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Tue Mar 10, 2015 12:34 am

modok wrote: RanX0R0X wrote: jamestwo wrote: on a V8 with a single plane, each cylinder has full benifit of of all four barrels, on a VW with one ventura per cylinder, each cylinder only draws from one barrel.

So on a V8 with a 65 cfm, each cylinder can draw up to 650 CFM, because only one cylinder draws at a time.

running four 325 cfm venturas on a VW would be the same as running a single 325 cfm carb on a single plane intake.

Yeah, I know, this post was from a long time ago but I'm curious to know how to think about the flow rate.

1 cubic foot/minute = 0.471 947 45 liter/second
Dell'Orto 40mm DRLA
266.4cfm*0.471 947 45
=~125L/second

1914cc/4=478.5cc which thanks to metric
=0.4785L

(2000RPM/2)*0.4785/60 seconds
=~8L/s

Does that look right?

Do keep in mind
Holley's carb CFM ratings were usually @ 3mm HG for regulear apps and 1.5mm HG for Hp apps. You can roughly convert 25 inches of water depression CFM to the lower carb CFM rating with a conversion you can probably find somewhere maybe.
I don't know what you are trying to figure out, for IR carbs most everybody just goes by venturi size. To get scientific maybe you could figure the flow coefficient of the vent size?? and use that to compare?

....Maybe this will help... maybe not....

Tue Mar 10, 2015 7:36 am

??????
the hollys were for 2 bbl&4bbl ratings(I think the 3 bbl was considered a 4bbl too) not realy hp ratings.

Tue Mar 10, 2015 8:52 am

Weber 32/36 - found statements of 220 to 300 cfm...not popular around SAMBA tho

26/27 mm venturi (most common?) = 270 cfm at 3.0 inches mercury

Tue Mar 10, 2015 4:27 pm

I found it listed as 280 CFM, holley model 5200 with 26/27 venturi

Thu May 21, 2020 10:42 pm

Nothing like a little bump ,
I just got some flow testing on some self ported heads and intake , What came out as a surprise was the 48 DRLA’s with 42mm venturis restricted the intake flow to just over 181 CFM at 25inches , yet at the start of this thread numbers over 300cfm are quoted for 48 IDAs with 44mm vents .

There is another quote off 388.5 for the DRLA but I’m now sure that was for both throats measured at 28inches .

I managed to get just shy of 230cfm at .500 intake and 175 cfm exhaust at .500 using 28 inches . 47/40 mm valves and a 40mm csa . Pretty happy with that .

Fri May 22, 2020 5:32 am

Wreck, they were flowing carbs alone, no heads attached😊😊.
Yes, even with 300+ cfm they will add restriction to your headflow tests. Congrats on your tests by the way! Thats impressive!

Also, thanks for bringing this thread back. It gives some great ideas. I got a few new sets of ida's I want to flow test...

Fri May 22, 2020 6:01 am

jpaull wrote: Wreck, they were flowing carbs alone, no heads attached😊😊.
Yes, even with 300+ cfm they will add restriction to your headflow tests. Congrats on your tests by the way! Thats impressive!

Also, thanks for bringing this thread back. It gives some great ideas. I got a few new sets of ida's I want to flow test...

So if you have a carb that is supposed to flow 300cfm and a manifold /port that flows 230cfm , how does the carb restrict the intake down to 182CFM ?

I gained 30WHP and almost 1/2 a second on the 1/8 by removing the 42mm vent and just running an auxiliary vent last year.

We tested that recently on the flow bench , running with no Venturi bumped the flow up to 210cfm at 28 inches . Removing the auxiliary Venturi made better gains . Up over 220 cfm . I'm guessing that is why JPM have developed their Venturi for the 48 IDA . I wish they did one for DRLA's .

Fri May 22, 2020 6:57 am

Yeah ,I'd buy a set of JPM vents for my 40 dells. Was the low down real bad without the vents fitted. I'm at 36mm in some 40's and its still good round town

Fri May 22, 2020 8:38 am

what ever happened to james2?

Fri May 22, 2020 2:54 pm

GARRICK.CLARK wrote: Yeah ,I'd buy a set of JPM vents for my 40 dells. Was the low down real bad without the vents fitted. I'm at 36mm in some 40's and its still good round town

It was drivable but the transition wasn't nice , I could've used a bigger idle jet to broaden the transition phase and make it smoother but then I'd loose my fuel economy . Since its a daily driver that's important .

How you would go with a turbo set up ,I don't know . You can only try , fit a richer idle , back the timing off and see what it does .

Fri May 22, 2020 3:07 pm

Yeah I'll try it see what happens. I luv trying to get the last hp out of a motor

Sun May 24, 2020 5:33 am

750 Demon gets the same MPG as the 680 Quick Fuel (28 MPG w/boost ) if I removed the rack up top and had a lighter right foot I'd probably get 30mpg, but that would be 'boring' :lol:

Sun May 24, 2020 4:40 pm

madmike wrote:
750 Demon gets the same MPG as the 680 Quick Fuel (28 MPG w/boost ) if I removed the rack up top and had a lighter right foot I'd probably get 30mpg, but that would be 'boring' :lol:

Thats impressive MPG for a very quick car , who says you can't have all :-)

Sun May 24, 2020 5:03 pm

Wreck wrote:

So if you have a carb that is supposed to flow 300cfm and a manifold /port that flows 230cfm , how does the carb restrict the intake down to 182CFM ?

because that's how things work man.
If a carburetor has ZERO pressure drop....how would it work? It would not even work. That is the energy that makes it work.

You put two resistors in series..... what happens? a 3 ohm and a 1 ohm
you have 4 ohms total.
If you run 12 volts through that, there will be 3 volts drop ACROSS the 1 ohm, and 9 volts drop across the 3 ohm. Which you can test with a meter.

Can you do that on a a flow bench? sure. You just did.
Now that you have added a carburetor, there is a few inches drop through the carburetor, and thus there is a few inches LESS pressure drop across the port.

In fact since you KNOW the CFM of the carburetor, and the total flow, you can calculate how much pressure drop there in in the middle.

Your flowbench instructions probably come with a chart or formula to calculate CFM at different pressure drops. Find out what pressure drop that is.

Sun May 24, 2020 5:06 pm

Wreck wrote: madmike wrote:
750 Demon gets the same MPG as the 680 Quick Fuel (28 MPG w/boost ) if I removed the rack up top and had a lighter right foot I'd probably get 30mpg, but that would be 'boring' :lol:

Thats impressive MPG for a very quick car , who says you can't have all :-)

Why stop with a turbo, thats kind of boring, add Nitrous to the Turbo😂😂

Sun May 24, 2020 5:11 pm

modok wrote: Wreck wrote:

So if you have a carb that is supposed to flow 300cfm and a manifold /port that flows 230cfm , how does the carb restrict the intake down to 182CFM ?

because that's how things work man.
If a carburetor has ZERO pressure drop....how would it work? It would not even work. That is the energy that makes it work.

You put two resistors in series..... what happens? a 3 ohm and a 1 ohm
you have 4 ohms total.
If you run 12 volts through that, there will be 3 volts drop ACROSS the 1 ohm, and 9 volts drop across the 3 ohm. Which you can test with a meter.

Can you do that on a a flow bench? sure. You just did.
Now that you have added a carburetor, there is a few inches drop through the carburetor, and thus there is a few inches LESS pressure drop across the port.

In fact since you KNOW the CFM of the carburetor, and the total flow, you can calculate how much pressure drop there in in the middle.

Your flowbench instructions probably come with a chart or formula to calculate CFM at different pressure drops. Find out what pressure drop that is.

Yes, perfectly said.

Sun May 24, 2020 5:38 pm

would you not want a carburettor sized correctly so there is a pressure drop into the centre of the Venturi do draw fuel but has the same total CFM as the intake system not to restrict it ? Is this the reason people started boring IDA's out to 51.5mm and still fitting 44mm vents ?

It would be good Jeff if you can flow your 48 IDA's and see what CFM per throat . My figure is just about 1/2 what is quoted. I think the figures given on page 1 are for both throats not 1 . I'm beginning to think the figures most likely come from V8 tuners that wanted to try putting duals on a 4 barrel manifold .

https://shoptalkforums.com/viewtopic.php?t=1949

Sun May 24, 2020 6:16 pm

Wreck wrote: would you not want a carburettor sized correctly so there is a pressure drop into the centre of the Venturi do draw fuel but has the same total CFM as the intake system not to restrict it ? Is this the reason people started boring IDA's out to 51.5mm and still fitting 44mm vents ?

It would be good Jeff if you can flow your 48 IDA's and see what CFM per throat . My figure is just about 1/2 what is quoted. I think the figures given on page 1 are for both throats not 1 . I'm beginning to think the figures most likely come from V8 tuners that wanted to try putting duals on a 4 barrel manifold .

https://shoptalkforums.com/viewtopic.php?t=1949

Yes Rabys CFM numbers were per carb.

Sun May 24, 2020 6:17 pm

no
200cfm head.... Why not use a 200cfm carburetor? doesn't that make equal sense?
:lol:

On a flowbench the more pressure, the more flow. That only happens up to a point. Then flow chokes

The real purpose of a flowbench is to see how efficient the flow is in an odd shaped thing.
A big hole flows more than a small one, but duh, you can just measure a hole. a 10" long tube has twice the resistance as a 5" long tube, hmm, no surprise there.
No point in flowing a venturi, if a venturi is made correctly it will flow exactly what it should.
If it was not true.....a carburetor would not work, nor would a flowbench.
You have to believe that carburetor jets and venturis and gravity, ect...... are 100% predictable, and....they pretty much are.

Engines operate at much higher pressure drops. Somewhere around 200 inches of water at peak power.
Engines do not operate in steady state flow.
These are the two main differences between an engine and a flowbench.

You want to size the venturi so it flows some% more than the head at over 200" of water pressure drop.

What % more exactly?.....that has to do with the additional volume between the venturi and port. Same as the related concept "Runner taper"

The valve only has to flow what goes into the cylinder. The pressure in the manifold drops low as flow begins, and as flow slows down the pressure rises. This is a sort of lag....so the venturi has to flow more than the valve in the same time period to make up this lag due to time and varying pressure. Basically due to the speed of sound.
So exactly what % more is a little tough to guess, but, you can tell it has to do with the volume and length of what is between the venturi and the valve.

So thinking the carburetor should have zero flow resistance, that's crazy, but maybe less wrong than the simpler idea that it should flow the same amount.

Sun May 24, 2020 6:40 pm

You need the pressure drop in the carb where the fuel comes out . Not under the butterfly in the top of the manifold . With everything gutted ,just a bare throat and butterfly ,it is still a restriction on my intake using a basic DIY Dave Vizard type flow bench . No CFM figures with it . But a velocity stack that also has a restriction only having a 46mm ID when placed on the manifold (50mm) does not .