Proper size carb

RJRENTON · Apr 13, 2021

440 Charger said:
Go bigger than the math look up the factory flow on a 6 pack.

The published Mopar Six Barrel engine using OEM Holley carbs is: 355 CFM (center carb) + 500 CFM (each end carb) = 1355 CFM at max open condition. Bear in mind that the end carbs open, in unison, based on the air flow thru the center carb, innitially, combined with the end carbs venturii pressure drop dignal, adding to the developed vacuum signal, to not over carb the engine's air flow requirements, until it can be tolerated. Just my opinion of course.
BOB RENTON

66Satellite47 · Apr 13, 2021

Bob, that's the 2bbl rating of those carbs. For a true comparison to a 4 BBL set convert to the 4BBL standard. It's about 950 CFM as I recall. Still a very impressive combination. With a little tinkering the 6 BBL can be really great.

RJRENTON · Apr 13, 2021

66Satellite47 said:
Bob, that's the 2bbl rating of those carbs. For a true comparison to a 4 BBL set convert to the 4BBL standard. It's about 950 CFM as I recall. Still a very impressive combination. With a little tinkering the 6 BBL can be really great.

Out of curiosity, since the carbs ARE 2 bbl carbs, why would you not combine their flow ratings? Don't 4 bbl carbs use a different pressure drop factor? As I recall, aren't 4 bbl carbs rated at 3.0" W.C. or are 2 bbl carbs rated at 3.0" W.C.? Maybe this will make comparisons easier (or not)....but relates to volumes at standard conditions. Without some standardization, its difficult to compare the end number, unless the method is known.
The pressure developed by the air velocity is called the velocity head, and it is affected by the density of the air. The density of the air, in turn, is a function of the local atmospheric pressure and the temperature. The equations that relate all these factors are:

Note that to determine the air velocity the density must first be known. This is the second equation and relates ambient atmospheric pressure and temperature to density. The temperature in degrees Rankine is an absolute reference and is T in degrees F + 460. Assuming average conditions of 70 F and a barometer of 29.92 In Hg, the density of air is 0.075 Lbs/Cu Ft.
Anyway.....apples are not oranges.....unless they are called fruit..... Just my opinion of course.
BTW...my 440+6 runs quite well....most of the time.....
BOB RENTON

joe smith · Apr 14, 2021

I Had my A12 car as a senior in High School... My Mechanics class instructor hooked it up to the scope for a class project one day.... He just laughed at it...

440 Charger · Apr 16, 2021

RJRENTON said:
Out of curiosity, since the carbs ARE 2 bbl carbs, why would you not combine their flow ratings? Don't 4 bbl carbs use a different pressure drop factor? As I recall, aren't 4 bbl carbs rated at 3.0" W.C. or are 2 bbl carbs rated at 3.0" W.C.? Maybe this will make comparisons easier (or not)....but relates to volumes at standard conditions. Without some standardization, its difficult to compare the end number, unless the method is known.
The pressure developed by the air velocity is called the velocity head, and it is affected by the density of the air. The density of the air, in turn, is a function of the local atmospheric pressure and the temperature. The equations that relate all these factors are:

View attachment 1097208

Note that to determine the air velocity the density must first be known. This is the second equation and relates ambient atmospheric pressure and temperature to density. The temperature in degrees Rankine is an absolute reference and is T in degrees F + 460. Assuming average conditions of 70 F and a barometer of 29.92 In Hg, the density of air is 0.075 Lbs/Cu Ft.
Anyway.....apples are not oranges.....unless they are called fruit..... Just my opinion of course.
BTW...my 440+6 runs quite well....most of the time.....
BOB RENTON

I would to see a dyno comparison between a holley 6 pack and dual holley 650s on a 440. Apples to apples' or close to it.

33 IMP · Apr 16, 2021

440 Charger said:
I would to see a dyno comparison between a holley 6 pack and dual holley 650s on a 440. Apples to apples' or close to it.

Hot rod did that about fifteen years ago (on a stout 383, however). Google mopar intake shootout part 2.
Much as i hate to admit it, being a sixpack lover..... the right tunnel ram will destroy a sixpack. Sixpack carbs really only flow under 1000 cfm if rated the same as a four barrel, and a tunnel ram will make a bunch more power than a two plane, which a stock sixpack is.
In the test mentioned above, tunnel rams beat the sixpack by more than 30 hp, on a 500 hp engine.

RJRENTON · Apr 16, 2021

33 IMP said:
Hot rod did that about fifteen years ago (on a stout 383, however). Google mopar intake shootout part 2.
Much as i hate to admit it, being a sixpack lover..... the right tunnel ram will destroy a sixpack. Sixpack carbs really only flow under 1000 cfm if rated the same as a four barrel, and a tunnel ram will make a bunch more power than a two plane, which a stock sixpack is.
In the test mentioned above, tunnel rams beat the sixpack by more than 30 hp, on a 500 hp engine.

But...aren't tunnel rams noted for their top end, because of the their open plenum and the realized equal runner length, and due to the expansion factor of the open plenum tend to kill mixture velocity characteristics at off idle to mid range transition? But again, why equate 6 barrel CFM flows vs tunnel ram CFM flows, unless flow characteristics are equalized to the same conditions as noted previously. Why can't 4 bbl carbs rated at the same conditions as the six barrel carb.....it would be interesting to see which method produces the bigger CFM number....assuming conditions are the same? Manufacturer's tend to use the bigger / biggest number that coincides with the point they are trying to maximize.....apples vs oranges???....maybe... Just my opinion of course.
BOB RENTON

66Satellite47 · Apr 17, 2021

True not an apples to apples comparison. I have no dyno experience with this comparison, mine is on the drag strip. I ran 6 Pak's over the years, different combinations of the Edlebrock manifold with various '69, '70 & '71 carbs. The '70 carbs seemed to work a little better for some unknown reason. I always liked the 6 Pak. But my Wieand tunnel ram with Holley 650 DP's 4777's made a bunch more power. at least .20ET & 3+ MPH better. I ran the tunnel ram on my 440' & my 400/452 motors. I even tried a set of matched 3310-1s on the tunnel ram. The 650's were better for those motors.
The SixPack was designed as primarily a street combination, the tunnel ram race.

33 IMP · Apr 17, 2021

The formula i found to rate a four barrel the same way a two barrel is rated, is multiply four barrel rating by 1.414. So...rated as a four barrel, the sixpack would be 955 cfm, verses the two 650s on the tunnel ram.
Rated as a two barrel, the 1350 cfm of the sixpack would have to compete against two 919cfm carbs, on a better manifold.

RJRENTON · Apr 17, 2021

66Satellite47 said:
True not an apples to apples comparison. I have no dyno experience with this comparison, mine is on the drag strip. I ran 6 Pak's over the years, different combinations of the Edlebrock manifold with various '69, '70 & '71 carbs. The '70 carbs seemed to work a little better for some unknown reason. I always liked the 6 Pak. But my Wieand tunnel ram with Holley 650 DP's 4777's made a bunch more power. at least .20ET & 3+ MPH better. I ran the tunnel ram on my 440' & my 400/452 motors. I even tried a set of matched 3310-1s on the tunnel ram. The 650's were better for those motors.
The SixPack was designed as primarily a street combination, the tunnel ram race.

IMO...
A tunnel ram with R3310-1 carbs (presumably mounted sideways or transversly) would be horrible due to the pressure drop or expansion factor yielding very low mixture velocity characteristics until the velocity approached the tunnel ram's "sweet spot". The smaller Holley 650 CFM carbs would work better because of the velocity factor being higher. Perhaps proving the addage that bigger is not better....
BOB RENTON

66Satellite47 · Apr 17, 2021

Well the 3310's were set up with a very quick opening vacuum spring and well adjusted accelerator pump to sort of match the double pumpers for response. This was on my drag car that was well above 4000 RPM instantly. Since the motor is an air pump, the demand varies with the RPM. The 780's did not improve performance, but were not "horrible" just not quite as good.
Your point is well taken, bigger is not always better.

BSB67 · Apr 17, 2021

The math is simple. Because the dP rating difference between the 2 bbl and 4 bbl is a factor of 2 ( i.e. 3.0" verses 1.5") the relationship is the square-root of 2, or its inverse.

When you talk about different carb flows, it's a comparison under one specific dP. It is very unlikely that the typical street car's engine will pull hard enough on a 4 bbl carb to actually see 1.5". If it does, it will likely make more power with a bigger carb.

There is nothing particularly magical about 3" or 1.5" of hg, its just a standard for comparison.

rumblefish360 · Apr 18, 2021

RJRENTON said:
But...aren't tunnel rams noted for their top end, because of the their open plenum and the realized equal runner length, and due to the expansion factor of the open plenum tend to kill mixture velocity characteristics at off idle to mid range transition? But again, why equate 6 barrel CFM flows vs tunnel ram CFM flows, unless flow characteristics are equalized to the same conditions as noted previously. Why can't 4 bbl carbs rated at the same conditions as the six barrel carb.....it would be interesting to see which method produces the bigger CFM number....assuming conditions are the same? Manufacturer's tend to use the bigger / biggest number that coincides with the point they are trying to maximize.....apples vs oranges???....maybe... Just my opinion of course.
BOB RENTON

RJRENTON said:
IMO...
A tunnel ram with R3310-1 carbs (presumably mounted sideways or transversly) would be horrible due to the pressure drop or expansion factor yielding very low mixture velocity characteristics until the velocity approached the tunnel ram's "sweet spot". The smaller Holley 650 CFM carbs would work better because of the velocity factor being higher. Perhaps proving the addage that bigger is not better....
BOB RENTON

Hey RJ. The tunnel rams are indeed known for there ability to scream power at high rpm’s. The throttle issues are tuning issues despite what you wrote. The breathing about a TR is the straight shot to the port and a long runner. Longer the runner the more ability it has to make torque. This is also why the shorter under hood low profile TR’s are not so popular.

The tunnel rams sweet spot, as the TR’s are cast today and some of the yesteryear models are designed & targeted for 2,000 or 2,500 RPM and up. Below I have have a shot of an Edelbrock TR5 with the short top that is an excellent street intake.

The amount of plenum the top portion of the TR has can be an issue for mello’er builds. If you tried one in the past with crappy results, it was probably unbalanced somewhere in the build.

I have to agree with the above statement that the 6bbl. set up is as good as it could get back in there day. Absolutely the best most flexible and maximum amount of carb CFM you can put on a dual plane for a multitude of engine sizes and performance levels.

To the left is a SB Victor.

rumblefish360 · Apr 18, 2021

Here is a friends W2 Pro Dominator. The plenum on this one is a little more generous. But not huge. Still very responsive.
The TR’s below it are M1’s with larger plenums designed strictly for racing.

Proper size carb

RJRENTON

FBBO Gold Member

66Satellite47

Well-Known Member

RJRENTON

FBBO Gold Member

joe smith

Well-Known Member

440 Charger

Well-Known Member

33 IMP

FBBO Gold Member

RJRENTON

FBBO Gold Member

66Satellite47

Well-Known Member

33 IMP

FBBO Gold Member

RJRENTON

FBBO Gold Member

66Satellite47

Well-Known Member

BSB67

Well-Known Member

rumblefish360

Well-Known Member

rumblefish360

Well-Known Member

Similar threads