I am trying to find out the next restriction on my engine.
It is probably the 48mm throttle body with an air foil. This is supposed to flow at about 820cfm.
But what about the intake tube that I have? It is 3". What would be the flow in cfm for this??

The opening of the throttle body itself is 4", maybe that would provide no restriction?

 

Hi LB9GTA,

3" tube will be OK for a (2x) 48 mm T/B.
It is a matter of surface comparison, provide that you make an smooth transition from round to oval.

To check we use mm numbers, it is easy for me.
A T/B with two 48 mm will give you a surface of 1809 square mm each, so the total surface would be 3618 square mm.
Now when you calculate the surface of the 76.2 mm (3") tube you will get a surface of 4560 square mm and that is lot bigger than the 3618 square mm of the T/B.

Now we try in inches.
2 times 48 mm T/B will give you a surface of 5.6 square inch. and a 3" tube will give you a surface of 7 square inch.

So it will not restrict the T/B

I know that the outside oval of the T/B can be around the 4" but that is due to the construction.
A Holley T/B has that size (4") but uses the 58 mm flaps that gives a flow of 1000 CFM.

I hope this help
Regards,
Cobra289

 

Yes that helps.
But what about a 52mm or a 58mm throttle body?
Would a 3" pipe be a restriction, then?

 

OK!
Lets see:
(2x) 52 mm has a surface of 6.58 Square inch
(2x) 58 mm has a surface of 8.19 Square inch

We know that 3" tube has the surface of 7 Square inch so the 52 mm is OK but i f you use it in the 58 mm the engine will be chocked.

The next tube will be 4" and that give a surface of 12.56 Square inch.

I hope this help.
Cobra289

 

Hi LB9GTA,

Please be aware that the T/B 48 mm or 52 mm and 58 mm are net dimensions of the blades. It means that the blade has a diameter of 58 mm or.....

Therefore when you use a pipe with and outside diameter of 3" you need to consider that the wall thickness should be subtracted.

Assume the pipe has a 3" outside diameter and it has a inside diameter of 2.9"
that 2.9" will have a surface of 6.6 Square inch.

How to calculate the surface is simple:
The formula is PI /4 x Dia^2
To make it simple 0.785398 x 2.9 x 2.9 = 6.6 Square inch's

I hope this help you.
Regards,
Cobra289