Author Topic: age old question. how cfm is to much?  (Read 3221 times)


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age old question. how cfm is to much?
« on: July 02, 2009, 06:21:15 PM »
I have a 67 mustang with very few engine modifications. I currently have :

     1.  Electronic ignition
     2.  headers (short)
     3.  2 1/4 exhaust
     4.  AOD transmission
     5.  2.80 gears in rear end

I want to replace the 2bl carb(autolite 2100) with a 4bl.  I currently have and edelbrock performer intake and holley 4bl 600cfm carb.  My question is, is the 600cfm carb. to much cfm with the above modificatations.  Wolud i need to add a cam with the holley 600cfm carb. (I assume the cam in the car now is stoke?, but not sure).  Or, should i not use the 600cfm and purchase a smaller carb. (ex. 450cfm , 500cfm, etc.)

Regardless of carb. set-up I plan on changing the gears in the rearend with 3.25 or 3.55.

New to the fourm so any advise would be appreciated.

Thanks sab67stang


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Re: age old question. how cfm is to much?
« Reply #1 on: July 02, 2009, 10:11:06 PM »
The gears out back aren't going to make any difference in the selection of your carburetor.  With the Performer intake and the shorties, you can probably squeeze by with the 600 CFM carb.  You can always trade it for a 500 or so if it proves to be too much.  Here is an explanation as told by an old red neck from another site that I used to moderate.  It makes perfect sense if you think about it. 

everyone either wants to upgrade their carb or has done it. same with exhaust. this lesson may seem a little wierd but it may help some of you understand why not go too large but to keep it big

First off, fill a large glass of water. Trust me . Take a small coffee straw and suck some'll see you can only get so little so fast. Think of your mouth as the engine and the straw as a small carb.

Next, take a larger straw, say normal size found anywhere, snd suck again and it will go at a decent rate and at a rate that you can still swallow without drowning yourself.

If you take a huge ass straw the size of your mouth, you may not be able to get anything to your mouth.

Now exhaust, same principle but backwards. Take the small straw and blow on the water, you see it move a little in the concentrated area, wooptie do, however you can probably keep blowing for a prolonged time due to the amount of backpressure, this is bad, too much back pressure on an engine creates problems.

The medium size straw will blow more air and still allow a little backpressure so you can keep blowing and the engine will actually gain torque and horsies while being able to exhale properly.

As for the big arse straw, the air only reaches as far as you can blow and may barely move the water at all. Plus due to the lack of backpressure you run out of air rather soon. This shows that with too big of exhaust, say anything over 2.5" for you small block boys like me, will end up losing power.

Now this is a little wierd but everyone keeps asking ?s like why cant i put a nice sized 600-650cfm on my 289...well you can, but you'd better make sure it has enough mods to need something that big.

if you all understand this, which i hope you do, maybe Soaring can move it to the FAQ for when more nubs come looking for this type of advice. 




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