Carb size, does it matter?

Randy

Well Known Member
Supporting Member 3
#1
I am currently running a Holley 750cfm in my 63, 409, 340hp. It is pretty stock, never been rebuilt. Petronix ignition in a marine distributor. Stock exhaust manifolds. Steel intake. Electric fuel pump. My question is , can I go to a lower cfm and still have the performance or will it hinder it by starving the engine of fuel?
I forgot to mention also about the vacuum, I need it for my power brakes, will that change with a smaller carb?
 
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Skip FIx

Well Known Member
#4
Smaller can work fine and give a little more throttle response. Is the 750 a 3310 vacuum secondary? The vacuum secondaries vs a mechanical secondary double pumper can give a little better response as the secondaries only kick in when need when tuned with the correct spring.302 Z28s a much smaller motor used a similar carb with down leg boosters.
 

Randy

Well Known Member
Supporting Member 3
#5
Smaller can work fine and give a little more throttle response. Is the 750 a 3310 vacuum secondary? The vacuum secondaries vs a mechanical secondary double pumper can give a little better response as the secondaries only kick in when need when tuned with the correct spring.302 Z28s a much smaller motor used a similar carb with down leg boosters.
Yes, it is a 3310s with manual choke and vacuum secondaries. Besides a Demon, what else would you guys consider?
 

1958 delivery

Well Known Member
Supporting Member 2
#6
I would use the Edelbrock 600-650 Performance version, always with vacuum operated secondaries
Considering the engines stock parts I would go with the Edelbrock Performer 600 1406
If you had performance intake and headers then the Thunder AVS 650, in fact if you had these performance add ons the 3310 would be a pretty good choice if you can live with a Holly.
 
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Skip FIx

Well Known Member
#7
The 3310 can be tuned OK in I had one on a fairly stock Pontiac 400 in the 81 TA my kids drove to high school. Put an electric choke on it get the vacuum secondary spring kit to dial in secondaries. PV number about 2" below idle vacuum most come with a 6.5 to dial in power circuit.

To get real close with any carb get a wide ban O2 sensor. Makes it easier to get jetting down.
 

Skip FIx

Well Known Member
#9
And running off the small primaries of a 3310 when most of the time the secondaries are a reasonable choice for a 409ci motor, or my mild 400. Although not as good a gas mileage when I swapped a Q jet back on it.

Trying to tune secondarys on an AFB/Edelbrock equivalent is pretty difficult with the weighted bar that opens them.
 

Carmine

Well Known Member
Supporting Member 6
#13
I installed a Q jet on my 340 hp a year ago. It was off a sbc 400, so took very little jetting changes. It usually runs low 14's, but one pass was high 13's. The July 14th race I went 5 rounds for the win. Mileage?? About 3 passes per gallon.
I think they are very good times for a 340/409. Did you do anything else to the engine?? What trans. and gears are you running?? Thanks, Carmine.
 

Ishiftem

Well Known Member
Supporting Member 2
#18
I would add to all this an edelbrock and a Holley rated for the same cfm do not flow the same. The edelbrock will flow less air than the Holley.
 

64ss409

Well Known Member
Supporting Member 9
#19
I think they are very good times for a 340/409. Did you do anything else to the engine?? What trans. and gears are you running?? Thanks, Carmine.
Carmine, The cam is an Isky no 920 from Show Cars, trans is stock Muncie wide ratio, gears 4.11. Last year I tried the Comp Cam, no 950 from Show Cars thinking a bit more lift and less duration might give better low end, but it didn't. It dropped 2/10 - 3/10 in the quarter, so back to the Isky.
Ron
 

425/409ER

Well Known Member
#20
I am currently running a Holley 750cfm in my 63, 409, 340hp. It is pretty stock, never been rebuilt. Petronix ignition in a marine distributor. Stock exhaust manifolds. Steel intake. Electric fuel pump. My question is , can I go to a lower cfm and still have the performance or will it hinder it by starving the engine of fuel?
I forgot to mention also about the vacuum, I need it for my power brakes, will that change with a smaller carb?
We too have a 340hp engine in our '63 Biscayne. I have tried several Holley's, Eddy's and a Demon carb on the '09. One day I decided to try the little Holley 570 Avenger carb on that '09 and it runs perfect. For years I though the old 3310 750 was the hot ticket on the old W engines but the car runs good and gets pretty good mileage to boot. We will leave it alone until my son decides if he want to put the dual quad setup on it with a pair of old 300hp 327 Carters. Kids, they just love their eye candy, LOL.