Picking a carb for my 393.

I've gotta ask...why does your uncle think his 750 is too small? Has he ever had the car dynoed and got an air/fuel mixture reading? I can't stress how bad the "i NEED a huge carb" mentality is. It's been that way since the musclecar days and people never seem to get it.

I'm not saying you DON'T need an 850....i AM saying talk to the PROS (because hey, i'm no pro, i've got too much carb myself) and find out what you really need before you buy something based on what anyone else says.

I would've bet that a 750 would do you just about right on an all motor 393 pushing 6500rpm...but again i'm no expert.

here's my 2 cents:

393 c.i. is .2274 c.f. and as a 4 stroke you are only intaking half that volume per revolution, thus .1137 c.f. and at 6500 rpm you are looking at 739 CFM, so i would check into a 750 CFM.

Good Luck,

Walz

EDIT:
Just remember that carbs are depend on velocity to work, this velocity is defined by the CFM rating of the carb.

Wallzy said:

here's my 2 cents:

393 c.i. is .2274 c.f. and as a 4 stroke you are only intaking half that volume per revolution, thus .1137 c.f. and at 6500 rpm you are looking at 739 CFM, so i would check into a 750 CFM.

Good Luck,

Walz

EDIT:
Just remember that carbs are depend on velocity to work, this velocity is defined by the CFM rating of the carb.

Click to expand...

Exactly. That's why too much carb is a bad thing. I'll try not to get too in depth on this one, but here's the "staw concept" on why too much carb is a bad thing:


Think of your typical McDonalds straw. You know that you can get a nice average size gulp of your favorite beverage from it with little effort. It's an average size, or at least it's a good size for getting your drink.

Now think about one a straw that's 1/4 the size (in diameter) of your McDonalds straw. You can still get your drink, but you're gonna have to really suck the hell out of it to get any liquid. This is the equivalent of having a carb that is too small. Your engine has plenty of power to suck air and fuel but it isn't getting much drink for it's work.

Now imagine a McDonalds straw that's 1 inch in diameter. You can still get your drink, but your cheeks are going to be numb after a couple sucks because of all the effort it takes to get the drink up the straw. You might get a big drink for your efforts, but it takes forever to get it and you're tired by the time you do. This is like having too much carb.

When air moves through a passage, the size of that passage plays a huge part in how fast the air moves, because air has to speed up to move the same amount through a small passage, and it slows down to move the same amount through a large passage. If you have a very small passage, air has to travel really fast to get to its destination, and it'll take a lot longer to get X amount of air from A to B. When air has a large passage, it slows down to move the same amount of air from A to B.


Too much carb means your engine is going to starve for air because air is moving really slowly into the engine (until it's running at such a high RPM that the demand is high enough). As a result your engine will be soft and unresponsive. Too little means it'll get plenty of air until the RPMs and demand get high enough that it can't move enough air through the smaller passage. The throttle will crack open super fast but it'll fall on its face in the upper RPMs.

You wanna be right in the sweet spot between these two. I could go on for days about this but i don't think anyone would read it

Those Holley HP's are nasty. I had a buddy who owned a speed shop and they basically built cars for fun since they got everything so cheap. He built a 347 and put it in this crazy all out coupe rolling chassis and put a box-stock 650HP on it and it ran 10s without even jetting it.

Have a 393 with afr 205 heads, victor jr intake. run a proform 750. 1.42 60 foot, 6.34 in 1/8 on 150 NOS. 750 is just right.

GSP393 said:

I found these two:
http://store.summitracing.com/partd...506943+4294902670+4294902704+115&autoview=sku
http://store.summitracing.com/partd...506943+4294902670+4294902704+115&autoview=sku

The differance is the booster type, I don't know which one I need or if these are out of the box carbs or if they will need tuning.

Click to expand...

I prefer annular because they're more efficient but also better for racing. You can typically get better mpg out of them and they're still badass for racing. That's what the HP carbs have.


Also i'll second the Proform parts....for being inexpensive Chinese parts, they make really nice main bodies.

Yeah. I am very happy with the proform. It wasnt but a little over $500 and it has billet metering blocks, main body, and throttle plate. Looks just like an HP and in my onpinion runs better. Not cold natured at all.