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Old 11-30-2019, 05:57 PM   #1
derek costello
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Default how many cfm for two carb flathead??

If we are to believe that a late model ford flathead is only supposed to require
about 300 CFM of carbaration and that is further reduced by an efficiency ratio of 82 percent, then we are to work with 247 cubic feet per minute for the design
of carburetors !
A typical Stronberg 97 is rated at 150cfm's and holly 94s are rated at 162cfm's?
So any combination of duel carbs is going to be way over the requirement of any but the wildest modified flathead.......? What gives???
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Old 11-30-2019, 06:23 PM   #2
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Default Re: how many cfm for two carb flathead??

Quote:
Originally Posted by derek costello View Post
If we are to believe that a late model ford flathead is only supposed to require
about 300 CFM of carbaration and that is further reduced by an efficiency ratio of 82 percent, then we are to work with 247 cubic feet per minute for the design
of carburetors !
A typical Stronberg 97 is rated at 150cfm's and holly 94s are rated at 162cfm's?
So any combination of duel carbs is going to be way over the requirement of any but the wildest modified flathead.......? What gives???

Any more than two carbs is strictly for looks and/or bragging rights! "Oh look! His engine has three carbs! It's got to be faster than that lowly flathead with only two carbs! Hey, here's one with four carbs. More is always better, right?"
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Old 11-30-2019, 07:25 PM   #3
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Default Re: how many cfm for two carb flathead??

I'm not sure what your getting at but, FH's are somewhat limited to 5500 rpm and by today's standard, they are small V8's.

The 94 carb is more like 125 cfm so, if you were to build a 276 CI engine and run it at 5500 rpm, using your .82 efficiency, it could consume 360 cfm and you could use three, 94's.

It's just how it is.
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Old 11-30-2019, 07:52 PM   #4
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Default Re: how many cfm for two carb flathead??

There are very few flatheads that run to 5500 and stay together for very long. Max power is usually below 5k for even the best of dtreetab.e cams The L-100 peaks out ariun 46/4800 and that was with 2 2GC's and 174 HP.
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Old 11-30-2019, 07:54 PM   #5
derek costello
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Default Re: how many cfm for two carb flathead??

hello frank... the factory book says that the 94 is rated at 162 cfm, so two would be 324 cfms. At 255 ci mercury engine at less than 4500 rpm these two carbs are over the top in requirements! but everybody still does it! So I'm thinking of reducing the internal ventrie
of the 94s to make them more compatible with MY engine size ??
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Old 11-30-2019, 07:54 PM   #6
derek costello
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Default Re: how many cfm for two carb flathead??

hello frank... the factory book says that the 94 is rated at 162 cfm, so two would be 324 cfms. At 255 ci mercury engine at less than 4500 rpm these two carbs are over the top in requirements! but everybody still does it! So I'm thinking of reducing the internal ventrie
of the 94s to make them more compatible with MY engine size ??
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Old 11-30-2019, 08:19 PM   #7
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Default Re: how many cfm for two carb flathead??

JWL has quite a bit of carb information at different configurations on the dyno. He showed modest gains with two 94 on a basically stock engine. With some engine modification the 3x2 will outperform a 2x2 at higher rpms, but the 4bl gives the best numbers. I'm not a 4bl fan on a flathead, so looks wins out!

Last edited by JSeery; 11-30-2019 at 08:29 PM.
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Old 12-01-2019, 01:10 AM   #8
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Default Re: how many cfm for two carb flathead??

twin triple quad its all about what floats your boat there are ways to make them work some better than others next time your at at meet have a real close long good look you'l be surprised how many twins and triples are actually only running a single the rest is there for looks its a wild flathead that can use that much carb
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Old 12-01-2019, 12:29 PM   #9
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Default Re: how many cfm for two carb flathead??

Would 81's at 125 cfm work better?
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Old 12-01-2019, 05:15 PM   #10
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Default Re: how many cfm for two carb flathead??

https://www.hotrod.com/articles/hrdp...old-smackdown/ Pete
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Old 12-01-2019, 07:30 PM   #11
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Default Re: how many cfm for two carb flathead??

Interesting article.
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Old 12-02-2019, 02:30 AM   #12
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Default Re: how many cfm for two carb flathead??

Article also mentions a 12.3/1 compression. Maybe I'm mistaken. That's a shit load. Race gas only.

You can run a lot of carbs with jets reduced and progressive linkage. It's a village, all needs to be in order. You have to at least have some displacement beyond stock.

I run just a single 97... jwl book was informative. Books are good. Great article link, h&h are great folks.


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Last edited by Tinker; 12-02-2019 at 02:54 AM.
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Old 12-02-2019, 07:42 AM   #13
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Default Re: how many cfm for two carb flathead??

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Originally Posted by 40 Deluxe View Post
Any more than two carbs is strictly for looks and/or bragging rights! "Oh look! His engine has three carbs! It's got to be faster than that lowly flathead with only two carbs! Hey, here's one with four carbs. More is always better, right?"
A little broad brushed, no?

Our own Walt Dupont runs a 4X2 carb'd flathead that gives him instant throttle response. Others have run various combinations of multi-carbs. Some with great results and some not so much.

Having just built a motor with one of the best in the business, I learned that every modification requires an equal reaction.

I think this multicarb myth still carries over for many folks just plop on a multicarb intake on to a relativity stock motor and are disappointed by the results. If you want to run multi-carbs, get good with a die grinder and practice and be willing to open your wallet for it takes some hi-po parts to make it all work.

Last edited by Tim Ayers; 12-02-2019 at 08:22 AM.
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Old 12-02-2019, 07:55 AM   #14
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Default Re: how many cfm for two carb flathead??

Then there are those of us who put in 3/4 cam and dual carbs cuz we just want to... Sound and visual stimulation was enough reason for me. My F1 was still not a race car, but looked and sounded sweet, and it helped muffle the tinnitus noises, but I could still hear my "little voices"... Chap
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Old 12-04-2019, 06:24 PM   #15
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Default Re: how many cfm for two carb flathead??

twin 81s would possibly work a little better being just that tad smaller they will actually have a slightly higher air velocity thru the carb and therefore give the carb better internal signals again there is endless constant debate on this. But if you take the time and do some digging looking at manufacturers who offer multi carb options you generally find that the muti carbs units only flow at around 75% of the single offering
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Old 12-05-2019, 05:26 AM   #16
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Default Re: how many cfm for two carb flathead??

The answer is "it all depends" . . . on the engine, the cubic inches, the ports, the cam, the heads, the usage (mild street or race), etc.. To make HP, you need air and fuel - so depending on a LOT of things in the combination, the air/fuel requirements (and carb sizes/numbers) will vary a lot.

Saying that any one combination is the "best" - makes no sense - just think about all the SBC engines out there . . . with mild ones running a single 2 barrel and making 175 HP (maybe) and fully prepped race engines with 850+ cfm (and that is small) - making 650 HP+.
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Old 12-05-2019, 09:34 AM   #17
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Default Re: how many cfm for two carb flathead??

This reminds me of when we were stupid ass kids in the early '60s. We had a great running '55 Chevy with a four barrel carb.So we figured by putting two four's on it will make it faster. Not so, it ran like crap and we put the four barrel back.
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Old 12-05-2019, 09:47 AM   #18
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Default Re: how many cfm for two carb flathead??

You think you were "stupid ass kids"; hell, you were amateurs. I had a friend with a good running 2 barrel 265 in a '55 Bel Aire hardtop. We stumbled across a 6-2 setup and installed it , expecting instant gratification. The only way we could get it to run was to use the center carb on each side. Surprisingly, it was at least driveable that way. Since we couldn't figure out how to adapt a progressive linkage, we ran a wire from the end 4 carbs into the passenger compartment. When we got this set up, we went out for a test drive. My buddy was driving, and right after he floored it, he yelled to me "Hit it!". I gave the wire a good pull and the engine immediately started to stumble badly and eventually died.

The next day we were at Carmichiel's Auto Parts (Junkyard), begging to trade the 6-2's for a 4-BBL and manifold. Ahhh, the follies of youth.

As to two carbs on a flathead? My recommendation (if you have a full hood), is a small base 2G on a bored out Merc manifold; 275 CFM and no problems.
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Old 12-05-2019, 10:44 AM   #19
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Default Re: how many cfm for two carb flathead??

In a copy of JWL's book, He goe goes through a number of multi carb instolations on the dyno. Just curious, has anybody thought of part throttle operation, and a 4 bl only runs on 2 , except when needed. And i had a ride in Walts 40 with the 4 carbs, ran great.
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