Re: Carb suggestions for FE 390
That 670 will likely be a significant improvement, but be very sure you are getting full throttle.
I'd be interested in what you are running for heads and headers though. If stock C8AE-H, C6AE-R or C7AE-A heads, that combo won't breathe too deeply and the 670 will be plenty, as it is likely a 5000 rpm peak and somewhere in the 360-375 hp range. However, if you have a good set of worked low risers, CJs, or Edels, that's easily a 400+ hp motor and it'll want to breathe deeper and longer, the 670 will still do fine, just at the bottom end of good, I'd rather see a 735 CJ carb or some flavor of 750 on it
I know this wasn't the question, but if you are running bone stock, or nearly stock heads, that engine will be a completely different animal if you get the intake ports in the 280 cfm range, either by better heads or by working yours. A pretty achievable number at .550 lift on any of the castings listed above if willing to spend the cash and the cam you have will take advantage of it. FYI as cast, those listed above are in the 220 cfm range and very turbulent, hard to break 400 hp, even if pulling on them hard with a big inch engine
FWIW, the good thing is that average port flow in an FE RPM intake is about 300 cfm, so anything you do on the head side adds power and RPM without hurting low end due to the small change in overall runner volume on a short FE head, port matching that intake to a med riser head is even better and will let that intake flow in the 325-340 range, it's a good piece.
Regardless, where you are now, you have a very nice combo, strong but smart street cam for a 390, tight quench, the right intake, carb's in the ball park now, as long as the timing curve is there and you have a decent exhaust system, it should run very well
EDIT: Just want to make sure I don't look like I am stepping on any toes. David's math is spot-on and matches every recommendation you will get out there from the carb shops, and in terms of cubic inches and RPM, those numbers are good. However, whether it's a harmonic issue of the long intake ports on an FE dual plane or some other factor, in my experience, FEs like it when you err on the big side.
Even the 428 CJ, which was undercammed, minimal overlap, low lift, improved but still poor exhaust manifolds, and a crappy stock exhaust was real happy with a 735. Granted, it had approx 32 more cubes than you, but sub .500 lift, no significant 5th cycle (overlap) effect, a poor timing curve, and in most cases, had to be able to run on any swill nationwide, so it was far less of a deep breather than you built
Remember, a carb is not an intake port. The intake port is fed out of the plenum which acts as a big reserve to dampen what happens in each port and in the case of an intake port, smaller (but adequate) helps. A carb only becomes too big when you do not have enough airflow across the booster to tune the main circuit properly at the RPM you are asking it to perform.
It is not an extension of the intake port, it is essentially an extension of the plenum. I don't want to get too crazy about it, because you are absolutely on the right track with your build, but among others, if interested, David Vizard wrote about "understanding boosters" in (I think), Understanding Holley Carbs, and explains it pretty well.
70 Sportsroof, 427 FE/489 cid, TKO-600, 31 spline 4.10, A/C. modified Mass-flo EFI/reprogrammed A9L/CnC ported Victor.
Last edited by My427stang; 08-14-2016 at 05:23 PM.
Reason: Added info