[d.b. cooper]

Here are my latest thoughts; Feel free to correct, criticize, condemn, or augment as you see fit.

Lately, i've been constantly struggling as far as what to do about my carb situation. Even before the problem and even if i DO manage to get it back to the way it was (we are close), i knew that the carb wasn't really "right" and needed to be replaced; It's actually calibrated for 305's and 350's----and i'm running a 262 inch v-6, 4.3. While it ran "ok" for the past 41/2 years, it never really idled "right" or "normal"

(Jeg's #15005 q-jet)

In my ongoing, extensive research (and we have a lot to talk about when this is all over....), i noticed that quite a few 350's and 327's of the sixties had 2bbls----the 2G----on camaros, impalas etc. Some of these engines produced ok output------for example LF7 327---210hp, L65 350---250hp. (Note, that my goal for my engine also happens to be around 250hp, moron this later)

a) Does a 327 or 350, or in my case 262 need that much cfm if the engine is not turned higher than 5000rpm? (we are dealing with stock bottom end)

b) It seems like the REAL advantage to 4bbs is more versatility---- possibly sharper, more responsive throttle at lower rpms----due to the smaller primaries vs. the 2bbl's throttle size---they can't be too small because there's only 2 of them vs.4?
Not that a 2bbl throttle response is sloppy at low rpm. Just not as sharp, especially compared to a quadrajet due to the very small primaries.

bi) Possibly better mpg, but i don't think it would be more than 1-2 mpg if that. Plus, i don't care about 1-2mpg improvement mpg right now.


But, in terms of flat out power, would a 2bbl (assuming it flows enough) be that much slower than a 4bbl if at all?

i'm just weighing all my options, because after ruminating so much lately, i think that carb has got to go because it was never right for the engine in the first place.

i've had success with rochester 2G's before i switched to q-jets.

i define "success" as
* The engine starts and runs normally.

*The engine idles normally----without having to crank it over 1000 rpm so that it will idle in gear at 600. (and really 1000rpm in neutral isn't good either, but i'm not good with carbs, so for me it's good)

*Doesn't stall out. This is with AC and all accessories on.

* MPG is reasonably in the ball park of the stock, comparable setup: For example if a camaro, from the factory, with a 350 gets 18 mpg, and you get 16-19---SUCCESS!


i noticed that the later, large base 2g's flow around 435 cfm's. Some may flow 500cfm. If we consider a relatively stock 350 with stock or close to stock cam, spinning max 5000 rpm, the 2bbl should be adequate for airflow?

 

[BillPac]

I'll probably get some flack for this, but . . . there is a formula for ideal CFM vs engine size, and it works pretty well for most applications. Some will argue that it doesn't represent real world conditions. Take it for what it's worth.

Engine size x max RPM x VE (volumetric efficiency) divided by 3456 (constant)

Most automotive engines run 75 -85% VE, though race engines may achieve 90-95%. On my car it works like this:
350 x 5500 x .85 div. by 3456 = 473

That means, if I use a carburetor that flows at least 473 CFM I should be OK. In my case I'm using a 625 CFM Demon carb, and it works great. I could probably get by with a 500 CFM 2 bbl, but I'll stick with what I've got. Hope this helps.

 

[eric32]

I myself over all of my years have always 99 percent of the time stayed in the 600 to 650 range size wise regardless of either a Holley vacuum secondary or a double pumper on my builds which has been 90 percent 350 builds and I have an old 383 and a current 377 which is a destroked 400 with a .030 overbore and I have always had excellent results with fuel mileage almost hitting 20 mpg highway and around 10 in town driving in my truck.

I did try a Holley HP 750 for a short time and the only benefit I got from it was the top end on things past 5000 rpm and I don't take it to the track or let alone race it or drive like a maniac on the highway and I like to be conservative on the size and I always just do some upgrades on my smaller cfm Holley carbs and for all of what I do they have never done me wrong and have always worked good even with getting into it. I know I leave some power on the table but I don't drive at 6000 rpm going down the highway lol so just something basic and simple but yet still provides me some performance is to me an all around good thing for me wise.

If you got a stock v6 I would say a 2g carb would work real well with that build. If a stock 350 can be given one then I don't see why it would hurt anything. I mostly stick to my Holley 600 vacuum secondary carb or Quick fuel and it has been the overall best all around performer that I have had the best results with and there are also so common that it makes it nice to get parts for at least that was before covid junk hit and they locked down everything.

I think the formula can still be good for an all around cfm rating wise to get a little bit of everything without limiting or choking your engine to bad in everyday driving conditions vs track only type of a deal.

 

[eric32]

Also on my engine 377 build a couple of years ago when I ran the Holley HP 750 my bottom end throttle response on mild and half throttle opening and hitting the throttle fast, the HP did not do as good as my Holley 600 or 650 carbs did. The smaller barrels provided a lot more crispiness on the throttle vs the bigger size of the 750. Ultimately that is what led me to sell the 750 and stick with my smaller carb as for me wise I enjoyed and better overall performance and signal it gave me on the bottom and mid range area of the tune and also while cruising it was a lot more nicer of a feel for what I like in my builds.

 

[BogiesAnnex1]

Yes! You’ve got the bull by the horns, if not the gonads.

Bogie