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Discussion Starter #1
I started a discussion similar to this in the drag racing forum regarding whether it's worthwhile doing a metering block conversion on a vacuum secondary 750. I got responses indicating it was wasted effort.

What I'd really like to know is why? If you modify the vacuum secondary carb by swapping out the metering block for a jet block, what's keeping the VS carb from delivering as much fuel/cfm as the DP??

I had previously assumed that not getting enough fuel (which should be fixed by swapping in the jet block & appropriate jets for the fixed size metering block) was the reason for the slow-down. Is this wrong?

Thanks guys! :)
 

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The mechanical card opens all the way by your choice. The VS opens based on the spring rate of the carb. If the spring rate is wrong it will open slower orto fast. I don't know if it would not open all the way. Another thing to check; are you that both open all the way. The rear barrels don't open all the way unless the fronts do on a VS carb. I didn't know that when I had one years ago. I assumed they did and it made a huge difference when I fixed that.
 

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The mechanical secondaries allow for less depression under the carb, resulting in higher VE.
 

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Discussion Starter #4
Now I'm seeing some of the light. :) Good info to have on file. Thanks!

Rather than spend the $200-400 for a new/used/rebuilt DP, I'm thinking of seeing how far I can get throwing a wee bit more change (<$100) into the VS I'm using now, which is why I am asking. If I can get me another 10th or two, I'd call it money well spent.

Mike Goble said:
The mechanical secondaries allow for less depression under the carb, resulting in higher VE.
Got any more detail on that comment Mike? I'm not 100% sure I understand the less depression part of your statement. I just wanna make sure I'm understanding properly what I think I'm hearing.
 

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What is it that you don't understand? If I close the throttles, I have maximum depression under the carb and the car is slow. If I open the throttles, the depression is less and the car is faster. The faster I can get the throttles open the faster the car will be. Which type of carb opens the throttles faster, the mechanical secondary carb or the vacuum secondary carb?
 

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Discussion Starter #6
Mike Goble said:
What is it that you don't understand?
A simple terminology clarification is all I was asking for. You appear to have provided it. Let just not assume everyone is as informed on hotrod lore and terminology as you might be. :rolleyes:
 

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The changing over to a secondary metering block alone will not get you mare power, but the ability to more easily tune the secondary's is the main reason to do this. With the added ability to mare easily and correctly tune the secondary side of your carb, you can now find more power by properly tuning your carb. There is alot of power to be found in properly tuning your secondary spring rate and making sure your throttle plates, both front and rear, are opening completely. Cars with manual trans seem to like DP's better, and automatics like VC's (on street and or street/strip cars, pure rce cars are a whole different beast). If your running a auto trans and your car slowed down when you changed to a VC carb, change to a secondary metering block, and add a quick change vacuum canister to the carb, because you have some tuning to do. You shouldn't have slowed down much if any.
 

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Discussion Starter #8
71SS454 said:
The changing over to a secondary metering block alone will not get you mare power, but the ability to more easily tune the secondary's is the main reason to do this. With the added ability to mare easily and correctly tune the secondary side of your carb, you can now find more power by properly tuning your carb. There is alot of power to be found in properly tuning your secondary spring rate and making sure your throttle plates, both front and rear, are opening completely. Cars with manual trans seem to like DP's better, and automatics like VC's (on street and or street/strip cars, pure rce cars are a whole different beast). If your running a auto trans and your car slowed down when you changed to a VC carb, change to a secondary metering block, and add a quick change vacuum canister to the carb, because you have some tuning to do. You shouldn't have slowed down much if any.
The car started out with my choice of a VC for probably misplaced fuel economy concerns, so I have no DP results for comparison. I'm looking for potential more than anything. I'd much rather blame my lack of tuning than a lack of potential in my hardware choices. :eek:

I've already got a quick change spring cannister installed, I just haven't played with it yet. It's got the recommended (from the Sallee Forum's ZZ4 threads) yellow spring in it. I'll try swapping it out on one of my next TNT nights. I know the floats are misadusted too. Concerning fuel delivery tuning, I'm thinking maybe the primary jets need to come up a size or two since the plugs look a bit light still. I haven't dug into the depths of the plug to see what's hiding under the plug threads to tell where I'm at secondary-wise on fuel delivery.

This discussion is probably a bit premature until I've fixed the problems I know are truly problems. :rolleyes: I've learned quite a bit recently though. I'm better armed to diagnose and fix fuel delivery/carb issues. It is truly appreciated.
 

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NovatoriusRex said:
TI'm thinking maybe the primary jets need to come up a size or two since the plugs look a bit light still. I haven't dug into the depths of the plug to see what's hiding under the plug threads to tell where I'm at secondary-wise on fuel delivery.
You can't see the fuel ring on the porcelain (tells you what the jetting is doing) without either cutting the threads off to see the very bottom of the porcelain or using one of those tools that Dr.'s use for looking in your ears.
 

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IMO if you intend to try and go fast you should just get a DP and be done with it. The vacuum secondary carb is a good street carb but just has many drawbacks for going real fast. Many dirt car classes actually control power by mandating the use of the unmodified vacuum carbs. On the dyno pulls the carb is wide open during the pull and you still see reduced HP compared to a DP carb. I'm not a carb guru but that alone tells me there is performance lacking in the V/S carbs so if you want to go fast why spend good money on something that will never give you what a DP will? JMHO, RM
 

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Discussion Starter #11
Real McCoy said:
IMO if you intend to try and go fast you should just get a DP and be done with it. The vacuum secondary carb is a good street carb but just has many drawbacks for going real fast. Many dirt car classes actually control power by mandating the use of the unmodified vacuum carbs. On the dyno pulls the carb is wide open during the pull and you still see reduced HP compared to a DP carb. I'm not a carb guru but that alone tells me there is performance lacking in the V/S carbs so if you want to go fast why spend good money on something that will never give you what a DP will? JMHO, RM
I found a good deal on a 650 DP last night. The VS will make a good spare. :D

DriveWFO said:
You can't see the fuel ring on the porcelain (tells you what the jetting is doing) without either cutting the threads off to see the very bottom of the porcelain or using one of those tools that Dr.'s use for looking in your ears.
I thought that the tip of the plug showed how your idle mixture is. The visible portion showed what your cruising mixture was and the "invisible portion" showed what your secondary mixture was. Is this incorrect?
 

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beware of good carb deals on ebay.. i got burned by a guy claiming to have these engine run tested rebuilt carbs for like half the price of new. i bought one. carb had crusty parts on the inside, but the outside was all done up with gold carb paint, and some green here and there.. all new bowl screws, and new butterfly screws...lol. the insides were all old, and the floats were crusty. when i told him this, he refused to refund, saying since i opened it, it voided his warranty. carb ran like crap till i cleaned it real good.. ended up buying a brand new holley 650 dp from alantic speedway for 350$ shipped. best carb ive ever owned..
 

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You can also add a bolt to the secondarys and Make them work just like a DP Then it won't matter what spring rate is in there. You can also change Out Both the butterfly shafts and Put i mech.shafts.
But also a Dp has Two Exl. pumps one on each bowl If i remember right. .
just my 2 cents of worth of What??????????? LOL
 

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Maybe we could take it straight from Holley?

QUESTION How do I know if a vacuum or mechanical carburetor is best for me?
ANSWER For street cars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a Double Pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.
http://www.holley.com/TechService/FAQ.asp
 

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You can also add a bolt to the secondarys and Make them work just like a DP Then it won't matter what spring rate is in there. You can also change Out Both the butterfly shafts and Put i mech.shafts.
But also a Dp has Two Exl. pumps one on each bowl If i remember right. .
just my 2 cents of worth of What??????????? LOL
THIS is a bad idea !!!!!! When you do this you get a massive LEAN bog. You need a secondary discharge nozzle to cover the additional air flow when you stab the carb wide open.
 

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I also have ran both carbs at the track , i ran a 3310 old school carb with vs (780cfm) and it ran 11.90 consistent. I had a buddy with a 750 barry grant dp . i swapped out the same jets and it ran 11.90 as well BUT picked up 1mph . I think you can tune the vs good if you have the time.
 

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I always have at least two carbs for my 406. I use a modified Holley HP 750 double-pumper on it but also have a tricked-out 750 VC with a secondary metering block installed and the adjustable secondary diaphram.

No matter what, the double-pumper always outperforms the VC. I used to run the VC carb on another engine but it's mainly just for back-up now and I use the VC mainly for a sanity check. If I ever think I'm having carb problems I slap the VC on to see if it really is the carb.

It does get a little better gas milage with the VC though and it's not bad performance-wise. Just not as good as the DP.
 

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How streetable is a DP mechanical secondary? I can see a DP mechanical for the strip where you want that full opening quickly. But for the street I would think you want a nice even application of power.

Am I off base here?
 

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How streetable is a DP mechanical secondary? I can see a DP mechanical for the strip where you want that full opening quickly. But for the street I would think you want a nice even application of power.

Am I off base here?
Just use a nice even foot...
 
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