I have been searching thru some threads looking for a similar engine as mine to decide what size and what type of carb would be best for my setup.

Here is the information that I remember back when I did the engine a few years ago..

1973 Mustang Mach 1
351 Cleveland bored 30 over 10 under, using 2 barrell heads that have been ported and polished, Lunati 292 cam with 560 lift and 230 duration. 10:1 pistons and normally runs good on 90+ octane. MSD 7al ignition and distributor. Nothing roller other than just roller tip rocker arms. Right now I have a dual plane alum. intake with a 750 holley/vac sec. Also has a C6 with a 2,800 stall. Running 3:42 gears in the rear.

I hope thats enough information for someone to help me pick the correct carb for this application. What I have read is that with that big of a hyd. cam that a vac. secondary carb may not be for me. Also, not sure on the size.. Holley site says I can use a 750 but from what I have read a 650 mechanical secondary might be the best carb..

Sorry for the long first post but hopefully there is a wealth of knowledge here like there appears to be. Here is a pic to maybe help with the style of intake I have because I am not positive which one it is without digging thru receipts..

 

Try this site, best Cleveland resource ever. If you are pulling low vaccuum definitely got with mechanical secondaries.

http://www.network54.com/Forum/119419/

 

Thanks for the link.. Here is a pic of my baby..

 

Id run a mighty demon 750 annular with mech secondaries, it will give the snapiness of a smaller carb but flow around 800 cfm, I run one on my 408 stroker. Better yet call BG and let them recomend a carb for your combo, I've had great luck with them and I buy mine remanufactured off ebay.

 

A dog, a boat and a car. All you had to do was throw in a picture of your wife with the rest of your expenditures and we would know where all your money is. LOL

 

A mechanical secondary is not a bad carb for street/strip application when you have a manual tranny. Mechanical secondaries are consistant, but worse on gas mileage and harder to tune. For an auto tranny street car, vacuum secondaries are going to be the better choice.

Yes you will have low manifold vacuum with that much duration, but the secondaries use the venturi vacuum as a signal, not manifold vacuum. Venturi vacuum is generated by the amount of air that is flowing through the carb. The more air, the more vacuum. Regardless of your vacuum levels, just like a power valve, you simply adjust the secondary diaphram to open when you want it to based on your engine's vacuum levels. Any carb on a performance car is going to need modification of jets, power vacuum, accelerator pump cam, vacuum secondary diaphram, etc. IMO, low manifold vacuum is not a reason to not use a vacuum secondary.

With 351+- cubic inch displacment, the largest carb you need is a 650. I have a 750 on my 383 and it is really too big. I should have gone with a 700. Bigger is not better when it comes to carbs. A mechanical secondary car is really going to give you fits in tuning if the cfm are too big, trust me I have one and it takes time to get it right.

I would get a nice 4150 Holley vacuum secondary 650cfm carb (0-80783C). I think it will be easier for you to tune and will run nice with your auto C6.

 

750 HP holley.
I had one custom built for me at BIGS Performance, but a stock HP out of the box is a great carb.

 

If you can find one, the 69 428CJ Holley is a great match for one of these. It is a 735cfm vacuum secondary carb. Holley "sleeved down" the primaries with special boosters to drop the 780cfm down to 735. The result is a dual line style carb with Ford linkage and left side fuel inlet like the Motorcraft 4300 and very crisp throttle response. I have installed these on everything from 350 Chevies to 460 engines in Lincolns. It also did pretty good on fuel economy. With low manifold vacuum, you need power valves that won't be open at idle.

 

For 351CID at 7000rpm and 100%VE you only need 700cfm. You will never run 100%VE with that car. High 80s to low 90s is more probable. You need 639cfm to feed a 351 at 7000rpm at 90%VE. That is why I say a 650.

When you go larger than you need cfm wise in a carburator, you will have tuning and performance problems. When you over carb, the carburator cannot atomize the gas correctly for the volume and velocity of air that is flowing through the carb. A similar issue happens when you put too large of heads on an engine. When you over carb, wet gas, or unatomized gas enters the intake and heads. This gas does not burn well, so you have performance issues. Also when you go too large on the carb, you have to jet the carburator WAY down or you run rich. The problem with that is the larger carbs are not made to run small jets, so you have more perfomance issues. You may also need to change out accelerator pump squirters and venturies if you WAY over carb. These problems will be most apparent at low thottle, which is where most street cars spend their time. For all these reasons, it is bad to over carb. You may think a larger carb is going to give you a performance advantage, but you will probably be going the wrong way with it.

Personally experience: I have a 383W running about 94%VE. The calculator shows me needing a 770 carb at 100%, so I bought a 750 mechanical secondary. Out of the box, my car ran fat as ****. I fouled the plugs in about 50 miles. I had to change out the power valve so it would not open at idle and jet down about 8 times. It is still running a little rich, but if I lower it anymore I get a lean hesitation. This is probably due to less than perfect atomization. If I had put a 700 on, it would probably atomize fuel better and the tune would have been much closer. As you see, I went a little smaller than calculated and it was still too big.

Do what you want, but I bet you will have a better running and better performing engine with a 650 than a 750. If you really want to over carb, I would stay away from a mechanical secondary or you will have hell getting the tune right.

 

No kidding... I bet you can guess which expenditure costs the most out of the 4 though... Not only that but the most expensive to get rid of...