mechanical secondaries?

[tybob81]

Should I run a vaccum or mechanical secondaries on my 68 swb 2wd? I had a 750 cfm E85 carb on it that was vaccum secondaries and now I'm wanting to go back to 92 octane due to the fact alot of gas stations are not keeping the E85 as much as they use to. I am wanting to run a 650 holley or possibly a 750 but I think the 750 will be to much fuel for a 351w. I was always led to belive that mechanical secondaries were for a lighter car with a manual trans or high stall.

[tybob81]

Cam specs are Comp 266HR 266/270 advertised duration 210/215 duration @ .050" .533/.533 lift 114* lobe seperation angle 1200-5200 RPM operating range. Heads are AFR 185 with 2.02 1.60 valves. Performer RPM intake.

[hotrodfeguy]

Smaller is better, and that goes double for mechanical carbs. So thats my pick.

[tybob81]

So a 650 holley with vaccum secondaries? or mechanical? Getting ready to buy one when I make up my mind on the size. Going to put the E85 750 QFT carb on my 75 F350 w/460/C6

[FORDification]

You definitely need vacuum secondaries on a street-driven vehicle IMO. A Holley 600 is more than enough carb for your engine and truck.

[DuckRyder]

http://www.holley.com/0-86670BK.asp

Your understanding is correct, double pumpers generally work better on light - manual transmission applications. A heavy vehicle with a small block and an automatic would be probably the worst possible application for a double pumper.

A V-secondary carb is also much more forgiving of sizing because withing reason it is "self sizing" due to the nature of the V-secondary. I personally would not go smaller than a 600 and I would chose the above linked 670 (it is available in other colors as well) although a 750 or 780 would probably do fine as well with some tuning.

[BobbyFord]

From Barry Grant carburetors:

"Our general rule of thumb is that we recommend a mechanical secondary carburetor on a car with a manual transmssion and cars with converters 3000 and above and the vacuum secondary carbs for cars with autos and the converter under 3000."

[tybob81]

Thank you for all the input. That makes picking the right carb much easier.