Thread: Help selecting a carburetor

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I just bought my first BBC. It’s a 289 block. Have no idea on the heads, cam, compression, etc... was pulled from a 1974 Pontiac Firebird & the guy wanted to put the correct motor back in. Was able to hear it run & set me back in the seat Only problem is he wanted the small carb ( Holly 650 ) & I need to get a new carb. Just don’t know what is the best one?? Thanks...

There's no quick answer to what is the best one.

Some are cheaper, some easier to tune.

I seem to recall that a benchmark for selecting size is CID X 2 =CFM.

Others will chime in and give more info, good luck.

Or, how about moving into the 21st Century and going with a F.I. retrofit system like FAST ? Or any of it's variants...

Originally Posted by 34_40

Or, how about moving into the 21st Century and going with a F.I. retrofit system like FAST ? Or any of it's variants...

X2, but if you're set on a carb Google "Carb sizing based on cubic inches" and you'll find the accurate calc relating them with rpm and volumetric efficiency.

From Speedway - The formula for calculating how much CFM (cubic feet per minute) an engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.

Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.

Nobody is wanting effiency------just RPM!!!!!!! altho CID x 2 does equal a CFM fiqure that is pretty good on a street PERFORMANCE vehicle-----it isn't an accurite mathematical formula---------

Using the fancy formula, @4000 rpm and assuming the 80% efficiency yields 289x4000x.8÷3456 = 267.6

That sounds rather small, am I doing something wrong?

The less accurate CIDx2= 578 which is a lot closer to the 650 it was running.

If the Holly 650 was doing it justice before, just get another.

Originally Posted by firebird77clone

Using the fancy formula, @4000 rpm and assuming the 80% efficiency yields 289x4000x.8÷3456 = 267.6

That sounds rather small, am I doing something wrong?

The less accurate CIDx2= 578 which is a lot closer to the 650 it was running.

If the Holly 650 was doing it justice before, just get another.

Well, the '73 block suffix 289 is a 454CI engine, and you want to use the maximum rpm, closer to 6000 than 4000. With those numbers and only 0.8 VE yields 630 scfm. Assuming it's better than bone stock, using a VE of 0.85 and 6K rpm pushes to 670scfm. The 650 was probably marginal at the top end.

I'm running a 750 on my 460 so a 650 on a 289 should work fine. If you liked how it ran before then buy the same carb and make it simple! If I ran cid x2 my bored out 460 would need 1000 CFM carb! At 4.19 a gallon here, It would never get run period!

Cool.

Now let's discuss holly vs edelbrock vs rochester vs Carter, etc.

I e run a lot of rochester and edelbrock. Rochesters can be modified from stock, and there are a million variations out there.

I seem to recall that Carter is really edelbrock or vise versa. Hollys seem to be preferred for ease of tuning.

Set me straight guys!

For out of the box preset ready to install with little to no adjustments Edelbrock. For the ability to modify Holly is my racing friend's favorite. For me simple works, but I'm running a pretty tame setup.I also don't enjoy playing with carbs! Just eating them!

Originally Posted by stovens

I'm running a 750 on my 460 so a 650 on a 289 should work fine. If you liked how it ran before then buy the same carb and make it simple! If I ran cid x2 my bored out 460 would need 1000 CFM carb! At 4.19 a gallon here, It would never get run period!

Guys, "289" refers to the last three digits in the block casting number, NOT the cubic inches of the block. The "289" block is a 454!!

And for what it's worth, the original poster stuck around for less than 15 minutes after posting his question, and hasn't been back. Once again.......

Well there is always is that Roger

For racing, Holley. For street cruising and light bruising, Edelbrock, with fuel pressure limited to 5 psi or less. Here is the new AVS2 with improved boosters. The whole point is to atomize the fuel into the tiniest little droplets (like a fog). This is what electronic fuel injection does and how it makes better horsepower than carburetors.
https://www.youtube.com/watch?time_c...&v=bNhOSX5lac8

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I really don't care if the OP has come back or not, I'm learning a lot.

Besides, we don't know his schedule.

Over the years I have basicly run carbs that were way bigger than most methods would work out---and the basic reason??????? well , they have been on trucks that I used to tow the real race cars---------and I wanted a set up that would give me more air on just midrange primaries for highway speeds ( 70-75 mph) without getting into the seconaries on the carbs----------had lots of discussions with the folks at Edelbrock( they balanced our California Charger dragster engines back in early 70s) there methods of sizing was for wide open max rpm areas and would just have a 600 cfm--------I ran an 800-850 that would give almost 400 cfm before sec opened-------I got 9-12 mpg whereas those smaller more effienct 600 carbs would only get around 6 mpg--------