Power valves. Some say half your manifold vacuum, some say 2 hg below. Who is right? - 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 11-20-2012, 01:35 AM
Registered User
 
Join Date: Jun 2011
Location: USA
Posts: 44
Wiki Edits: 0

Thanks: 3
Thanked 0 Times in 0 Posts
Power valves. Some say half your manifold vacuum, some say 2 hg below. Who is right?

Half the people I talk too say you cut your idle manifold vacuum in half, and round down to the nearest .5, so with 10hg you want a 4.5pv, etc. The other half of the people I talk to say you want 2 hg below your idle manifold vacuum, so with 10hg you would want a 7.5pv.

Who is correct, and why?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 11-20-2012, 09:25 AM
Registered User
 
Last photo:
Join Date: Jan 2011
Location: Hopewell, Va
Age: 62
Posts: 1,193
Wiki Edits: 0

Thanks: 0
Thanked 117 Times in 113 Posts
Power valves. Some say half your manifold vacuum, some say 2 hg below. Who is right?

I read an article where a vacuum gauge is installed in the car and you use a narrow band O2 moniter. You cruise around in your basic operating rpm checking your gauges. Keep a record of your range of vacuum levels from regular driving. You want your PV number below these levels. The A/F ratio will be at 14.7:1 at cruise to 12.8:1 under power which is when the PV opens.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 11-20-2012, 10:18 AM
Registered User
 
Last photo:
Join Date: Jan 2011
Location: Hopewell, Va
Age: 62
Posts: 1,193
Wiki Edits: 0

Thanks: 0
Thanked 117 Times in 113 Posts
Quote:
Originally Posted by Zerocyde View Post
Half the people I talk too say you cut your idle manifold vacuum in half, and round down to the nearest .5, so with 10hg you want a 4.5pv, etc. The other half of the people I talk to say you want 2 hg below your idle manifold vacuum, so with 10hg you would want a 7.5pv.

Who is correct, and why?
If you use example #2 what if your engine vacuum is 15 hg at idle and you -2 hg the largest PV number you can get is 10.5PV Everytime your engine vacuum drops below 10.5 the power valve opens feeding more fuel. If driving in a hilly countryside and the vacuum stays at 8.5 cruising this would be rough on gas mileage. If using example #1 with the same engine vacuum and 15 hg*2 = 7.5PV. If driving in the same conditions the PV never opens until 7.5 hg so you have a lean condition which could cause severe complications.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 11-20-2012, 10:36 AM
Registered User
 
Last photo:
Join Date: Jun 2009
Location: Camaro land
Posts: 357
Wiki Edits: 0

Thanks: 1
Thanked 1 Time in 1 Post
also remember the larger the pv the eailier the pv will open,so earlier your in rich ment will be example, a 10.5 will open before a 6.5, because as the throttle opens and vacume drop's, and your running 15g's as you can see as the vacume goes down, it will reach the 10.5 before the 6.5...You want to split it in half or sligtly above.. if your running 15 at idle, half is 7.5, or a 8.5 depending on the rest of the setup
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 11-20-2012, 10:37 AM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,764
Wiki Edits: 0

Thanks: 4
Thanked 427 Times in 366 Posts
Quote:
Originally Posted by Zerocyde View Post
Half the people I talk too say you cut your idle manifold vacuum in half, and round down to the nearest .5, so with 10hg you want a 4.5pv, etc. The other half of the people I talk to say you want 2 hg below your idle manifold vacuum, so with 10hg you would want a 7.5pv.

Who is correct, and why?
Both, I hate this; "but it all depends". Sounds like Einstein talking about relativity doesn't it?

It gets down to more than one way to set up the carb. To simplify; if the main metering is rich then you can delay the power circuit to a lower vacuum before it's necessary to turn it on. If you're running on the lean side say for good cruise gas mileage, then you may want the power system to come on earlier to cover power and anti-detonation requirements say when pulling a trailer or heavy load in this case having it come on earlier but not under cruise vacuum would be desirable.

If your idle vacuum is only 10 inches then this speaks to a large cam. Large cams speak to high performance so you want the power enrichment to come on earlier. But not at cruise if this is a street strip machine; so you need to get your cruise vacuum reading as well, which might be higher than idle vacuum. This is an effect of gearing which is a big player in this, therefore, a major reason why hard, fast rules are dangerous to apply.

A full up race engine that never sees the street tends to block off the power system and use really rich main metering instead, but these engine are either idling or at WOT so in-between metering like cruise isn't a consideration while eliminating the power circuit is one less potential failure point.

Certainly if you can do all of this tuning with an exhaust gas analyzer all the better?

Bogie
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 11-20-2012, 03:54 PM
Registered User
 
Join Date: Jun 2011
Location: USA
Posts: 44
Wiki Edits: 0

Thanks: 3
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by oldbogie View Post
Both, I hate this; "but it all depends". Sounds like Einstein talking about relativity doesn't it?

It gets down to more than one way to set up the carb. To simplify; if the main metering is rich then you can delay the power circuit to a lower vacuum before it's necessary to turn it on. If you're running on the lean side say for good cruise gas mileage, then you may want the power system to come on earlier to cover power and anti-detonation requirements say when pulling a trailer or heavy load in this case having it come on earlier but not under cruise vacuum would be desirable.

If your idle vacuum is only 10 inches then this speaks to a large cam. Large cams speak to high performance so you want the power enrichment to come on earlier. But not at cruise if this is a street strip machine; so you need to get your cruise vacuum reading as well, which might be higher than idle vacuum. This is an effect of gearing which is a big player in this, therefore, a major reason why hard, fast rules are dangerous to apply.

A full up race engine that never sees the street tends to block off the power system and use really rich main metering instead, but these engine are either idling or at WOT so in-between metering like cruise isn't a consideration while eliminating the power circuit is one less potential failure point.

Certainly if you can do all of this tuning with an exhaust gas analyzer all the better?

Bogie
Think I'm gonna get my buddy to hold the vacuum gauge while we take a ride today. Get some numbers to work with.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 11-20-2012, 04:41 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 8,995
Wiki Edits: 0

Thanks: 17
Thanked 315 Times in 295 Posts
idle vacuum doesn't have a damn thing to do with PV selection. Drive your car on the highway at your normal cruising speed on a level road, what is your vacuum at that moment? Now subtract 3 or 4 and you'll have a good number to start with. Going smaller will slightly help mpg's going higher will slightly help power.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 11-20-2012, 05:11 PM
gearheadslife's Avatar
MentalMuffinMan
 
Last photo:
Join Date: Nov 2012
Posts: 2,215
Wiki Edits: 0

Thanks: 108
Thanked 284 Times in 266 Posts
Quote:
Originally Posted by ap72 View Post
idle vacuum doesn't have a damn thing to do with PV selection. Drive your car on the highway at your normal cruising speed on a level road, what is your vacuum at that moment? Now subtract 3 or 4 and you'll have a good number to start with. Going smaller will slightly help mpg's going higher will slightly help power.

lol'n guess you've never had a nasty engine that idles real ruff
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 11-20-2012, 05:17 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 8,995
Wiki Edits: 0

Thanks: 17
Thanked 315 Times in 295 Posts
Quote:
Originally Posted by gearheadslife View Post
lol'n guess you've never had a nasty engine that idles real ruff
guess you never heard of carburetor tuning or an "idle circuit".
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
89 tbi 5.7 running poorly no power at low rpm or half throttle monsterchevy355 Modern Electronic Engines 3 03-10-2012 07:25 PM
glass fuel filter half full of half empty. tomb Hotrodding Basics 4 09-22-2011 11:46 AM
Ported Vacuum or Manifold Vacuum? Regal Beagle Engine 13 09-05-2008 12:30 PM
ported vacuum vs. manifold vacuum ko.17 Engine 4 06-13-2006 01:53 PM
delima = vacuum, valves, and timing dopsdad Engine 3 10-11-2003 01:41 PM


All times are GMT -6. The time now is 01:59 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.