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Critical data for carb size choice

 
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Old 06-11-2019, 10:12 AM
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Critical data for carb size choice

Pmuller mentioned on another forum that the 300 has 34% more cylinder volume than an equivalent v8. That is an amazing fact, and it explains why forum members have routinely found that a 600 cfm carburetor works well on the 300.
Rule of thumb for calculating the cfm needed for a carburetor has been to multiply the cubic inch displacement times 1.5. With that formula we come up with a carburetor with 450 cfm. I'll bet that works well for most v8's. However, if we add in an extra 34%, either to the c.i. displacement or to the cfm needed, the calculation shows that a carb of just over 600 cfm would be ideal.
 
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Old 06-11-2019, 11:41 AM
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Ideal for what, though, is the question. Drag racing? Most of the online CFM calculators are setup for ***** to the wall RPMs. Pickup trucks are setup and geared for hauling heavy loads. The factory set them up with small carburetors, in part for crisp off idle acceleration from a stop. Ford only offered a 245 CFM 2 barrel in their 292 c.i. Y-block equipped F100 thru the big trucks (for example) It depends on the intended use.
 
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Old 06-11-2019, 02:16 PM
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I like this calculator as you can use your rpm limit and estimated volumetric efficiency.
Calculate the size of carburetor based on specific engine size and RPM

If we put in 300ci and 5500 rpm at 100% VE we get 477 cfm. Those values are pretty hard to get for most people on the 300. I don't see how 600 cfm is a great choice, I think it would be too large and lead to low signal to the carb and low end would suffer. I know the 625 cfm street demon I have seems very large for the application, but the small secondaries make for great low end. That 625cfm has made 500hp on the dyno on a v8.
 
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Old 06-11-2019, 09:56 PM
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Originally Posted by jason832 View Post
I like this calculator as you can use your rpm limit and estimated volumetric efficiency.
Calculate the size of carburetor based on specific engine size and RPM

If we put in 300ci and 5500 rpm at 100% VE we get 477 cfm. Those values are pretty hard to get for most people on the 300.
With a stock exhaust system 85% efficiency is probably in the ballpark. Then plug in a typical highway cruising RPM. Those figures will usually net a result very close to the factory supplied carburetor size. Again it's important to select components for the intended use.
 
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Old 06-12-2019, 09:07 AM
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I've often wondered about this, because in every formula I've ever seen, cylinder size is never a component. Unlike aftermarket stuff, which is almost always geared toward V8s, carb CFM size is used for every engine, with all configurations.
Then, as all the pistons are moving up and down, it creates a steady airflow in through the carburetor, not individual pulses through the carb for each cylinder. It's simply pulling in 300 cubic inches of air (minus whatever for vacuum) for every other RPM.

That's where the number 3456 comes from in the CFM formula. It's just converting cubic inches to cubic feet, then multiplied by 2, since a 4 stroke only pulls in air every other rotation. 12 * 12 * 12 * 2 = 3456
There's no accounting for number of cylinders or their size.


That said, what you say always seems to be true. I've always had better luck with carbs that are bigger than the 300 should be using. My 390cfm had way less performance than the 600cfm I had on there.
Most have better luck with 500 cfm Edelbrocks than they do 200 - 300 cfm carbs.
Etc.

Makes you wonder
 
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Old 06-12-2019, 10:06 AM
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Your experience is correct especially with a modified 300 six.
It has to do with being able to supply the peak flow of the larger cylinder rather than the average flow.
The plenum volume also comes into play here.
The less volume is available in the plenum to supply peak flow the more flow responsibility is shifted to the carb.

What we are seeing with recent builds using a big valve ported head and cams with .050" durations in the 220 degree range is huge amounts of torque from 1000 rpm and flat torque curves out to 4500 rpm.
Those using less than 600 cfm carbs cause the torque curve to fall off early and the power band gets shorter with no gain in low end torque.

The question being examined is whether or not a vacuum operated secondary has any advantage over a mechanical secondary for the 300 six.
The feeling is that a late opening secondary is costing some engine response.
 
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Old 06-12-2019, 03:48 PM
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The 300 is a different animal. It likes big carbs and big cams.
 
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Old 06-13-2019, 12:39 AM
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Originally Posted by fordman75 View Post
The 300 is a different animal. It likes big carbs and big cams.
I think you're right. What I noticed with the 600 cfm carbs I have run, two Autolites and two Summit 600's, was that with the relatively large primaries, I did not notice a smaller amount of zip down low. What I did like was that the 600 cfm carbs gave a more enjoyable DD experience. The primaries could supply all the juice I needed, whereas with the Quadrajets I have run and two 485 cfm Autolites, they fell short in the fun throttle and had to open the secondaries to keep the power coming. Because of that the 600's got better mpg, unless I never got on it enough to open the secondaries.

*Holy smoke! My secondaries stuck open today. I have the QF adjustable secondary diaphram. I think I spun about 8k!
 
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Old 06-15-2019, 04:01 PM
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FYI for people reading this trying to figure out what size carb and if v2 or 4v just know for Holley carbs that measure the 2v & 4v carbs at different HG's.
IIRC the 2v was half the HG's of a 4v carb when setting the CFM so don't go thinking a 2v might be too small at 500 CFM.
I can tell you a 500 CFM 2v Holley is a little to big on an AMC 258 straight six but ran great on a AMC 304 v8 motor.
Dave ----
 
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Old 06-15-2019, 07:22 PM
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Originally Posted by FuzzFace2 View Post
FYI for people reading this trying to figure out what size carb and if v2 or 4v just know for Holley carbs that measure the 2v & 4v carbs at different HG's.
IIRC the 2v was half the HG's of a 4v carb when setting the CFM so don't go thinking a 2v might be too small at 500 CFM.
I can tell you a 500 CFM 2v Holley is a little to big on an AMC 258 straight six but ran great on a AMC 304 v8 motor.
Dave ----
As far as I know, this goes for ALL carburetor manufacturers.

1 barrel and 2 barrel carbs are measured at 3hg of vacuum. 4 barrel carbs are measured at 1.5hg of vacuum.
This causes a disparity of the square root of 2 (or 1.41421)

So, a 500cfm 2bbl is equivalent to a 353cfm 4bbl.
Likewise, a 465cfm 4bbl is equivalent to a 658cfm 2bbl.
600cfm 4bbl = 848.5cm 2bbl

etc
 

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