Sounds like a newb question, I know, but after finding out my 670 SA is only a 600, i feel like i've been nubbed.

So I contacted Holley about that 670 and was told: "the 670 CFM rating is based on a Dry flow calculation measured on a slightly different pressure drop (vacuum). Typically Holley utilizes a wet flow rating closer to 600 CFM, but given the fact that most people put on too large of a carb (to the detriment of drivability and
performance) we elected to go with a dry flow rating on this model to encourage consumers to buy the next size smaller."

Bitches.

Now I know why my motor ran like **** with the 236/242 cam in it...and im pretty sure that 600cfm is too small for it now. I am looking for suggestion for my next carb (*or if anyone has one for sale*). Holley recommended a 750DP, but i think that it too much for the motor...and more gas than I care to spend.

If anybody has a 700DP for sale I would gladly take that, but I think my choices are narrowed to 650DP or 750VS...what do yall think.

(btw...I got 50 bucks says 5-7 tells me the 650... )

 

I was right about that SA!! 600 cfm...thanks Holley for your

We all know five7's recommendation. Mine is for a 750 VS. That is what I have on my 383 and I am liking it. Out of my go-to book on carbs and intakes (How to Build Horsepower vol. 2 by David Vizard) it has a dyno test of a 650DP vs a 750VS and on a 383. I will quote the caption from the graph:

"This 383 Chevy motor was equipped with ported Dart II heads, a 9:1 CR, a 280 Comp Cams hydraulic flat tappet cam and 1 5/8 inch headers. The point to note is that the 750 vacuum secondary carb gave better low end and more top end than the 650 Double Pumper carb. Vacuum secondaries are really good for the street."
(emphasis added)

Another quote:
"A 3310 style vacuum secondary carb will give the necessary drivability and response on the street, and you can size it up to 100 CFM larger than a comparative mechanical secondary carb. This offers a performance advantage on the high end without low-speed losses."

If you want to learn A LOT about carbs and intakes, that book is highly recommended. Since mileage is a concern, that would also favor a VS. Or you may even want to look into a spreadbore, like a Q-Jet if you are not set on a Holley, or try Holley's spreadbore.

Just my

 

Thanks for your information Bunk, I think i'll get a 750VS.

Originally I had asked the Holley tech if I could share this information with you guys, but then he didn't get back to me soon enough and I decided to share my findings anyways. He finally replied telling me NOT to "publish" my findings...too late! Here is what I told him:

In most cases, where the user knows nothing about carb selection, I could see why Holley would do this, but what about people that DO have a clue about how to choose a carb? Had Holley at least taken ALL of its customers into consideration and WARNED us that it was dry-flowed, then I would have gotten a 770 so that I had an actual 700cfm...what a joke. Now I HAVE to get another carb because this one is not just border-line small, but IS TOO SMALL. The shame is that I don't know that I can trust this brand again. I could go on ebay and get a 650DP or 750VS for cheap, but selling my 300 dollar "670" isn't ganna fetch me 175 dollars. I can see why you wouldn't want this to be published, plenty of people would be pissed. People on my message boards NEED to know that the SA line cannot be accuratly compared to other carbs, so that they can choose correctly.

*holley loses another customer to BG*

 

750 DP.

Where's my $50???

 

Bah, you know DAMN WHELL what I meant...but either way, just because I happen to have fifty dollars that *said to me* : "hey, he's ganna say the 650DP..." DOES NOT mean that you get it

:-P

 

I think Demon rates their carbs wet flow. I like my 650 DP

 

According to the Holley website, the SA's are "100% wet-flow tested and street calibrated".

Wet-flow tested and wet-flow rated apparently are two different things. And, the vacuum at which the flow is rated is obviously critical, as we know with 2bbl vs. 4bbl industry "standard", but nothing requires anybody to abide by those standards.

Regardless, I've never had much regard for the SA line, one of the reasons being the line is more of a marketing ploy than addressing a technical niche. If you really think you want vacuum secondaries, get one of the universal models (I have an electric choke 3310-type 750 VS sitting in the garage attic right now).

 

I don't know what all it means, but after calling Holley out on it, they have decided to "work with me on this" *shrugs*. If I can get free 4160 - 4150 conversion for my nice new shiney 80508 then i'd be happy!

 

Where'd the 80508S come into the picture?

Did I miss a memo????

(That's what's in the garage attic, by the way, with 4150 conversion, quick change secondary spring kit, and Quick Fuel adjustable secondary housing...)

 

I won it (and paid) a few hours before reading your post *kicking myself still). My dad needs a bigger carb, how much do you want for it?

 

My reccomendation is the 650 DP. When tuned right it's plenty of carb for a 383. Had a similar setup on the engine dyno a few weeks back. 383, 6 inch rods, Vortec heads, 650DP, 236/242, Performer RPM. Made 453HP@4200 and 475TQ @ ?? RPM (sorry, can't remember exactly). Not sure what size rods you've got, but if they're shorter (5.7" or 400 rod) you'll notice the power a little lower in the RPM band. I like the DP b/c of the Mech secondaries. For a car that's bracket raced they tend to be more consistent.

 

453hp at 4200!! WOW.

I got 6.0 rods too.

Thanks for your input, too late cause I already got the 750VS...maybe DP will come another day...

 

The VS will be fine, maybe alittle better on the street but I think 750 will be too much. You can pick an intake that will make the engine think it's got a smaller carb though.

 

See, i origionally wanted a 670...but it turns out, get this, that Holley's 670 is REALLY a 600!!

I dunno, 750 may be too much, may not...the VS should compensate I would think.

 

i think it will be too much, especially with the small cam. Choose your manifold wisely.

 

Correct, your secondaries will only open as much as your engine needs. It is "impossible" to overcarb with a VS at WOT because of that. If it is too big then you'll get reduced booster signal at low rpm's and drivability will suffer, but that is usually not a problem until you get into the 850 CFM range. The 750 VS on my 383 pulls hard and I am getting better city mileage than my well tuned Q-Jet...but my Holley has a few tricks though. If you have a dual plane intake (and you should) 750 VS will be perfect.

 

I felt the same way shortly after my 950HP purchase and found out it only wet flows 830cfm

750VS will work out great, when you get ready for less mpg and more go fast, 750DP all the way!! Been there dont this with 6 different Holley carbs ranging from 600VS to my 950HP all on the same old 355 I started out with- 9:3.1 compresion, 230/230 .480/.480 110LSA hyd flat tappet, dual plane intake and with every increase in carb size the car got quicker AND faster until I swapped from Holey race 830DP with a few mods to this 950HP I'm running now, but the switch to the HP series gained considerable consistency over all the rest!!!

 

I didn't really think that a 750VS would be too big. I have an RPM air-gap manifold so I would assume that, even with that lil notch, the signal will still be strong.

 

I think the 750 vs will do just fine performance wise and as far as gas mileage it will do even better.