Results 1 to 5 of 5

Thread: Ported/Timed vacuum & PCV port conversion?

  1. #1
    Join Date
    Dec 2013
    Location
    Lake Orion, MI
    Posts
    13

    Default Ported/Timed vacuum & PCV port conversion?

    We currently have a 363-inch Boss 302 project engine in the early build phase and there is a lot of desire to run the 750 Ultra XP carb in Hard Core Grey (List # 0-80803HBX) for its tunability, low profile, and several “built-in” features. However, before going much further, I’d like to float a few questions to the larger community for some constructive feedback:

    First, unlike the familiar 750 Ultra (List # 0-76751HB), the Ultra XP has no provision for a timed/ported vacuum (spark) port. Is it possible to make this conversion by modifying the existing front metering block or is there an upgrade metering block kit available to recover this feature?

    Second, (and again in contrast to the 750 Ultra), we would like a PCV provision available on the base plate. The Ultra XP base plate design has a boss “that can be consumer machined for vacuum source” but there is no information on how this is to be properly accomplished and the available real estate appears to be very minimal to the casual observer. Has anyone ever accomplished this conversion and /or found any documentation that walks this process through?

    Any help would be greatly appreciated!
    Sven Pruett
    Lake Orion, MI

    1970 Boss 302 -
    http://1970boss302.blogspot.com/
    1956 Thunderbird
    1988 Mustang Hatchback - NHRA Super Pro

  2. #2

    Default

    First, unlike the familiar 750 Ultra (List # 0-76751HB), the Ultra XP has no provision for a timed/ported vacuum (spark) port. Is it possible to make this conversion by modifying the existing front metering block or is there an upgrade metering block kit available to recover this feature
    Timed/Ported vacuum comes from the base plate, not the metering blocks. You can get full manifold vacuum from you adding a fitting into the plenum of your intake. Requires drilling & tapping your intake.

    Second, (and again in contrast to the 750 Ultra), we would like a PCV provision available on the base plate. The Ultra XP base plate design has a boss “that can be consumer machined for vacuum source” but there is no information on how this is to be properly accomplished and the available real estate appears to be very minimal to the casual observer.
    It's pretty straight forward. Take the base plate off of your carburetor, and drill the appropriate sized hole in the boss for the nipple fitting you intend to use. Just depending if it's going to be a press-in fitting or a screw in fitting. I guess it's all about how comfortable you are with taking your base plate off, getting in setup in a vice of some sort, centering your drill bit, and then proceeding to drill a hole in that brand new carburetor base plate.

    I too, wanted to use the same carb as you for my car, but was advised this is a purpose built race carburetor, and is intended for such. My thought was to see if the base plate from an HP carb would work on the Ultra. That would require a call to Holley Tech for that. My guess is that they'll say no.
    Last edited by 69cam; 09-08-2015 at 09:17 AM.

  3. #3
    Join Date
    Dec 2013
    Location
    Lake Orion, MI
    Posts
    13

    Default

    Quote Originally Posted by 69cam View Post
    Timed/Ported vacuum comes from the base plate, not the metering blocks. You can get full manifold vacuum from you adding a fitting into the plenum of your intake. Requires drilling & tapping your intake.

    It's pretty straight forward. Take the base plate off of your carburetor, and drill the appropriate sized hole in the boss for the nipple fitting you intend to use. Just depending if it's going to be a press-in fitting or a screw in fitting. I guess it's all about how comfortable you are with taking your base plate off, getting in setup in a vice of some sort, centering your drill bit, and then proceeding to drill a hole in that brand new carburetor base plate.

    I too, wanted to use the same carb as you for my car, but was advised this is a purpose built race carburetor, and is intended for such. My thought was to see if the base plate from an HP carb would work on the Ultra. That would require a call to Holley Tech for that. My guess is that they'll say no.
    Actually, ported vacuum comes from _above_ the throttle plates, and is channeled through the front metering block on Holley Ultras. Chevy's often use full manifold vacuum for advance, and that's an easy connection to the base plate. That's where the problem comes in.

    I have no real problem doing the machining, but the base plate is not machined for the vacuum port in the throttle bore crossover area. That's what has me scratching my head a bit. That, and I can't find a hint of info from Holley on doing this.
    Sven Pruett
    Lake Orion, MI

    1970 Boss 302 -
    http://1970boss302.blogspot.com/
    1956 Thunderbird
    1988 Mustang Hatchback - NHRA Super Pro

  4. #4

    Default

    Actually, ported vacuum comes from _above_ the throttle plates, and is channeled through the front metering block on Holley Ultras. Chevy's often use full manifold vacuum for advance, and that's an easy connection to the base plate. That's where the problem comes in.
    I understand where ported timed vacuum comes from, and that vacuum is only sourced from above the throttle plates. I understand on different models of Holley carbs, this is accomplished several different ways. One coming from the metering block as you stated.

    I guess I should have been more specific, in regards to the base plate I was referring to. If you look at the Street HPs, you can see that the ported/timed vacuum is routed from the base plate. That was my thought in thinking the base plate from the street HPs, would be as close as possible to fit on the ULTRA HPs. Just not sure if the throttle body has the provisioning needed.

    I looked high & low as well, might be worth a call to Holley, or one of the race carb facilities local to you.
    Click image for larger version. 

Name:	Holley HP.jpg 
Views:	1143 
Size:	54.2 KB 
ID:	1379

    Another approach would be to get the Street HP or Classic HP, and get the billet metering blocks for the tunability aspect. I know there are some other nice comfort features the Ultra has that you give up, but that might be as close as you can get, if you want the vacuum ports.
    I live in the Houston, Texas area, and we have a couple of custom carb shops. You can see what Davinci carbs do to one of there custom built carbs here:
    http://davincicarb.com/750.html

    Or there is also Patrick with Pro systems as well:
    http://www.prosystemsracing.com/proseries4150.html
    Last edited by 69cam; 09-09-2015 at 02:11 AM.

  5. #5
    Join Date
    Dec 2013
    Location
    Lake Orion, MI
    Posts
    13

    Default

    Ah! Have not ever seen a Street HP, so I learned something new with your pic! Thanks! As much as I hate to, based on past experience, I guess I will have to break down and call Holley's tech line. Ironically, the last two times I called looking for info on the Terminator EFI setup, they sent me here!

    Really appreciate the feedback. Wish me luck!
    Sven Pruett
    Lake Orion, MI

    1970 Boss 302 -
    http://1970boss302.blogspot.com/
    1956 Thunderbird
    1988 Mustang Hatchback - NHRA Super Pro

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
About us
Holley has been the undisputed leader in fuel systems for over 100 years. Holley carburetors have powered every NASCAR® Sprint® Cup team and nearly every NHRA® Pro–Stock champion for four decades. Now, Holley EFI is dominating the performance world as well as our products for GM's LS engine. Holley's products also include performance fuel pumps, intake manifolds & engine dress–up products for street performance, race and marine applications. As a single solution, or partnered with products from other Holley companies - Hooker Headers, Flowtech Headers, NOS Nitrous, Weiand, Earl's Performance Plumbing, or Diablosport - Holley products can give you the edge you need over the competition.
Join us