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Better performance with 750 or 850

  • 750 Vacuum

    Votes: 3 60.0%
  • 850 Vacuum

    Votes: 2 40.0%
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1969 Chevelle 2 Door Sport Coupe Malibu COPO clone
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Discussion Starter · #1 · (Edited)
:cool: ZZ454, ZZ502 Roller Cam (upgraded 1.72 Rocker Arms), Hedman full length 1.75 primary tubes, 3 inch collectors into 2.5 headpipes / tailpipes / tips, X Crossover, straight thru mufflers (2.5 in and out). 700R4 (2300 stall), 3.73:1 Posi.

More grunt with Holley 750 or Holley 850 (both vacuum secondaries)?
 

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If you run a 750 VS and it does not have the little tube coming out in the LF pri barrel the secondary's are not going to open well. Just fixed several of mine and test last weekend and the 750 now works great. I know the 850 Holley's do have that tube.
 

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1970 El Camino SS396, VortecPro 467, ATI TH400, Moser 12 bolt /TruTrac 3:42, Hedman headers
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I see the “bog boys” (AKA..the double bumper brigade) are out early pushing their favorite product. If this is a street driven motor a good vacuum secondary carb is the preferred way to go, the factories had it right when these cars were built, vac. secondaries. As a rule the 750 cfm carb has better street manners, idle, mileage, etc. vs the 850, especially if using street gearing with an automatic. If you have the full boogie street race motor and 4.56’s, etc. MAYBE a double doucher. Your $$$$$, yours to live with.
 

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To answer the OP's question between a 750 vacuum secondary and 850 vacuum secondary - as vortec710 says, for the street, I would go with the 750.

I'm running a 770 Ultra Street Avenger vacuum secondary on the street on my 467, and it has more than enough grunt to annihilate the tires just by simply giving the gas pedal a stern look. And it gives great street manners. Any bigger carb would serve no purpose, unless I was at the track all the time, and even then, I'm not sure it would make any difference, as my engine was dyno'd with this exact carb and produced awesome power.
 

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I see the “bog boys” (AKA..the double bumper brigade) are out early pushing their favorite product. If this is a street driven motor a good vacuum secondary carb is the preferred way to go, the factories had it right when these cars were built, vac. secondaries. As a rule the 750 cfm carb has better street manners, idle, mileage, etc. vs the 850, especially if using street gearing with an automatic. If you have the full boogie street race motor and 4.56’s, etc. MAYBE a double doucher. Your $$$$$, yours to live with.
Every DP I had and I had many had great street manors. Only the manual choke of yesterday was the shortfall but now you can buy what you need to convert it or buy it that way. I think vacuum secondary Holley's are awful and I ripped them off every new car I had and went DP and my payment was power, not delayed power and I tried every trick and spring out there. Today I run modified Rochester's and for street power IMO is the way to go.
 

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I see the “bog boys” (AKA..the double bumper brigade) are out early pushing their favorite product. If this is a street driven motor a good vacuum secondary carb is the preferred way to go, the factories had it right when these cars were built, vac. secondaries. As a rule the 750 cfm carb has better street manners, idle, mileage, etc. vs the 850, especially if using street gearing with an automatic. If you have the full boogie street race motor and 4.56’s, etc. MAYBE a double doucher. Your $$$$$, yours to live with.

My "bog" happens at any MPH. BTW, the "bog" is instant tire smoke. At any speed , in any gear. Gosh those double pumpers suck!

Maybe some of us can tune a fish, and some can't! ;)

To Bruce's qustion, sir, I've never heard of anyone getting an 850 vacuum carb to work well, but assume pros can. Go 750, more forgiving. Pure balllz = 850 DP. Guaranteed! . Thats what I said.

Easy to rig and get to work out-of-the box: 3310, and they work OK.
 

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I have run 11.9 thru 12.1 @111-113 MPH with a 780 cfm vacuum Holley.
On DOT tires. No headers. 3.31 gear,LS6 intake.

850 vacuum slowed the car down no matter what I did to it. Tried out of the box calibration, tried blocked rear power valve and jetted up,different secondary spring,check ball,no check ball etc. None of it helped..made the car seem slightly sluggish no matter what. Very crappy transition from primary to secondary.
 

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I have run 11.9 thru 12.1 @111-113 MPH with a 780 cfm vacuum Holley.
On DOT tires. No headers. 3.31 gear,LS6 intake.

850 vacuum slowed the car down no matter what I did to it. Tried out of the box calibration, tried blocked rear power valve and jetted up,different secondary spring,check ball,no check ball etc. None of it helped..made the car seem slightly sluggish no matter what. Very crappy transition from primary to secondary.
What did you run with a 750DP. I was running a 650 DP on my 70 z28 and my buddy was running his stock 780 on his LT1 and I blew him away. He changed to a 750DP and I only took him by two lengths. He borrowed a 700DP and I was a length ahead. We didn't have another 650 DP to test but it was obvious which carb was the strongest for our modified LT1's and they were mechanically twins but he thought BIGGER was better. Nope. . .
 

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I tried a 750 double pumper last year, jetted 74/82 and it slowed down enough to go back to the 780 vacuum which is jetted the same.I don't have my notes in front of me but the 780 rocks on my car.
Works flawless and class rules specify that I use a vacuum Holley.
 

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I ran a highly modified 750 DP on the street and track for years and it was my best performing carb until I bought 2 used Mighty Demons. Both of those carbs blew that 750 away at the track by 3 tenths and I was really impressed by them and still use to this date. The repaired 750HP street VS carb last weekend ran very well also and that would make a great street carb if you do not need a choke. If you run a street car with not much stall speed stay away from the 850 stuff.
 

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1969 Chevelle 2 Door Sport Coupe Malibu COPO clone
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Discussion Starter · #18 ·
If you run a 750 VS and it does not have the little tube coming out in the LF pri barrel the secondary's are not going to open well. Just fixed several of mine and test last weekend and the 750 now works great. I know the 850 Holley's do have that tube.
:cool: Please tell me more about the tube. I assume the tube is mounted in the carb base plate? What does it do? TY.
 

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If you run a 750 VS and it does not have the little tube coming out in the LF pri barrel the secondary's are not going to open well. Just fixed several of mine and test last weekend and the 750 now works great. I know the 850 Holley's do have that tube.
Here's some good info on that tube mod :

 

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