Looking at going back to a carb!!

[*13*]

600 is not too small, if you don't plan spinning higher RPMs. In fact, a 600 cfm carb will have a snappier response.
RPM range is really the key. If you are trying to pull large amounts of air through the carb(higher rpm) you will need larger cfm. If you have a larger cfm, lower volumes of air have a more difficult time pulling fuel out of the Venturis(poor response).
So, the carburetor needs to be selected for its use, not max RPM

[AL427SBF]

Originally Posted by *13*

600 is not too small, if you don't plan spinning higher RPMs. In fact, a 600 cfm carb will have a snappier response.
RPM range is really the key. If you are trying to pull large amounts of air through the carb(higher rpm) you will need larger cfm. If you have a larger cfm, lower volumes of air have a more difficult time pulling fuel out of the Venturis(poor response).
So, the carburetor needs to be selected for its use, not max RPM

Good advice, it's the throttle response vs. peak HP trade and what RPM band you will do most of your driving. Seems the cam also plays a role, I know my builder got with comp cams to discuss and agreed that a 650 willy's was the ticket for my 427w.

[Frank Messina]

Whether you decide to stay with the EFI or go carb it sounds like you'll need some help tuning. A 750 on a 427 is definitely not too big but a 600 0r 650 is for sure going to be smaller than what the engine wants. Don't think you're going to take it out of the box, plop it on there and live happily ever after. People like to say that but reality is they all need some tuning to get them exactly right. I have a Holley HP 750 on my 351 and give up nothing anywhere. No stink, idles well, pulls strong, no stumbles or hesitation and gets decent gas mileage. The point is you have to tune for what the engine wants. If you've been tuning your EFI yourself you can most certainly learn to tune a carb. It is not hard.

Frank

[Cobraqcca]

Originally Posted by AL427SBF

Good advice, it's the throttle response vs. peak HP trade and what RPM band you will do most of your driving. Seems the cam also plays a role, I know my builder got with comp cams to discuss and agreed that a 650 willy's was the ticket for my 427w.

Excuse my ignorance but what is a "650 willy's" Sorry I Googled Willy's and now know!!
I wonder why they recommended a 650cfm when most people are saying a 750cfm here?!
Hmmmmm!!!!

[AL427SBF]

Originally Posted by Cobraqcca

Excuse my ignorance but what is a "650 willy's" Sorry I Googled Willy's and now know!!
I wonder why they recommended a 650cfm when most people are saying a 750cfm here?!
Hmmmmm!!!!

Because a Willys 650 flows more than 650 cfm but still meets IMCA 427 spec motor criteria.

The most potent carburetor currently available for the IMCA spec motor, carbs deliver instant throttle response and more torque. Available in gasoline or alcohol versions, carburetors use gloss black powder coated Holley HP 650 main bodies with 1. 250" venturis, 750 base plate assemblies with stainless steel butterflies, Teflon® bushed throttle shafts, and Willy's exclusive, externally adjustable metering blocks.

Give 'em a call and talk to them about a couple of their 427 race motors using the 650, that should answer any questions you have about 750 vs. 650. Not cheap either, start at $1200 and then if you add some trick options more $$.

[AL427SBF]

Originally Posted by Cobraqcca

Excuse my ignorance but what is a "650 willy's" Sorry I Googled Willy's and now know!!
I wonder why they recommended a 650cfm when most people are saying a 750cfm here?!
Hmmmmm!!!!

... short answer, most people here are over-carbed, and in some case ridiculously over carbed