I completely agree on the carb sizing formula being nothing more useful than a baseline for carb sizing. But when it comes to a street driven vehicle, a smaller carb is always better than one that's bigger, especially when it comes to a mech secondary carb.
Thanks, D.Hearne. I was going to ask woodsnake about which secondary was on the Shelby 715. And that coincides with what I read, that larger vacuum carbs are more forgiving, whereas your performance will suffer if your mechanical carb is too big.
And I decided that I don't know enough about my car yet (need more info on my gears, cam, cylinder bore) to determine whether my 650DP is "too big" or whether or not a VS or MS carb would be a better fit.
1965 coupe, 30 over 289, Crane Fireball-1, hi-po 289 heads, 9.5 cr, Boss 302 rods, tri-y's, Cobra intake with devider cut down 3/4", Accell vacumn distributor set at 6 btdc, 650 D.P, 2.78 1st top-loader, 3.25 8" differential and 215/60 Euro T/A's, shift at 6500 rpm..........5-65 mph=5.5 sec. Enough said.
i have 2 vacuum secondary 600 cfm 1850 holley's i can send you, i'll rebuild both of em, if you would like to trade for the 650. im building a 302 thats gonna see some heavy dose's of nitrous, and is more of a street strip application, big cam and such. one was running on my 289 '66, my daily driver last year, it ran great, fired up every time, and held a tune great. i'll even pay pal you 25 bucks for shipping.
I've been reading this post with a lot of interest also. I recently bought a 650DP for my build and am now wondering if I made the right choice.
I've got a roller 347 with Trick flow heads. Edelbrock Air-Gap intake, Comp pro magnum roller rockers, its got a comp cam - don't have specs in front of me. But told builder I wanted something that can still make enough vaccum and not be overly agressive. Running a C4, plan on using a 2400 stall and have 3.44 trac- loc. Was told compression should be 10.5 to 1. Will be a street car but not overly concerned about gas mileage.
650 should be fine but is mechanical secondaries going to be a problem?
I've been reading this post with a lot of interest also. I recently bought a 650DP for my build and am now wondering if I made the right choice.
I've got a roller 347 with Trick flow heads. Edelbrock Air-Gap intake, Comp pro magnum roller rockers, its got a comp cam - don't have specs in front of me. But told builder I wanted something that can still make enough vaccum and not be overly agressive. Running a C4, plan on using a 2400 stall and have 3.44 trac- loc. Was told compression should be 10.5 to 1. Will be a street car but not overly concerned about gas mileage.
650 should be fine but is mechanical secondaries going to be a problem?
No, you're sized a tad on the small side, which is better than being too big. You may need to change the jetting though to compensate for the increased vacuum pull at WOT. A 700 or 750 would have been the ideal size for generating the max HP on topend, but no it's big thing. If you're not concerend with fuel mileage, a DP carb was an good choice. If you want better mileage (and topend), a change to a vacuum sec 750 would be the way to go
Thanks - just out of curiosity - what would be the gas mileage difference. Although I say not too concerned if I'm getting 5 mpg that could be a problem.
Thanks - just out of curiosity - what would be the gas mileage difference. Although I say not too concerned if I'm getting 5 mpg that could be a problem.
Now that's a question only you'll be able to answer once you've got it together. And it'll all depend on how disciplined your right foot is and keeping that s--t eating grin off your face everytime you nail the go pedal I just drove the Comet to work and back today and it got 13-14 mpg with it's 331, C-4 transmission, 3.50 geared 9" and 26" tires. And that was turning 3500 rpms@ 80 mph. The intake setup is a mech secondary 3x2, basically a 750 cfm "triple pumper" But I rarely ever kicked the secondary carbs in.