Stop in for a cup of coffee

[ScampMike]

How do I select the correct CFM carburetor for my application?
Answer: The carburetor CFM is a very critical choice in your build. Too large will cause low RPM issues and a rich condition. Too small will restrict the engine performance at high RPM. A simple formula is commonly used by our techs to select the proper CFM:

(Cubic inch of engine X Max RPM) / 3456 = maximum CFM required

Ex. (350ci X 6500RPM)/3456=658.28 CFM (a 650 CFM would be ideal for this application)
which is interesting - because that would lead me to a 650 - but my engine guy and the dyno people both suggested 750...

 

[straightlinespeed]

might just get a 5 dollar craving box from the bell ...taco bell

its really a deal, a burrito supreme, chulupa thingy, nachos, and a taco ....oh yea and don't forget it comes with a pop.

All that individually ...would be way more money and that will last me a day and a half

We had Taco Johns today... That was pretty damn tasty!

 

[Princess Valiant]

I get that part, but when a 340 engine is running 3 carbs for a total of 960cfm @ WOT, how does it function properly?? Now granted I dont know if all 3 carbs open together or not, but that is what Im trying to understand.. It doesnt make sense to me.

higher compression ....the more efficient an air pump is, the more CFM its going to pump. Those original 340 motors that got six packs were supposed to be a lot more than a "regular" 340

 

[straightlinespeed]

How do I select the correct CFM carburetor for my application?
Answer: The carburetor CFM is a very critical choice in your build. Too large will cause low RPM issues and a rich condition. Too small will restrict the engine performance at high RPM. A simple formula is commonly used by our techs to select the proper CFM:

(Cubic inch of engine X Max RPM) / 3456 = maximum CFM required

Ex. (350ci X 6500RPM)/3456=658.28 CFM (a 650 CFM would be ideal for this application)
which is interesting - because that would lead me to a 650 - but my engine guy and the dyno people both suggested 750...

My dyno guy liked my 750 as well, but I could tell my engine was much more snappy with that 600 on it.. I could break the tires loose in 2nd gear which the 750 would not allow.

 

[Princess Valiant]

you assume your air pump (engine) is around 65% efficient with those calculation ...the higher the compression and the better the air flow to closer you get to 80% because you will never reach 100% but 80 is pretty dang good

 

[dukeboy440]

No, I dont recall getting a link... Was it on here or FB?

here.
let me see if i can find it

 

[Princess Valiant]

We had Taco Johns today... That was pretty damn tasty!

never had that because we don't have one of those here but the "john" part is very de-appetizing ...you would think they could at least make it taco Juan's

 

[dukeboy440]

No, I dont recall getting a link... Was it on here or FB?

xtremepenetrator - MidwayUSA

 

[straightlinespeed]

you assume your air pump (engine) is around 65% efficient with those calculation ...the higher the compression and the better the air flow to closer you get to 80% because you will never reach 100% but 80 is pretty dang good

I believe the 340's had 10:1 compression if I remember right. But people buy the manifolds and throw them on there engines and are able to get the them to run good. So I dont necessarily believe that the higher compression is what allowed it.

 

[straightlinespeed]

never had that because we don't have one of those here but the "john" part is very de-appetizing ...you would think they could at least make it taco Juan's

 

[ScampMike]

never had that because we don't have one of those here but the "john" part is very de-appetizing ...you would think they could at least make it taco Juan's

or Jacks...

 

[straightlinespeed]

xtremepenetrator - MidwayUSA

Holy crap I have a whole bunch of phillips bit tips laying around.. Im going to make them fit my .223's LOL.

 

[ScampMike]

I believe the 340's had 10:1 compression if I remember right. But people buy the manifolds and throw them on there engines and are able to get the them to run good. So I dont necessarily believe that the higher compression is what allowed it.

are all three of the trip carbs the same size?

 

[dukeboy440]

Ok guys, I need help wrapping my head around something and I didnt want to make a thread. So explain to me why when you use a carb sizing calculator it gives your what your ideal carb size is.. For example my engine is spec'ed to use a 625cfm carb. Ok now you have a factory 340 six pack and from my research is around 960cfm total.

I was running a 750 on my car and it ran good, but when I threw on a 600cfm carb it was much more responsive..Regardless of that, what I dont understand, how is it possible to oversize a carb?? Why could I not run a 800cfm carb on my car and have it run like a six pack? Is what Im saying making sense?

jetting has a ton to do with that, as does cam keys and accelerator pumps. my 408 believe it or not, also runs better down low with the 600 that's on it, but begs for more CFM over about 4500, which in a truck, I'm never there so I leave the 600 on it.

 

[dukeboy440]

Holy crap I have a whole bunch of phillips bit tips laying around.. Im going to make them fit my .223's LOL.

 

[straightlinespeed]

xtremepenetrator - MidwayUSA

Its definetly a different type of bullet. Im going to have to look into those and watch some videos on them.

 

[dukeboy440]

are all three of the trip carbs the same size?

i believe so, but they are jetted differently, the center is the main carb and has the biggest jets I do believe

 

[Ddaddy]

I believe the 340's had 10:1 compression if I remember right. But people buy the manifolds and throw them on there engines and are able to get the them to run good. So I dont necessarily believe that the higher compression is what allowed it.

The six pack is a very different animal than a 4 barrel. The center carb is the primary and the end carbs open later in the rpm curve. It works so well and high cfm because it is staying in balance across the rpm range not just at the top end. If they had simply used a 4 barrel at that cfm it would have been a complete dog at the lower rpm.

 

[straightlinespeed]

are all three of the trip carbs the same size?

From what I can find the center is a 350cfm and the outers are 500cfm

 

[Ddaddy]

The jetting also came into play so that the amount of fuel was optimized to keep the A/F ratio in a tight range across the rpm curve.

 

[ScampMike]

From what I can find the center is a 350cfm and the outers are 500cfm

bud - that's friggin' 1200 cfm!
wait - you changed that!! LOL which is your answer!?
so it's 1350cfm??

 

[straightlinespeed]

The six pack is a very different animal than a 4 barrel. The center carb is the primary and the end carbs open later in the rpm curve. It works so well and high cfm because it is staying in balance across the rpm range not just at the top end. If they had simply used a 4 barrel at that cfm it would have been a complete dog at the lower rpm.

That is essentially what a 4 barrel does though correct? The primary opens first then the secondaries follow.

 

[straightlinespeed]

bud - that's friggin' 1200 cfm!

I know, the info is all over the board for them. 960-1200. Im basing it off of this.

carb-0-4790 OE number=3577185 1970-71 340
3x2 (Outboard Carb.) . cfm-500

carb-0-4792 OE number=3577183 1970-71 340
3x2 (Center Carb.) A/T choke-Remote cfm-350

 

[Princess Valiant]

bud - that's friggin' 1200 cfm!

that is about right on a 440+6 but I am not sure a 340 can do that much.

I can tell you from experience that a 440+6 with that much is a blast to drive ....YOU will hesitate and back off the throttle long before you will ever feel the engine running out of carb and manifold like with a 4 bbl. or other set up

 

[straightlinespeed]

Its just interesting to me and hence why Im struggling understand it fully.