruise and will report back with my findings.
Thanks for the help fellas.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 09-04-2008, 10:46 AM
DoubleVision's Avatar
Not Considered a Senior Member
 
Last photo:
Join Date: Oct 2002
Location: Heart Of Dixie
Age: 40
Posts: 10,655
Wiki Edits: 0

Thanks: 15
Thanked 59 Times in 56 Posts
To give a slight explanation of it, I have delt with some GM vehicles that ran better with it on a ported source, but most ran best with manifold. Idle and off idle cruise mixtures are lean, and lean mixtures burn slower than rich mixtures, this is where the extra advance comes into play. When I was running my cutlass, I had it on a ported vacuum running a mild 350 chevy. The thing was a slug on low end, I kept wondering why it had no bottom end power, it had a RV cam and 9.5:1 compression, headers, a dual plane intake and a small 625 demon jr carb. it had really poor throttle response and drank gas. Then one day on here I complained about it and was told to set the base timing with the vac advance disconnected and plugged to 12 degrees, then reconnect the vac advance to a manifold source, I did so and it felt like it picked up 50 horses, it was really sensitive throttle response wise and a pleasure to drive. Fuel economy went up as well. Since then I`ve stuck to manifold vacuum unless it runs better on a ported.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 09-04-2008, 11:04 AM
Jmark's Avatar
Registered User
 
Last photo:
Join Date: Sep 2003
Location: phoenix
Age: 61
Posts: 4,808
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
This is a subject that always draws out both sides.

Personally, I always ran ported on my truck. When I really got into researching it, I decided to do some real world testing. I "T'd" in a vacuum gauge and went for a long drive. Up some hills, down some hills, mild acceleration and hard acceleration. I took good notes on speed and my best guess on pedal position and vacuum readings on hills and cruising, following the same route one time on ported and one time on manifold vacuum.

My bottom line was that at cruise, I had almost identical vacuum readings, and identical timing added, whether on ported or manifold. During idle, of course I had max advance on manifold and no added advance on ported. At WOT, there was no vacuum signal so neither added anything.

When going from an easy cruise to a harder acceleration, on manifold vacuum, the vac. signal falls off very quickly, which is good since the mix goes rich quickly once the metering rods OR power valve are now open and giving a rich mixture.

From all I could tell, the biggest advantage is the added timing at idle, which, depending on cam selection, can be a great bonus in idle quality and a boost in idle vacuum. For me, the biggest drawback is when emissions testing comes around. Idling with 18 degrees of combined timing, my HC% levels are through the roof. If I just pull over before going in and yank the hose, my HC% is always within spec.

So for now, i'm running manifold vacuum for a little better idle quality since my cam is a tad tall for my converter stall.

and now back to work!

Mark
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 09-04-2008, 12:23 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,764
Wiki Edits: 0

Thanks: 4
Thanked 426 Times in 365 Posts
Quote:
Originally Posted by Regal Beagle
I was adjusting my idle mixture today and speaking to a mechanic friend for some assistance to be sure I got it right. My distributer was hooked up to the ported vacuum port but my friend says it needs to be hooked up to the manifold vacuum side of my Edelbrock Performer. I switched it over and adjusted the idle and got the car running and performing well much the same as when it was hooked up to the ported side.
My question is which side do I need to have my distributer hooked up to for maximum performance?
All ported vacuum does is withhold exposure to manifold vacuum from the vacuum advance at idle. It exposes the cannister to vacuum as the throttle blades move into the fuel transition slot. From that point on the vacuum advance functions the same whether ported or direct. Low compression or highly cammed engines can benefit from more idle advance, one way to get this without running the base timing ahead to where the engine is a difficult load on the starter is to connect the advance directly to manifold vacuum.

Vacuum advance is not a part of "maximum performance" if by that you mean most horsepower at WOT. Vacuum advance is there to increase performance and efficiency at part throttle cruise where the mixture density in the cylinder is low. This causes the burn to proceed to slowly and the exhaust valves open before max temp and pressure is achieved. This throws unburnt fuel out the exhaust. The vacuum advance is intended to provide a longer burn time so combustion can be completed before the e-valve opens.

Bogie
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 09-04-2008, 07:12 PM
Regal Beagle's Avatar
Registered User
 
Join Date: Apr 2008
Location: Calgary, Alberta
Posts: 199
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by oldbogie
All ported vacuum does is withhold exposure to manifold vacuum from the vacuum advance at idle. It exposes the cannister to vacuum as the throttle blades move into the fuel transition slot. From that point on the vacuum advance functions the same whether ported or direct. Low compression or highly cammed engines can benefit from more idle advance, one way to get this without running the base timing ahead to where the engine is a difficult load on the starter is to connect the advance directly to manifold vacuum.

Vacuum advance is not a part of "maximum performance" if by that you mean most horsepower at WOT. Vacuum advance is there to increase performance and efficiency at part throttle cruise where the mixture density in the cylinder is low. This causes the burn to proceed to slowly and the exhaust valves open before max temp and pressure is achieved. This throws unburnt fuel out the exhaust. The vacuum advance is intended to provide a longer burn time so combustion can be completed before the e-valve opens.

Bogie
Thanks for the reply Bogie. What do you consider a highly cammed engine?
And do you hook your advance to ported or manifold vacuum?
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 09-04-2008, 07:49 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,764
Wiki Edits: 0

Thanks: 4
Thanked 426 Times in 365 Posts
Quote:
Originally Posted by Regal Beagle
Thanks for the reply Bogie. What do you consider a highly cammed engine?
And do you hook your advance to ported or manifold vacuum?
When a cam is getting around 220 - 230 degrees of duration, you're getting to the practical limit of having vacuum advance. However, a cam with less LSA is more likely not to make effective use of vacuum advance than a similar duration cam with a 112-116 degree LSA. This, also, depends a lot on vehicle weight and overall gearing as well as type transmission and if automatic converter stall.

Stiffer gearing has less need for vacuum advance as the engine is revving higher which makes it easier to have the centrifugal do the work. Higher gears and a heavier vehicle can use vacuum advance if street driven as the engine isn't spinning so fast. But you will need to customize the vacuum advance.

You can push vacuum advance further into a long duration cam, but you need to have an adjustable "can" so you can make it sensitive to the realistic manifold vacuum numbers. There's no sense in having a vacuum advance that functions with 18 inches of vacuum on idle when you have an engine that only makes 10.

A loose converter lets the engine rev up which once again allows the use of the centrifugal system at low speeds that also allow high engine RPMs.

However, regardless of cam, if you spend a lot of time cruising low and slow and don't want to constantly run in the lower gears to keep the revs up, a vacuum advance can be effective to squeeze the best performance possible under these conditions. But again, as the cam gets bigger, the vacuum will be less, which means the systems response needs to be customized to reality.

Whether or not to use manifold direct or ported vacuum is really dependent on set up. A lot of factors become involved these include the effectiveness of squish/quench, better needs less advance; location of the spark plug, centered in the chamber needs less; mixture turbulence, more needs less, mixture ratio, to a point more needs less, the weakest mixture in a cylinder needs more, this is usually the lower plane of a two plane manifold, single plane manifolds require specific solutions; headers will want more advance as they tend to over-scavenge the cylinder, however, with anti-reversion cones probably less is best. All these kind of things come into play, which makes exact specifics a learning situation.

Full up race engines tend to dispense with advance mechanisms starting with vacuum not because they have some trick unknown to Detroit' engineers, rather it's because as manifold vacuum goes down a vacuum system becomes less and less functional and it brings durability problems and failure modes that out weight it's usefulness. In far out motors, the mechanical advance can be eliminated again for durability and reliability issues that out weigh the usefulness of an engine that runs so fast so much of the time it just needs full advance. These type engines usually separate ignition and fuel flow from cranking, such that at start-up the starter is just spinning the motor without fuel or ignition. This eliminates the chance of backfire when just starting to crank it. Then the fuel is turned on to prime the motor then spark is added to fire it. With modern electronics the choice is there to have a starting retard or running full advance, or to even have electronic advance which has proven to be much less failure prone than mechanical mechanisms on the race track.

Bogie
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 09-04-2008, 07:59 PM
Regal Beagle's Avatar
Registered User
 
Join Date: Apr 2008
Location: Calgary, Alberta
Posts: 199
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks everyone for the replies. Its amazing how much I have learned over the last 2 days.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 09-05-2008, 10:21 AM
Member
 
Last photo:
Join Date: Nov 2004
Location: Lakeland FL
Age: 65
Posts: 4,110
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
Regal,
I agree with Old Bogie, it really depends on all of the car/motor/drive train specifics to determine which of the two ports many possible different benefits are most needed...

here's a Pontiac 455 side by side (different build's) article link on manifold versus ported to add some insight....

http://www.highperformancepontiac.co...ech/index.html
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 09-05-2008, 12:30 PM
Registered User
 
Join Date: Jun 2008
Location: Gardner, KS
Posts: 18
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
IMHO ported for the street. none for every thing else.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
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 for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Port or Full time vacuum cool rockin daddy Engine 129 10-05-2010 03:00 PM
Vacuum adv.. underdog305 Engine 58 01-30-2007 11:19 AM
305 Sbc kringold Engine 57 10-15-2006 09:50 PM
Ported Vacuum Reading at Idle? JohnTN Engine 29 11-09-2005 04:41 PM
vacuum advance help batman09 Engine 14 09-09-2004 09:42 PM


All times are GMT -6. The time now is 02:01 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.