Friends,

Please forgive me if this a re-run, but I've had some health issues and the memory is not quite what it used to be. When I finally get the '57 Mercury 312 in my wrecker, I have a factory four-barrell manifold (Later type, square bore) and I need some suggestions on what size and brand carb to use.

For some reason, a 450 or 500 Holley keeps coming to mind, but a friend who is helping me suggests an Edelbrock 450. The truck has the original 4:56 gears and I plan to keep those.


Also, I scored a pair of ram's horn exhaust manifolds for this project.

Oh, it's a '56 F-350 wrecker.

Thanks in advance,

 

With a 312 in it I'd go with a 600 CFM and vacuum secondaries anything else I believe would be too small for the cubic inches and would starve the engine of fuel.

 

In your application I would lean towards less CFM. I'm not a big Holley guy for street applications but that doesn't mean they don't work well. I just prefer the AFB (Edlebrock) design. Not sure if they are compatible with the OEM 312 Intake Manifold.

500 CFM is probably your sweet spot for throttle response and drivability. I've had good luck with the Strip Kit which allows metering rod changes in less than a minute..

No disrespect for the Holley - I've had a million of them. I just like the AFB for the Street.

Just my .02

 

John and E-Man,

Thanks for y'all's responses. My friend who is doing the engine swap has much more carb experience than I do. Just because carburetor(sp?) rhymes with radiator doesn't mean I know about them!

 

The original Holley would have been a 4 barrel with governor and would have been a 390 CFM on the 272 V-8. (Interestingly enough, the carburetors on the Lincoln 317 engine used before 1956 used a Holley 2 barrel with governor. One thing for sure, a 272 does not need more than 450 CFM. With a smaller CFM carburetor, your low speed throttle response will be better although the secondaries will come in a bit sooner.

The Edelbrock Carburetors (assuming you do not want to have it exactly original) are easier to set up than a Holley and both, IMHO, are very pricey. Either will work, but the smallest Holley you can find now is a 390 CFM and it would be a good choice. From there, the size jumps to 550 CFM and you can find Edelbrocks and Holleys in that size. Don't go any larger than 550 CFM. Since it now is a 312, it could use perhaps 450 CFM if it were revved over 5000 rpm.

Regards,

Alan

 

I would go with 600 vacum secondary holley. I thimk model # 1850 simple and adjustable.

 

The 1849, it is a 550CFM just like the 1850 which is a 600 CFM. These were old style non smog carbs and are legal for these years. They are hundreds of dollars less than other Holleys or for any Edelbrock. As far as setups, they are very easy to set up, if you learn a little Holley stuff. With Holleys they will come with metering plates on the Secondary End and a metering block on the Primary. You can change jets and the power valve on the primary. You can convert the carbs to have a metering block on the secondary for about $70. The Edelbrock Carbs are quite hard to change, being an old Carter AFB, they use metering rods and jets which have ranges. Edelbrock does ask you all about your use, where you live, and they set up the jet and metering rod sizes and metering rod spring for you. So with the Edelbrock you get a nicely tuned carburetor, but you pay for it. Since I am a tuner, I would not pay their price, and the Holley is a lower restriction carburetor that is much better for performance use but is still good for stock engines. I use an engine size cfm chart and a 312 at 5000 rpm only requires 316 CFM with a stock cam and even if you pushed up the volumetic efficiency with a rebuild with higher compression, it would only need 360 at 5K rpm and 380 at 5.5k rpm. So the carb size doesn't need to be above 390. 4 barrels are rated at 1.5 inches of vacuum, and 2 barrels at 3" of vacuum. This is to relate the vacuum each can maintain at WOT (wide open throttle) You don't get all the CFM due to air cleaner restrictions, and in some cases port sizes and port matching, but you don't push your truck engine up past 5.5K rpm either... Now in a loaded truck with a Holley you want a power valve that is fairly low since your vacuum at cruise won't be too high and you want the power valve to be closed at cruise, so I would set it up with a 65 (6.5 inches of manifold vacuum) If you engine can pull a higher vacuum at cruise you can go to an 85. Jetting for low altitude should be pretty good out of the box. Probably 56 primaries should work fine. The rear metering plate is for the secondaries during high throttle working and they can be pretty rich. I don't think you would have to bother with them......