Max CFM for 351C 4v
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Junior User
Joined: Sep 2006
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Max CFM for 351C 4v
I am wanting to spend some money for a new carb for my 351c 4v. This is for a '70 model engine with stock 4v quench heads (10.7 to 1, to the best of my knowledge), and a stock square-bore 4v manifold with a stock motorcraft carb. I need advice on what would be best brand carb and how many cfm this engine will handle. I'm also running a 268 Comp cam and Dura ignition.
This engine is in a '64 Fairlane, with a t-10 4speed, and 4:11 gear.
Any advise will be greatly appreciated!
I just hate spending a bunch of money only to find out what I had worked just fine.
This engine is in a '64 Fairlane, with a t-10 4speed, and 4:11 gear.
Any advise will be greatly appreciated!
I just hate spending a bunch of money only to find out what I had worked just fine.
Posting Guru
Joined: Jul 2007
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uftball, this engine heads have huge ports for higher rpm, a 650 for his aplication would restrain it, first iv wouldve used a bigger cam due to the compression, maybe a xe274h, anyways, better use a 750cfm carb....
Lead Driver
Joined: Mar 2004
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I had a '72 Torino with a Cleveland years ago. I had closed 4v heads, Edelbrock single plane intake, big cam, 4 speed, etc. A 600 was too small but it ran ok. A 750 was all around too big for the street. I ended up with a 650 on it in the end. A 700 probably would have worked ok as well but I never got to try one on it. I'd pitch the stock intake when you do the carb swap. Try to find a dual plane Edelbrock of something.
Posting Guru
Joined: Jul 2006
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From: America
i'm working on it... haven't got them yet. ill let you know when it happens or if its not going to happen
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Laughing Gas
Joined: Jan 2006
Posts: 883
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From: New-brunswick,Canada
With street engines, volumetric efficiency is typically around 75 to 80 %. Boost the performance and VE goes up to 95%. The best indicator of engine performance is an engine dynamometer. Here's how to pick your carb size:
VE % x Cubic-inches x maximum rpms divided by 3456= proper carburator size.
I got an example from ford part interchange. They built a 460 that performing strong on dyno. The dyno figures tell them 85% VE. Take 85% VE x 460ci x 5500rpm=732.06018. Because they want 85% of 732.06018, that's 622.25cfm. opt for a 650 cfm carb.
VE % x Cubic-inches x maximum rpms divided by 3456= proper carburator size.
I got an example from ford part interchange. They built a 460 that performing strong on dyno. The dyno figures tell them 85% VE. Take 85% VE x 460ci x 5500rpm=732.06018. Because they want 85% of 732.06018, that's 622.25cfm. opt for a 650 cfm carb.
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Posting Guru
Joined: Dec 2002
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From: chicago burbs
the cam is too small for the combo. at the moment i have a 268H in mine, and 4:11 gears. it runs out of steam pretty quick and it isn't the 650 double pumpers' fault! i've run the 268H with 2:75 rear gears and stock converter no problem.
my combo was Crane F238/246 solid cam, 4V open chambers, Offy Port-O-Sonic intake and 3000 stall in a wide ratio manual valve C6...
i pulled it apart to swap in 1 piece valves and found the dizzy gear and cam gear were chewed up bad so i put it back together as a 2V/268H top end to keep it running while i re-do the 4V top end. turns out i picked up a blower set-up.
anyway, i'd say you either need to pull the heads and get them done up to handle enough cam to tug on the 4:11's, or drop the gear down some to match the rest of the combo.
other than that, i like the QF680 idea on a torker with more cam and 4:11's
my combo was Crane F238/246 solid cam, 4V open chambers, Offy Port-O-Sonic intake and 3000 stall in a wide ratio manual valve C6...
i pulled it apart to swap in 1 piece valves and found the dizzy gear and cam gear were chewed up bad so i put it back together as a 2V/268H top end to keep it running while i re-do the 4V top end. turns out i picked up a blower set-up.
anyway, i'd say you either need to pull the heads and get them done up to handle enough cam to tug on the 4:11's, or drop the gear down some to match the rest of the combo.
other than that, i like the QF680 idea on a torker with more cam and 4:11's
Posting Guru
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As i stated before, I agree, defenitivley needs a bigger cam and i would still use a 700-750cfm carb
Posting Guru
Joined: Oct 2005
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From: Longwood, Fl
spread bore
What a coincidence! I have a 70 mach I, same engine, FMX tranny myself 
I suppose you know that it is not as simple as just a carb swap--! The ford 4300 spread bore carb is not the same spreadbore pattern as the Quadrajet spreadbore, and a squarebore holly will be a bad mismatch almost certainly requiring an adapter, which will cause numerous complications. Do a google on 4300 carburators, and get some info. They flow in the 600's.
I have already bought an edlebrock alum DP, just letting it "season" in my garage. Now I find out that the edlebrock may not be up to the flow task. Apparently a weiand may be better. Edlebrock is a good street manifold apparently. After about 5 years, its pretty well seasoned--
. I have numerous "pre-owned" Hollys ( 750/850 squares and a 4010 spreadbore replacements) that will go on it----providing I ever get the #$@% thing running again. LOL
I suppose you know that it is not as simple as just a carb swap--! The ford 4300 spread bore carb is not the same spreadbore pattern as the Quadrajet spreadbore, and a squarebore holly will be a bad mismatch almost certainly requiring an adapter, which will cause numerous complications. Do a google on 4300 carburators, and get some info. They flow in the 600's.
I have already bought an edlebrock alum DP, just letting it "season" in my garage. Now I find out that the edlebrock may not be up to the flow task. Apparently a weiand may be better. Edlebrock is a good street manifold apparently. After about 5 years, its pretty well seasoned--
. I have numerous "pre-owned" Hollys ( 750/850 squares and a 4010 spreadbore replacements) that will go on it----providing I ever get the #$@% thing running again. LOL
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From: chicago burbs
i was thinking still more cam than the 274, more like the 280H or 292H, or the 282S or 294S.
phredE, if you do have the 10.7:1 that you suspect, you need to cam it appropriately or you'll have crazy high cylinder pressures, insane octane requirements and/or a major detonation problem.
does the engine rattle under load at all now?
phredE, if you do have the 10.7:1 that you suspect, you need to cam it appropriately or you'll have crazy high cylinder pressures, insane octane requirements and/or a major detonation problem.
does the engine rattle under load at all now?
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