I need some input as to what kind of power gains I should expect if I go from a 650 cfm edelbrock truck carburator to a 750 double pumper?
My engine is a 350 with a comp cam extreme energy 230/236 .520 lift cam, trickflow heads, roller rockers, performer rpm intake, LT headers, etc. I shift at 6500 rpm. I think the 650 is not sucking nearly enough air. How much do you guys think a 750 carb will help powerwise? A good friend has lent me 750 holley double pumper but I won't be able to put it on for a few weeks. I'm hoping to pick up 2 tenths in the 1/4.

 

You should pick up a tenth or two. Pending on the condition of the Holley 750 dp. Doesn't really have anything to do with sucking air. The Edelbrocks are not real good performance carbs.......but very reliable.

 

IMHO, a 750 is too big for a 350. Talk to any custom carb building shop, and they will say the same thing (unless this is a full out drag motor, NOTHING else). Driveablity will suffer, as will throttle response.

I had a custom carb built for a 350 with all the usual goodies with aluminum heads (pretty much like yours), and the custom carb shop recommended a Holley 650HP. After they worked their deal on it, it flowed around 680 ~ 685 CFM. It was the best carb I've ever owned.

Bigger is not always better. Is that 750 DP box stock, or has someone changed jets, PV, squiters, pump, cams, etc, etc, etc?

 

IMHO, you can't overcarb an engine. It will only suck in what it needs. Now that is within reason, you can't stick a 5,000cfm carb on a 2.0L engine. Anyhow, I think you would gain some from the 650 you have now, but I would agree with Xcyz, and check in to a carb shop and have them get you one tuned for your engine.

 

A very good friend and I each have a fastburn 385 + HOT cam (430hp) in our cars. His is a few hundred pounds lighter than mine and has a holley 650. I have a 750. He runs 12.6's @110 and I run 12.8's@108. Both of us run the same street tires and get very similar 60' times.

Once that 650 is set up properly, it will perform just as well as a 750.

 

Ditto, a 750 maybe a lil too big and may even hurt performance. Depends on the metering and jetting. Best way is to dyno it with both carbs and set adjustments as needed.....

I was running a Holly 750 and switch to a E 650. I get better driveability, MPG, and lil more power.

 

Funny, we're currently running a 750 Edelbrock on my Dad's '73, but we've got a 650 Holley double pumper we want to try on it!

I don't know how accurate this is, or how well known it is, but in a performance magazine I've got, they give a formula to figure out the CFM requirements of an engine (based on 100% efficiency, although most motors they say are less than 100%, usually ~80 - 85% for a street engine):

engine size (CID) X max RPM / 3456 = CFM required, @ 100% volumetric efficiency

ie: 355 CI X 6500 RPM = 2,307,500 / 3456 = 667 CFM @ 100% v.e.
667 CFM @ 85% v.e. = 568 CFM, so in theory, just a 600 CFM carb would do, according to this article I've got.

 

Ive got the 750 on my 350, i just had it leaned out, and jetted correctly, and you're right a lot of people say its way to much for a 350, and i havent really tested it on the track, but after putting the new pump and regulator i did notice a bit of a decrease in performance, im just going to go get it tuned at a shop.

 

I'm driving a 79 with a rebuilt 355, mild cam, stock 882's. I started with a stock replacement holley 650 new from Summit. It was horrible, the motor stumbled and choked up everytime I accelerated. So I figured it was getting too much gas so I downsized my squirter a few sizes and it made it worse. So I upsized and it made it a little better. I went to a 50cc pump and 72 Main jets from the 68's that came on it, and it made my acceleration smoother but it still stumbled on hard acceleration. So I bought a 750 Holley from a friend for $100.00. It came with 72 Mains and I down sized them to 70's. Put my 50cc pump on, and tuned the heck out of it. Im getting ZERO problems with acceleration and performance. Only problem I have now is about 12mpg.

 

Thanks for all the input guys, I think the 650 edelbrock is great for daily driving but I just don't think it pulls past 5600 rpm like a 750 will, my motor was built to go to 6500rpm and the 650 chokes out. I tried a 750 and although I didn't tune it right it definately pulled harder at the higher rpms. I ran a 12.6 et with the 650 but it was set to stock and it ran lean, in a week I will try the 750 again, this time at the track.

 

I finally put the 650 Holley on the '73 last week.

So far?? Well, the car is 100 times better for driving on the street! You can actually pull away smoothly from a set of lights without it breaking down and stumbling across the intersection like it did with the 750 Edelbrock. In the mid-range, it seems a little.....boring?.....but up top (once you hammer it and get all 4 barrels open) then it pulls good to ~6500 - 7000 rpm still!

It's hard to say though, 'cause once the car was moving, it seemed to work good and pull really strong up high with the 750, but the tailpipes don't look so black now with the 650.

 

Dude , stick with the 650 performer carb and you will be happy.AND YES YOU CAN OVER CARB A MOTOR!!! It's just like you can over cam it.What is good about the the 650 performer it can be rejetted and you don't even have to take the carb off if you don't want to.Hollys are a pain in the a**.But if you get a demen or barry grant they are good carbs.I have a 383 stroker and i have 660 cfm three 2 set up and it runs perfectly.......my 2 cents. PS i shift at 7000 rpm......

 

650 will net you more torque out of the hole (which could mean more tire spin...fine line trade off.) But the 750 will give you more upper hp. Pick your poisons. I run a 750 hp pro series holley. I would not trade it for nothing. My car is not fast. But throttle response is crisp. Best time is 8.209 in the 1/8th with a 60' of 1.84 Drag radials

 

I run a professionally built sb 406 (Gerald Dyer and Gene Fulton built) and they both told me to run a 650 double pumper but i went with a 725 Demon and toned it down a little. Listen to these other guys or your gonna lose time on bottom end over any gains you would see at high rpm's which would be minimal anyways.