Fuzzy Math

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Captainkirk

Captainkirk

Old School Mopar Warrior
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I know I'm opening a can of worms here. But I brought popcorn, so it's all good.:popcorn:

Regarding carb size on a 340 build...going by the CFM formula in the old Holley Carbs by Mike Urich and Bill Fisher, my calculation come up with a CFM requirement of somewhere in the 600 cfm rating for a 340 cube engine. Most people seem to be running carbs with quite a bit higher air flow ratings. With max RPM being in the 6500 range for a hydraulic grind and 4-speed trans, would a Holley 4777 (650 double pumper) be "adequate" for a street performance build, and why or why not?
I realize I'll probably get a thousand opinions from a thousand different people, but that's OK.
I brought extra popcorn...
 
A lot of guy's are going 750 but depends on your build. Staying close to a stock build a 650 Holley or Eddy carb would work. It would still run good on a 600 but imo it'a conservative. Ohh, I'm no expert.
 
Throw in the fact that most box-stock carbs actually flow considerably less than the numbers they're rated at, and 650 on a mild build and 700-750 on a hot build suddenly doesn't seem too far off base.
Remember, the factory put TQ carbs on stone stock engines that flowed up to 1000 cfm, and the six packs that were rated at 1300...
 
Maybe for a Holley. Factory 340 AVS was 630 cfm and 71-73 TQ was 750 cfm. I've run a 71 TQ 750 cm on a 273, 4 speed, and loved it. Sold all the Holley carbs years ago.
 
Professor Fate said:
Throw in the fact that most box-stock carbs actually flow considerably less than the numbers they're rated at, and 650 on a mild build and 700-750 on a hot build suddenly doesn't seem too far off base.
Remember, the factory put TQ carbs on stone stock engines that flowed up to 1000 cfm, and the six packs that were rated at 1300...
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Yes, but...
Both the Six Pack and TQ used vacuum operated secondaries (in the case of the TQ, the air valve was vacuum operated) So the engine doesn't flow more cfm than needed at any particular RPM range. The DP Holleys use mechanical secondaries so "over-carbing" can become a very real issue.
 
Fuzzy Math joke

A help wanted ad was placed by a firm.

"Accountant Wanted"

3 people showed up.

Hiring manager interviewed each one. He had 1 question for each.

"What is 1 + 1?"

The first applicant said "2"
The manager said thank you we will be in touch.

The second applicant said "2"
The manager said thank you we will be in touch.

The third said "What do you want it to be?"

The manager hired him on the spot.

SORRY! JUST HAD TO!
 
Captainkirk said:
Yes, but...
Both the Six Pack and TQ used vacuum operated secondaries (in the case of the TQ, the air valve was vacuum operated) So the engine doesn't flow more cfm than needed at any particular RPM range. The DP Holleys use mechanical secondaries so "over-carbing" can become a very real issue.
Click to expand...
Just another reason that the old "cfm formula" can get thrown out the window. It is just a generalization, anyway.
Heck, @318willrun is running a 1200 cfm dual quad on a mild 318.
 
All I can tell you is a great carb changes everything how your engine runs. I tried all kinds of carbs. And believe me that 273 took every bit of that 750 cfm TQ. I do love air valve secondary carbs. You can dial them right in. Holley makes the Street Demon which appears to be a square bore TQ. They even use original TQ floats.
 
I’ve typed this so many times lately I’ve worn out the keys on my keyboard.

Buy your carb based on venturi diameter and NOT CFM. Carb manufacturers think we end users are so stupid that they use CFM so it’s simple. But it’s a piss poor way to size a carb.

Without typing out 40 pages, the upshot is all 4 venturi carbs are tested at 20.4 inches of water. There isn’t an engine out there that pulls that kind of average depression.

For a relatively stock or a “warm” small block you should have a 1.250-1.310 Venturi and 1.688 throttle blades. That’s all you need, unless you want to use annular boosters and tune for that.

You’ll have the best drivability, the lowest fuel consumption with that architecture and still have plenty of air at WOT.
 
Professor Fate said:
Just another reason that the old "cfm formula" can get thrown out the window. It is just a generalization, anyway.
Heck, @318willrun is running a 1200 cfm dual quad on a mild 318.
Click to expand...
Semantics. What sort of "dual quads"? Vacuum secondaries? Quite plausible, especially if they are flowing 500 cfm, give or take. Even the smallie 'Vettes had dual quads. A VS 2X4 setup that never opens the secondaries is probably not flowing much more than 600 cfm...ever. Even the Six Pack running about town on the center carb is only using 350cfm. A 2X4 manual secondary setup will probably lose you a lot of races unless you do a lot of dyno tuning and jetting work. Same goes for a MS triple deuce setup.
 
Bear in mind that vacuum secondaries or "air door" designs like QJ or TQ throw CFM numbers out the window. Also remember that Holley 4bbls and 2bbls are rated at different vacuum settings, and Carter are rated different than Holley
 
Captainkirk said:
Semantics. What sort of "dual quads"? Vacuum secondaries? Quite plausible, especially if they are flowing 500 cfm, give or take. Even the smallie 'Vettes had dual quads. A VS 2X4 setup that never opens the secondaries is probably not flowing much more than 600 cfm...ever. Even the Six Pack running about town on the center carb is only using 350cfm. A 2X4 manual secondary setup will probably lose you a lot of races unless you do a lot of dyno tuning and jetting work. Same goes for a MS triple deuce setup.
Click to expand...
Professor Fate said:
Just another reason that the old "cfm formula" can get thrown out the window. It is just a generalization, anyway.
Click to expand...
See YR's post above.
 
yellow rose said:
For a relatively stock or a “warm” small block you should have a 1.250-1.310 Venturi and 1.688 throttle blades. That’s all you need, unless you want to use annular boosters and tune for that.
Click to expand...

How are you measuring venturi size? Center bore?
 
Captainkirk said:
How are you measuring venturi size? Center bore?
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It should be listed for the carb. If I can’t find it I use a piece of solder and stick it in from the bottom of the car to the bottom of the booster and bend it up in there so it goes across the bore. Then you pull it out and measure it.
 
yellow rose said:
It should be listed for the carb. If I can’t find it I use a piece of solder and stick it in from the bottom of the car to the bottom of the booster and bend it up in there so it goes across the bore. Then you pull it out and measure it.
Click to expand...
Now why didn't I think of that?
 
Captainkirk said:
Semantics. What sort of "dual quads"? Vacuum secondaries? Quite plausible, especially if they are flowing 500 cfm, give or take. Even the smallie 'Vettes had dual quads. A VS 2X4 setup that never opens the secondaries is probably not flowing much more than 600 cfm...ever. Even the Six Pack running about town on the center carb is only using 350cfm. A 2X4 manual secondary setup will probably lose you a lot of races unless you do a lot of dyno tuning and jetting work. Same goes for a MS triple deuce setup.
Click to expand...
A pair of 600's hooked up direct (1 to 1). Mechanical throttle plates, vacuum air door. I would go with a 750 double pumper on your 340.
 
318willrun said:
A pair of 600's hooked up direct (1 to 1). Mechanical throttle plates, vacuum air door. I would go with a 750 double pumper on your 340.
Click to expand...
Watched your video. Quite interesting. I wonder why there are 2X4 manifolds for just about every smallblock out there, but no LA smallblocks?
 
Captainkirk said:
Watched your video. Quite interesting. I wonder why there are 2X4 manifolds for just about every smallblock out there, but no LA smallblocks?
Click to expand...
Through the years, I've found the 750 DP to be my "carb of choice" for low end of cost carbs. I have one now on my 360.
 
I'll throw one more point in. Gains in power from decreasing restriction to airflow are most noticible in the higher rpm ranges. There is a point of diminishing returns which depends on the quality of cylinder to cylinder fuel distribution and charge stratification. Large plenum volumes and more direct flow paths make it easier to obtain even fuel distribution under wide open throttle conditions. Exhaust and engine of course play a role too.

In line with what others have already posted here, check out the first two posts here: 100 cfm more with anular and downleg boosters can't beat AFR
The most important thing for maximizing WOT power is get the timing and fuel mixture right.
If the carb is ballpark correct to begin with, then really all that's needed is some jets and a 1/4 mile drag strip.
 
TQs, AVS, AFB & QJs are all mechanical secondary carbs, ie when the throttle is stomped, all 4 t/blades go to the WOT position. With a Holley vac sec carb, the sec t/blades do NOT go the WOT position when the throttle is stomped. Yes, a bit confusing but it was Holley who coined the phrase 'mech sec' when they introduced that line of carbs. All carbs work on vacuum or negative pressure.
 
Captainkirk said:
I wonder why there are 2X4 manifolds for just about every smallblock out there, but no LA smallblocks?
Click to expand...
There were LA TRs made.
Photo 002 here [SOLD] - Mopar Performance M-1 Tunnel Rams
One installed on the team Coming from Behind Dart's 318.
road-race-dart-2-jpg.jpg
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dart-arse-freeze-a-palooza-24-hours-of-lemons-626x426-jpg.jpg

originally posted at https://blog.caranddriver.com/wp-co...eeze-a-Palooza-24-Hours-of-LeMons-626x426.jpg

 
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