carburetor choice

[Anonymous]

my buddy just got his head redone for his 250. (71 maverick 2door) they put 40 over valves in it and it just occured to me that the stock carburetor might not be the best match for the new setup. right now he has a carter model 0-2241 rbs. this is the carb that has the huge ugly round fuel bowl on the front. no one i've called has a kit for it. all of their paper catalogues skip over this model. finding a rebuild kit for it so far has been frustrating to say the least. my question is:
.
should i bother rebuilding the old one barrel or should we opt for a larger one barrel?
.
i know that the intake is cast onto the head and the size of the hole in the top is fairly small. i have heard stories about mounting a 2 barrel on an adapter plate, but does it really make that much of a difference if the size of the original hole in the intake is not increased in size? this is kind of a side question. my main question is whether or not i should fix the old one or get a bigger one to match the 40 over valves.

we plan to get a header for it eventually but we just need it on the road for now. does anyone know specific calculations for fuel flow needed with the bigger valves? also, does anyone know the fuel flow of the carter 0-2241 rbs carburetor?

thanks in advance for any replies.
signed,
me

 

[Hot 6t Falcon]

Wile,

First, I wondering if the shop put in valves with .004 oversized valve "STEMS" or if they put in valves with .040 larger FACES. If they are larger stems to get rid of sloppy valve guides - then there is no change in the amount of air they will flow.

If they are larger faces (intake from 1.76 to 1.80 and exhaust fron 1.39 to 1.43) It will flow a little more air and fuel. BUT not enough to "need" a bigger carb. HOWEVER, it is a good excuse to get a bigger carb! HERE'S WHAT WE ALL WANT KNOW, IF THIS IS THE CASE; WHERE DID THEY BUY THOSE OVERSIZED VALVES!!!!!

Finally, there will probably lots of learned discussion about adapting a two-barrel to a one-barrel hole. Do this. Hold the choke open on any carb and then flip the throttle wide open. Look through the barrel(s). There is one heck of a lot of "Crap" for the air and fuel to work its way around. Now consider that the carb bore hole in the manifold is a wide open 1 3/4 inch hole with NO obstruction. That wide open hole would probably flow 500cfm - but as far as I know - no one has ever flow tested the throttle bore. So don't go quoting me on the cfm of a one-barrel log. Putting a two barrel adapter on an early log with a 1 1/5 inch hole would not be as effective - but practice has shown it still works!

Some have used another method of adapting the two barrel. We mill the carb flange off flush with the head and use a "homemade" adapter of 1/2 aluminum plate. Once the maniflod is flat we elongate the carb bore to approximate the carb bore. The aluminum adapter tapers just a bit - but not nearly as much as a two into one adapter. You'll need a heat-spacer on top of the adapter to get clearance for the throttle linkage.

This picture is from page 28 of The Falcon SIX Cylinder Performance Handbook

 

[Anonymous]

Hi wily,

Couple of thoughts for you. The stock 250 is a "high" torque, low rpm engine designed to move some heavy cars (Granada, Fairmonts, etc.) around town reasonably well (love the torque). So, the RBS didn't need to flow high CFM's (small bore = high velocity = better torque). In 1971 the stock 250 was rated at 4000 rpm, the same engine was rated at 3600 rpm in 1971.

As a side bar the auto industry went to "net HP" rating in 1972. Prior rating was on an engine dyno (open header, no accessories, etc). Net was as installed in the car. So, the 1971 rating is unreliable. I would assume the 1972 numbers are a better baseline, i.e., 95HP@3600 - 181 lb ft @ 1600.

Back to the point. Carb flow can be calculated as follows: ((rpm/2)xCID)/1728. So, for the 250 we get ((3600/2)x250)/1728=260cfm. This number assumes 100% volumetric efficiency (VE). In the real world the stock 250 won't get there. 75% is probably more like it. So, 260cfm x 75% = 195cfm. The stock RBS flows approx 193.

Now what? Your engine will turn faster than 3600. The cam is probably good for 4000 - 4500. The head will flow better than 195cfm. At 4000 it will need 220cfm. At 4500 it will need 240cfm. You say you plan to get a header. This will increase your VE. For your set up I would go with the Holley 5200 from Stovebolt (same as the Weber DGE 32/36), which will flow as high as 320 at 3" Hg. (low of at 270 at 1.5" Hg). Being a progressive 2V carb it will run on one throat until 70% throttle, at which point the second throat will start openning. Keeps your intake velocity high at low rpm (torque) and provided higher flow at 4000 / 4500 rpm. If you continue to mod the engine (cam, port, etc) you will run out of carb soon. The next step up would be the Weber DGV series (38/38 - 370cfm), which will fit on the same adapter.

About the adapter. The one I recommend is the one from Clifford. It is a quality product made for this specific purpose. The "generic" types available form others (Stovebolt, Summit, etc.) are not user friendly. As for the 2V carb on a 1V hole, look at it this way. The surface area of the 1.75" carb bore in your manifold is 2.4 sq. in. The suface area of the 32/36 throats (26/27 venturis) 1.77 sq. in. The 38/38 is 2.42 sq in. A reasonable match for the manifold, and the adapter has a nice taper. However, there is no substitue for the direct mount, if you have the skills and the equipment.

Ingition. An electronic ignition upgrade will improve performance quite a bit. You have 2 choices. The Pertronix Ignitor II with a Flamethrower2 coil or the DSII. The Ignitor just drops in the stock distributor, the DSII is a conversion (little more time and work to install). IgnitorII with Flamethrower2 is about $115. The DSII is a little more. Both work well.

Probably more than you wanted, or needed, but what the heck. Just wanted to pass on some hard learned lessons.

Steve

PS - I highly recommend the Ford Falcon Performance Handbook. http://falconperformance.sundog.net

 

[strat1960s]

Wiley,

Should you decide to go with the Weber 38 DGAS carb on your motor you have one of two choices for mounting it.
1. Get an Aussie head and have an adapter made to mount the carb to your intake.

-or-

2. Get the adapter from Clifford.

Both options are good, but buying a new head may be a little more than you were wanting to spend. If that is the case, you can have my adapter for the cost of shipping, probably somewhere in the $5 to $10 area. I never used my adapter, because I switched to the Aussie head after ordering my Weber 38 DGAS carb kit from clifford.
Let me know if you are interested.

Ted

 

[Anonymous]

you guys are great. steve, you gave me an overwhelming ammount of info! just what i needed and i can't thank you enough. ted, don't forget about me. we might be getting in touch with you really soon about that adapter. give me a little while to let this new information to sink in real good. i'd like to take this to my engine buddy for his input as well. thanks again for all the great stuff!!! rock on fordsix.com!!!
signed,
me