Where does the added CFM come from, the back barrels or a combination of both front and back? I had noticed that some 650s and 750s (not sure what particular models, just an observation) had the same primary bore size but the 750 had bigger secondarys...anybody know if this is true for the S/A carbs? Wouldn't this mean that one should get the same gas mileage (keeping the back barrels closed) with a 770 as a 670?

Think the extra 100CFM is worth it?

 

I didn't find the specifics on the SA's, but in general 650 vs. 750 with Holleys is a function of venturi diameter, not throttle bore diameter.

Any particular reason you're thinking of the SA line? That's the part that I wouldn't consider "worth it".

 

I just happen to have a 670 already and was using it for the sake or arguement. Most people with 383s seem to slap 750s on them and I was wondering if it is worth swapping for 80-100 CFM. I would rather reserve my mileage so would only consider it if bigger secondarys is where the extra CFM came from.

 

If the SA 670 was even a 650 (will address in a sec), a 650 will probably not give up much in HP and the smaller venturi will create a stronger and better signal at the boosters. This will result in better mileage and throttle response at the cost of restriction at the top end. CFM of Holley's is a combination of venturi diameter, throttle bore diameter, and booster type. Venturi's are the greatest contributor to cfm though. But in looking at carb books, increasing throttle bore alone is worth additional cfm. And for boosters look at a 3310-1 Holley vs 3310-2. Holley lists the 3310-1 as a 780 CFM while the 3310-2 is 750 CFM. Everything I have mentioned is the same (venturi and throttle bore), except the booster type differs. The 3310-1 has less restrictive downleg boosters while the 3310-2 and later carbs have more restrictive straight boosters.

Knowing all that, look at holley's web site at the "670". It's boosters, venturi size and throttle bores are the same as carbs rated at 600 cfm. I have a holley book that lists a few at 650 with everything the same, but I would go with the majority (600). I was in the same dilema, I had a "670" sitting around for my 383. But when it looked clearly like a 600 in print, I said "no way" and got myself a 750. If you want to "have your cake and eat it too" then get a 750 and have annular boosters installed. That will get you the stronger signal as if you had a smaller venturi and still be less restrictive, giving up less hp up top.

HTH

 

On the 396 a couple of years ago, I went from a 750 VS to a 650 DP and took 2 tenths off the 1/4 mile ET w/o any reduction in MPH. I then put in the Proform 750 mainbody and didn't really pick up anything.

VS carbs will only open as much as the engine demands. A 100% VE 385 won't flow 670 CFM until 6000 RPMs or so. You probably aren't even opening the secondaries fully until 4000 RPMs or so.

You'd see more improvement going to a double pumper than you would a larger VS carb.

 

SO what about this talk of the 670 only being 600cfm...HOW can you seel a 670 but it only flow 600...*is confused*

 

Yeah it is confusing, but it's Holley, they can do as they please. You can say a 600 flows 700 if you want, just put in the fine print that you flowed using a different method. If you do a search on here you can find someone who talked with a Holley tech who also said it was really a 600 too. "670" is just a marketing gimmick like Splitfire's magic spark plugs.

Careful, you are using the ol' "formula" which if you read the fine print on that, it is for calculating needs of a single plane intake only. If IROC_385Z has a dual plane you want more cfm since you cut the carb in half effectively (if you want max hp that is).

 

But still assuming 100% VE, which rarely is the case.

The "formula" doesn't make any distinction between VS & DP, either. And, for peak power, it won't make any difference. However, DP will consistently pull harder in RPM ranges where the VS secondaries are not fully open.

I believe Holley is trying to be more consistent with their CFM ratings. Note that the SA and HP series are wet flowed, unlike their earlier model "universal" carbs. I guess BG got to them - nothing like a little competition.

 

Well, I have a raped 750VS laying around...missing a bunch of part no doubt...could I marry the mainbody of that to the metering blocks and bowls of the 6"70"? Does that sound like a good idea or am I just sounding like a fool hoping to not buy a new carb...

 

Well, remember that your engine will run, and probably run well with the SA. If you are trying to just use a carb that works for now, go for it. The only issue is that it might "run out of steam" if you are racing. But if it is like your daily driver, who cares?

Now on using the 750, I'd want to have the main body, throttle body and metering blocks that all match. The metering blocks on a SA are probably matched to that carb and might not work well with the 750's air bleeds etc. I wouldn't chance it. So basically the only parts to salvage would be the bowls, and vacuum and choke housings. When I did the math when I was in your shoes, it was a better idea to just sell it on eBay since you can get 750 VS Holley's for $80. And I think I sold my SA for more than that. You definately aren't a fool though. Also keep in mind that a rebuild kit will run you $25, incase you are trying to save money and that SA is already ready to run.

 

It is a daily driver and I try not to get *too* caught up in the HP bug...BUT, a 650DP is starting to sound like a good idea...especially since I can probly sell the SA, buy a DP an have enough left over to rebuild it, and then some...ya know, when you've spent so much on the motor (time and money) it just seems foolish to NOT spend the extra (again, time and money) to just do it right...catch 22 baby...catch 22...*sighs*

 

...And THANK YOU guys for the advice!! MUCH appreciated.