Exhaust Pipe diameter flow rate??

[Brummmie]

Hi Guys, i want to make my exhaust note a bit quieter, I currently have 2x 2.5" silencers underneath, but i am looking at y piecing underneath into one larger volume box. It is siamesed further down so lets not consider the "X" factor, what size single pipe flows the equal of 2x2.5" for the same distance. Eg 500mm seems a common length, and there are a number of people making 3.5" bore boxes. Cheers Brum.

[diycerb]

Hi Brum,

Haven't got an exact answer but if you look at it from an cross section area point of view.

2.5" pipe = 4.9 square inches, * 2 = 9.8 square inches.

single pipe of 9.8 square inches is:-

9.8 / pie = 3.125

square root of 3.125 = 1.76 inches (radius)

1.76 * 2 = 3.53 inch dia pipe.

I think an exact answer would look at circumference so you'd be looking at 5" dia!!! From one of my tuning books 400-500hp should have a pipe dia of 4" and 480-630hp should have a pipe dia of 4.5".

Hope this helps for a starting point.

Rich.

[HarryW]

Brummie some useful Vizard rules of thumb I've noted in the past;
Straight open pipe flows about 115cfm of gases without restriction per square inch (CSA).
Each HP needs about 2.2cfm of unrestricted flow before you start to affect power.

Therefore your twin 2.5" will allow about (560cfm) 260hp per bank to flow without restricting it. So the combined CSA of 9.8" as already worked out below for a 3.5" pipe will allow about 520hp to flow no problemo. Close enough given its a rule of thumb.

Other minor points I would look out for; ensure the two pipes when they merge do so at least at 60degs to each other and that the merge section is not a restriction in itself (there is an ideal merge ratio but I can;t remember that). This merge would also act as a balancer pipe so its length from the 4-1 will affect delivery long is good +30" if you can get it. If you are then feeding all this into a single off the shelf muffler type then ask what the cfm rating is for it, unless its a straight through cherry bomb type, as you are looking not to restrict your 2x560cfm flows with this one box . If in doubt get the next size up i.e. 3.75".

Edited by HarryW on Monday 17th March 23:11

[Brummmie]

Cheers Harry, talking to a guy on this stuff he also says that too big an opening will cause the gas to tumble as opposed to flowing, he suggested 2x2.5 into 3", its a black art this stuff!!!

[HarryW]

Brummmie said:

Cheers Harry, talking to a guy on this stuff he also says that too big an opening will cause the gas to tumble as opposed to flowing, he suggested 2x2.5 into 3", its a black art this stuff!!!

I would have said (as below too) that 3.5" maybe better given your power output.
Although 3" may not be too restrictive it may even give more torque at the expense of a bit of top end, might be worth dynoing both if possible. If not possible I would plumb for 3.5"

[HarryW]

Just run the 3" diameter through those Vizard rules of thumbs I posted earlier, the computer says its good for 370hp before becoming a restriction, even the 3.5" actually comes out at just over 500hp.