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  #1  
Old 09-09-2005, 09:51 PM
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Default Mechanical or vacuum secondaries?

I'm running a 340 in a 3000 lb streetrod with a hughes 1423 cam, headers, edelbrock performer RPM manifold. Transmission is a manual Tremec TKO 5 speed. I have had a lot of opinions on a carburetor. Some say I should run a vacuum secondary carb. Others say a mechanical secondary carb is the way to go. I currently have a 4778 Holley. (700CFM with mechanical secondaries)
I thought I would post this question and see what other opinions I get on this. A few years ago I ran a 700 CFM race carburetor with vacuum secondaries and it seemed to run better. I gave that carburetor away! Wish I had it back.
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Old 09-09-2005, 10:16 PM
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Quote:
Originally Posted by coupe32
I'm running a 340 in a 3000 lb streetrod with a hughes 1423 cam, headers, edelbrock performer RPM manifold. Transmission is a manual Tremec TKO 5 speed. I have had a lot of opinions on a carburetor. Some say I should run a vacuum secondary carb. Others say a mechanical secondary carb is the way to go. I currently have a 4778 Holley. (700CFM with mechanical secondaries)
I thought I would post this question and see what other opinions I get on this. A few years ago I ran a 700 CFM race carburetor with vacuum secondaries and it seemed to run better. I gave that carburetor away! Wish I had it back.
What are the specs of this cam? Please provide duration at 50 spec and valve lift.

Also, what are the rear end gears and tire size (height/diameter) in inches
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  #3  
Old 09-09-2005, 10:20 PM
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duration @ .050 is 214, 223 Valve lift is 470, 504 350 rear with 28" tires
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Old 09-09-2005, 10:44 PM
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The car is light enough that a mechanical secondary/double pumper carb should work, but..... with those gears and tires, and relatively mild cam, a vacuum secondary would probably be marginally better, and easier to tune.
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Old 09-09-2005, 11:39 PM
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Thanks Dave for your input.
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Old 09-10-2005, 08:17 AM
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I agree with Dave. I usally recomend a mech. secondary carb with a manual trans, but that rear gear and tire height is a tall set.
Another good call by Dave.
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Old 09-10-2005, 08:12 PM
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Hey Rumble...There's 5 bucks in the mail to buddy
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Old 09-11-2005, 09:48 AM
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Quote:
Originally Posted by dave571
Hey Rumble...There's 5 bucks in the mail to buddy
????? Ahhhh, missing the "U", LOL I was sayin, whos buddy?

Coupe you have plenty of underhood room right. Just wondering if you worked with spacers and how much filter you have on the carb?
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Old 09-11-2005, 11:39 PM
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Quote:
Originally Posted by rumblefish360
????? Ahhhh, missing the "U", LOL I was sayin, whos buddy?

Couope you have plenty of underhood room right. Just wondering if you worked with spacers and how much filter you have on the carb?
Yes, I forgot the "you"

Doesn't matter, the 5 bucks is Canadian, so it won't even buy you a gallon of gas...LOL
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  #10  
Old 09-12-2005, 06:35 PM
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I have a 14" X 4" K & N fliter on it. Ive got plenty of under the hood room. Thanks
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  #11  
Old 09-13-2005, 07:25 PM
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OK, cool. I was thinking of a suggestion of a K&N extreme top.
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Old 09-13-2005, 08:04 PM
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Well, Ive got a little problem with my 32 coupe's hood. The center hinge of the hood leaks water when it rains onto the engine. I thought about a K & N filter on the top also but I don't want to chance rain getting into the engine when its running. It is basically a sunny day car but sometimes I get caught in the rain when I take it out of town. The hood doesn't leak real bad but probably bad enough during a hard rain. Thanks for all the suggestions everyone.
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  #13  
Old 09-15-2005, 10:43 PM
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Default rule of thunb

run a vac seconadary carb on engines under 400 cubes and mechanical sec on engines over 400 cubes.
Biggest advantage of vacuum secondaries is you can run more cfm than you otherwise could becuase it stays out of operation till you need it or more correctly till the engine can use it. uh uhu now you double pumper guys , dont get too excited. tis true.
The six pack had 1250 cfm and worked well . It would have been impossible to drive with a 1250 double pumper or darn near. Cetaijnly would have had a huge bog .
Once you get over 400 cubes the extra airflow gives back more than it loses . in the smaller engines airflow is lazy and the carb does not respond well till enough flow is established to make all the circuits work properly. The vac secondary concentrates all flow in the primaries untill flow has reached the level where adding the secondaries will not hurt anything.
How come i am so sure?
A few years ago we took several carbs to the track and three different engines.
We had a 350 chevy , a 400 firebird and a 440 dart. The chevy didnt like the double pumper the 400 could go either way and the 440 loved the dp.
These were all bracket cars. I felt it was a pretty fair test.
A Afb or edelbrock os BTW a vacum sec carb even though it is done differently than say a 3310 holley the effect is the same. In the test we used a 750 vac sec holley 3310, a 770 double pumper and a 850 double pumper.
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  #14  
Old 09-15-2005, 11:41 PM
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No way. Carb size is more important than weather or not the engine is big or small. Lazy air flow is the intake & heads problem followed by cam size. Carb size, when properly matched and tuned properly, will perform great.
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  #15  
Old 09-16-2005, 02:44 AM
Kevin Garceau Kevin Garceau is offline
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Quote:
Once you get over 400 cubes the extra airflow gives back more than it loses . in the smaller engines airflow is lazy and the carb does not respond well till enough flow is established to make all the circuits work properly. The vac secondary concentrates all flow in the primaries untill flow has reached the level where adding the secondaries will not hurt anything.
Interesting...

So my 1000 hp small block needs more cfm than my 330 hp big block?

You my friend are out to lunch on this theory. Your test proves nothing, cubic inches are very small fraction of the equation.

My test, my friends 358 cubic inch small block chevy. Very short stroke even for SBC, but heads that flowed over 420 cfm, used more carb than my 440 in the same weight car adn we ran the same..... go figure. According to your test it was all wrong.

Everything comes into play, in this case for a street driven car, I would suggest a Vacuum secondary carb. You have the CFM for when the engine wants it, but have a little milder drive ability when doing your daily cruising.

If you had the cash, electronic fuel injection is the way to go. Best of every world. I so want to put electronic fuel injection on the dragster... few years it will for sure.
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Old 09-16-2005, 02:46 AM
Kevin Garceau Kevin Garceau is offline
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Quote:
The six pack had 1250 cfm and worked well
the six pack set up ran OK at best. Put a good 4 bbl on that car, and you will generally see a nice gain in ET at the dragstrip. But the 6 pack has the cool factor going for it. I drove a friends 440+6 dart for him one season at the track, thing was terribly inconsistent no matter what we did with it.
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  #17  
Old 09-16-2005, 07:33 AM
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Quote:
Originally Posted by Kevin Garceau
the six pack set up ran OK at best. Put a good 4 bbl on that car, and you will generally see a nice gain in ET at the dragstrip. But the 6 pack has the cool factor going for it. I drove a friends 440+6 dart for him one season at the track, thing was terribly inconsistent no matter what we did with it.
I had heard the best way to get the 6pak to go best is with a change over to a mech. set up once offered. Anyone know if it is still out there?
I know 6 pak tuning isn't easy for most people, but I think the potential is there for a great street strip set up.
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