306 carb size.

[cas69aus19]

i am building my 306, but am not sure of what size carb to use. I have tested the calculators out there and they give me a 450cfm, but all the guys at work tell me that it is too small.

I will have .10/.10 under crank, hyper pistons, ported and polished heads with roller rockers & racier springs. edelbrock torker intake or weiand stealth.

plan on turning about 6000 with 2200 rpm stall converter.

any help is greatly appreciated.

 

[5.0ina66]

What heads? What cam? Roller or flat tappet cam? Assuming 351W heads and a modern flat street grind, I'd go for a 600cfm 4bbl in the brand of your choice.
HTH
--Kyle

 

[Max Power]

450 is too small. 600 would be a good bet.

 

[steel1212]

I have the 670 on about 311 cubes and its perfect. See mods in garage.

 

[ddonaca351]

imho.... 600-650 with vacuum secondaries. (holley / deamon / etc)

 

[12sec67]

650 dp!

 

[65ShelbyClone]

A Holley 600 or 650 double pumper would be about right. Mechanical secondaries are fun .

My old '65 notchback had a similar setup as yours only with a 289 & 4 speed and a 600 pumper was well suited to it. I have alot more in my current engine and a 650 is about as large as is practical for a mostly street driven car.

 

[cas69aus19]

ok that helps, but in choice of a vacuum secondary or mechanical secondary. A couple guys at work tell me that vacuum sucks for racing because they sometimes hesitate, and that mechanical is the best way to go. What kind of experiences have you guys had.

i will be mainly drag racing and running open course races like the pony express or gamblers run in nevada.

 

[302 coupe]

double pumper would work best for you, with mechanical secondaries. Vacuum secondaries are for street cars where mileage is a concern.

 

[Max Power]

Vacuum secondaries are the perfect variable venturi carb. If they bog, they aren't tuned correctly. Both would work fine for your application, but some people are reassured knowing for sure that the secondaries are opening, DPs are good for that.

 

[ddonaca351]

ok that helps, but in choice of a vacuum secondary or mechanical secondary. A couple guys at work tell me that vacuum sucks for racing because they sometimes hesitate, and that mechanical is the best way to go. What kind of experiences have you guys had.

i will be mainly drag racing and running open course races like the pony express or gamblers run in nevada.

imho... then the experts at work dont know how to tune vacuum secondaries as well as they think they do.

Now I do feel that for a race car, a dp is a better choice. HOWEVER the 1% or so, power that you loose with the vac sec is BY FAR made up for by easier drivability and tunability for the street.

Again this is just my .02, and Im sure others will disagree.... but in my near fourty years the above has been my experience.

If you already have a dp, I wouldnt think the difference would be worth the cost of replacing it (but you might be able to trade for free...) but if you are going to buy one, I would HIGHLY reccomend the vs over the dp for a street driven car.

Dave-

 

[redhotcomet]

FWIW...I just bought a Holley Street Avenger 570 for my 306. Right in the little instruction booklet it explains how to tune vacuum secondaries. Something to the tune that if you feel a surge of power when the secondaries open, it is not tuned properly. The power should rise steadily. The surge is actually a result of the motor bogging when the secondaries open too soon, and the engine struggles to catch up and once it does the power increases again.

 

[cas69aus19]

what you just explained there is what my co-worker said happened to him when he had his vs carb on his mustang. sweeet thats a lot of help.

 

[ddonaca351]

Sounds like a different spring to hold the secondaries to open just a little slower would be in order, or possible a slightly longer profile on the accel pump cam could help too. Might also be a little lean on the primaries, and as the sec open it compunds the problem... either way proper tuning would be the solution, NOT a different carb. But again just mho.

DD