i have a 289 with an edelbrock 650 carb and an edelbrock intake,some body told me that carb is way too big..i have a 1971 mercury comet 2dr that im converting to a GT.can anybody help me
It depends on your whole combination, engine and drivetrain mods, not just displacement. 289 Shelby Mustangs came with, what, 730 cfm Holleys or something like that.
The motor from my Maverick has had one of each an Edelbrock and a Holley, both of the 650 variety. It should be fine for an application such as yours.
Oh and I should add, except for the cam and intake it was relatively stock. Not even a set of headers. The intake was a single plane (very old) and the cam real close to stock.
I will try to explain some things based on your engine combination as discribed. A 289 with stock heads and near stock cam will begin to peak in hp at somewhere near 4800 to 5200 rpm. At those peak rpm numbers the motor won't pull much more than 400 cfm considering the flow losses and poor exhaust. Add to this the old single plane intake that may help power in the higher rpm range along with the cam helping a bit as well as the big carb.. The single plane is poor for low end throttle response and along with the large carb makes it worse yet. For compairison, the larger 302 at 6000 will hardly pass 500 cfm in stock form. From this you should see that the 650 and the single plane is not good for overall throttle response in a smaller engine with no real performance build. Yes your motor will run with about anything bolted on but is not matched for the best overall performance. It's a matter of capacities and airflow. The Mustang with 750 was on a motor that was equipped with a solid lifter cam, intake and better heads and exhaust that would rev to near 7000 rpm and did use a 750 to eliminate any possible flow restriction at WOT and peak rpm for the drag wars that were encountered back then but still was 'technically' too large for general street use. So some of what is selected is purpose driven and does not fit all applications. You need a dual plane 289 for a stock type manifold and a 500 cfm vacuum secondary carb to improve throttle response and overall driving pleasure with the combination you now have. Using a carb that is larger than what the motor will pull at peak power only reduces the last small amount of flow losses at the carb and does nothing to improve airflow thru the intake or heads. This can be tested by watching a vacuum gage while running the motor at it's highest rpm. If there is no vacuum registered, then the carb is not causing any restriction. On a street motor with an optimum size carb, you would see maybe 1/2 to 1-1/2 inches of vacuum at WOT with a near correct sized carb. Trying to get a motor to the last amount of power in the only time a larger carb should be considered and to reduce the to near zero the last detectable amount of vacuum caused by the carb venturies. Using large carbs presents jetting problem, and poor throttle response. What happens with to large a carb is the airflow thru is slowed down, makes fuel atomizing worse and needs larger (not smaller jet sizes) to attain enough jet cross section to allow slower airflow to pull fuel from the bowls. This is where the poor throttle response comes from.
