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Don't see a large issue

November 26 2011 at 9:18 AM
Ken  (Login 68Lancer)


Response to Tough decision...

Paul,
Good morning. Didn't mean to imply that the 750 CFM carb would create either a rich or lean condition at any given RPM. As you know, you can change the A/F mix to your liking.
The engine will, in fact, get only what it needs based on air flow through the venturi system... but, that only holds at a given RPM and loading.
When you move the throttle, everything changes.
If you can imagine the carb body as a restriction (which it is) in the air flow, and the engine vacuum as a big function of carb bore area, you can also imagine a flow curve for a given carb. That curve varies with RPM and load.
Flow change is "sensed" by the carb and fuel is added or reduced based on carb design.
Now imagine two identical engines running under the exact same conditions... but one has a 600 CFM carb and the other has a 750 CFM carb.
When you move the throttle to increase power, the vacuum in the intake drops, and mother nature tries to balance the difference with increased air flow... which results in more fuel being atomized. Any given flow change is a greater precentage of the powerband in a smaller carb and that carb will respond quicker to the flow change.
What can happen? Oversize carbs can result in "flat spots" in the powerband as well as surging, etc. I've seen over-carbed engines simply stall from a standing start when the throttle is suddenly thrown wide open... the fuel delivery is too far behind the flow change. Guys with this issue spend their lives fiddling with the accelerator pump and never figure out what's going on.
Will any of those things happen if you use the 750 CFM carb? Hard to say. But, why use it if you don't need it?
The Edelbrock carb is a knock off of the old Carter AFB used on all the high performance Chrysler products in the 60s/70s and I'm far from knowledgeable on them. They've been around for 50 years. Some guys swear by them. Just as some guys swear by Holleys or Rochesters. I'm going to guess you won't find many FE guys who know Carter carbs very well... but maybe.
As you've noted, opinions are often worth what they cost. happy.gif
All I can say is that I'd use the smaller carb and minimize possible issues.
I wouldn't lose any sleep over it... if it doesn't work out to your satisfaction, carbs are easy to change. Just keep telling yourself that's half the fun.
Good luck with your decision.

Regards, Ken

 
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