hmmmmm...now I even have more to think about. So the 6 two barrel setup on ebay for a FE would be nice to have???? I know I'd like the looks of it...I always thought that more carbs meant more pain in the A$$ to set up...LOL. From Bears comments it sounds like that is the best way to get low end response and still have the high end...is that a correct interpretation??

Thanks,

Tracy

 

No, I think you'd be better off with the 750cfm 4 barrel. That 3x2 configuration will have mechanical linkage, and lose all the advantages of having vacuum secondaries.

 

The throttle bores don't control the air flow at WOT (well a little bit, but bigger just makes low end throttle response worse), the venturi size does and the smaller the venturi the higher the air flow the better the throttle response. The 500 2V is an performance carb just like a 3310 780 (well they call them 750s now). They are not intended for the average street engine. The "Tim "the Toolman" Taylor" attitude of "More Power" would work except he is also a believer in "Bigger is Better" and in carburators that just isn't so. Because it works on a race motor, pretty much means on the street it won't work worth a damn.

 

Check this out ebay #4602767528. He says it's 6 2 barrel carbs....isn't that 6 1 barrel carbs?? Anyway...looks cool...but I think I'll pass as I'm broke right now...LOL.

Tracy

 

Yes, regardless of what of what Rusty thinks. The reason the Europeans ran a lot of carbs was to have low end throttle response along with the big breathing for the big end.

 

Excellent thread! Feel like I am at Carb school. (which I definitely need!)

 

You can run almost any carb on almost any engine. However the more mismatched, size wise, the carburetor is the worse the engine will run. I've run the same 600cfm carb on a medium 289 mustang and a mild 400 truck. It worked well in both situations. Why? Light cars, high reving engines, vacuum secondaries and numerically high(drag) gear ratios all permit bigger carbs than ideal. Heavy trucks, low reving torquey engines, mechanical secondaries and numerically low(highway) gears all hurt driveability with too much carb.

A 750cfm on a 352, in a F250 should lose low end torque. If it's mechanical the difference will be drastic. You'll likely have trouble with lean mixture at part throttle, or rich mixture at full throttle. You'll also spend a lot of time with your foot in it to compensate for the loss in low end torque, meaning very poor gas mileage.

Try the 750 if you want to just for kicks, then get a 500-600cfm. Use a dual plane intake.

 

I will look for the formulas but what I found when researching my project was:
You need 125% of the engines max airflow for a high performance build.
For my 396cid FE it was 625cfm at 6000rpm.
So, 625x1.25= 781.25cfm.
I used a 750cfm carb.
This is off the top of my head. So I don't remember the VE value I used at the time. I will find the article and post a link.
For a stock build I could see a 600cfm being a good choice.

 

Fair enough. I just looked it up in my book.

Venturi diameter:
750cfm 4 barrel - 1 3/8"
500cfm 2 barrel - 1 3/8"

Busted!

In case you're wondering, "Super Tuning and Modifying Holley Carburetors," by Dave Emmanuel, page #122-123.

Now, I do agree that these larger bore diameter carbs will cause problems on the street. It just amazes me that some people (not you Bear) who think a 500cfm 2 barrel is perfectly acceptable think that a 750 is way over the top.

 

Rusty what is the advantage of vacuum secondaries? I know there must be some reason, because almost all you see in auto magazines are the vacuum type. But I had a 1966 Plymouth Satalite with a 383. It had a carter 4 barrel and at 80,000 miles I changed it to a Holley 750 dual pumper with manual linkage. I couldn't believe the difference in power. When you punched it to the floor, it would almost turn around in the road. And the gas milage went from 13 mpg to 19 mpg.

 

rusty70f100, if it makes you feel better, I'd say both the 2V 500cfm and 4V 750 are wrong. Too little and too big respectively. Something in the middle should work quite well.

76BIG/ORANGE The advantage of vacuum secondaries are that you can run a bigger carb(even bigger than ideal) without bogging when you punch it at low rpm. The secondaries shouldn't open unless you have some vacuum. You can actually measure the correctness of your carburetor's size by a similar principle. Run with a vacuum guage reading manifold vacuum. Street engines should show about 1" of vacuum at high rpm at full throttle. More than 2" means you need a bigger carb. 0" means your carb is too big. (assuming everything is working properly.)

"But I had... a 383. It had a carter 4 barrel and at 80,000 miles I changed it to a Holley 750 dual pumper with manual linkage. I couldn't believe the difference in power... gas milage went from 13 mpg to 19 mpg." I have heard that carters are very hard to get running right. I have also heard they are pure junk. I have not personally tried one so I don't know. There's a good chance the carter needed rebuilding and/or tuning.

EDIT: Just found this link.
This shows that you have to have some vacuum to open the secondaries. This is how they prevent bogging.
On second though, why not read the article for self education?
http://rodandcustommagazine.com/tech...135_0402_carb/

 

Derherr65, that is exactly correct. And you're right, both would be wrong in this application, it's just the 750 with vacuum secondaries is somewhat less wrong. Close enough to give it a try anyway.

 

As promised.
Max air flow:
displacement x rpm x ve / 3,456 = Max air flow.
Example:
396 * 600 * .90 / 3456 = 618.75
Now volumetric effency is a whole other discussion. Probley why it was left out of the above article.
The carb should exceed the needs of the engine by 10 to 25 percent depending on the kind of engine it is. I.E. drag or street or something in between.
I was building a street/strip truck when I did my build so I used the 25% more value. Example:
618.75 * 125 = 773.4375cfm.
So I got a Edel 750cfm.
Now that this engine is in a big 4x4 the carb still works just fine no bog off idle or flat spots in the acceleration. The only problem I have is with my cam I have a hard time keeping the carb in the idle circuit. But I can fix this by drilling the butterflies.
Can someone post up ve specs for different heads or how to calc them?

 

Yes, it works because the Edelbrock (really an old Carter made to look pretty) has mechanical throttle plates, but air flow controlled air valves. Which really control the metering rods and how much air the engine sees. Another form of vacuum secondaries is what they are.

 

Randyb12: Does your 396 really hit 6000 rpm? What's the bore and stroke on those engines? The vacuum secondaries in an edelbrock should eliminate any bogging problems you could have.

Normal car engines, including our mild engines run about 85% VE. 90% is close enough if you are building a good street/strip engine. 100% is used for all out race engines. CID x max rpm x VE ÷ 3456 = CFM. Then multiply by 1.35 for dual plane intake or 1.2 for single plane. Some examples:

308 x 6250 x .90 ÷ 3456 = 500cfm *1.35 = 675cfm. So, a 700cfm carb should work nicely on my stroked, balanced, solid lifter '289' 65 mustang.

The old daily driver 289 engine needed 293 x 5500 x .85 ÷ 3456 = 396cfm * 1.35 = 535cfm. A vacuum secondary 600 worked fine.

My 77 F150 is 408 x 4500 x .85 / 3456 = 452cfm x 1.35 = 610cfm. A 600 works fine, but a 650 probably would give me more horsepower. However since this is a truck engine, and rarely sees more than 2500 rpm, the 600 gives better throttle responce, gas mileage, and (probably) better off idle torque. Plus I had a spare 600cfm.

Oh, I said I'd heard carters were hard to get running right earlier... I meant Q jets. My edelbrock is easy to get running.