What carb would you guys put on a 306?

[Blown331LX]

Engine is in a 1990 GT, .030 over forged flat tops, stock rods, Patriot Heads, F303 cam, BBK longtubes, full MSD ignition & dist, Holley red pump, Victor Jr Intake, World Class T5, 4.10 rear, naturaly aspirated for now but will have the bottle before too long, and will only be 75 to 150 shot. Car currently has a Carter AFB 600 or 650 with vacuum secondaries. Just the other day I discovered that the secondaries dont open up at all, even all the way up to redline. The car has not ever felt fast enough and this would be the likely cause. Im currently looking at putting a Holley 650 dp with mechanical secondaries on it. Some say they have done this on a almost stock 302 without issues and it would be great on a 306, others say it will be too much carb for the car and will wash the rings out?????????????? ..... What do you guys think???

 

[Blown331LX]

Engine is in a 1990 GT, .030 over forged flat tops, stock rods, Patriot Heads, F303 cam, BBK longtubes, full MSD ignition & dist, Holley red pump, Victor Jr Intake, World Class T5, 4.10 rear, naturaly aspirated for now but will have the bottle before too long, and will only be 75 to 150 shot. Car currently has a Carter AFB 600 or 650 with vacuum secondaries. Just the other day I discovered that the secondaries dont open up at all, even all the way up to redline. The car has not ever felt fast enough and this would be the likely cause. Im currently looking at putting a Holley 650 dp with mechanical secondaries on it. Some say they have done this on a almost stock 302 without issues and it would be great on a 306, others say it will be too much carb for the car and will wash the rings out?????????????? ..... What do you guys think???

 

[white 89 GT]

I would use a 650 and most likely a vac. sec.

 

[85_SS_302_Coupe]

650 double pumper and don't look back. Mine has served me well from a mild 302 to a healthy h/c/i 302 to my current 347. I wouldn't waste my time on vacuum secondaries if you plan on running a healthy cam. Fox cars just don't weigh enough to justify VS especially when you run a good sized cam.

 

[Blown331LX]

650 double pumper and don't look back. Mine has served me well from a mild 302 to a healthy h/c/i 302 to my current 347. I wouldn't waste my time on vacuum secondaries if you plan on running a healthy cam. Fox cars just don't weigh enough to justify VS especially when you run a good sized cam.

I hands down agree on the mechanical secondaries, Ive been running the vacuum secondaries and your right with even my .512 lift cam it doesnt do well. A friend on mine use to have a very similar engine in his car with the 650 Holley DP and he stated when he would mat it to the floor you could here the 4 barrel kick in like it was in your lap and it would shove you in the seat. My car almost identical to his, he said feels like its pulling a trailer.....

 

[85_SS_302_Coupe]

Vacuum secondaries have their place...like in a bone stock 302 daily driver. If you want performance, mechanical is where it's at...at least for light cars like Fox's. If you drove a big 4500lb car then vacuum would be better but then again the cam and engine vacuum come into play again.

 

[white 89 GT]

Vacuum secondaries have their place...like in a bone stock 302 daily driver. If you want performance, mechanical is where it's at...at least for light cars like Fox's. If you drove a big 4500lb car then vacuum would be better but then again the cam and engine vacuum come into play again.

Well my 650 demon is vacuum secondary and at 11.39 (spinning) I hardly think of it as holding me back. The secondary`s are relying on air passing by to create the necessary vacuum to open, not engine vacuum as some would think.
A mild 302 don`t need a double pumper, unless you don`t know how to make vac secondary`s work.
I use the lightest spring possible in mine and it works very well, but I would like to try a 750

 

[cjones]

as white 89 gt eluded to...have you tried tuning the carb??

different vacuum springs and jets. a properly tuned 650cfm is plenty for a 306 unless you're spinning over 7,000 rpms. you can run up to a 350cid engine to 6000 rpms with a properly tuned 650cfm carb. and a tune kit costs less than a new carb.

i wouldn't worry about a dbl pumper unless you'll be racing more than driving on the street.

 

[85_SS_302_Coupe]

So let me ask you guys....what's the point of using the lightest springs possible? Aren't you just pushing your vacuum secondary to be as close to mechanical as possible? You get to the point where it takes so little to open them that you may as well just run mechanical and save yourself the work and hassle of messing with the springs. It's not like a double pumper flys open as soon as you touch the gas, i would venture to say it's no faster than your super light spring VS setup, except with mine i don't have to worry about a poor signal due to having a huge cam. Also mechanical secondaries are still adjustable. Then there's the mpg thing....it's been my experience that i can actually feel through the pedal when the secondaries start opening because the pedel effort gets harder...so it's very easy to drive my car and stay on the primaries. This puts me right along side any vacuum secondary carb as far as mpg is concerned since my secondaries don't open unless i want them to....so in the long run if i worried about it i could probably knock down slightly better mileage since you can't stop yours from opening unless you play with the spring...and then you can't have the best of both worlds like i can.

 

[white 89 GT]

You use the spring to tune the carb to the engine, for me at 331ci I can use every bit of my 650 and to ensure it opens as far as possible I use the lightest spring.
It opens very soon and will not create a bog as the dbl pmpr will although I could use a 650 dbl pmpr on my engine and I'm sure it would perform very well, but let me tell you its easy to get rid of off idle hesitation with vac sec. My engine revs very crisp and does not stumble as I leave the line off idle.
I also can tell when the secondarys start to open (on the street anyhow) not at the track because they open very quick. The size of the cam makes no difference, have you ever watched a vacuum guage when you go wot, it goes to near zero with all cams.

 

[85_SS_302_Coupe]

So why do people run mechanical secondaries if vacuum is so good?

Also, i don't have and have never had any off idle stumble or bog when the secondaries open. You get rid of this with proper jetting and power valve sizing. When my 650 was box-stock it didn't have any off idle stumble or bog and that was on a bone stock 302 long block. I just think a lot of this stuff is myth, or its people throwing an 850 double pumper on a 302 and expecting it to run right.

Another thing is, your car is an automatic...that changes a lot of things. With a stick you just put it in the right gear and mash it and it does what it's supposed to do. With an automatic, if you're on all 4 barrels in the wrong gear then performance will suffer. For your car a tuned VS is probably best, but for a 5 speed you couldn't pay me to run VS.

In fact right now i'm having some ignition problems with mine and it doesn't want to idle right. I had feared it was something with the carb that i didn't see so i swapped on a buddy's 650 Holley, which happens to be VS. I don't know what spring is in it but it didnt open up until like 4k RPM and was very noticably softer pulling than my DP. There's a lot of tuning that could change this but i just don't see the point when my DP runs so good.

 

[white 89 GT]

Im not saying its so good, im saying I know how to make it work with a given application and it will work very well.
See I have allways thought that at 2k-3k rpm it is not helping to have all 4 barrels fully open unless you have a very small carb and the engine can use all that air.
My engine could use a 750 cfm and possibly pick up a mph or so, but it damn well can`t use all 750cfm when I leave the line so why would I want a dbl pmpr, besides its called a "dbl pumper", because it needs all that fuel to mask over the sec opening and im to believe that just because I dump all that fuel and air at once the engine needs it, no way can my engine use all that air and fuel at lower rpm`s .
Put the yellow spring in that vs 650 of his and see if it works better, my guess is it will perform very good.

 

[90 lx convert]

I would be reluctant to bash VS. The point is the tuning needed to get the max from the carb. With your setup a well tuned 750 VS will out perform a 650 DP. If all you want is to "feel" the secondaries come in then keep the DP. Keep in mind that feel you get is the secondaries crashing in, vacuum goes to 0 and the air velocity in the carb staggers to halt. That's what you're feeling. Mike