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I have a similar 306 build with just a smidge more cam. I ran an edelbrock 600 and a Summit 600 and I love the Summit. Its got the annular fuel boosters and was easy to get the idle right.
 

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You're missing a great learning experience if you don't try the 570 for a while. If it doesn't take off like you want then up size the squirter, if it gets a lean pop on the upper end upsize the jets. After that if it starts acting like a rev limiter [email protected] RPM in 3rd gear then you know its too small. Take notes then decide later if that loss in off the line, part throttle pep was worth it all0:):yoho:
 

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After that if it starts acting like a rev limiter [email protected] RPM in 3rd gear then you know its too small. Take notes then decide later if that loss in off the line, part throttle pep was worth it all0:):yoho:
I have a 570 on two different 302’s....one is a bone-stock 5.0 pull out, and the other is a fairly stout, 9:1 engine with Trick Flow heads.

On both engines, I would get that lean “surge” as the secondaries came in. I had to use the black spring (stiffest one) to make it go away.

According to Holley, using then stiffest spring should barely allow the secondaries to open on a 302, but I put a zip-tie on the secondary linkage, and found it pushed all the way down after a hard run. This indicates the secondaries are opening all the way....strange.

Overall, they are a great carb for a street driven 302.
 

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The difference between 570, 600 and 650 cfm is mostly tomato/tomato, especially since they all flow different numbers (than stated) on a test bench. Tuning, booster type and style will all play a much larger role in how the carb feels and the power it makes. In theory, a QuadraJet Carb (small primaries and bigger secondaries) makes the most sense in almost all applications short of competition, but they have their drawbacks/compromises as well.

Figure out what's important to you, be honest with yourself and driving style, and pick accordingly.
 

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I have a 570 on two different 302’s....one is a bone-stock 5.0 pull out, and the other is a fairly stout, 9:1 engine with Trick Flow heads.

On both engines, I would get that lean “surge” as the secondaries came in. I had to use the black spring (stiffest one) to make it go away.

According to Holley, using then stiffest spring should barely allow the secondaries to open on a 302, but I put a zip-tie on the secondary linkage, and found it pushed all the way down after a hard run. This indicates the secondaries are opening all the way....strange.

Overall, they are a great carb for a street driven 302.
If you had to restrict the secondary air (tighter secondary spring) to cure a lean surge, I'd make the opposite case: it wasn't getting enough secondary fuel flow for engine demand.
 

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autolite 4100? or sticking with holley?
 

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If you had to restrict the secondary air (tighter secondary spring) to cure a lean surge, I'd make the opposite case: it wasn't getting enough secondary fuel flow for engine demand.
I would disagree. You can only get secondary fuel when you’ve got enough air speed.

The lean surge occurs when the secondaries open too soon, and you don’t yet have enough air speed to pull fuel through the boosters.

Increasing secondary jet size would allow it to pull MORE fuel, but I don’t believe it would allow it to pull fuel SOONER.

Also, my AF meter was showing my mixture being right once I got past the lean surge. So, to maintain that, I had to delay the secondary air flow.
 

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Discussion Starter #30
Good info sharing going on. I didn't mention that my compression ratio is 10:1, used Keith Black Flat top pistons with the smaller chamber volume 165 heads. 600 CFM is looking to be a good size between the 570 and 650, I did notice one build using a 625 cfm when I did a search, I was just looking at my Holley 570 4 barrel which is basically new. I guess I could sell it, if I end up not using it.
 

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Good info sharing going on. I didn't mention that my compression ratio is 10:1, used Keith Black Flat top pistons with the smaller chamber volume 165 heads. 600 CFM is looking to be a good size between the 570 and 650, I did notice one build using a 625 cfm when I did a search, I was just looking at my Holley 570 4 barrel which is basically new. I guess I could sell it, if I end up not using it.
On the 9:1 306 with Trick Flow heads I mentioned above, I started out with a 650 double-pumper, on the suggestion of my engine builder. As others have suggested, he’s a drag race guy.....enough said.

I was never happy with the way it behaved in the real world, even in a VERY light car (2,000 lbs) and a manual trans.

I’ve since switched to a 570 Street Avenger, vacuum secondary, and while I can’t say it’s the PERFECT carb for my application, I’m much happier than I was with the double pump.

As someone else said, be honest with yourself about what you’re going to be doing with this car.
 

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On the 9:1 306 with Trick Flow heads I mentioned above, I started out with a 650 double-pumper, on the suggestion of my engine builder. As others have suggested, he’s a drag race guy.....enough said.

I was never happy with the way it behaved in the real world, even in a VERY light car (2,000 lbs) and a manual trans.

I’ve since switched to a 570 Street Avenger, vacuum secondary, and while I can’t say it’s the PERFECT carb for my application, I’m much happier than I was with the double pump.

As someone else said, be honest with yourself about what you’re going to be doing with this car.
But, did you have that lean surge with the double pumper? :0p

Of course, that's not the end all, be all - and there is a lot more to all this than how a lean spot was resolved. I am glad you solved the problem and are happy with the results!
 

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But, did you have that lean surge with the double pumper? :0p

Of course, that's not the end all, be all - and there is a lot more to all this than how a lean spot was resolved. I am glad you solved the problem and are happy with the results!
So, let’s hear it, what would you suggest for curing a lean surge?
 

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So, let’s hear it, what would you suggest for curing a lean surge?

Without being there, knowing your needs and driving style, etc - I would have started by adding fuel. The previous double pumper's secondary accelerator pump was probably enough, and while mentioned you had other issues/things you didn't like it, I'm guessing a lean surge with that carb wasn't one of them.

Your way obviously worked too - ultimately you either take away air (stiffer secondary spring) or add fuel (accelerator pump on the secondaries). :)
 

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Add an accelerator pump on a vacuum secondary carb?

That would be tricky ;-) If jetting up the secondaries, adding a power valve, or changing that plate didn't work, I'd go back to a double pumper. Maybe not the one you had, of course. There are options.

But if I had a warmed up small block with good flowing heads and everything else that took advantage of the improved flow (cam, intake, headers, etc), I'd be looking at a double pumper anyway. Call me a drag racer too :grin2: but your engine guy wasn't wrong.
 

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Double pumpers are awesome on the strip. But vac secondaries work sort of like a 'variable venturi' carburetor. At low RPMs, when the engine is not pulling much vacuum at WOT, you get excellent atomization through the primaries. Then, as needed, you get more from the secondaries - again, with proper atomization.

If you're running a double-pumper with steep gears, and/or running a manual transmission, you get into higher RPMs so fast, there's not too much of a penalty for the poor mix quality; you build RPMs and then the carb can atomize well even with all 4 venturii roaring along.

On an automatic running highway gears with a stockish torque converter, you are often put in a situation where the initial spurt of gas from the accelerator pumps gets burnt, all 4 barrels are open, but the engine is still at low RPMs, and not drawing much air. At this point, most double pumpers will be raining big fat drops of gas down the intake, and even if you don't have a bog, that's not great for performance or for economy.

For an automatic-equipped car, destined for use on the street, I much prefer a carb that's going to deliver snappy performance no matter what my RPMs are at. I've seen some really nasty strip terrors that could not break the tires loose in first gear, from a rolling start (without dipping the clutch). I've used a lot of carburetors, chasing performance, economy, or both - but always with street use in mind. My favorite by far is the old Autolite 4100 and its newer descendants. I used to really make my buddies mad. Lining up against their 350 Rocket '72 Cutlass Supreme, 73 455 Buick Riviera, 71 400 GTO convertible, and '70 Super Bee 389, my little 302 would always stomp 'em, and they never could believe how eager my car was to 'go' from any speed. Though to be fair, Dave always really babied the clutch and granny-shifted his Bee. ;)

A huge part of that was my old Autolite, because I sure didn't have the money for a torque converter or hot cam at the time.
 

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Discussion Starter #38

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Discussion Starter #39
On the 9:1 306 with Trick Flow heads I mentioned above, I started out with a 650 double-pumper, on the suggestion of my engine builder. As others have suggested, he’s a drag race guy.....enough said.

I was never happy with the way it behaved in the real world, even in a VERY light car (2,000 lbs) and a manual trans.

I’ve since switched to a 570 Street Avenger, vacuum secondary, and while I can’t say it’s the PERFECT carb for my application, I’m much happier than I was with the double pump.

As someone else said, be honest with yourself about what you’re going to be doing with this car.
I have been experimenting with different carb calculators and as you stated the 570cfm is what seems to be a good place to be since I'm not planning on spinning over 6000 rpms. The reason I went with the AFR 165's is because of how well they flow under .500. My cam Lunati 218/218 rpm range is 1800-5800, with the 1.7 rockers it gives me a little more lift so I'm thinking that the redline of the cam has shifted to at least 6000 rpms. With 10:1 compression ratio I'm thinking that I have plenty of power through the mid-range and top end. This is a C4 setup with a GV overdrive and 3.55:1 traction lock differential gears. 2400 stall converter

It will be used on the street and for spirited driving, but will never see a 1/4 mile strip or road course. Been there done that with my other vehicles and bikes. I just wanted a hopped up stang with 350-400hp.
 

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I appreciate the thoughts on keeping my existing Holley 570, I went to the site and used the carb calculator and the range is from 570-650. What about this carb? At least one hotrod build used it and it seemed to put out some come good numbers.

https://www.holley.com/products/fuel_systems/carburetors/demon/road_demon/parts/RDA-650-VS
Unless you are on a tight budget Get the quick fuel. Way nicer carb for a little more money. VS and electric choke Like this one. https://www.holley.com/products/fuel_systems/carburetors/quick_fuel/super_street_series/parts/SS-680-VS
 
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