Why do tunnel rams use so much more CFM?

FYI..hemi super stocks use 770 cfm holleys. not 650's..

Think in terms of pressure drop across the carbs. 4bbl carbs are flowed at 1.5"Hg (20.4" H2O). Using the standard formulas, if you picked the right carb you would have 1.5"Hg pressure drop @ WOT. A bigger carb would give you less pressure drop, increased density of charge in the cylinder. More air+fuel=more HP. Same reason to use bigger cams is to get the charge density up at high RPM.

I'm sure the SS Hemi's could use bigger carbs and would gain even more power if they could run two Dominators. I think there was an interview with Harry Holton about ten years ago and he said the carbs were a limitation. They are almost 200 HP higher now than when Timex made 900 HP.

I'm no carb wiz but here is what I understand- all you need is proper fuel atomization for good power at the minimum rpm you run. A street car may need to go WOT at 2500 rpm. Lets say the biggest carb on our 440 v8 than can do that well is a 750.
to atomize fuel properly, the carb may restrict the air coming in by about one third what it will at high rpm. The true air flow will be maybe 300 cfm, and the true vacuum will probably be only about .6 inchs of vacuum. But the carb needs enough air flow ( and restriction) to do its' job of pulling fuel out of the boosters and atomizing it. Go below a certain air speed, and no fuel comes out.
Tunnel rams run at much higher rpm as a rule, and being race only (for the most part) will only need to function well from about 5,000 rpm up. So the TR set up could use two 750s, for 1500 cfm and get away with it, also reducing total vacuum and gaining power that way plus the tuned length of the manifold itself.
You can also increase the size of the carbs some with boosters that work better at lower rpm and modify the fuel curve to fit the situation better.
Vacuum secondary carbs get around this problem some, allowing use of 800 cfm carbs on small blocks of only 340 cubes.
Fuel injection goes a step further, providing the energy to atomize the fuel with a pump. So the system will function with much more airflow. Total vacuum can be reduced to almost zero when you can run a "hat" that flows over 3,000 cfm. My Terminator deal with 2100 cfm struggled to pull .5 inch vacuum at max rpm on the dyno.

And carbs don't flow what they are stated to flow either.. Stock carbs, especially eddy carbs, flow a lot less than the stated cfm...So be careful when you start broadly comparing "CFM ratings" of carbs.

Just like a factory engine that has been blue printed, a carb also needs attention to detail and a good going thru. Don't even assume that SS carbs are out of the box with all the factory workmenship flaws that yours might have. I assure you, they are not in 90% of the cases. Attention to detail should not be limited to the heads on an engine. Heck I even port my header flanges to makes sure they flow as good as they can.

With that said, my efi Tunnel Ram flows 2100cfm on my 484 Hemi, and it is pretty awesome for throttle response.

I'm guessing that you were refferring to "3-two barrells" on a tunnel-ram? You have my interest peaked! I would like to install an old Offenhauser tunnel-ram that I have made a 3x2 top for, but still havent descided how to go at it carburetor wise - do I get 3 NOS six pack carbs, or get 3 regular 2300's with metering plates and run a non-vacuume linkage? How did you do it? jetting recomendations? B.T.W. it will be on a Victor headded 500 c.i. low deck about 10:5/ 11:1 compression unit.

one of the reasons for the big carbs is that your tunnelram works alitle like a I.R manifold each barrel feeds one cylinder and the plenum should realy only be there to smooth out the pulses that most american performance carbs dont like to be subjected to. fuel atomization is a much smaller problem when the fuelairmixture dont have to make any 90degree turns betwen the carb and the intakevalve.

with the straight shoot from the carbbarrel down to the intakevalve you DONT want any restriction forcing the cylinder to draw air and fuel from more barrels than neccesary.

with the right combo tunnelrams work great even on the street and will be increadibly quick repsonding to input from your right foot. forget about all the bull about "its a raceonly intake that only works from 12565215454rpms and up" just get the combination right and they are great

I can testify about BB Mopar motors with standard 440 heads and M.W. size heads, the more air you give them the faster they go It must be something in the designs Not the genes, jeans, geans all kidding aside a drag motor is not the same, even drag racing a stock motor, as the stock motor and the way there driven I have had a bunch of other brand(Chevys and other GM motors) racers tell me that I'm crazy on the size carbs I use on my motors, street and strip motors. Theory is one thing, actual results in the real world are many times not the same Test,test and test some more to learn

Recently I read an interesting article by a carb guru who made the point that as long as your booster is providing enough signal for metering, the bigger carb will always make more power. The amount of carb signal from the booster can vary by a factor of more than two.

There is only so much atmospheric pressure pushing the air into the engine, and the less pressure drop between the outside air and the inside of the cylinder, the more air you will get in there.

A couple of years ago another series of dyno tests showed that the closer the vacuum under the carb gets to zero, the more power the engine makes, and there is quite a bit of power going from a 1.5" depression to a 0.5" depression.

This is also the reason why 350 cubic inch injected engines have 1000 cfm throttle bodies.

I believe the tunnel ram gives the carbs more signal to work with so the larger carbs are driveable and, of course, make more power.

R.

Let's call this thread "the blind men and the manifold".

I am no expert either. As I undersatand it, the longer straight runner Packs the air behind the valve .
Allowing more air and fuel to enter the cylinder when the valve opens .similar to forced induction on a small scale.
if you can get the air moveing in one direction it does not just stop when the valve closes it packs at the valve .
I have seen tunnel rams make better power at lower rpm than even a torker intake . it is all based on runner length and size verses rpm.