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  1. #1
    Join Date
    Jun 2015
    Location
    San Diego
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    158

    Holley Ultra XP 850 vs 950?

    I have a 67 Camaro with a LSX 454 which has ported heads, LPE GT21 cam (700 lift), 11:1 CR, and a tall intake manifold. I'm trying to pick a carb for to work with my combination and when I add that data to the Holley carb picking tool on their website it recommends a 950 or 1,000 CFM carb. When I use the equation that most hot rodders go by which for me is 454 x 6500 x 1.05 (efficiency correction factor with 243/259 duration and race ported heads)/3456 = 896. This will be a street car with some events and street racing. The 2 tuners I spoke with said the 850 is going to be the best even though the 950 will make a couple more HP up top. Any advice or shared experiences would be greatly appreciated.



  2. #2
    Join Date
    Oct 2016
    Posts
    176
    Sorry no advice but I have a 950 not the XP but a custom built carb that I bought and is way too much carb for me. I'll sell it cheaper than an XP if your interested. I could send u the build sheet if interested. Lmk if interested.

  3. #3
    Join Date
    Jun 2015
    Location
    San Diego
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    158
    does anyone have any experiences with the new XP carbs in 850 or 950 CFM on a modded LS motor?

  4. #4
    Join Date
    Apr 2017
    Location
    TX
    Posts
    139
    I can give you some real world advice that I have learned over the years. A smaller carburetor will give you better throttle response and more torque at a lower RPM. With the smaller carb your car will be more enjoyable to drive and feel faster. With the larger carb it will probably be a couple of hundredths quicker in the 1/4 mile but its not that much. My advise is to go small if it is mostly street driven. I have a SBC 406 with a 750 hp series and it runs awesome even though it could make more power with a 850.

    Dan

  5. #5
    Join Date
    Jun 2015
    Location
    San Diego
    Posts
    158
    Quote Originally Posted by badolds View Post
    I can give you some real world advice that I have learned over the years. A smaller carburetor will give you better throttle response and more torque at a lower RPM. With the smaller carb your car will be more enjoyable to drive and feel faster. With the larger carb it will probably be a couple of hundredths quicker in the 1/4 mile but its not that much. My advise is to go small if it is mostly street driven. I have a SBC 406 with a 750 hp series and it runs awesome even though it could make more power with a 850.

    Dan
    thank you for the insight, I'm going to go with the small carb.

  6. #6
    Join Date
    Dec 2004
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    HILLBILLY HOLLYWOOD, TENNESSEE!!!
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    2,042
    Good choice as most folks tend to over carb their engines as has been said to me many times by the Holley engineers.
    Mike

    Remember, "Drive Fast, Turn Heads, Break Hearts!"

    www.musclecardeals.com

  7. #7
    Join Date
    May 2013
    Location
    San Diego, CA
    Posts
    238
    Country Flag: United States
    Not sure if this is the right one or not but it's inexpensive:

    https://ls1tech.com/forums/parts-cla...-carb-msd.html

  8. #8
    Join Date
    Jun 2006
    Location
    Katy,TX
    Posts
    1,670
    All Holley HP carbs are rated at a different vacuum than "normal" Holleys. Kind of liek comparing head flow numbers from one bench at 15" and another at 28".Barry Grant started that back in the day. To really compare carbs you need to look a venturi diameter and throttle blade size.
    The HP 1000 has the same size as the old Holly 850s-yes the contoured air horn picks up some but not that much. The "
    old" 950s had 750 venturi size and 850 throttle plates.That was started many years ago my some of the carb gurus liek DaVinci and others. A friend just got another 950 HP that was basically the same venturi/throttle as the 1000HP/850DP size so not sure why it was only rated 950. Had down leg boosters and not the large annulars that can decrease cfm. He actually swapped the main body out for one with annulars and liked it better.

    Also many of the carb tweaker guys do not like the new HP metering plates as they moved the idle restriction feed higher up and seems to create some transition issues for many motors.
    1978 Black Trans Am 455 Edelbrock heads [email protected] through mufflers on pump gas
    1981 Trans Am 400 stock type motor
    79 Camaro getting a 500" 695 hp IA2 Pontiac motor
    1965 GTO project car
    470ci/Chevy dual quad 409 604 HP 64 Impala SS project
    2004 Pulse Red GTO

  9. #9
    Join Date
    Jan 2018
    Location
    CA
    Posts
    4
    I have a 950 HP that I run on my 427w that runs great. I did have to make the modifications to the metering blocks with different power valve restrictor size, idle feed restrictor size, and change the throttle plates (w/o holes).

    I also tuned an 830 HP for the engine, but the 950 HP actually drove better on the street and made more HP and TQ. If you're willing to put in the work to fine tune it, I think the bigger carb is the better way to go. But it did take me drilling and tapping the metering blocks and making my own restrictors with set screws.
    1969 MaCh 1 - CHP 427w 10.8 comp
    3140 lbs. - 460 rwhp / 490 rwtq

  10. #10
    Join Date
    Oct 2016
    Posts
    8
    I have read that the ls motors alway like bigger carbs. Know a few people running 830 on ls3 mild builds. So do not think the 950 is that far off, check the lsx forums.

  11. #11
    Join Date
    Sep 2010
    Location
    Orange County, CA
    Posts
    509
    Quote Originally Posted by badolds View Post
    I can give you some real world advice that I have learned over the years. A smaller carburetor will give you better throttle response and more torque at a lower RPM. With the smaller carb your car will be more enjoyable to drive and feel faster. With the larger carb it will probably be a couple of hundredths quicker in the 1/4 mile but its not that much. My advise is to go small if it is mostly street driven. I have a SBC 406 with a 750 hp series and it runs awesome even though it could make more power with a 850.

    Dan
    This is pretty good advise

  12. #12
    Join Date
    Aug 2011
    Location
    Connecticut
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    926
    Country Flag: United States
    Id say maximum carb would be an 850cfm! I have a high compression 408ci LSX and its only got a worked 850cfm


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