Street 351W: 500 CFM too much?

Skymarshal · May 17, 2006

I have a lot of drivability issues with my 69's rebuilt Motorcraft 2100- rough idle, hesitation, stalling on hills, etc. I've adjusted the timing, idle speed, mixture, and most of the other settings, but still haven't been able to get it running smoothly. One of my shops says a lot of the rebuilt carbs are crap right out of the box, and suggest getting a Holley or Edelbrock replacement.

My question is: for a purely weekend-driver, is the 350 CFM 2bbl enough carb for a 351W? I know this engine flows about 580 CFM, which adjusting for volumetric efficiency is right around 500 CFM, but one gearhead swears 500 CFM is too much for a stock 351W and will only increase gas consumption.
I'm aiming for smoothness, throttle response, and fuel economy versus additional horsepower.

69Rcode_Mach1 · May 17, 2006

Get a 4 barrel 600cfm holley with vacuum secondaries!!!!!

Edbert · May 17, 2006

While I'll agree that for cruising around town in a mild 351W a 500cfm carb might work well, it DEFINITELY is not too large!

Max Power · May 17, 2006

A Holley 600cfm carb would be perfect. The primaries are actually smaller than your Motorcraft, with vacuum secondaries you have the perfect variable venturi, and you get an electric choke if you want, which is way better than the heat tube style on the original Motorcraft. The only downer is that you have to buy a manifold as well.

But no, it is not too much cfm. Anybody who relies on that chart has a very numb seat of the pants.

Realmongo · May 18, 2006

Back when I had my '69 with a 351W and original Autolite 2V, I bought the Holley 500 CFM 2 barrel. It bolted right on with no problems and the car ran much better afterwards.

rbohm · May 18, 2006

500cfm 2bbl carb too much on a 351w? nope. in fact its about right.

D.Hearne · May 18, 2006

The Holley 500 is perfect for what you want. 4 bbl Performance in a 2 bbl carb. The 350 would give you better mileage though. A 600 cfm 4 bbl would too, but only if you keep your foot out of it, but you'd also have to factor in the cost of the intake swap.

HistoricMustang · May 18, 2006

69Rcode_Mach1 said:
Get a 4 barrel 600cfm holley with vacuum secondaries!!!!!

This is what I run on the street car and it is a 289. No problems. Your 351 will be OK.

Vacuum secondaries is the key as people try to bolt on a 600 double pumper and run into trouble.

HistoricMustang
www.historicmustang.com/tech.html

jbuening · May 18, 2006

I'm running an Edlebrock 1405 (600cfm) and my motor is fairly stock, with the exception of a slightly bigger cam. I'm actually looking to get a Holley Street Avenger 670cfm because the Eddy doesn't seem to keep up. 500 cfm is definitely not too big, if anything it is a tad too small unless you are looking for fuel economy and not performance.

danny clemens · May 18, 2006

Ford even put 600 cfm's on 352's and 289 hi-po's. Oh yeah, more power.

Stangninjak · May 18, 2006

jbuening said:
I'm running an Edlebrock 1405 (600cfm) and my motor is fairly stock, with the exception of a slightly bigger cam. I'm actually looking to get a Holley Street Avenger 670cfm because the Eddy doesn't seem to keep up. 500 cfm is definitely not too big, if anything it is a tad too small unless you are looking for fuel economy and not performance.

Yes the OP is looking for feul eco and drivability, so the 500 cfm is by no means to big. I run a 670 on my setup, and its dam nice. The carb comes with three springs you can swap out, so you can always get what you are looking for.

See my setup in my sig.

Sicarius428 · May 18, 2006

500 would be about right but if you want some growing room a 600 would be definately the way to go.
Kevin

69Rcode_Mach1 · May 18, 2006

A four barrel will get better economy as well as more power. The two barrel 500 will also get you more power but it is flowing more gas and air the whole time which will hurt economy.

Focker67 · May 18, 2006

I always thought the formula was Cid x max rpm /3456 would give you the carb cfm you need and then you can round up or down depending on your mods and availability.

That is just what I was told......

bnickel · May 18, 2006

if you are looking for drivability and economy and want to stay with a 2 barrel carb go with the 350cfm holley. the 500 2 barrel is too big for decent economy and will actually get worse mileage than a 600 cfm 4 barrel carb that is driven nicely (staying out of the secondaries). if you want to stay with a ford carb get an older autolite instead of the motorcraft. look for an older autolite from a 2 barrel 390.

D.Hearne · May 18, 2006

The Holley 500 is built with perfomance in mind, however you can jet it down slghtly if you want better economy, it would also help too, to replace the 50 cc acellerator pump it comes with with a 30 cc unit. As for it being capable of mileage, it can do that too, but it, like a 4 bbl 600 will depend on your foot. The 500 is exactly half of a 750 cfm double pumper Holley (375 cfm) but they rate the 2 bbls at a different pressure differential, any given engine will exert a stronger pull at WOT on a 2 bbl vs a 4 bbl hence the reason it's rated at a higher cfm than half a 750 cfm 4 bbl.

Max Power · May 19, 2006

D.Hearne said:
The 500 is exactly half of a 750 cfm double pumper Holley (375 cfm) but they rate the 2 bbls at a different pressure differential, any given engine will exert a stronger pull at WOT on a 2 bbl vs a 4 bbl hence the reason it's rated at a higher cfm than half a 750 cfm 4 bbl.

That's interesting, I didn't know that.

D.Hearne · May 20, 2006

Max Power said:
That's interesting, I didn't know that.

This is also the reason why that carb sizing formula isn't 100% correct. They rate 4 bbls at a 1.5 inches of mercury drop, but not all engines pull that amount of vacuum with all sized carbs. The 2 bbls are rated at 3 inches (think thats correct) A larger engine can actually pull more cfms thru any given size orrifice than a smaller one. That formula is just a base line to start with in choosing a carb. Chences are the carb you choose will not actually flow that cfm on the engine you put it on.

86bluecobra · May 20, 2006

I would be getting a 4 barrel manifold and going with holley street avenger in the 600cfm range.

Max Power · May 20, 2006

D.Hearne said:
This is also the reason why that carb sizing formula isn't 100% correct. They rate 4 bbls at a 1.5 inches of mercury drop, but not all engines pull that amount of vacuum with all sized carbs. The 2 bbls are rated at 3 inches (think thats correct) A larger engine can actually pull more cfms thru any given size orrifice than a smaller one. That formula is just a base line to start with in choosing a carb. Chences are the carb you choose will not actually flow that cfm on the engine you put it on.

I knew that, I just didn't realize the spread on the Holley 500 was that big.

Another thing to consider, Holleys are tested wet, Edelbrocks are tested dry. Since Fuel displaces air, a Holley 600 and an Edelbrock 600 do not flow that same amount of air. The holley will flow a little more.

Street 351W: 500 CFM too much?

Skymarshal

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