Re: 305 Block
Ok... looks like I learned something today but I have a question...
If a 450 cfm carb and a 650 provide the same amount of fuel then why isn't the 650 running extremely lean when it's using ALL of it's flow capabilities?
That just doesn't make sense to me. Is it all done by vacuum?
Also I couldn't tell you how many times I've heard of people, statistics, ratings, etc claiming less mpg with a 4bb over a 2bbl (in cars).
I'm not even old enough to know anything about carbs. I'm a efi guy so bare with me.
1st of all why do you think the 650cfm would be lean when it's using its flow capabilities...I guess you need to explain that more...
like I said all a carb is...is a "controlled fuel leak" you are moving air thru the carb created by vacuum from the pistons pulling air into teh chambers...
Now we know that gas should be mixed with air at around 14:1 for normal driving...notice I said normal we aint going into power mode or anything like that but eh same applies...
so the carbs job is to put enough fuel into the stream of air to create this 14:1 value...no more no less... now engine efficiency, venturi size, booster shape and size all play factors too...as well as the intake and head port velocities...
now just givin the carb it doesnt know,nor does it care what engine it is on to make the given AFR (Air Fuel Ratio) alot of carbs are set of with the correct AFR on a flow bench (Edelbrock anyone?) and they will run pretty much on any engine....
the reason it wont run lean after the engine exceeds the cfm rating is that is still produces the same AFR..that is a "Constant" the carb will then act as a Restricter and the engine wont be able to accelerate much beyond...I shouldn't say that because it will but a smaller carb will limit Maximum RPM...
Now when you go from a 2 barrel carb such as the 2GC Rochester carb to a 4 barrel carb such as the Weber 650 cfm AFB style carb there is NO way you can get worse mileage if you run both carbs just on 2 barrels...in fact the 4 barrel should see a little gain in both performance and efficiency due in fact that the Primaries on the 4barrel are smaller than the 2 barrel and thus the signal will be stronger at the lower RPM range and the carb will meter fuel more efficiently...
now if you tip into the 4 barrel you will only use more fuel once you exceed the capacity of the 2barrel carbs rating...basically because the 2 barrel was the restriction and the 4barrel will run right past that restriction and allow for more power...now in terms of fuel consumption you will only see more fuel consumption if the jetting is wrong on the carb Primary and secondary side combined OR the RPM is higher than the 2 barrel was/is at WOT....
Carbs are ONLY designed to be efficient in the middle range of use on either end of the RPM scale they are very different...mainly due to booster design...one that has great idle quality will often lack high RPM potential and efficiency....and vice versus
this is where fuel injection rules but only in a Dry system with Sequential fire systems...
Fuel injection like TBI and MPI and Tuned Port (MPI and Tuned Port are batch fire injection) are very easy and simple once you understand them...only thing most lack is the tools to set them to be variable...there are no mixture screws like carbs...the O2 sensor and MAP/MAF are the mixture screws and the cold start sensor is the choke....
