Which Size Holley DP ? - Chevelle Tech
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post #1 of 20 (permalink) Old Sep 8th, 19, 2:57 PM Thread Starter
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Which Size Holley DP ?

Getting ready to purchase a carb for my GM crate engine #366250 454 based on the LS6 . Its rated at 10.2 compression with nice open chamber heads , dual plane Intake , forged pistons and solid lifter cam . Im gonna run a 4 speed trans with 3.71 ratio axle and headers . I want to run a Holley double pumper on It . I have a 4779-2 750 I bought years ago and was thinking about running It because the guy at the local speed shop recommends a 750 . When I do the Holley cfm calculator It comes up with the 850 and my gut feeling Is the 850 would be a better match . I know these came with a 780 vacuum secondary carb originally In 1970 . What do you guys think ? Would an 850 be too much carb ? It (850) comes with 80 primary and secondary jets , and 31 primary and secondary nozzles , a little too fat on the primary side out of the box ? I don't mind spending the money , I want the best match Holley DP . Thanks for youre help guys .
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post #2 of 20 (permalink) Old Sep 8th, 19, 3:20 PM
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Re: Which Size Holley DP ?

Iíd initially run what you have. Rebuild it and return all of the specifications to stock so you have a clear baseline for any needed changes.

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post #3 of 20 (permalink) Old Sep 8th, 19, 5:16 PM
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Re: Which Size Holley DP ?

4779 is perfect. A 4780 or 4781 would work too but might not be as crisp at part throttle.
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post #4 of 20 (permalink) Old Sep 8th, 19, 5:42 PM
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Re: Which Size Holley DP ?

I have run any DP from 650 to 1000 on my 454's and most of them have all been with in a couple of tenths and a few mph of each other. If this is a street engine and not going to the moon on RPM then stay with a 750 DP you will be happier. The larger carburetors tend to be a little fussy sometimes. I ran a QF for the first time at the track yesterday and it was a 750DP and despite having to tinker with it for a short while it did run pretty good, no flat spots and pulled until my valve spring issue reared it's ugly head.
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post #5 of 20 (permalink) Old Sep 9th, 19, 12:10 AM
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Re: Which Size Holley DP ?

My 454 runs the best with a 4779 750DP on the street and the track, crisper on bottom end. See my signature.
August 9th my car ran 12.14, 12.14, 12.10, 12.18, 12.13 with a well tuned 850 Mighty Demon.
My best outing was with my 750DP, 11.97 and 11.96 at 110.
Bigger is not always better.
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post #6 of 20 (permalink) Old Sep 9th, 19, 2:18 AM Thread Starter
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Re: Which Size Holley DP ?

Sounds like the 750 would be fine then . It comes with 70/80 jets , 28/31 nozzles , 6.5 power valve and pink cams . Any changes to fit my application ?
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post #7 of 20 (permalink) Old Sep 9th, 19, 7:08 AM
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Re: Which Size Holley DP ?

Quote:
Originally Posted by byf41 View Post
Sounds like the 750 would be fine then . It comes with 70/80 jets , 28/31 nozzles , 6.5 power valve and pink cams . Any changes to fit my application ?
Those heads are huge for a 454. I'm sure somebody here can advise how to tune with rectangle port heads ?

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post #8 of 20 (permalink) Old Sep 9th, 19, 7:32 AM
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Re: Which Size Holley DP ?

Quote:
Originally Posted by byf41 View Post
Getting ready to purchase a carb for my GM crate engine #366250 454 based on the LS6 . Its rated at 10.2 compression with nice open chamber heads , dual plane Intake , forged pistons and solid lifter cam . Im gonna run a 4 speed trans with 3.71 ratio axle and headers . I want to run a Holley double pumper on It . I have a 4779-2 750 I bought years ago and was thinking about running It because the guy at the local speed shop recommends a 750 . When I do the Holley cfm calculator It comes up with the 850 and my gut feeling Is the 850 would be a better match . I know these came with a 780 vacuum secondary carb originally In 1970 . What do you guys think ? Would an 850 be too much carb ? It (850) comes with 80 primary and secondary jets , and 31 primary and secondary nozzles , a little too fat on the primary side out of the box ? I don't mind spending the money , I want the best match Holley DP . Thanks for youre help guys .

Here's theone I'd go with, they do make a 800CFM as well just in case 850 CFM is too large.

https://www.jegs.com/i/Holley/510/0-76850HB/10002/-1
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post #9 of 20 (permalink) Old Sep 9th, 19, 2:34 PM
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Re: Which Size Holley DP ?

Quote:
Originally Posted by byf41 View Post
Sounds like the 750 would be fine then . It comes with 70/80 jets , 28/31 nozzles , 6.5 power valve and pink cams . Any changes to fit my application ?
u may have to go up a couple sizes on the front squirters and the front jets
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post #10 of 20 (permalink) Old Sep 9th, 19, 2:49 PM
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Re: Which Size Holley DP ?

Quote:
Originally Posted by byf41 View Post
Sounds like the 750 would be fine then . It comes with 70/80 jets , 28/31 nozzles , 6.5 power valve and pink cams . Any changes to fit my application ?
u may have to go up a couple sizes on the front squirters and the front jets, and drop down in power valve depends on how much vac u have at idle
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post #11 of 20 (permalink) Old Sep 9th, 19, 7:04 PM
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Re: Which Size Holley DP ?

Idle vacuum has no bearing on power valve. The power valve feeds the main well, not the idle or transition circuits. You can take the power valve completely out and the engine will idle just fine until it runs out of gas. Cruise vacuum is what you should be using to determine power valves.

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post #12 of 20 (permalink) Old Sep 9th, 19, 7:39 PM
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Re: Which Size Holley DP ?

save you money, run what you get. it's fine.
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post #13 of 20 (permalink) Old Sep 9th, 19, 8:09 PM
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Re: Which Size Holley DP ?

I agree with both Tom's above!
Cruise vacuum -2"
I've done this with good results.
If you do the 1/2 cruise method you'd have to go pretty deep into the throttle to get to PV enrichment.
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post #14 of 20 (permalink) Old Sep 9th, 19, 9:39 PM Thread Starter
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Re: Which Size Holley DP ?

This Is a ground up build and I would like to get the carb close to light It off , but don't know what kind of vacuum to expect to be honest . Sounds like most guys feel the 750 Is a good match and I already have a nice clean one as I said . I was thinking of bumping up the primary jet to maybe 75 or 76 , leave the original 80 In the secondary , bumping the primary nozzle to a 30 , leave the original 31 In the secondary . Maybe a 8.5 power valve ? What do you tuners think ? Thanks again for youre help guys .
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post #15 of 20 (permalink) Old Sep 9th, 19, 9:56 PM
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Re: Which Size Holley DP ?

I always start with stock specs. Holley typically wasn't far off with the stuff that is changeable. My 4781 has stock jets, squirters, cams but had to add TSRs and change emulsion holes. I think I probably wouldn't have needed to do that if I were at sea level.
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