Exhaust sizing

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Pmelsness90

Well-known member
I'm currently getting a plan together for redoing my exhaust from manifolds to my muffler, my initial idea is to use a merge collector that has 2 1/4" inlets from each manifold to a 3" outlet. My current setup is a walker y pipe and it's too poorly put together to make modifying it worth it. My EFI manifolds have been cleaned up on all the runners and the outlets have been opened up to 2" outlets. If 2 1/4 pipe isnt ideal I'd like to hear suggestions.
 
To mentally visualize the air relative velocities, get the area of each size, and establish a ratio between the two pipe sizes. This helps determine if changes in air velocity from one pipe size to another is reasonable.
Area = 2(diameter) x .7854. (A simpler form of Pi x radius squared). Since we don't need the actual area, drop the .7854 multiplier and simply square the pipe size. for example, two 2 1/4"pipes into a 3" pipe.
2.25 squared = 5.06. Times two pipes=10.13 . A 3" pipe =9. So 10.13 over 9 = 1.125. So a single 3" pipe flows 12.5% less than 2 2 1/4" pipes. EXCEPT, the smaller pipe has more surface area producing more wall-friction. Also gas velocity continues to diminish as it cools. So (IMO) two 2 1/4" pipes into a single 3" pipe would be a well balanced, nearly equal velocity flow ratio.
 
I'll wade in here, if your about an NA 300 six with two outlet exhaust manifolds, consider that each outlet is being fed by 2.5 litres, so with that in mind a 2inch pipe from each will be sufficient. From there you go into a collector of sort, as long as the collector has a smooth entries and exits you will be OK. From there to the silencer, 2.5 would be a minimum, if you want to go bigger, that is Ok. What Ive found generally is that a single 2.5 on a 5litre engine is good for about 250bhp, and with a turbo type silencer is fairly quiet. If however you are talking about a blown application then you need bigger all through. All the chatter about "tuned" pipes is OK for race cars , but on the road with a silencer low restriction is all that is needed, and that general means turbo type silencers and bigger pipes. good examples of a well designed high performance systems are found on BMWs and Mercedes, they know how its done.
 
I may end up changing my muffler out but for now it's a flowmaster super 44 single inlet dual outlet. I'll be using a merge collector from stainless headers and piping from them as well.they build very nice header components.
 
Pmelsness90 said:
I may end up changing my muffler out but for now it's a flowmaster super 44 single inlet dual outlet. I'll be using a merge collector from stainless headers and piping from them as well.they build very nice header components.
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Since this has a dual inlet, why not pipe up two inlets to one outlet, no collector needed, probably easier to do, and easy to fit a decent sized tailpipe. That silencer looks like a turbo style, keep it. here is a good guide for sizing: www.lukey.com.au/pages/exhaust-size
 
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My current setup is a single 2.5" inlet dual 2.5" outlet. My thought was to just cut the pipe off the muffler and go 3" into the muffler if I can I'd like to keep the remaining exhaust since it's pretty nice already.
 
Pmelsness90 said:
My current setup is a single 2.5" inlet dual 2.5" outlet. My thought was to just cut the pipe off the muffler and go 3" into the muffler if I can I'd like to keep the remaining exhaust since it's pretty nice already.
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You can do that for a tone change. The effect of that change will reduce power, not help it. But only a dyno could see the small difference. A solid 2.5" front to rear is as good as a mild 300 engine can get. Making it larger in mid-stream will slow the velocity, increasing backpressure.
 
The reason I'm thinking of doing 3" back to the muffler is cause it is more than a mild 300, I do have it procharged and I am trying to get into the 500hp range with it.
 
Pmelsness90 said:
The reason I'm thinking of doing 3" back to the muffler is cause it is more than a mild 300, I do have it procharged and I am trying to get into the 500hp range with it.
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oh for goodness sake- my bad! Forgot the thread context.
 
Haha happens to the best of us. You are correct about 2.5" on a standard 300 it was a significant power increase on my 240 when I swapped to EFI manifolds with a single 2.5" pipe on it.
 
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