I read on here in the forums that the smaller cfm carb you get, the better your driveability and mileage. I also read hat the larger cfm carb you get, the better. Period. Then I read someone says a 650 cfm carb is perfect for his 350, more driveable and much nicer car. Then I read that a 350 should need "at least 725 cfm".

There's not so much concensus going on here... is there a clear answer or is it just one of those things where everyone has their own way of doing it?


I've got a mild-ish 350 I'm building around some slightly worked Vortec 906 heads and a comp xe262 cam. Im thinking it should be a great street setup, which is what I'm after.

From what I know from reading everywhere, the Q-jet seems like the best carb for me, but I dont like the idea of having to scrounge juckyards to find one and I definitely dont want to deal with a computer controlled carb. I dont want to deal with the computer at all. And yes I know that means I need a torque converter switch and new distributor.

So Im thinking just for the sake of availability and commonality, i'll get a Holley with vacuum secondaries. You guys think that would be a good idea for my purposes? I havent realy looked close at any model numbers or anything but I was hoping something would pop up on the classifieds. Just hoping to get a ballpark cfm number... I was thinking I'll just go with 650, but it'd be a shame to spend all this money on these new vortec heads and all these other performacne parts and choke it with a small carb. And then I hear people say something about carbs with vacuum secondaries you need to treat the cfm number differently from ones that have mechanical secondaries...

It's just a bit much for me to take in in little isolated pieces and be able to put it together nicely without some clarifications.

 

We typically ask questions up front such as "What have you done to the engine/car" and "What are you going to use the car for". Those answers will change the recommendations.

There are reasons a smaller carb improves driveability and a larger carb improves power. There are only compromises when you try to accomplish both. The q-jet is about the best example of compromise you can find, and even then you'll have issues in some area no matter how well you account for things.

If you aren't out for all-out performance or economy, then a Holley vacuum secondary can be another justifiable compromise. The only thing about CFM and VS is that you can "get away" with a carb that is larger than needed with VS unlike mechanical secondary carbs, which can kill you if you go too large. By the age-old CFM formula, (CID x RPM)/3456, you only need about a 575 CFM carb - but that assumes 100% VE and a single plane intake manifold. Since a dual plane intake works better on the street, bump that up a little and 650 is a pretty "safe" size. The primaries won't be so big that driveability or economy will suffer, or in total so small that power would suffer. If you consider DP, you only need to avoid going so large that you below the minimum RPM for going WOT for the engine and carb size (read: torque converter stall speed) - for a 350, a 750 is safe for around 1500 RPMs, so you're still okay with a stock torque converter. Back in the 70's, I had a 302 with 650 DP on a single plane intake and 1200 stall torque converter - it was a dog off the line, but once you got above 2500 RPMs, it would fly.

Personally, I'm partial to mechanical secondaries once you go away from the q-jet, but I wouldn't stop talking to you if you went VS.

 

"I've got a mild-ish 350 I'm building around some slightly worked Vortec 906 heads and a comp xe262 cam. Im thinking it should be a great street setup, which is what I'm after."


Just something on the practical side of high performance. I'm hoping to have a car that gets 15-20 mpg but as long as it's over 10 I'm satisfied. I hear it has more to do with right foot moderation than anything else, but Im sure the carb has some effect on that.






Will I see a huge gas mileage decrease with mechanical secondaries then? I just figured vacuum secondaries woudl be the more practical setup. But if it's not a huge difference I'll go ahead and go that route.

 

It's all about tuning. Theoretically, a DP will get the same gas mileage as a VS if you only run on the primaries. I have heard that DP has different power valve passage sizing than VS, but I've never seen that backed up. If you jet the primaries for a good cruise air/fuel ratio, size the power valve for good idle/cruise to power transition, and jet the secondaries for power, there's no reason a DP can't get good mileage. Theoretically. Most DP owners aren't as concerned about mileage, though.

(BTW, my first comment about usage was in response to what you said about reading what appeared to be contradictory carb sizing approaches, not that you hadn't given sufficient information yourself.)

 

There is a non-cc q-jet eh? That avoids the computer. You can buy them aftermarket, as well as most of the parts aftermarket, if you're afraid to get your hands dirty at the JY. I prefer to get my parts "previously loved", and dirt cheap from the JY. I like my non-cc q-jet, as far as "vacuum secondary" carbs, it's one of the best. (It's technically not a vacuum secondary carb, but....)

The main diff in vac sec vs mech sec is that on a mechanical, when you punch it to the secondaries, a double pumper will allow you to snap open the secondaries when desired, and it'll just dump in the fuel to compensate. If the carb is too big, it'll basically flood it, or overwhelm it. So it'll bog. Either way, you can see how this might lead to excessive fuel consumption right?
A vac sec only opens the secondaries as fast as the engine needs it. So if the engine is smaller, it doesn't need it as soon, so that's how a vac secondary carb can be bigger than needed, and not bog. This way you don't need the secondary accelerator pump, it just pulls fuel from the secondary jets, as the secondaries open slowly.

I think this leads to a double pumper having slightly better throttle response when you first smash the gas pedal, and if you're lifting your foot to shift your standard tranny car, you get that faster response at the beginning of each gear. If it's an auto, or you're powershifting, it's not that noticeable of a difference between the carbs. That sound about right five7?

 

Pretty much.

A too-big DP doesn't "flood" the engine. What happens is the flow area is so large that the air velocity through the venturi is too low to draw fuel through the booster venturi - hence, a lean mixture and "bog".