Twin 32/36 Webers On A 4k??

[Felix]

A 4K with 2 webber's and you're hitting 400cfm on a 1.3L motor. This will run a mild 308 or 253 happily.

 

Not really as you are effectively running a dual plane manifold. Due to the fact that the manifold is split into two, the cylinders can only see half of the 400cfm, ie. 200cfm/cyl (or the same as running a single DGV). Any flow improvements would be through slightly better manifolding and less turbulence under the carbs.

 

Likewise running twin 40mm sidedrafts on a motor. Each cylinder can only see one barrel of the carbs. So say your 40mm sidedrafts (with say 36mm chokes) have a flow rating of 175cfm per barrel, or 750cfm total (for both carbs), each cylinder only sees a max of 175 cfm. Note that the cfm/cyl would be less than if running dual DGV's, except for the fact that downdrafts have a much less efficient intake path.

Edited August 20, 2008 by Felix

[beerhead]

750cfm, you're dreaming. Don's law states to find approximately what size carb an engine will require at 7000 RPM double the cubic inches

 

So a (1.3L)79 cubic inch motor at 7000 rpm needs 160 CFM

 

So a 350 Cubic inch V8 needs 700 CFM @ 7000rpm. That enough flow for 400hp.

 

I know for a fact you can't flow much past 200hp with 40mm weber's.

 

A twin carby K manifold is not a dual plane manifold, for that to be so cyl 1 and 3 and 2 and 4 should be linked to allow the inlet pulses to come after eachother on each carb.

 

 

 

 

From the graph you can see that you'll have 80cfm per port. The twin downdraft weber will deliver 100 cfm per port, but will flood the crap out of the engine down low, damn my old 4K was a shocker, and that was only a single carb.

 

 

Not really as you are effectively running a dual plane manifold. Due to the fact that the manifold is split into two, the cylinders can only see half of the 400cfm, ie. 200cfm/cyl (or the same as running a single DGV). Any flow improvements would be through slightly better manifolding and less turbulence under the carbs.

 

Likewise running twin 40mm sidedrafts on a motor. Each cylinder can only see one barrel of the carbs. So say your 40mm sidedrafts (with say 36mm chokes) have a flow rating of 175cfm per barrel, or 750cfm total (for both carbs), each cylinder only sees a max of 175 cfm. Note that the cfm/cyl would be less than if running dual DGV's, except for the fact that downdrafts have a much less efficient intake path.

[Felix]

750cfm, you're dreaming. Don's law states to find approximately what size carb an engine will require at 7000 RPM double the cubic inches

 

I never said that the motor sees 750cfm. Only one cylinder is on it's intake stroke at a time (plus some overlap).

 

I just did the same thing you did and added the flow of the carbs together for total carb cfm as you did with your example of running 2 DGV's to get 400cfm.

Notice I said on a motor with 40mm sideys, on each cylinders induction stroke it has 175 cfm of carburetion available. Port and manifold inefficiencies obviously make things worse.

 

bare carb flow figures for a 40mm DCOE sidedraft with 36mm venturis is 175cfm per barrel at 1.5 in/hg. A 32/36 DGV is 230 cfm (for both barrels) measured at the same pressure drop.

 

Don's law states to find approximately what size carb an engine will require at 7000 RPM double the cubic inches

 

David Vizard recommends for carb selection between 2 to 2.2 CFM per hp (bare carb flow measured at 1.5 in/hg). So a single 32/36 DGV carb is best on a motor making between 100 to 120 HP.

 

A twin carby K manifold is not a dual plane manifold, for that to be so cyl 1 and 3 and 2 and 4 should be linked to allow the inlet pulses to come after eachother on each carb.

 

Yea, it is probably best described as a Divided Single-Plane Manifold:

 

 

The twin downdraft weber will deliver 100 cfm per port

 

No. Each port is connected to BOTH barrels of one of the webers... So each port has 230 cfm of carburetion available to draw on. This doesn't mean it will, as each port is only on it's intake stroke for 180degrees out of every 720 degrees of crank rotation (or roughly a 1/4 of the time). What I am getting at is that hyperthetically if the motor could use the full capacity of the carbs it would draw 230cfm in a minute, 115cfm per carb, or 57.5 cfm per cylinder.

Edited August 20, 2008 by Felix

[beerhead]

The graph you have is for a bare carb with a vaccum device attached free flowing. A peak flow is just a magic number, its like buying a 350hp peak turbo and complaining that your motor doesn't make 350hp. Real world values are the only ones that count.

 

The graph I have there is carby fitment guide, which is closer to the real flow when a carby is being used

 

The numbers you use just don't add up

 

 

32/36 choke surface area:

220 CFM/

1822.123 mm2

 

=0.121 CFM/mm2

 

DCOE40 choke surface area :

350 CFM/

2034 mm2

 

= 0.172 CFM/mm2

 

 

Yes I know there are ram tubes on the carby, and they are much better than a downdraft, but do you really think a DCOE outflows the downdraft by 40% per mm2 of area, seems a little outrageous, love to see a reading on a DCOE 4K under vacuum.

 

Doesn't sound right to me

[Felix]

The graph you have is for a bare carb with a vaccum device attached free flowing. A peak flow is just a magic number, its like buying a 350hp peak turbo and complaining that your motor doesn't make 350hp. Real world values are the only ones that count.

 

Yes it is, the figures for the DCOES are free flowing, likewise are the estimated figures you were using for the DGV's.

 

The graph I have there is carby fitment guide, which is closer to the real flow when a carby is being used

 

Yea I've got the David Vizard book that the graph you have came from.

 

As for the flow figures you have there, they are comparing the flow of two barrels of the DCOEs to the flow of a DGV. Seeing as DCOE's are used on individual runner manifolds so an intake port can only ever see one barrel of flow (half of the figure you show). On a DGV each port sees both barrels of the DGV. Doesn't mean the port uses more flow with a DGV, as downdraft manifolds are usually a lot shittier.

 

Think about it, imagine you are in the port of the motor looking up the manifold to the carb butterflies.... How many butterflies could you see with a DCOE, now what about a DGV. With the DCOE there is only one butterfly, but with the DGV there are two, but around more bends and twists.

 

Here is a better flow comparison making a note of the individual barrel flows of the sidedrafts (note the 28/36 DCD weber which flows 5 cfm less that a 32/36 dgv):

Edited August 20, 2008 by Felix

[Felix]

Yes I know there are ram tubes on the carby, and they are much better than a downdraft, but do you really think a DCOE outflows the downdraft by 40% per mm2 of area, seems a little outrageous, love to see a reading on a DCOE 4K under vacuum.

 

Noticed an error in the calcs for the DCOE. Should be:

 

32/36 choke surface area:

220 CFM/

1822.123 mm2

 

=0.121 CFM/mm2

 

DCOE40 choke surface area :

350 CFM/

2514 mm2

 

= 0.139 CFM/mm2

 

Looks a bit more realistic now I guess.

[snot35]

Excellent debate! As a slight detour off topic, as the Vizard book seems to deal with A series engines do they delve into selecting single side draft carbs? I've got a single weber setup that's going on the k powered rolla, and if there is some tasty info about selection and tuning I may have to grab a copy of this book.

 

Cheers

[beerhead]

I did my calcs off the size of the restrictor, not the size of the butterfly. The butterfly comes after the choke dude.

 

Noticed an error in the calcs for the DCOE. Should be:

 

32/36 choke surface area:

220 CFM/

1822.123 mm2

 

=0.121 CFM/mm2

 

DCOE40 choke surface area :

350 CFM/

2514 mm2

 

= 0.139 CFM/mm2

 

Looks a bit more realistic now I guess.

[Felix]

snot35 yea it has plenty of tasty info. As you said, it is based on the A series mini motor, though a lot of the info can be applied to our motors. There is heaps of carb selection and tuning info.

 

I did my calcs off the size of the restrictor, not the size of the butterfly. The butterfly comes after the choke dude.

 

From what I can see you based your calcs on the area of the throttle butterfies on the DGV, and with the DCOE you based the calcs on it having 36mm venturis(chokes).

 

A 32/36 DGV has 26mm primary and 27mm secondary venturis.

[snot35]

snot35 yea it has plenty of tasty info. As you said, it is based on the A series mini motor, though a lot of the info can be applied to our motors. There is heaps of carb selection and tuning info.

 

 

 

From what I can see you based your calcs on the area of the throttle butterfies on the DGV, and with the DCOE you based the calcs on it having 36mm venturis(chokes).

 

A 32/36 DGV has 26mm primary and 27mm secondary venturis.

 

Great Felix, thanks. So this is the "Tuning the A-series" book??

 

Thanks again.

 

Michael

[Felix]

Yes that's the one Michael. My book is an earlier edition and is titled "Tuning BL's A-series Engine".

 

You're welcome.

 

Doug

[brendan]

hey mate how do u still have the webbers