Originally Posted by DavO_LTD
I'm currently rebuilding a 351 from a FC LTD wanna know what mainfold u guys would recondmend and what size holly.
it is 2V open heads, and we are mildly porting them, kb hyperutectic pistons, crow cam IN .530" EX .548" valve lift , twin 2.5" exhaust, pacemaker extractors prolly tri-y but unsure.
so yeah, what size holly and what mainfold would you guys reconmend? would a 750 DP be over kill?
Its my first car and will be a 95% street use car
If I can offer a bit of advice...you may want to consider your extractors and carburetion based on demand/needs. What duration (at .050") is your camshaft? What is the LSA? Is this a hydraulic (or other) camshaft? Are your valve diameters stock? Are you running screw-in studs and guideplates? Roller rocker arms? Is the engine bore +.030" or more/less?
Are you running an automatic or manual transmission? What differential gears are you using? What tire diameter? Are the pistons flat tops/dished/domed? (I'm guessing flat tops with valve reliefs) How much does your car weigh?
A 95% street car using an auto trans needs power between 1500-5500 RPM. You gear/tire the car to match the output of your engine. Given an existing gear/tire combination, you match your engine to the chassis.
I would strongly consider a dual plane intake manifold and a vacuum secondary carburetor. They are very, very simple to tune, though very few people seem to know how to do it. A VS also tends to be a lot less expensive and you may find someone here with a bit more racing blood to part with theirs for a good value opportunity for both of you.
You'll probably want to use the smallest diameter extractors available for your engine. Something around 1-5/8ths primaries in a Tri-Y design will be great for your low-end performance.
If you are running a manual transmission, you can choose to go with the same combination as an automatic, or a bit more wild: A single plane with a small 4-bbl, like a 650DP.
I would NOT recommend a 750 CFM carburetor for this application, at least based on what I know about it and what I suspect is true about it. I definitely feel that a 750 DP is overkill and an inappropriate choice, though I do not have complete details on your combination.
Check out: Carb CFM Calculator
...for your engine. If you use: 357 as your CID and 5500 as your maximum RPM, you'll come up with 483 CFM for a street engine. If you push it up to 6500 RPM, you're at 571 CFM for a street engine. Both of these are significantly below a 600 carburetor, which is usually available very inexpensively as people often upgrade from these when building "stronger" engines.
If this car is your primary means of transportation, you'll want to be more conservative in your carburetor sizing and probably use a vacuum secondary carb for better fuel economy. A lot of guys say that they don't care about fuel economy when building a "performance" engine, but the truth is that for any carbureted engine, the best economy for it means that it is burning just the right amount of fuel and not wasting it out of the exhaust, which also hurts the environment and makes passing emissions testing more difficult, where applicable.
I would seek out a good dual plane, a 600-650 Vacuum Secondary carb and spend some quality time tuning it. I would likely convert the VS carb to a "dual metering block" using the add-on kit, which gives you fine control over the secondary metering. Also, I would probably switch to a quick-change vacuum secondary spring kit so that it is easier to swap out your secondary spring--though this is only useful while setting it up and tuning, if you're not hitting the track from time to time. If you do visit the track, you can quickly swap springs for (possibly) more performance/better ETs.
A double pumper will waste a lot more fuel when driving around town. You can baby it and get it to be fairly economical, but few drivers actually do it. A vacuum secondary will be "nice" at all reasonable demands placed on it during operation.
I built a 383" stroker with a dual plane and a 750 VS carb. It made 460 HP and 430 TQ...before changing it over to dual metering blocks...and using the carb exactly as it came out of the box. The engine ran 9.5:1 compression ratio and had a solid roller camshaft of .610" lift and 254* duration with a 3.89:1 rear end and automatic transmission. It idled very nicely and ran like mad through the mufflers. I mention this because it is easy to make good performance with a VS carburetor and a dual plane if you understand the RPM band where they're going to be doing their work for you. It would have made more total power with a single plane intake, but as it was, it made over 400 foot pounds of torque from 2000-5000 RPM peaking at something like 436 TQ at about 4500 RPM. That is just where you want a street engine to run. Make the best possible torque that you can as early on in the RPM band as possible. If you really think that you're going to be making 6000+ RPM blasts up through the gears in your car on the street, then consider a single plenum and a double pumper.
Here is a good test. Take out any other car you can use, the more factory stock the better. Drive it "hard" accelerating at every light as quickly as possible up to the maximum speed limit keeping an eye out for the peak RPM. Note how many times it exceeds even 5000 RPM and be sure to note when it (if ever) exceeds 5500 RPM. If you can't count the number of times, then that's how often it is likely to happen in your car when it is properly tuned/matched for best street performance. When you build a hot street car, you want to keep the same RPM range and just reduce the time (by making more power) that it takes to get up to those RPMs. You'll likely find that as you make more power that you'll overpower the chassis/tires on the street very easily. At that point, no amount of more power is going to be helpful. On the street, you're likely to find it happens long before 4500 RPM so that there is no need to "build" your RPM into the 6000+ range. No sense in wasting the fuel down low trying to get up there, either.
Some guys will build an "RPM" engine and then stick a big gear into it to make it work better down low. This is a lot of fun and makes for a very "nasty" car that has a radical idle and drinks (fuel) like all your mates at an open bar.
There are any number of ways to go with any combination. By stating your intention of 95% street, I'd keep that in mind when putting the rest of it together. Also, before you don't consider it, use a closed element air filter and duct cold air to it. Don't run an open element filter that will just suck up a lot of hot underhood air blowing straight at it from the radiator/fan.