I hope Adam comes in quick from painting his engine bays because I'm kinda scared I'm gonna baffle you all with BS!
If you hate imperial, don't read this, brothers!
As for you Yellow Stang, you'll want to copy this lot and check it out with a turbo expert and get proper quotes. And as ever, someone else shold comment on this before I lead you astray!
1. To answer your question, yes, the figures I gave were were for non-turbo sixes which had poor head flow. They are only GM Holden, after all!
2. Everyone needs to know that Holley, Carter, Edlebrock, Rochester, Autolite, Motorcaft, Impco, OHG, or whatever use ratings at 1.5 (20 inches of water), 2.0 (28 inches of water), or 3.0 inches of mercury (which is a little over 40 inhes of water) because they are trying to replicate the flow condition of the engine the carb is likely to end up on. Since a carb, head or manifold is just a pipe with a venturi in it, you can increase the cfm flowed, by just bumping up the flow drop or pressure.
Example A:-One head machiners may claim their cylinder head port flows say, 300 cfm at 500 thou lift, but if that was at 28 inches of water( 2.0 inches of mercury, or Hg ) then the one that flows 290 cfm at 500 thou lift and at 20 inches of water (1.5 inches of Hg), would clean up the 300 cfm one for maximum power. Its just like a hose. Up the pressure, up the flow.
Example B:- if you use the common carby formuae of
(rpm*cubes) all divided by 3456,
then a 400 Ford reving to 4000 rpm is likey to be flowing more than 462 cfm if it has 100% volumetric efficency! In practice, they had a VE of more like 75% at 4000 rpm, so the stock 2 barrel they used would have needed to flow 347 cfm just to yeild maximum power. Thats 217 hp net, tops
Note that VOLUMETRIC EFFICIENCY IS REALTED TO CARB, CAM, HEAD,COMPRESSION. You either apply the intake loss and forget about the VE, or estimate the VE and forget the intake flow loss... do not add both!
If you wrongly applied both, you'd get a conservative 25% flow loss on that awfull 2V intake they ran, then the peak cfm is now down to a lowly 278, then its likely the peak net flywheel hp would have been 278 divided by 1.6, or 174 ponies.
In practice, VE is the governor, and it takes a bit of work to figure it out! If the 2-barrel carb they ran was rated at, say, 450 cfm at 3.0 inches of mercury, then the actual pressure drop at the carb can be calculated. That calculated cfm by the formuae at the top is critical. If the carb does 450 cfm at 3.0 inches Hg , then at the calculated 347 cfm the pressure drop is likely to be 450/347 cfm, which is (1.29683 squared) less than the 3.0 inch figure. The 1.29683 must be multiplied by itself, and then the result is divided from the number 3.0. That is, the actual pressure drop on the 400 engine was 3.0 divided by ( 1.29683*1.29683), which is 1.78 inches of mercury. This was no where near the rated CFM at 3.0 inches.
Example C: A 650 cfm Holley carbed NASCAR used in the Winston Cup back in the early 1990's could churn out 650 horsepower at the flywheel at 7500 rpm. The 357 Windsor/Cleveland SVO block had alloy heads, the best Jack Roush intake manifold, and a volumetric efficiency of over 100 % because of the mild tunnel ram of those single plane style intakes. Doing the sums again,
(357*8000)divided by 3456 = 826 cfm needed. That only gives 516 horses calculating by dividing the cfm by 1.6. But they only had a 650 carb. Thats only 406 hp! Something screwy? Nope. They actually ran a pressure drop of over 2.56 inches of mercury, enough to flow 1040m through that little 650 cfm carb! The intake flow was less than 10%, and it was cancelled out by the mild supercharge from the almost vertical intake runners! So the 650 hp was still cfm divided by 1.6, but there was so much suction that it was 1040 cfm. Take actual rated carby cfm at 1.5 inches, divde by formulae cfm (826 cfm)... this gives us 0.7869 to square. Now divide he 1.5 rating standard by 0.7869*0.7869...its 1.906 inches of Hg. Dividethat by 1.6, and we only get 516 hp. Clearly, only the VE or the revs can change in the formula, so those NASCAR engines would have had to run theoretical volumetric efficiencies of over 125, and that 1.6 CFM to HP value derived from a engineer siting at a dyno cell was pessimistic for these special Holley carbs. At 200 mph, the air pressure at the carby would be more than atmospheric, and even the collector pipe exhast would have had a scavange effect placed on it from the moving air.
2. Turbo power calculations:-these are sorta different when you do net flywheel figures from the rated cfm at 1.5 inches of mercury (gage pressure drop from inlet of carb to the downstream flow) readings.
Two reasons. In an old fashioned draw through with a 600 cfmVac Sec Holley carb, a turbo pumping in 15 pounds per square inch of pressure is raising the flow rate at the carb from its normal level. It's no longer 1.5 inches of mercury pressure, but something different!
Example A: Unturboed. On A 200 cube six, a non turbo 2V engine with good forged rods, bolts, and a 256 cam wouldn't get within a bulls roar of 200 hp. I'd say a 390 cfm carb would be only a small oversupply on a 200 cube six, and I think that you'd only get 173 to 180 hp with it only using 277 cfm of its peak flow at 1.5 inches Hg. It's likely peak pressure drop would be 390cfm rated divided by 277cfm as calculated below, gives a flow drop of only 1.408*1.408 from 1.5 inches of Hg, which is 1.5 divided by 1.982, and therefore only 0.757 inches of flow drop at wide open throttle. But don't worry. Most engines can handle being 40% over carbed. 351 Cleveland V8's only need 550 cfm to get 340 hp, but the always like a 780 cfm #3310 much more!
The actual amount of Hp would be rev dependant. A 256 with a nice 1.75 inch intake and 1.44 inch exhast may hit the 175 cfm at 480 thou lift mark at 28 inches of water(2.0 in Hg). Chevy OffRoad and Marine Engineering in Aussie say that with a hydraulic cam, a little V8 265 SB Chev with 175 cfm of intake flow can hit a maximum power of 175*2*0.80 equals 280 hp net. Since a 200 is 3/4 of a V8, 210 is all that is likely unless you raise the cam lift and take a loss in drive-ablity. Often you get less. All those Aussie sixes ran hotter than 256 duration cams. A turbo or Nitrous is needed for streetablity. My calcs say, unturbo'd, with a 256 cam, and 175 cfm at 28 inches H20 head, you'd get need a cam that would rev to 6000 rpm, with power at 5500 rpm. Then you'd maybe hit 180 horses. There would be plenty below 3000 rpm in the torque department, maximum torque at 4000 rpm. But even single barrel 202 racing HQ Holdens have 260 duration and 170 Hp. Your loosing power for a mimimal drivablity improvement. To get 210 Hp, you'd need a 272-292 degree cam with around 500 thou lift. Go for more lift, you get more cfm flow, more power. At 5500 rpm, with 200 cubes, divide by 3456, multiply by 0.87 for your VE, you'd need 277 cfm at 1.5 inches of Hg, and could hit 173 hp. To raise VE, go for a 272 to 292 cam, and loose torque and races from the lights!
Example B: Using a turbo, you could go for a draw through or blow through installation. Draw throughs are not the idea for a streeter as the carb never seams to atomise the fuel air mix enough before it hits the turbo. You'd need a bigger than 390 cfm carb if you were running more than 9 pounds per sq inch boost. ( I'd prefer a 600 cfm Vac Sec) At 9 pounds, the peak flow drop on a draw through as described above in Example 2A, can be calculated. Its boost in pounds plus 14.7 all dived by 14.7 pounds (the air pressure at sea level). The boost ratio is 1.62. That means, as long as little heating occurs, you have a theroretical cfm increase from 277cfm to 449 cfm. The good thing is, your 256 cam would love this.Now, your 390cfm might just love that too, but your engine is now behaving like a 327 cube Chevy, and could use the extra carburation of a carb rated 449 cfm at 1.5 inches Hg, or larger. Definately no smaller. The worst thing you can do to a turbo is run it out of fuel. In practice, the heat added into the engine reduces the intake flow, and thus the peak hp, required from the carb. The potential rise from 180 hp to 290 hp (1.62 times) wouldn't occur ..it'd be more like 260 horses. This is why people use an "intercooler" on blow through turbos. I've also heard them called aftercoolers...depends which culture you hang out with, I guess.
Example C: Using the 390 carb mounted on the intake, not ahead of the turbo, we can pressurise it to 9 pounds. Various things need to be done to the carb...throttle spindles need presurised air to the shaft by drilling holes into the cast alloy adaptor under the carb to stop the turbo pressure coming out (boost referencing, its called), the fule pump needs to either raise its pressure to 9 pounds or face prblems, the float needs to be heavy duty brass or nitrophol, the operation of the carbs secondary circuit needs to be changed. In reputable Australian magazines, there are often photos of turbo V8's with aluminum or stainless steel sealed boxes around the carb. Lots of work here. But the results are worth it!
Example D: Use the Aussie 4.1 Cross-Flow EFI system, spend US $1000 on doing the extra inch or so of cast iron welding around the lifter gallery on your 200 I6, and fit the Oz cross flow. The AIT/Mike Vine/HKS/NormalAir Garret turbo system was develpoed in the 1980's for this engine, and it doubled the power of a stock carb six.
Example E: This is my pick. Grab just the Aussie cross-flow alloy head EFI intake manifold and injectors and thorttle body, and drill the 2V ports for the Aussie fuel rail. Then turn the EFI manifold around 180 degrees and make a tubing intake header from 1.5 internal diameter steel thbe bends. Then ask Jack about the Simple Fuel Injection system he uses on his Aussie EFI 250 1966 Stang. Then use a Garret T04B turbo to blow through to a in front of radiator Mitsubishi Evolution 5 intercooler, then back to the injection throttle body.