carb CFM?

[bigmac73]

what is the CFM rating on the factory Quadrajet carbs that came on out trucks,(72 C-10, 350 4 barrel)??

[Coley]

...they could flow up over 700-750cfm...but generally delivered less, since its very hard for a 350 to use over 600cfm....unless it really revs out.
Coley

[mmiddle]

^^^^YEP^^^^

Some CBOP 455/473's were 800CFM.

[bigmac73]

okay so the reason i am asking is, the crate motor I installed recommended a 600 CFM carb, I put a 600 DP on it but it seems like something is holding it back when i get on it , I was thinking upgrading to a 650 DP Holley with a four corner idle setting,

so if the manufacture suggested a 600 on a rated 385 HP small block, but the factory stock engine QJ was a 750, and if i look at other top end combos at summit they recommend a 750 CFM carb ( most of them are 375 HP and higher.

Just thinking the 650 would give me a little more juice

[geezer#99]

Put a 750 vac secondary 3310 on it and be done with it.
Years ago I tried swapping my factory type 3310 on my 70 LS6 to a 650 dp and it felt doggy, put the vac sec carb back on. World of difference with the secondaries opening slower.
Your 2000rpm TQ is holding you back.

[bigmac73]

Geezer,
can you explain the Converter theory of holding me back

[luvbowties]

Originally Posted by bigmac73

Geezer,
can you explain the Converter theory of holding me back

...someone told me to just imagine how it feels when u take off[running a stick shift tranny] while holding the clutch pedal partially still in--clutch is slipping & not delivering full power to the pressure plate/flywheel.
Sam

[bigmac73]

well the recommended stall converter for my engine was 2200 to 2500, since my cruising rpm is around 2500 on the highway, i went for a slightly smaller stall speed.

[geezer#99]

Originally Posted by bigmac73

well the recommended stall converter for my engine was 2200 to 2500, since my cruising rpm is around 2500 on the highway, i went for a slightly smaller stall speed.

At 65mph you should be turning closer to 2900 rpm if you're running a 28" tall tire.
To turn 2500rpm you need a 32" tall tire.

[jhwkns]

When it comes to carburetors, bigger isn't necessarily better. A 600cfm carb is more than enough.

[Lattimer]

I have a 625 cfm Street Demon on my 355, and it freaking screams. When you go too big you gain a little at WOT, but part throttle and tip in get doggy and will bog out.

I'm running a manual trans, so I need it to run well right off the bottom for low speed take off. I had to adjust my secondaries a lot and add a larger accelerator pump nozzle, but now its running awesome.

600-650 cfm is plenty until you get up to a 383.

[GMCJOE]

It would have been a 750 quadrajet. I recommend running a quadrajet on it from SMI Carburetor. They do great work. Tell them the specifics of your engine, and they build it to perform to your motor. They have excellent reviews. And if you need any help while installing, you can call them up.

http://www.smicarburetor.com/product...ID2/9/sfID3/32

[bigmac73]

Originally Posted by geezer#99

At 65mph you should be turning closer to 2900 rpm if you're running a 28" tall tire.
To turn 2500rpm you need a 32" tall tire.

I am running at 31 x 10.5 x 15
so my factory tach says 2500 at around 65 give or take

[67ChevyRedneck]

Are you sure it's tuned properly? Both carb and timing? If so, you can try rejetting the secondaries for some more power.

[bigmac73]

timing is set to specs for engine and I adjusted the A/F mix with a vac gauge, i have not changed any jets out yet , might try that, there is a speed shop in town that will tune it, change jets ect ect. in on the dyno for about 200 bucks a session.

[burnin oil]

Qjets flow 750 for the side entry and front entry were 800 cfm. The reason that GM got away with so much carb is that a Qjet does not act like any other carb. That secondary choke flap looking thing is an air door. It will only let as much air as the motor use get through. So theoretically you could put it on a 231 cid motor and have no issues. It just wouldn't pull much through the secondaries. The primaries of the standard GM qjet flow around 250 or 270 cfm. The exact number escapes me but the primaries are like the size of a dime which is why they run so nice down low. The 3310 Holley carbs act similiar with the stiffer secondary springs. The heavy spring doesn't even open the secondaries all the way! The spring kit comes with a neat little chart that shows this along with the approx RPM the secondaries will start to open and full secondary operation happens. Swapping springs makes a world of difference because of this.