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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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Also, what are the rear end gears and tire size (height/diameter) in inches |
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duration @ .050 is 214, 223 Valve lift is 470, 504 350 rear with 28" tires
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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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Thanks Dave for your input.
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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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Hey Rumble...There's 5 bucks in the mail to buddy
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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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Doesn't matter, the 5 bucks is Canadian, so it won't even buy you a gallon of gas...LOL |
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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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OK, cool. I was thinking of a suggestion of a K&N extreme top.
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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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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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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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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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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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