| ole_Bill | -- 10-02-2010 @ 7:07 AM |
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Trying to figure out the CFM requirements for my engine. It is 239 bored .080 using a Mercury 4" crank = 266ci. The formula to determine CFM for a particular engine requires an estimate of max RPM. I guessed 5000RPM - that too high? I'm planning on running 2 Holley EAB 94's on a Edelbrock Super manifold but worry I might be over-carbed. Can someone tell me what the CFM of a 94 is? ....and if the 2 94's are too big, is there a way of reducing the CFM of these carbs to better fit my requirement? I'm thinking NASCAR restricter plate here. As always, thanks for the great technical assistance I get here on the forum.
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| supereal | -- 10-02-2010 @ 8:52 AM |
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Bill: The formula for determining carb size for maximum volumetric efficiency is figured by multiplying the displacement in cubic inches by the maximum rpm planned, and then divide by 3,456. The result is the optimum cfm of the carburetor(s) to be used. Thus, in your case, 266 times 5000= 1,330,000. Divided by 3,456= a cfm of 385. 5,000 seem high for street use, as the power band would be closer to 3,000. A restricter plate reduces the vacuum thru the carb, but the volumetric efficiency is determined by the venturi size, which affects fuel atomization. That is why using a four barrel with the proper size for the primaries allows good low speed performance, and the secondaries open when the engine gets spun up. I'm not a fan of multiple carbs, as most tend to work well in a very narrow band, and you either have plenty at the top, of the engine won't idle much below 1,000. As to the cfm of the '94, I don't recall it, but someone else does, I'm sure.
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| Stroker | -- 10-02-2010 @ 9:13 AM |
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I'd contact Edelbrock, since they are currently building 94's. As an aside, I see they now list a rebuild kit for all 94's, which presumably has better quality "innards" than some of the import stuff that's out there. Having said that, I don't believe you will have too many issues with 2-94's. If they turn out to be too much for your particular motor, you can always drop back to a couple of 81's. I don't believe a restricter plate has any merit, as it is used by NASCAR to limit top end RPMS. If you are having "bogging" problems with your combination due to too much venturi area, it will be because there isn't enough airflow velocity through the venturi to properly meter fuel. A restricter plate won't cure that velocity problem.
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| Stroker | -- 10-03-2010 @ 8:15 AM |
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While I have no experience working on NASCAR powerplants; I believe that the so-called "restrictor-plate" is simply an administrative device like a body template that limits the throttle bore size without having to write paragraphs of detailed specifications. I believe that if you could probe the throttle body to intake manifold interface, you would see no evidence of the restrictor plate other than a thin "stripe" on the bore surface which would have been carefully sized, and blended to perfectly match the dimensions of the plate. In other words, the restrictor plate doesn't stick out into the path of the air stream. It simply mandates the size of everything leading to it, and away from it. Supereal has the best solution if you want smooth drive-ability. It's clear that your primary objective though, is to have a "classic hot-rod" appearance. I'd try the 97's on your Edelbrock Super, and if you are unhappy with the "fussy" throttle response, drop her back to a pair of Stromberg 81's. 81's were stock on V8-60's, and you have to look really close to see that the're not 97's. I've heard that there was a Holly "81" built towards the end of 60 production, but I've never seen one, so that may be the stuff of "legends". The Strombergs are quite common though. The key is to have your point of greatest restriction AT the venturi, not before, nor after. Only a smaller venturi area (like an 81) will achieve that in your 2-2bbl set up.
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| TomO | -- 10-03-2010 @ 8:51 AM |
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One of the problems with using the 94 carburetors instead of the Stromberg is the power valve. Vacuum with 2 carburetors is lower at the carburetor than with one carburetor. The power valve on the 94 requires vacuum to keep it closed. If it is open, the mixture will be rich. Tom
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| supereal | -- 10-03-2010 @ 11:35 AM |
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Many years ago, we used to have "driveaway plates" that were used when a vehicle, mostly a truck, was driven from the factory to the dealer. It was a flat piece of metal with a hole over each intake manifold port that was placed between the carb and the mnaifold. It was designed to prevent "hot rodding" the vehicle when it was new and being delivered. It effectively worked as a governor. As a prank, we used to slip them under an unsuspecting owner's carb and tease them when the car would only run to about 40 mph, then perform a "miracle" fix by removing the plate.
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| carcrazy | -- 10-03-2010 @ 1:30 PM |
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According to a table I received from an old carburetor engineer who worked at Ford the maximum flow rate for a Holley 94 Carburetor ranges between 150 and 165 CFM.
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| Stroker | -- 10-03-2010 @ 3:03 PM |
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Carcrazy: I'm going to make a note of that, as I've never seen any published CFM numbers for these vintage carbs. By any chance, does your chart include 48's, 97's or 81's?
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| supereal | -- 10-04-2010 @ 10:40 AM |
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That sounds about right. A 239 ci engine at 2,500 rpm would require 173 cfm. The 2,500 is the center of the power band. However, 150 to 165 seems a bit low. As TomO has pointed out, the Stromberg 97's were popular for multiple carbs to eliminate power valve problems, and they are very easy to rejet. That is why we used them in our dirt track racers.
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| carcrazy | -- 10-04-2010 @ 3:46 PM |
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The chart shows the following CFM ratings for these Carburetors: Carburetor Model Venturi CFM Rating Variation Stromberg 48 1.031 175 167/182 Stromberg 81 0.812 135 130/142 Stromberg 97 0.969 150 144/155
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| Stroker | -- 10-04-2010 @ 5:03 PM |
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Carcrazy: Duly noted. Thank You!
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| trjford8 | -- 10-05-2010 @ 6:30 AM |
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Bill, in my estimation the best setup for your flathead would be a 4 barrel carb. In my 40 coupe I had a Merc flathead about the same cubic inches as yours. I used a 4 barrel manifold and a Edelbrock 500 carb. It performed flawlessly throughout the rpm range and the mileage was around 17 mpg. I was running a C-4 automatic behind the flathead. If you are running the stock 3 speed your mileage could be better.JMHO
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