# A quick comparison of 4" vs 5" duct fittings.



## dbhost (Jan 28, 2008)

I am getting questions about why I am going with 5" ducting instead of staying with 4" mains, and it is all about air flow. 6" is IMHO just too much for a HF 2HP DC even with the Wen impeller mod, so I am going with the largest size mains as recommended by the charts on Bill Pentz' research pages...

But I still get the comments that 5" just isn't that much bigger.

Well...

Judge for yourself, side by side comparison of a 4x2.5 and a 5x5 ABS Branch wye.


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## Rebelwork (Jan 15, 2012)

It's up to you. I don't know Bill Pentz..


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## rrich (Jun 24, 2009)

4-inch pipe is about 12-½ square inches. A 5-inch pipe is about 19-5/8 square inches or over a 50% increase in capacity.


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## BigCountry79 (Jun 2, 2021)

4" pipe internal area 2x2 x 3.14
5" pipe internal are 2.5x2.5 x 3.14
...math...

5" pipe has 56.25% more cross sectional area.

Put another way, 5" pipe can move 312cfm at the same airspeed that 4" pipe can move 200cfm

Put another way, to pull 500cfm, the 4" pipe needs to move it 1.56 times faster. Because air resistance is relates to the velocity squared, the total resistance in a 4" system is 3.125 times higher than the total drag in a 5" system (1.56 x 1.56 = 3.125)

Put still another way, a 5" system with 30' of ducting and 6 45 degree bends will have similar performance to a similar 4" system with 10' of ducting and 2 45 degree bends.


...so why not go to 6" duct and really keep the velocity and drag to a minimum?
Because you need a minimum airflow speed to keep the dust and chips suspended in the air.


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## dbhost (Jan 28, 2008)

BigCountry79 said:


> ...
> ...so why not go to 6" duct and really keep the velocity and drag to a minimum?
> Because you need a minimum airflow speed to keep the dust and chips suspended in the air.


And if what I am reading is correct, and there is a good bit of debate about that, but I have to go with SOMETHING, a 2HP blower just isn't up to the task of providing sufficient air speed through a 6" duct, which is why the majority of them are fitted at the factory with 5" inlets. And even if, I am also factory fitted with a 5" outlet to the filter ring, and I do not want to mod that ring as I already have the Wynn filter etc...


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