holley 650dp for my 355 - Page 2 - Hot Rod Forum : Hotrodders Bulletin Board
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  #16 (permalink)  
Old 03-21-2013, 08:05 AM
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Ive used DP's on the street and the track many times with no issues....big converter and enough gear and you are there....primary jet needs to be AT LEAST a 70, I would go with 72's in the front and 80 or so in the back. Agree, the pump squirt too big, back it down, and i know that PV is not right. A little tweaking and you should have it.....couldnt agree more on the timing, and I think the min clearance over the air horn is like 3/4", and that is marginal at best. A few inches needed for sure.

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  #17 (permalink)  
Old 03-21-2013, 11:44 PM
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What I liked about the double pumper was that it gave you the same metering every time. It was completely mechanical and no door opened variably for load. That meant part-open throttle points were consistant. This gave the vehicle a solid predictable factor that you could not live without at the track. Some of the roughness you might not want driving around, in those cases a vacuum secondary works better.

My tastes have always favor mechanical secondary carbs. Not double pumpers , but Qjets, AVS, and edelbrocks afb. For that normal muscle car performance, they work fantastic. Not that anything else doesn't.
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  #18 (permalink)  
Old 03-22-2013, 01:54 AM
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I compare a DP to a stick shift- it takes some finesse, you can't just rug it anytime and always expect to accelerate seamlessly. But once you learn how to drive w/a DP, it's not any harder than any other carb IMO.
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  #19 (permalink)  
Old 03-23-2013, 10:48 PM
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650 dp = 283 carb,even with and auto and 3.08 gears it wont bog.a 750 is perfect for a mild 355,even with stock converter,,,I used a 750 on my old 283 in my 65 Acadian,it never bogged.I now use a 950 dp with 3.50 gears,@ 60 mph I can floor the gas in fifth gear and it does not bog,,,,,
having air bleeds is a very good mod if you are seriious about tuning
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  #20 (permalink)  
Old 03-24-2013, 08:54 AM
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Tend to agree with vinnie......hopped up 355 wud go with a 750 for sure.....
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Old 03-24-2013, 10:00 AM
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750cfm carbs are for the guys that want to go fast.

600's and 650's are for guys that are scared to go that fast.

If you want to go fast, the 750cfm carb is always the better choice on a high performance SBC.
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  #22 (permalink)  
Old 03-24-2013, 10:47 AM
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When my 440 had the .509 it made like 10 " of vacuum. I don't think it could have pulled the vacuum secondary open. The 750dp dumped it down the throat..
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  #23 (permalink)  
Old 03-25-2013, 04:19 AM
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Quote:
Originally Posted by spinn View Post
When my 440 had the .509 it made like 10 " of vacuum. I don't think it could have pulled the vacuum secondary open. The 750dp dumped it down the throat..
Vacuum secondary carbs do not work on manifold vacuum.
This is not how they work. The vac secs will open fine when you rug it.

Venturi vacuum is no the same thing as manifold vacuum.
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  #24 (permalink)  
Old 03-25-2013, 02:16 PM
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Agreed, Venturi vacuum is generated with air velocity in a carb.
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Old 03-26-2013, 07:23 AM
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The vacuum secondary's open progressively based on the vacuum signal produced by the engine, regardless of throttle position.

Tuning the secondary pod you would start with a light spring, and go heavier until the stumble from opening to quickly went away. I dont think you could make it open faster, only keep it from opening too fast. Watching the vacuum secondary pull open on the dyno is quick, but not like a mechanical.

Last edited by spinn; 03-26-2013 at 07:37 AM.
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  #26 (permalink)  
Old 03-26-2013, 08:20 AM
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Wrong my friend
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  #27 (permalink)  
Old 03-26-2013, 09:45 AM
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This how it works
Attached Images
File Type: bmp Vacuum secondary.bmp (1.46 MB, 35 views)
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  #28 (permalink)  
Old 03-26-2013, 11:37 AM
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You are wrong. The pistons moving on the thing called a rotating assembly causes negative pressure. That is in every text book.

A venturi has no vacuum on its own.

I don't know if you are talking about an engine. You seem to be continuing to fixate on the venturi.

The piston pulls the air threw the venturi . The amount of pull by the piston is affected by the ivo/ivc.

Please diescet post 25. What are you saying is incorrect.
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  #29 (permalink)  
Old 03-26-2013, 12:10 PM
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The comment on not being able to open , was a descriptor/illustrator for the idle quality. I do not want you to think the engine was actually not able to pull the secondary open to a tunable degree.

The comment on dumping , the fuel is still being induced in a metered fashion. Not actually dumping.

Now, please explain what you think is in error.
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  #30 (permalink)  
Old 03-26-2013, 12:20 PM
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Quote:
Originally Posted by lg1969 View Post
Agreed, Venturi vacuum is generated with air velocity in a carb.
What? That is pretty weak.

A venturi pulls a fluid with air velocity...yes

The air velocity is created by the piston downforce...yes.

The air door is controlled by vacuum.. yes

Last edited by spinn; 03-26-2013 at 12:25 PM.
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