Just one last note. A 670 Avenger will work fine on his existing motor, although for reasons I don't want to get into I don't like those carbs. The 4779 Holley 750 will work fine too on his existing motor and will work fine on the motor he plans to build with a roller cam and Brodix heads, so he only has to buy one carb

 

100 CFM over recomended is a good rule of thumb. Also agree with using a wideband A/F Meter in real driving conditions for best tuning. Not taking anything away from the Quadrajet but I have always preferred a Holley. All GM's high performance engines used Holley carbs.

I have a 4778 Holley Mechanical secondary DP in the garage taking up space. It is a 700 CFM. It will need a kit and tuning but it is nice and all there. For a 350 with a hydraulic roller which will limit you to around 6000 RPM, a 336 gear and automatic, I think it would be perfect. $75.

 

Thanks motorhead and mako for the info and the offer, But should i stay with a vac secondary for the automatic?

 

Carb sold to a guy with an automatic that read this post. Bump for you.

 

In 62 minutes. Nice.

 

It's been a while since I ran one, if you need all the vacuum ports and you want vacuum secondaries and a choke then you are going to have to look around the Holley site for the appropriate carb.

Also annular booster are better for the street, you can buy them separate if the carb has spun in boosters. Hope this helps.

 

Not that hard to do. I get 293 hp with a 600 CFM DP on a 331 at 9.5:1 with a hydraulic 327/350 cam.



BTW, Summit Racing just came out with their own line of carbs. They look to be compatible with Holly DP stuff but have no gaskets beneath the fuel level. They have 600 and 750 CFM vac and mechanical sec models and retail for ~$275.

 

you are bang on, I'll post a scan tomorrow - only at 4500-4900rpm was I at 100% VE or greater

VE was highest at max torque 4600rpm (100.5 % and 448tq)

VE was 96.4% at max hp 5700rpm and 444hp

at 6000rpm she hits the wall:

5800rpm.............442hp/400tq.............VE% 95.5
5900rpm.............403hp/359tq.............VE% 92.2
6000rpm.............372hp/326tq.............VE% 88.4

 

Obviously never read my post or just didn't get the point I was trying to make

Point was there was nothing wrong with the parts I was putting on my mild 355ci

 

i had a stock 81. original everything .stock carb, heads, 2.78 gears the lot !!! . bought all the parts i needed for my rebuild which was GM fastburns,demon 750 , air gap.roller cam, transmission ,bla bla bla but before i did the swap i stuck on the demon 750 DP just to see what it did. ran like a big fat pig. way too much fuel getting in. it drove but man it was a slug.

bolted on all my other bits and pieces and been hammering ever since. if your budgeting for less then 280hp at wheels stick with the 600-700 carbs. if you have 280+ at the wheels go the 750 . thats my opinion.

 

I thought the point you were making was that you needed 750 cfm to make those power levels, and that is just not the case.

 

Interesting... so your max air flow rate is 561 cfm. A 750 is then 189cfm more than you need, or 32% more!

 

Not exactly their own. It's their version of the old Holley 4010/4011 (long discontinued), which was a knock off of the Autolite 4100. They're not bad carbs by most standards and are very well suited to anyone seeking "out of the box" performance.

 

Yeah two completely different motors and you compare only the carbs, makes sense to me. Yours probably would have made more power with the 750 Holley

 

if the 750 was too big you would see it with a hit in the midrange - not so as I'm making 400+ tq at 3200rpm.

I don't believe that I'd have made 444/448 with a 600cfm carb.

At some point you have to put the calculator down and go with what works

Good Luck, Ed

 

If the 750 was too big you would NOT see it in the mid range, as long as it was set up properly. I have never said you can not use a 750, I just said that you don't need it that big.

You might not believe it, but just because you think something doesn't make it true You never tested with a 600DP (or 650, 700, 800, etc).

And again, this is my point. You say go with what works... but you don't know that a 600 or other carb won't work. Clearly a 600 has the capability of flowing as much air as you need. So why doesn't it work? You just don't know if it works.

 

I have just invested in several carb books to teach me how to tune my Holley... and in every book they say the same thing... bigger is not always better... Additionally, they spend a lot of time dispelling racer conventional wisdom...

I think we might be seeing the "If 650 is good, then 750 must be better right?" syndrome. If your engine only pulls 600 cfm... then a 750 will not help right? Won't you just have to jet the carb down a whole lot?

Again, despite any other engine stats, if your engine uses, say 561cfm... anything over is a waste right?

 

Sometimes racer conventional wisdom comes from actually trying an idea and seeing what makes the car go faster.
You can work all of the math equations that you want, but if the car goes faster with a "too big" carb then you need to reevaluate your theory.
Tell me this, what does the carb cfm formula do to account for pumping losses caused by high vacuum levels in the intake at WOT?
Small carbs are fine on your daily driver, but if you are going racing you are giving up a little ET.

 

Wrong. Volumetric efficiency can be higher over a wider range with a slightly larger carb than required due to less air flow restriction at the only entry point for air coming into the motor. That is where the rule of thumb 100 CFM bigger than required comes in. An engine is like a pump. The more efficent the flow in and flow out the more power that can be made. That is why headers make more power than iron manifolds. Less restriction of flow. That is why big flowing heads have more capacity to make HP than smaller heads. If you go too big it will reduce atomization of the fuel, it will drop out of the mix. Same with a carb. That is why racers run a little bigger carb than required. A street used engine might use a slightly smaller carb to keep the fuel suspended in the A/F mix better. All the Q-jets are 750 CFM. The Holley carbs installed on the factory high performance, solid lifter, high RPM motors were all 750 CFM or bigger. Even the 302 Chevy in 1969 used a 780 CFM holley. The early dual Quad corvettes used 2- 400 or 450 CFM carbs for a total CFM of 800 or more on a 283. They were just jetted down. Why would you put a 600 CFM carb on a performance build? It will probably work fine with a 327 hydraulic cam motor but a 350 and up all had 750's installed by GM even during the gas crunch when they were looking for economy.

 

ok then, show me a performance built 350 or 383 with a 600cfm carb

as Mako said, why would you put a 600cfm carb on a performance motor? Because your calculator told you so?

When I said go with what works I meant go with what has been proven - personally I have yet to see a 450hp 350 or 383 with a 600cfm carb.