Carby selection

[Jayro]

Hey guys,

Curious how you guys decide which carby to go for. Not so much brand but size. When do you opt for a 600 or a 650 or 750 holley etc?

Is each size good for hp to xyz or not the case? I’m just curious to know thats all.

Cheers
Jase

 

[c9zx]

It depends on how the engine is built and how you want to use the car. No matter what size you select pay extra, if needed, to get as much adjustability as possible (changeable air bleeds, idle channel restrictors at a minimum). I prefer Quick Fuel, Holley, or Summit carbs. Custom units are available from AED or Pro-Systems. Chuck

 

[71ProjectJunk]

On a 351 Cleveland I would use a 750 CFM vacuum secondary carb.

 

[Hemikiller]

What Chuck said, it all depends on intended usage. If it's an ice cream cruiser, then a run of the mill 600 is just fine. If you're chasing some performance, then you'll be heading to a 750cfm.

 

[mgaeta12]

There is a formula for that, which takes into account the engine build and intended driving style. My stock 289 calculate out to 320CFM, and that’s about what Ford put on them, my Aurolife 2100 is 300 CFM (1.14 Venturis) Check out this link: https://performanceimprovements.com...lating how,volumetric efficiency of about 80%.

 

[steves73]

Like mgaeta12 said, there is a math formula. For my 351C, I don't go over 6000 rpm so a 670 is best. Bigger isn't better, because the fuel/air flow needs to be matched with the carbs design.

 

[jscott]

When I bought my car 2+ years ago it had a Holley 750 that drank gas. I tried a Holley 600 and they engine did not like it, went with a Summit 750 and it now runs great and, I think, gets decent gas mileage. Through my research I found that many say the 351c likes a big carb although all charts for CI size say different.

 

[Bentworker]

Displacement, camshaft, elevation, intended RPM range all play into the magic formula.

For me I have a 379” Cleveland and use a 750 CFM Vacuum secondary Holley based carburetor. I agree with Chuck to buy one with adjustable (replaceable) air bleeds. Mine did not have replaceable air bleeds and I ended up tearing down the carb and machining it for them.

No matter what you get the odds are not in your favor that it will be perfect out of the box. Plan accordingly. An air fuel ratio gage for tuning is handy. A jet and air bleed kit is good to have, along with an assortment of accelerator pump cams.

 

[giantpune]

There are calculators and formulas you can use to see exactly how many CFM your engine will need. 351CID spinning 7000RPM running 85% volumetric efficiency will need to flow 600CFM. Same size engine, running 5000 rpm, and even running down at 75% volumetric efficiency only needs 380CFM. Its not really about horsepower. Its picking a carb that is big enough that it can flow as much air+fuel as your engine will ever want.

Once you figure out what size you need, then you decide if you want vacuum or mechanical secondaries and if you want electric or manual choke. Holley recommends only using mechanical secondaries for manual transmission cars. On my 351, I'm running one of the holley brawlers, 600CFM, with vacuum secondaries and the transparent windows in the bowls for easier tuning. On my chevy 350 and my ford 302, and even on my ford 429, I'm running edlebrock carbs, around 650CFM, with electric chokes.

 

[mrgmhale]

I find that with with a moderately built 351 (W or C) I have done well with a 750 or even 780 CFM Holley as long as it has Vacuum Secondaries. The Vacuum Secondaries keep that large a carb from being "too much carb" until the RPMs are high enough to be able to make use of the added flow when the secondaries do open up fully. It is a very flexible (forgiving) design feature. Long ago I listened to an "expert" who told me my street/strip built 351W (1969 Mach 1) would run better with a 650 CFM carb. I was miserable. The vehicle was noticeably slower and lacked the ability to breath enough to get to the same kind of performance I had with the 780 VS Holley. Now I read and listen to other opinions, and may even follow certain kinds of advice. But never again will I choke a 351 with a 650 CFM carb. They deserve better, and the 351C with its much larger ports can make even more use of it than my past and current 351W street/strip built engines.

 

[Idaho Chris]

Everyone has an opinion on brand of carb and proper size. Next to oil selection, it is a common conversation at any gathering of car guys; online or in person it is always fun.

As far as sizing, I generally go with what the calculators/formulas recommend. Unless you are going to regularly play in the 5000+ rpm range, on a street build cruiser with your 351C, in my opinion 650CFM is more than enough.

Next question- vacuum or mechanical secondaries. In most all cases I am a believer in vacuum secondaries. The factory engineers went this way for a reason. That said, if you are running a manual transmission with a high ratio rear end you can certainly have more fun with the immediate action of a good old Holley (or other) mechanical secondary double pumper.

Now you're down to the choke question. In almost all cases I am an electric choke advocate as they are simple and reliable.

On my 351C, running factory HO 4V cam, T5 manual transmission and a 3.89 rear end I have been very happy (and have a lot of fun with) a Holley 650 double pumper.

I also play around with a '69 428CJ. On that car I run a vacuum secondary 780 cfm Holley.

 

[will e]

I couldn't decide, so I went with three.

 

[Airich91]

The tri-power, I love it man!