My 1967 Delmont 88 has a slightly built 455 (ported C heads, 10:1 compression, headers and a ***** Thumpr cam [moderate lift with fairly long duration]). After building the engine I installed a 750 cfm vacuum secondary QuickFuel carburetor. It always seems that the engine didn't have the power that one would expect from a 455. I tried re-jetting that carb but it just didn't seem to be enough. Since the car has a 2.73 or 2.78 rear I attributed the lackluster performance to that. Recently I changed it to a ProForm 850 cfm double pumper and it runs like a different car. Acceleration is now a fairly violent event--which is what I was hoping for all along.

I just wanted to mention this in case someone is in a similar situation. The Summit cfm calculator is what led me to buy the 750 originally.

Chris

 

Good post, your combination goes against all recommended advise. Maybe it's good to try something contrary, especially when it has good results. I'm sure the experts would be scratching their heads. Does the bigger carb perform better in all driving modes; idle, cruise and full throttle? Good luck.

 

You cannot always rely on a new carb being what it should be. I have used the holly double pumper 650 cfm spreadbore carb on 425's and it performs quite well with its mechanical secondaries. The 850 is not overkill on a 455. Glad it worked out for you.

 

The performs well; however, the idle is a little rich. From what I've read you just have to increase the idle air bleeds to cure this.

 

That's too small but most likely your problem was a slow opening secondary. If it was jetted right it should've still run fairly well.

 

thats so funny I have a 65 425 motor and I also started with the brawler 750 double pump and tried jegs fuel injection system and that was a disaster I then just got the pro form 850 drag carb it’s been falling on it face under normal acceleration then after an inch of gas it will kicked in and go like hell I learned it needs 6.5 psi and I’m using a new Carter and they only give 3.5 and sometimes about 5 psi I’m hoping that’s the problem I’ll know soon I just ordered a Holley fuel pump

 

This is a very old thread, but my suggestion is still the same. Buy a wideband and quit guessing.

 

i have two wide bands one for each side on a triple gauge with a vacuum in the middle also it runs at 13 to 14 on the gauge and 12 vacuum the pro form 850 comes with a 4.5 power valves o i put a 6.5 in that i had on hand and it didn't clear up the condition idle is 13/14 as soon as i touch the gas it shoots up to 17/24 after i give it an inch of gas it comes back to 13 i'm waiting ona fuel pump because this carter mechanical doesn't even put 5 psi out i've seen it put out 3 after it warms up so thinking fuel pressure problem but if i nail it there's no problem