Carb cfm with ported heads?

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IMO, the original computation was a good gauge of the smallest acceptable carb for an engine
Key word, “SMALLEST ACCEPTABLE.”
 
Go 950, if the cam and GEARS were bigger I'd say 1050cfm.
The 3" exhaust will have it pulling where it normally runs out and extend the rpm range.
It's very noticeable.
I just put a jegs 3" header to bumper pipe set on my car,used straight through dyno max ultra flows...major league difference and very small noise increase that is very much out weighed by the power increase.
I'd tell you to get the exhaust first so you can see for yourself, but it is fair to say the 750 Holley is too small. Fwiw the demon 750 "aka 780" is a bit better ...but with that cam you have a 950 would be best.. Thing is your 750 could be what' helping it run decent with those weak gears you have out back.
 
The formula people use ( cid x rpm / 3456 = cfm ) doesn't give carb size but how much volume an engine is at rpm.

If we assume 100% VE an engine only has intake stroke once every other revolution so a 410 would be 205 cubic inches per revolution or 0.119 cubic feet. At 6000 rpm it would be 1,230,000 cubic inches per minute or 714 cubic feet per minute.

Since air is compressible any carb can pass any amount of air, so a 650,750,850 etc.. would all pass the 714 cfm just at different velocity and vacuum (restriction) just like different header sizes.

On a pure race engine you can give up velocity for less restriction, opposite for pure street and street in between.

On your engine your 750 should be slightly under a vacuum level of 1.5 if the formula is correct, which is pretty conservative. But like I said measure the vacuum level and decide from there if its worth going larger and by how much. Or see if you can borrow some larger carbs and try it out.
 
What did it go when you tuned it? What's the deal with the electric choke? Just wondering is all:)

I ran out of track time last year. I work most weekends. I need to get the bungs welded to my headers while the car is down and get my AFR installed.

I got my choke out of whack when I had to drive the car to work for a week in sub-freezing temps. Flooded the car a bunch of times messing with it, and got aggravated trying to get it right again. I didn't get to drive it again before it was time to pull the heads for porting.
 
yesterday at Pamona the NHRA stock class winner was a 1969 340 Dart in E/SA = 10.7 seconds with - STOCK : 625 AVS carb, intake, rockers, heads, compression ratio (blueprinted), stock cam lift, weight, 9" slick. I run 11.6 with 360 cubes and 2.5" Edelbrock mufflers. an $ 650. ? carb is worth what, .10 seconds?
you forgot to mention what gears that car has in it, dam sure aint 3:23 `s.
 
The formula people use ( cid x rpm / 3456 = cfm ) doesn't give carb size but how much volume an engine is at rpm.

If we assume 100% VE an engine only has intake stroke once every other revolution so a 410 would be 205 cubic inches per revolution or 0.119 cubic feet. At 6000 rpm it would be 1,230,000 cubic inches per minute or 714 cubic feet per minute.

Since air is compressible any carb can pass any amount of air, so a 650,750,850 etc.. would all pass the 714 cfm just at different velocity and vacuum (restriction) just like different header sizes.

On a pure race engine you can give up velocity for less restriction, opposite for pure street and street in between.

On your engine your 750 should be slightly under a vacuum level of 1.5 if the formula is correct, which is pretty conservative. But like I said measure the vacuum level and decide from there if its worth going larger and by how much. Or see if you can borrow some larger carbs and try it out.
the problem w/ this is, there`s an intake stroke every 90 degrees. If u divide by two, my 505" wedge would need a 500 something cfm carb. Everyone knows they require 950 to 1200 cfm. Don't divide by two, and it comes out 1047 . What say you ?
 
So you think now the exhaust will be the choke point?
My exhaust guy won't do 3" over the axles. That and the 3.23's are pretty much non-negotiable. Mainly a street car, but would like to crack 11's at the track.
Will the 750 hold me back with the added flow? I want a bigger carb, but do I really need one?

You can buy a bolt on over the axle 3"

The 3.23's are real tall for that combo.

When you have better heads the carb cfm becomes less critical to hp production. with better heads on a street car it is advisable to run a slightly smaller carb. If you are looking for all out performance go with the 950. For a primarily street driven car with those 3.23's I'd stick with your with the 750.

I run a 950 proform on my combo. It's a great carb. In contrast my fathers truck makes 500hp with a 750 carb. My circle track car makes 463hp with a 500cfm 2 barrel.
 
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Between the ported head and a larger carb, does anybody see a .66+ drop in et.?
 
the problem w/ this is, there`s an intake stroke every 90 degrees. If u divide by two, my 505" wedge would need a 500 something cfm carb. Everyone knows they require 950 to 1200 cfm. Don't divide by two, and it comes out 1047 . What say you ?


Thats the problem, people mix with what an engine consumes with carb size. If your engine consumes 800 cfm of air doesn't mean you need an 800 cfm carb. Cause carb ratings are somewhat arbitrary, manufacturers pick 1.5 in/hg a long time ago for rating 4 bbl. But we know thats overly conservative leaving power on the table for no reason. By that formula most of are engines we have wouldn't needed anything over 600 cfm, but obviously that would be a minimum choice for any V8, Some pure race engine can go close 0 in/hg cause only WOT is only important. A hot street machine or street strip you might want around 1 in/hg give or take. So if you rated carb around 1 in/hg instead, a 1200 cfm @ 1.5 in/hg it might be about 800 cfm @ 1 in/hg. So in other words whatever carb you put on your engine becomes the size your engine needs basically but vacuum level (restriction) will change accordingly.

As to the formula its only adding the volume of your engines cylinders at speed (rpm) and converting from cubic inches per minute to cubic feet per minute (cfm). So your 505 @ 6500 would be 1,641,250 cubic inches per minute or 950 cfm.

505 x 6500 = 3,282,500 which you divide by 2 cause of intake stroke ever other revolution equals 1,641,250 and to convert that to cfm you divide that by 1728 which equals 950 cfm. The reason the formula uses 3456 instead of 1728 which is twice number so you don't have the extra step of dividing by 2. cid x rpm / 3456 = cfm which is the size of the engine at speed (rpm) not carb size. 505 x 6500 / 3456 = 950 cfm. Also any engine that comes out to 950 cfm is the same size.
Eg. 202.5 x 13,000 / 3456 = 950 cfm.
 
The carb doesn’t become bigger or smaller but the engines demand does. The carb can’t change size. The way it is rated is the given size. What it can flow at different ratings changes. Suggesting the carb will flow what ever the engine needs is misleading because by that singlular sentence, a 600 cfm carb is fine. The vacuum draw is not an issue via that statement. Argue that it is a restriction and you contradict yourself.

You write that often and it is confusing to people.
 
You can buy a bolt on over the axle 3"

The 3.23's are real tall for that combo.

When you have better heads the carb cfm becomes less critical to hp production. with better heads on a street car it is advisable to run a slightly smaller carb. If you are looking for all out performance go with the 950. For a primarily street driven car with those 3.23's I'd stick with your with the 750.

I run a 950 proform on my combo. It's a great carb. In contrast my fathers truck makes 500hp with a 750 carb. My circle track car makes 463hp with a 500cfm 2 barrel.

What more impressive is the 500 cfm 2 bbl is even smaller than the 500 cfm would lead us to believe because of the 2bbl being rated at 3 in/hg instead of the 4bbl 1.5 in/hg. If rated like 4bbl for a more apple to apple comparison it would be more like 390 cfm @ 1.5 in/hg. Shows an engine will pull most of what needs even through the smallest of carbs but at a big penalty of power cause of the over restriction. I bet if you dyno some 4bbl on your engine, even a 650 would give a huge boost in power than every 100 above would give less and less returns, which if this was a street engine you'd have to decide where to draw line for gains vs sacrifices as an eg. to the OP.
 
The carb doesn’t become bigger or smaller but the engines demand does. The carb can’t change size. The way it is rated is the given size. What it can flow at different ratings changes. Suggesting the carb will flow what ever the engine needs is misleading because by that singlular sentence, a 600 cfm carb is fine. The vacuum draw is not an issue via that statement. Argue that it is a restriction and you contradict yourself.

You write that often and it is confusing to people.

Im not saying carb size changes. What Im saying in that example his engine consumes 950 cfm cause thats the actual displacement at speed (rpm). If you try a 750/850/950/1050 or even 2100 cfm carbs on that engine what would change besides hp would be vacuum levels. I don't have time to make this mathematically correct so it will be a poor guesstimate. And yes the variation in hp cause of the variation of restriction would effect the 950 cfm number but for this eg. we'll say 950 cfm is a consistent. So on this engine a 950 cfm carb should have a vacuum of 1.5 in/hg so say the 850 is 2 in/hg and a 750 is 2.5 in/hg and going the other way a 1050 1 in/hg and a 2100 is 0.3 in/hg. Now if you converted the cfm to there new vacuum level those carbs all should come out rated to 950 cfm.

And thats my point the rating of the carbs somewhat arbitrary and only good for judging restrictiveness to each other just like header size.

Every carb is a 750 at the right vacuum level or a 950 again at the right vacuum level etc..

It might better if they rated carbs by barrel size kind of like headers or give 3 cfm ratings.
Something like mild @ 1.5 in/hg, hot street strip @ 1 in/hg and full race @ 0.5 in/hg.
 
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The carb doesn’t become bigger or smaller but the engines demand does. The carb can’t change size. The way it is rated is the given size. What it can flow at different ratings changes. Suggesting the carb will flow what ever the engine needs is misleading because by that singlular sentence, a 600 cfm carb is fine. The vacuum draw is not an issue via that statement. Argue that it is a restriction and you contradict yourself.

You write that often and it is confusing to people.

A 600 cfm will flow 950 cfm just it will cost you power to do so, way to restrictive. Thats why a late model stock car still makes power with 2 bbl just less than a proper size carb would. A 600 cfm doesn't stop flowing above 600 cfm air is compressible. Im not saying you dont know that but seems a confusion for some.
 
What more impressive is the 500 cfm 2 bbl is even smaller than the 500 cfm would lead us to believe because of the 2bbl being rated at 3 in/hg instead of the 4bbl 1.5 in/hg. If rated like 4bbl for a more apple to apple comparison it would be more like 390 cfm @ 1.5 in/hg. Shows an engine will pull most of what needs even through the smallest of carbs but at a big penalty of power cause of the over restriction. I bet if you dyno some 4bbl on your engine, even a 650 would give a huge boost in power than every 100 above would give less and less returns, which if this was a street engine you'd have to decide where to draw line for gains vs sacrifices as an eg. to the OP.

I have to run a 2bbl in the race class that engine is used in. It would certainly have a bigger carb if allowed and would make more power.

That engine has Dart heads on it, 12.5:1 compression and runs on race gas. The cam is nasty too.
 
Between the ported head and a larger carb, does anybody see a .66+ drop in et.?

There are a few other factors at play that make me think I can get there-

Tires - I've only been able to launch at 2500rpm on the Cooper Cobras with 18 lbs in them. Any higher and I spin 'em. I am looking into getting some Nitto drag radials on there.


My driving- on the 12.66 run I hit the rev limiter shifting to third with the factory console shifter, and lost some time. I will be running a B&M Quicksilver this year. I think the car could run better as it was. It has trapped at 107mph on other runs. I have some room to improve my driving, lol...

Then there is the carb...I will make some runs with the 750 and try to dial it in with the AFR meter and see what's up first.

I think it can be done, but only time will tell.

Here's the 12.66 time slip.

20170609_202244.jpg
 
If you aren't running in a class that require radials or DOT tires get some real tires under that thing.

I think you could go 12.20's, maybe.. Trap speed speaks pretty loudly about HP and ET potential.
 
Thanks 273, you just proved me right. Thanks again.
 
Has anyone else gone from box stock to fully ported closed 63cc chambered heads, and if so what kind of gains did you see in HP or E.T.'s, without other major changes?
 
it has around 5.13 gears-are u saying a car with 3.23 gears wants more carb than one with 5.13s?
No i believe he is saying velocity at the carb is what is probably helping his combo run with those shitty gears, the only thing lending to any snap it probably barely has under 3000 rpm...is the tiny carb.
Just like all those 1990's hot rod guys who put 440's in a bodies I've seen with a .509 3.23 and a 600 eddy sitting on top.lol
 
There are a few other factors at play that make me think I can get there-

Tires - I've only been able to launch at 2500rpm on the Cooper Cobras with 18 lbs in them. Any higher and I spin 'em. I am looking into getting some Nitto drag radials on there.


My driving- on the 12.66 run I hit the rev limiter shifting to third with the factory console shifter, and lost some time. I will be running a B&M Quicksilver this year. I think the car could run better as it was. It has trapped at 107mph on other runs. I have some room to improve my driving, lol...

Then there is the carb...I will make some runs with the 750 and try to dial it in with the AFR meter and see what's up first.

I think it can be done, but only time will tell.

Here's the 12.66 time slip.

View attachment 1715141620


My 9.5 comp 340 4 spd spun tire to 104 mph while blubbering down the track too rich in the 1/4 and chassis dynod to an equated 392 hp.

What cid again?
 
intake is a LD340....T&D roller rockers... cam lift is .480 with about 260 duration at .050.....aint much stock in a stocker....
last time I had an nhra rule book stockers had to run a stock intake and stock rockers and stock cam lift, are U mixing in super stock or have the rules changed?
 
last time I had an nhra rule book stockers had to run a stock intake and stock rockers and stock cam lift, are U mixing in super stock or have the rules changed?

Stock Elim has the stock lift (laughable) and a ton of duration. Rockers are no longer an issue in Stock either.
 
There are a few other factors at play that make me think I can get there-

Tires - I've only been able to launch at 2500rpm on the Cooper Cobras with 18 lbs in them. Any higher and I spin 'em. I am looking into getting some Nitto drag radials on there.


My driving- on the 12.66 run I hit the rev limiter shifting to third with the factory console shifter, and lost some time. I will be running a B&M Quicksilver this year. I think the car could run better as it was. It has trapped at 107mph on other runs. I have some room to improve my driving, lol...

Then there is the carb...I will make some runs with the 750 and try to dial it in with the AFR meter and see what's up first.

I think it can be done, but only time will tell.

Here's the 12.66 time slip.

View attachment 1715141620



Pass on the Nittos. They are junk.
 
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