How Many CFM's for a 347

90lxcoupe said:

How do you figure these things out before choosing a cylinder head or cam

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He already knows that it is a 347. He should know whether it is a street car or a race car (or dual purpose). He already knows what trans he has ( mt or at).
He should know what RPM band he intends to typically be in. ( i.e. street car....typically spends its' life in the 1500-5500 range). Unless and until he intends to go beyond that, Heads and a cam will not affect ANY of those numbers. Obviously, if he goes beyond that RPM, he'll need better components to allow that to happen, but again, he'd know what RPM he wanted to turn the thing to before he even bought a single thing.

Cam and head choices will affect where power is made w/i that power band, not what CFM a carb has to be.

straight off the holley web site:
You need to know the CUBIC INCHES of the motor. You need to know the maximum RPM the engine will be spun. You need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPM) are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it could consume, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. Most stock production and street performance engines will fall around 75%-85% of volumetric efficiency.

The math formula is: CARB CFM = CU.IN. X RPM divided by 3456 X VE% (already been posted previously)

that said, if the motor is 347 , he intended to spin it 5500 rpm, and he had a "typical street performance" ve% 85%,... his carb size should be 650 CFM

srtthis said:

carbs are a lot easier then people think

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I know it isn't that bad, I can read plugs and tune a carb. I've just gotten lazier as I've gotten older and prefer to make adjustments on a laptop as opposed to leaning over a fender and futzing with a carb.

Personally, i wouldn't bother with duel carbs or a tunnel ram unless you're planning on running crazy RPM. Get a Victor Jr and a custom carb from one of the carb shops (Pro Systems, Quick Fuel, The Carb Shop etc). My understanding about CFM needs was blown out of the water when i ordered my Pro Systems carb and they built me a 780cfm for my 347.

In related news, Barry Grant just went out of business....so i wouldn't consider one of their carbs...and i wouldn't anyways because for what i priced them at, i only paid $30 more for my custom carb and it's super nice.

85_SS_302_Coupe said:

Personally, i wouldn't bother with duel carbs or a tunnel ram unless you're planning on running crazy RPM. Get a Victor Jr and a custom carb from one of the carb shops (Pro Systems, Quick Fuel, The Carb Shop etc). My understanding about CFM needs was blown out of the water when i ordered my Pro Systems carb and they built me a 780cfm for my 347.

In related news, Barry Grant just went out of business....so i wouldn't consider one of their carbs...and i wouldn't anyways because for what i priced them at, i only paid $30 more for my custom carb and it's super nice.

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Yes I agree, I have a 750cfm quick fuel modified 800cfm by Williams carb on my 331. It is actually too big and will be changing the venturies to 780 or maybe even 750 to eliminate the off idle lean condition. I think with the better flowing cylinder heads of today that formula is no longer valid. I would have to have a VE of 1.15 or better to run that big of a carb.

I would not put a dual quad on your 347.
BTW does anyone even make a dual quad for a Windsor?

Scott

HotFox said:

Yes I agree, I have a 750cfm quick fuel modified 800cfm by Williams carb on my 331. It is actually too big and will be changing the venturies to 780 or maybe even 750 to eliminate the off idle lean condition. I think with the better flowing cylinder heads of today that formula is no longer valid. I would have to have a VE of 1.15 or better to run that big of a carb.

I would not put a dual quad on your 347.
BTW does anyone even make a dual quad for a Windsor?

Scott

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#1. is your carb a vacuum or mechanical secondary? because 780 CFM carbs have traditionally ALWAYS been vacuum secondary. (for 85 ss 302)
#2. Dual 450's or 600's are also "on demand" vacuum secondaries. They will not flow 900 or 1200 cfm unless (which I doubt it ever would) the engine demanded it.
#3 I'd have two carbs on the 2v engine I'm building in a second if there was an intake that would allow it.
#4 there is definitely a 2 4 intake for 351 w Edelbrock Performer RPM airgap

madmike1157 said:

#1. is your carb a vacuum or mechanical secondary? because 780 CFM carbs have traditionally ALWAYS been vacuum secondary. (for 85 ss 302)

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mechanical made by Quick Fuel.

Scott

My 780 is mechanical secondaries...Holley HP main body, flow bench tested.

Yeah I'm thinking that the formula listed is outdated as well. I did mine and it said I should run a 740 cfm carb. 7200x418/3456x.85=740.

Well...my guess is that when you start running strokers and big cams (like mine is) then needs change a lot. If we're talking about stock displacement and mild cams, you don't need big carbs. As a few others have mentioned, the heads and cam make all the difference in the world. One 347 might be happy with an out of the box stock 650, while another might run a 1050 Dominator because it has 13:1cr, .700 lift cam with crazy duration, and 10k RPM. There's more to it than total displacement and RPM range.

OK. I'll agree to the VE thing being a little too ambiguous to truly nail down a carb size due to varying combinations ( net result is recommendations for minimum carb size) but,...seems to me that if you look at the original posters' question, he was trying to determine if he could run dual 4's w/ a big cam on a TR. (I assume on the street, he bowed out at reply "way the hell back")
He originally was trying to determine how big the carbs should be based on that.

The TR idea was killed off by every one. The dual 4's, in any other form killed off by another 99%.

Common concensus: everybody replying to this thread is running 750-800 CFM when running a carb on their 347. (at least from what I can tell) and (other than running a single 750-800cfm carb on his 347, If poor ole dude runs either of the two vacuum carbs recommended for multiple carb street use, his engine will use what it needs, and leave the rest of the available CFM in a partially closed secondary. (probably using between 700-800 CFM in total)

It will probably NOT make any more power than a single, It will be more of a hassle than a single. It'll certainly cost more than a single, BUT,... it'll be what YOU want.

I'll bet you guys are thinking...."I'll be glad when this guys' hand heals, maybe then he'll have a purpose other than spouting know it all"

Well...i'm guilty of giving more advice than is asked for, but i never thought that was a bad thing?