carb sizing
Thread Starter
|
Freshman User
Joined: Jun 2001
Posts: 26
Likes: 0
carb sizing
I have a freshly rebuilt 390 4bbl with a mild cam in a 78 F250 4x4 4 speed with 4:11 gears and I was wondering what size carb I should use, I have a 780 Holley dual feed single pump, is this too big? I also have a Edelbrock 650 single feed single pump, I have had both on the truck each had its own problems, I am having trouble getting the holley jetted right, I have 72's in it right now, The Edelbrock seemed to have a dead spot right out of the hole, I didn't try adjusting it yet because they aren't nearly as easy as the holley. Also what initial timing are you guys using with 89 octane fuel.
thanks, Neil Kinsley
thanks, Neil Kinsley
Elder User
Joined: Aug 2001
Posts: 962
Likes: 2
From: Collierville, TN
carb sizing
The general formula for carb sizing is: (cid x max rpm x V.E.)/3456. That will tell you how much air your engine pumps at max rpm, and your carb should be rated to deliver that and a little more. Assuming you run as high as 6000 rpm, and have a V.E. of 85% (pretty good estimate for a mildly built FE), that works out to 575 cfm. A 600-650 cfm carb would be plenty. Going bigger won't give you any additional benefit.
Senior User
Joined: Jul 2001
Posts: 298
Likes: 0
From: Oztralia
carb sizing
a simple rule of thumb
2 * cid = cfm ( maxpower double pumpers)
2 * 390 = 780
a 780vac would be good - though I've found them hard to get economy
a 650 double pumper worked well for me and gave good economy eventually
2 * cid = cfm ( maxpower double pumpers)
2 * 390 = 780
a 780vac would be good - though I've found them hard to get economy
a 650 double pumper worked well for me and gave good economy eventually
