slumlord444 (12/13/2011)
As I recall I saw flow figures on the dual quad carbs a long time ago and they were 350 CFM. That sounds realistic considering they made a 500 cfm 2 barrel. That comes to 700 cfm. I am thinking even if they were only 300 cfm that adds up to 600 cfm. I can see where the better 4 barrel manifold would be better but I doubt the numbers. I am wondering what the jetting was in the dual quads? Both sets of original dual quad carbs that I have had '56 Ford single quad jets in them when I got them. Way too rich.
The comparision chart only shows the dual quad setup going to slighly under 6000 rpm. Mine goes to 6500 with no problem and has gone to 7000.
slumlord444 (12/15/2011)
I don't see where engine size has anything to do with how many rpms the engine will turn. That would be determined by the camshaft and valve springs. Still wonder what the jetting was on the dyno test dual quads. My manifold has had the ports matched to the heads using the intake gasket as a guide. I am assuming this helps flow somewhat. Still can't see the 45 horse loss with the dual quads. I know when I canged out my original factory single 4 barrel to the dual quad setup the improvement was not near 45 horses.
When dyno testing, it’s typically only performed to the point just past peak power being made. Any more than this simply stresses the engine although it’s capable of considerably more rpms.Regarding the richness of the single 4V Teapot carbs being used on a 2X4 intake, the power valve actuation springs must be modified. With dual quads, the vacuum signal to each carb is essentially cut in half and this is enough to keep the single quad power valves from closing at idle with any kind of performance camshaft being used. The power valve springs and diaphragm that come with the new carb kits are designed for single quad use and not dual quad use. Hence the need to modify the tiny ‘ball point pen’ springs on the new units before they are installed in the carbs. And then if you want the secondaries to open at the factory specified rpms, the spring tension in the secondary diaphragms must also be reduced. That same ‘half vacuum signal’ scenario plays in with this also. The long yellow springs supplied in the Holley spring kits is a good starting point for this.
While camshaft and valve springs will ultimately dictate what the valve train limitations will be, where the peak horsepower and torque numbers actually come into play can be determined by a variety of other factors. It’s one thing for an engine to be able to twist past 7000 rpms but entirely another for it to make good power numbers at that same rpm. The 2009 EMC Y engine was twisted repeatedly at the Engine Masters Challenge to 7400-7500 rpms but the power numbers fell like a rock after 6500 rpms. This was an engine that still required significantly more air flow than was being supplied by the fully ported 113 heads that were on it. The aluminum heads cured that issue in the 2010 EMC competition.
As the cubic inches goes up, so does the air flow requirement to maintain the power increases in the upper rpm band. If the overall air flow is not correspondingly upsized along with the cubic inches, then the flow becomes more restricted as the rpms do go up. In these instances, the torque numbers increase at a lower rpm but the horsepower drops off earlier in the rpm band. If increasing the head flow, then the intake flow must at least match the head flow but this is typically at the cost of some of the lower rpm band torque numbers. In those instances where the intake manifold and/or carburetor are matched closely to the stock head flow, then porting the heads where they flow considerably more will not see the same improvements as using intakes that also flow appropriately more. There will be an improvement but not near as much as if the whole system is ‘balanced’ or matched throughout.
But back to the main question. The aluminum heads simply require an intake and carburetor combination that matches the air flow requirements in the upper rpm’s. For this I’ve reposted the HP graph below. Up to 4300 rpms, the 2X4 intake and the Mummert 4V intake are similar in HP values. It’s after 4300 rpms that the Mummert intake on the aluminum heads starts to run away from the 2X4 intake. It’s not that the 2X4 intake is all that bad, it just that the Mummert intake and 750 carb that were being used are just that good. Had a pair of carbs with more cfm capability been used on the 2X4 intake with the aluminum heads, I suspect the HP values would have been closer. Keep in mind that the original 2X4 carbs that were used in this test are smaller in cfm than the standard run of the mill Teapot carbs that are used on the single four barrel applications. Those original carbs were suitably matched to the factory heads and camshaft as well as ‘just’ 312 cubic inches.
Lorena, Texas (South of Waco)