V8 Diet: Big Carbs or Little Carbs?

I have a 1966 361cid big block with a 750cfm Edelbrock carburetor on top. A car-guy buddy says “that’s too much carb, install a 625cfm Carter and it will perform better”. I wanted to hear a logical explanation that made sense, or maybe even science-based, but no luck thus far. What say ye?

How are things working out with the 750? How is the idle quality? any flat spots off idle? In short, is there an issue with how the car is running now that needs attention?

The story is deeper. The 750 was on the Charger since 1976. Besides that and the dual exhaust, the car is strictly stock. The car ran smooth, reliable, good idle, but not an impressive performance for a V8. Two years ago I installed the original 2-brl. carb (the original condition of the car is its showing feature); performance changed like one expects. This past fall a buddy listened to the car and said, “this engine is lazy, ever change the timing chain?” No. “while your at it, put a 4-brl. back on it, but the 750 is too much, go with a 625”. The timing chain was severly stretched, and the new one made a nice difference. The used 625 has a leaking problem . . . bummer. So, I could either put the 750 back on, or swap for a good 625. I guess the 2-brl. is still an option.

I like the car being very original, but I’m ok with tactfully increasing the fun factor while remaining in the Survivor/Historical show class. But the question is: Why would one claim this engine will perform better with a smaller carb?

You certainly can bring together incompatible engine components, no question to that but from previous experience you are able to say that running the 750 cfm carb did not cause yor car to run like a vehicle that had too much carb on it. My feeling is that at one time this 750 was tuned and modifed internaly to be an acceptable carb for your engine.

A carb rated at 750 cfm would not be a good choice for your car straight out of the box,but it could be modified to work well on your engine and since you say it did work well I must conclude that it was re-jetted to work with the other components of your engine. You are not running a true 750 anymore, that 750 must have been worked over to make it suitable for your engine. This is the only way I can explain your report that the 750 worked out well for years.

The formula for finding the correct carb size is: CARB CFM = Cubic Inches x Max RPM’s / 3456 x VE%

That’s about as scientific as it gets.

Your carb size falls in the 450-500cfm range.

A bigger carb dumping in more fuel doesn’t always translate to more power. The engine can only handle so much fuel the rest goes out the tailpipe. If the carb is too big it likely runs ok on the primaries but may get too much fuel when the secondaries kick in.

Matching the carb with the intake manifold can make a difference too. Just putting the smaller carb on the current manifold might work, or it might not. You need to find a good race engine builder who specializes in MoPar.

You can do trial and error tuning but it is hard to judge results without a dyno to actually see what you are gaining and where in the rpm range you get more torque and hp.

Wow, really? Smaller than the 625. Tell me about VE%. That variable(s) I don’t know. Thanks, missleman.

I once again point out that the OP says this carb called a “750 cfm carb” did work without issue. This is what leads me to believe it has been modifed. I certainly agree that 750 cfm is much to much for this engine but somehow the setup works(as reported by the OP).

Changing the CFM rating on a carburetor does not necessarily mean the performance will improve or even degrade and the answer is not that simple. Those CFM ratings are figured at wide open throttle. How often do you drive around with all 4 barrels open? Very seldom.

In normal driving the back 2 barrels would seldom open and the engine would be running on the primary barrels. Any difference would be based on the size and shape of the primaries. In this case, going from a 750 to 625 would be negligible at best.
It is also correct that a small block engine will run fine with a 500 CFM carb.

The 361 is a good engine but it’s also not one of Ma Mopar’s most powerful screamers. Many years ago a friend of mine owned a '66 Sport Fury with a modified 361 (cam, intake, etc.) and my bone stock 383 Roadrunner would blow it into the weeds. On occasion we would swap cars for the weekend so I had a chance to play around with it and had firsthand knowledge of what it would or would not do.
I think the carb is getting too much of the blame for the engine design characteristics.

VE is volumetric efficiency. Volumetric efficiency is the measurement of how close the actual volumetric flow rate is to the theoretical volumetric flow rate. Most engines fall in the 80% range according to my sources. Some hemi engines have a larger flow rate.

As Uncle Turbo said, the engine is running on the primaries most of the time. I suppose a good test would be to floor it—if it bogs or feels like it’s choking at full throttle, the carb is wrong-sized or needs different jets. (or the ignition timing could be off too) If the bogging is accompanied by black smoke out the back, it’s good indication the carb is dumping more than the engine can digest.

Yes a 750 could be too much carb. Most guys have too much carb on their engines. The engine can only handle so much …the other guys stating that the 750 could have been modded by using different primary jets…thus limiting the fuel dump. There are calculations to find out how much carb and cfm your engine needs. Too much would inhibit clean crisp running…and is all too common.

The 361 is also not a big block. Mopar is it? No wait thats a 360…

Thanks, HB. About the 361, it’s an older Chrysler engine. Most folks come to the same conclusion, that I meant the better-known small block 360, but this is actually a big block 361cid, used in DeSoto cars, maybe others. 1966 was the only year it was available in the Dodge Charger.

oldschool, I’m checking with my older brother who owned this car since 1973 (I’ve had it since 2003). He installed the 4-brl. I’ll get back with you.

WOW…I will look that up. Sounds interesting

Heres the info…COOL…Chrysler’s B and RB engines are a series of big-block V8s which in 1958 replaced the first-generation Hemi engines. The B and RB engines use wedge-shaped combustion chambers.

Design features include 17 capscrews per cylinder head, a cylinder block that extends 3 inches (76 mm) below the crankshaft centerline, an intake manifold not exposed to crankcase oil on the underside, stamped-steel, shaft-mounted rocker arms (race versions used forged steel rockers), and a front-mounted oil pump driven by the camshaft.

361The 361 cu in (5.9 L) B engine was similar to the 350 except for a wider 4.125 inches (104.8 mm) bore. The Plymouth version was called the Commando, variants of which included the Golden Commando and SonoRamic Commando. In 1962 the Dodge Polara 500 came standard with a 305 hp version of the 361 that had a four barrel carburetor, dual-point distributor, and dual exhausts.

“Two years ago I installed the original 2-brl. carb”

The fact that this came originally with a 2 bbl carb means it had the lower-performance intake manifold, heads, cam, valves, and exhaust manifold/system. All lead to an engine that doesn’t need lots of breathing. So a smaller 4 bbl would work well. That said, I’d agree with the others, if your 750 was working fine no reason to change it, just get it rebuilt.

And you’ve put the 4 bbl on a 2 bbl intake manifold? The 4 bbl matched up OK?

No, I have two intake set-ups: 2-bbl and 4-bbl, from the intake manifold to the air cleaner wing-nut. The engine was 2-bbl. from the factory. The car does have dual exhaust now.

Ouch, this could be a long story, I’ll try to keep it short.

I have worked with carburetors for many years (and still do).

If You have a car You only use as a daily driver/cruiser/whatever and never take it on a track to try to beat the other guys and You - hopefully - never drives it to the max on the streets, then read on.

What You need is a carburetor which meet your engine capacity (cui) at max rpm + 5 to max 10%.
Max rpm is when the valves float, the rev. limiter kicks in or where you are saying “I’ll never exceed xxxx amount of rpm”.

That’s the number you need.

Just for the fun. Let’s say that your engine can take 6000 rpm (neutral, petal to the metal), then your engine consumes a little less than 627 cfm. I have a hard time to see that engine revving that fast.

I could easily see that engine doing really good with a 550 or 600 cfm carb., but if you have a 625 in good working condition, I’d try that first.

It all boils down to: airspeed through the venturi so you can get the fuel broken up in the air as much as possible. THAT’S what creates power and torque, not just buckets of fuel poured into the engine.

As a little fact, my friends -69 Camaro 396 CUI was not running perfect. Loads of power - above 3500 rpm, would have pulled Exxon Valdez off the rock, not doing well under. Changed the 750 cfm to a 650 cfm carb. Now it will pull the ship apart and that engine is running so perfect now at any rpm. Top speed is the same, though the last 5 miles is a tad slower, but the increase in the torque in the lower rpm,s is awesome.

Good luck

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Thank you, asterix; I believe I’m understanding the science of why a big carb can be no good . . . velocity where it counts to atomize the fuel, balancing the air volume through the venturi with the volume of fuel being delivered.

My Charger is a sunny-day pleasure ride, a go-easy, take-care-of-historical-value car. But I do enjoy some fun. This past summer she saw a Christmas tree for the first time. No, she’ll never see a set of slicks; just wanted to see where the car was with an untouched engine (85,500 mi.) and the original 2-bbl carb setup. It was pitiful, dailing a 17.14 sec., not even 80 mph. The timing chain and gear have been replaced to get the valvetrain in sync with the pistons (lagging about 8 deg.) and a 625cfm Carter sits on it now which will be modestly covered with a OEM 4-bbl air cleaner. In the spring we’ll see how the track time goes . . . this is entertainment racing, nothing serious; just me verses the lights. If I get drag crazy, I’ll get another classic muscle car more appropriate. Man, that would be crazy.

The present issue, which has been answered, was what to do with this 625 carb that is leaking. Replace with the 750 carb sitting on the bench? No, I will seek to exchange the 625 with the same.

The 750 was on the car for many years, and while it ran good, it was a mediocre performer, poor performer in the low rpm range. The exhaust residue has always been sooty black and it was easy to flood in certain conditions. The information in this dialoge is piecing together some understanding. Thanks again.