What Carb are you running and are there reasons one is better than the other? I want to know the good and the bad, Tech support also? With so many different carbs to choose from it can make your head spin! Anyone have Pro Systems or Quick Fuel or Biggs? This could be used for future reference for all! Right now I'm going with a Holley 650 Double Pumper...upgrade to come so I await some of your input here! Thanks, Richard
I run a Holley 600 dp. I run it because it's what I had laying around. This carb has been good on many motors and always performs well. I'm sure I could get beter performance out of a new hp series or other performance carb but that's a good chunk of money I don't want to spend. When I dynoed my motor the air fuel ratio showed that I need to adjust the air bleeds but the carb doesn't have that ability. The HPs do. If I was to buy a new carb it would be a hp series carb.
You need to match the carb to the rest of your engine. A 347 turning 7000 rpm needs a bunch more carb than a stock 302. Cu" x RPMmax / 3456 = cfm at maximum rpm For drag racing use CFM x 1.5 as maximum size for the carb in CFM For street use CFM x .9 for size of carb in CFM For hill climb/road racing (not street racing) use CFM x 1.2 for maximum carb size in CFM. Using a smaller carb will give you better throttle response at the cost of proper mixture at the high end. Using a larger carb reduces the low end response but provides a more even air/fuel ratio in the mid to high end.
Ok Paul so how would converter diameter/stall speed come into play on this? Assume that all the other parts are in the package of course!
If you go to holley.com they have a carb calculator and one parameter you have to enter is minimum rpm at wot.
The converter acts like a slipping clutch until it reaches the stall speed. Your car still moves as you press on the gas pedal but under load it will slip a little. The less load you put on it the less slip you get. It should help the low end so you can use a slightly larger carb (see drag racing formula) depending on the other criteria. You never want to use a carb that is 1.5 times your maximum CFM unless you are drag racing and need that top end mixture. You could use an extra .1 added to your carb selection or just use the largest in your selected range. I tend to select the carb on the small range just because a lot more time is spent getting to your top speed than you spend there.
dual plane vs single plane intake manifold plays a role too. You can get away with more carb on a dual plane....Used to run an 830 on a really mild 302 in my old Maverick. Perfect for it. Excellent low end, great top end and good MPG There's a whole lot more to selecting a carb than CFM.... personally, if I had the choice I'd go EFI or MFI long before going back to a carb.
Agree with what mavman said, but just as a referral. I ran a 750 quickfuel for years on my 347 running 6.50's in the 1/8 and I loved it! I gave them all my specs, they built the carb. Was pretty much dead on right out of the box.
Pinky - what are the rest of the specs for your engine and what is the primary tune for? (racing, street?)
A lot of good information here, but the real "proof of the pudding" is to put the motor on a engine dyno, or the car on a chassis dyno and tune from there. As was said, street and drag racing tuning is like night and day and a carb I'd run on the street I'm not sure I would run at the strip.................I guess what I am saying is that trying to build a dual purpose car can be done..................but you need to make compromises and on the street I'd rather go too small than too large................conversely, for a strip only car like mine...................I would rather be too large than too small. The Maverick has a CarbShop reworked 850 that supposedly has been reworked to flow 1020cfm. If I was to do the math I should be running about 1250cfm.....................but the motor responds fairly well at lower rpms and pulls like there is no tomorrow at the top end, of course the cam has a lot to do with that, but she wants to fly on the top end. Again, like was said, your motor will dictate what size carb you will use, and then the fun part begins with the fine tuning............IMHO
Look into a ProSystems carb or a Demon from BG. Just from the ET and engine size shown under your avatar I'd venture to guess (and it is a guess) that you're pretty close in the CFM range at 650. Any extra power or elapsed time you might gain will be from better metering or better response. Either of those manufacturers can help in those areas.
