My system, 1200 CFM on my 306 Ford is a factory setup. 2 600 cfm carbs with vacuume secondarys. Designed by Ford in the early 60s. As it is in a light car the bottom end suffers a little but the top end is GREAT!!!!
I'd like to put an Oxygen sensor in that, and data-log the mixture results at idle and part-throttle cruise.
I get a kick out of those types of comments.......no further explanation, no correction, no references to charts or tables....nothin' What IS the correct figure/method to which you allude ?
There is no strict answer to this question. There are far too many variables. They way I select them will likely not be relevant to others. I choose, and subsequently tune for maximum efficiency, at the RPM operated at, in top gear, at average freeway cruising speed. I use electronics to tune the living hell out of them. That's just me, though. For me it is not an art. It's a science.
Peak RPM as well as cubic inch is a major factor in determining carb. CFM. For example, a stock 302 C.I. Ford with a 2 barrel carb that only revs out to around 4,500 RPMS only requires a 350 CFM carb... Now take another engine, for example a 302 Chevy that came in Z-28 Camaros same C.I. but revs out to around 7,500 RPMS requires a 600 CFM carb.... Fact is that most carbureted engines were slightly overcarbureted to begin with. Here's a online calculator from Holley for guys like me that hate Math... It seems pretty accurate. http://www.holleycarbs.com.au/chooseCarb.htm
That sum's up the other half of the OP's thread. The true intent of the operation of motor ='s the cfm.
It is pretty rare these days that my tuning of a carburetor equipped vehicle, built by someone else, that does NOT begin with putting a smaller carburetor on it. My best record improvement 7 MPG, increased to 21 MPG. Thermostat temperature, initial timing, and plug heat range were the only other changes. Bigger? Almost never better, on the street.
Yo' gimpy. I just put a nice 600 cfm Edlebrock on a haunted OT Jimmy 4X4 (it snowed, I drive it now) it will not start without gas poured in. It would hard start w/ the old worn out Q-jet. Is the pump fading away? This 4x4 is fighting every improvement.
Check the float level. Edelbrock's are almost never correct out of the box. If Fex Ex shipped it, it certainly won't be. If it is used, assume it is not. Pull the top and set it, either way, and see if that does the trick. Clean out the bowl of any junk found in there, too.
Gimpyshot rods said it best. Way to many variables. How many carbs, intake style, intended use for the mill, internal spece etc etc etc etc. i gave a simple answer to a simple statement. Theres tons of threads on here about cfm for a single 4 barrell all the way up to 8 twos. Why should i re write already presented info ? Sent from my DROID device using the TJJ mobile app
How do you figure for elevation? I live at 6000 feet. Here is wyoming and in the rocky mountains I always see some mild 350 sbc being drowned by everyones favorite go fast Holley 750 double pumper or they dont understand why the tunnel ram with 2 four barrels on it doesnt work. I always pick the smaller carb with proper jetting.
There is an old story here in Missouri about a young hillbilly who had never owned a pair of shoes. On the young man's 17th birthday, his uncle offered to buy him a new set of boots as a birthday present. So the young man walks to town, and later walks home wearing a shiny new pair of boots. The uncle asks him what size they were. The young man replies "Well, the salesman told me my correct size was 9, but 10's felt SO GOOD, I bought 11's"! More often than not, carburetors are bought using the exact same "logic" Even Santa got in the act! Eight reindeer were insufficient; he had to have 9! Merry Christmas. And if you celebrate away from home, be safe on the way home! Jon.
Jon, reminds me of a old saying, " never judge a man until you have walked a mile in his shoes.... this way you will be a mile away and you will have his shoes! " MERRY CHRISTMAS from Traditions Racing!!
If you want to confuse things even further a 350 cfm 2 bbl. carb doesn't flow the same as a 350 cfm 4 bbl. (measured at different vacuum) And do 2 600 cfm carbs actually flow 1200 cfm? Anyone seen a formula specifically for multiple carb set ups?
"Read" your tailpipes....That's what we used to do back when we were young kids learning about performance. A dry grayish brown buildup was good/a black or sooty wet film was too much.....dripping was ,well bad.
Yup that's mixture. That and this is another "tuning method" that has WAY too many variables to be used accurately too. Tubing diameter vs. engine displacement, number of exhaust tubes, crossover, length of entire system, overall volumetric effeciency (scavenging effect), cam timing, cam profile, overlap, ignition timing, etc. It is close enough to call it guessing instead of tuning.
correct=It could be either......too much gas from too much carb is still the same as a flooding carb.....
All of the engines in the quiz used either a single one-barrel carburetor or two one-barrel carburetors. I figured many here would not have thought of engines of this size; and might be interested in the fact these were (according to many) WAY "undercarbed". As several have alluded to in this thread, MANY street builds have too much carburetion. And the same is often true (although to a lessor extent) for race engines. I wish I had a nickel for everytime some true-blue brand enthusiast told me "Yup, "xxxx" (you can fill in the x's) engines REALLY like CFM. We have often suggested a SMALLER carb for their race car, and they have gone quicker (although not always faster). Sometimes a larger carb will equate into a higher trap speed, but also slightly higher E.T. Maybe in this situation, a better chassis setup would have run quicker as well as faster, I don't know. Jon.
It that were true, there would be a lot of pissed off people. Edelbrocks have a proven reputation for being one of the best "out of the box" carbs made. Maybe YOUR FedEx guy shook yours, but he isn't shaking all of them.
Maybe, but I have installed over 100. Most might be considered "close enough", which might be fine for you, and others, but not for me, my customers, or my reputation. I make my money being correct, not close enough.
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