Have a built 289 with 10.5 to 1 cr, fairly wild cam, roller rockers, headers, electronic ignition, ect. First went with a Holley 650 and had horrible idle quality. Bad lope at idle, piss poor mileage, and transition hesitation. Got it dialed in, but stilll not what it should be. Stepped down to a 625 Demon, and the drivability was better, but the mileage still sucked, still needed to keep a fairly fast idle, and it still had a pronounced lope (more than it should), but it was a 100% improvement over the Holley. Finally picked-up a Holley (standard, not double pumper) 390 cfm and bolted it on. Transformed the whole car! Got the mileage up around 16 mpg, idles slow, and just a hint of cam lope. Acceleration off the line is crisp, and without a trace of hesitation. I know we're always thinking bigger is better, but in my case the magic came in a smaller package.
You're right, bigger isn't always better. Everyone told me my GTO needed an 850 to run right. I had a 600 Holley on it and it would run mid 12's with a mild 400. Bolted on a loaner 750 and it went slower!
Theres alot to be said for air velocity and charge distribution,as well as atomized fuel remaining in suspension. Small carbs...Small wonders. I have to ask though...How rich/lean does it run with the small carb and a fat cam???
Exhaust gas is showing 14.3 to 1 which is just a hair off and could just be a calibration error. Plugs are showing proper combusion at extended idle and wide open throttle @ shutdown. With the 390 I lose a little top end and you tell can tell its running out of breath, but overall, especially for the street its worked out to be the best all around choice. If low rpm drivability and mpg wasen't important and I wanted to maximize 1/4 mile time, I'd stick with a 650 because it really comes into its own from midrange to top-end.
I had a 500cfm Holley on my 289. The engine had a mild,said for extra torque for towing,cam and tube headers. These were the only mods,except the carb,and it ran GREAT. I never had a sniffer check,but the plugs ran clean and no pinging. Gas mileage was 18 in a 68 Mustang with 3spd on the floor.
Does your AFR stay good all the way through the run? You mentioned that it is running out of breath, but some times, it can be the fuel bowls running out of fuel at the top end. Just a thought.
AFR stays constant. It's just a factor of cfm limitation at the end of the rpm range. Someone new to the car would most likely not even pick-up on it, but I can tell the difference just becuase I've driven it with the larger carbs.
I'm not sure how you expected a decent idle with a "fairly wild cam" And double pumper Holley's were never intended to deliver decent gas mileage. :16suspect But instead of the 390, you should have tried a 570 or a 450. But it's also no surprise the 390 works, afterall, that's what many Nascar guys are limited to.
Normal cam lope is acceptable, what is not is the increased idle speed required to offset the over rich mixture from the larger carbs. Where did you read anything about a double-pumper? The only similarity between the vac. scondary square bore 390 and what the NASCAR guys are running are the throttle plates. MPG is a thermodynamics issue, and the cam is only one part of the puzzle, and in this case (because of the altered valve timing) it will actually improve volumetric efficiency, so I'm not sure I understand your point.
Its all in how the big carb is tuned, B.G. sent a wet flow sheet with mine and it was over 1100 cfm, I can get 17+ mpg if i drive it rite. A bigger carb to me is easier to tune due to the slower air speed thru the carb. You cam is likely your gas mileage culprit. A lot of overlap (narrow lobe separation) Wastes fuel. But yes if you don't like to tune, And just want the bolt on and go a smaller carb usually works better.
So what is "normal cam lope" I'm not sure I understand what you mean here either. Most guys who choose a "fairly wild" lopey cam, expect just that, that's why I was questioning your statement. So the 650 you had wasn't a double pumper ? Sorry for the misunderstanding, Most guys running a 650 Holley are using the double pumper variety. Very few are aware that they make a 650 VS carb (the 80783) The Nascar 390 is an HP series carb, nothing like the std 390 single feed, off the shelf carb. The only similarity is the cfm rating and the brand name stamped on the carb. I've also come to the same conclusion as you have, in running my 331 with the 3x2 setup. Even though the advertising for the Z303 cam is a bit optimistic in it's description, the street manners with it, combined with the dual plane intake and the 250 cfm carbs is excellent. Idles at 500 rpms in gear with a healthy lope. Some of it I also attribute to the wide LSA (112*)
Your are right, let me try and be more specific. With the bigger cfms, the idle speed had to be set higher to compensate for the a/f mixture being delivered. With the cam's profile, it hit anything but a sweet spot that went from a nice lope like you would expect, to a violent lope. As the rpms came up it would go away, but you could never get the engine to idle in the 500-600 rpm range. Going to the Demon made a huge difference from the 650, but it was still too much carb for daily street use. The 390 eliminated three problems. The first was the ability to set the idle down to an acceptable range. The second was that the cam lope was what you would expect - a healthy, but smooth lope at idle. In other words, you hear the lope more than you feel it. The third was the increase in mpg. The original 650 was getting about 11, the Demon was better at 13, but the 390 is clocking in at 16 which is very acceptable for what the car is. Is it maximized for performance, no. It is what it is, a nice street driver that will still throw you back in the seat, but a little easier on the wallet. If the budget was unlimited, I would opt for your set-up. I think its the best of both worlds with 500 cfms in reserve when you need it.
Keep your eyes peeled and pickup a used Ford 2x4 intake and another 390 Holley and try that. That could be the ticket to a better topend, while keeping the bottom end too. All without busting the wallet. Use it with a progressive linkage so it'll run on a single carb most of the time.
Your missing the association between cam duration and carb throat area! A long duration cam means the intake valve is open some number of crank degrees as the piston comes up on compression. This "pumps" back pressure into the intake making the vacuum level low, unstable and causes a loss of cylinder pressure when the intake finally closes. Low vacuum level and low airflow through a large ventui will not draw all the needed fuel out of the bowl and the RPM wanders all over the range. Therefore you have an idle speed/fueling problem at idle, hesitation etc apart from just the long cam duration or 2+ problems at the same time, contributing to the total issue. As far as carb size vs performance, the engine displacment and RPM determine the correct size the carb needs to be on the top end of the RPM range you run. Taken a step further, at wide open throttle and max RPM the intake vacuum should be just above or at 0 to get max airflow and operate the power valve for additional fuel richness. The near zero vacuum at WOT means the motor is taking all the air it can at that point unless the RPM goes up, then vacuum shows as a restriction to airflow and begins to limit power. It sound like you want the sound of a hot motor with the long cam, want fuel mileage and good idle. You can't have all these at the same time but have to sacrifice something to make the motor idle first then you lose top end airflow for power. The carb type to consider to meet all these conditions would be a unit with small primaries and larger secondaries (spred bore) or a very well matched and tuned vacuum secondary carb of suitable sizing for smooth operation over the whole range. The usual 289 HP motor from Ford back in the 60's used a 750 but with gears and a high winding motor where low rpm and fuel mileage was not the intent and the owner put up with it for the high end performance. There is a lot to understand about what to expect from a setup and many don't get it right and end up with the same sort of issues you are concerned with. Btw, the larger the carb, the larger the jets tend to need be because the airflow is lower and needs larger jet 'cross section area' for the low airflow to act accross. It dosn't sound logical but it's the physics of it. Using a 390 cfm carb greatly raises the air speed making idle and atomization satisfactory, intake vacum a bit higher and tames the long cam a bit but you pay for it with loss of top end where the motor can't get enough air to make the power it is capable of. Hope I have given some understanding of your issues and why. Good luck.
