Y-pipe flow and restriction
Thread Starter
Joined: Nov 2003
Posts: 6,216
Likes: 236
From: Wichita KS / Rancho San Diego
Y-pipe flow and restriction I know this isn't really "advanced", but I haven't been able to get similar questions answered in External Engine, so here goes ...
What is the relationship between the size of the the dual tubes coming off the header and the restriction at the "Y?" In other words, how does the size of the dual tubes affect flow as long as their sum is greater than the restriction at the merge? If the I pipe joint on the Y is a single 2.75" pipe(many are that size to mate with the stock I), what difference does it make if you have two 2.5" tubes or 3" tubes going into it?
It would seem to me that overall flow is going to be no better than the narrowest restriction in the system, and that having too big of pipes going into the merge could in fact hurt flow by causing excessive turbulence at the point of greatest restriction.
I'd like to get the physics here ... maybe from somebody who knows their fluid dynamics.
What is the relationship between the size of the the dual tubes coming off the header and the restriction at the "Y?" In other words, how does the size of the dual tubes affect flow as long as their sum is greater than the restriction at the merge? If the I pipe joint on the Y is a single 2.75" pipe(many are that size to mate with the stock I), what difference does it make if you have two 2.5" tubes or 3" tubes going into it?
It would seem to me that overall flow is going to be no better than the narrowest restriction in the system, and that having too big of pipes going into the merge could in fact hurt flow by causing excessive turbulence at the point of greatest restriction.
I'd like to get the physics here ... maybe from somebody who knows their fluid dynamics.
Most ypipes have the downfall of not having a true merge. Lets build a mock scenario:
You have 1 7/8" long tube headers with a 3" collector. From that 3" collector you run 3" pipes to a "y connection". Flowmaster, among others, offer a 2x3" to 4" merge. This merge that they offer is a true merge. Power is gained from having a true merge since the exhaust gases are actually "merging" and not "colliding". This collision of exhaust gases promotes raspy exhaust tones. With a true merge, this is eliminated in most cases.
Now that the exhaust is merged, one would run a 4" pipe back to the axle to gain as much power as possible. Then the exhaust is necked back down to 3" over the axle and into the traditional catback exhaust. That is the formula to achieve the best power with a catback system.
One area to visit are the option of having a dumped exhaust before the axle. This is the best flowing exhaust since it is basically headers to ypipe to muffler to dump. Flow-through mufflers like the Bullet or spiral flow or split flow are the better ones out there.
A last vaiation is to add a cutout into the true-merged catback exhaust. Adding a cutout right after the merge will allow for some gains.
To answer your question about restriction, with a 3" true-merge to 4" exhaust, there is no restriction. I don't think most of our engines flow enough to require any larger of an exhaust. Some big blocks require larger exhausts.
These are my opinions
Andy
You have 1 7/8" long tube headers with a 3" collector. From that 3" collector you run 3" pipes to a "y connection". Flowmaster, among others, offer a 2x3" to 4" merge. This merge that they offer is a true merge. Power is gained from having a true merge since the exhaust gases are actually "merging" and not "colliding". This collision of exhaust gases promotes raspy exhaust tones. With a true merge, this is eliminated in most cases.
Now that the exhaust is merged, one would run a 4" pipe back to the axle to gain as much power as possible. Then the exhaust is necked back down to 3" over the axle and into the traditional catback exhaust. That is the formula to achieve the best power with a catback system.
One area to visit are the option of having a dumped exhaust before the axle. This is the best flowing exhaust since it is basically headers to ypipe to muffler to dump. Flow-through mufflers like the Bullet or spiral flow or split flow are the better ones out there.
A last vaiation is to add a cutout into the true-merged catback exhaust. Adding a cutout right after the merge will allow for some gains.
To answer your question about restriction, with a 3" true-merge to 4" exhaust, there is no restriction. I don't think most of our engines flow enough to require any larger of an exhaust. Some big blocks require larger exhausts.
These are my opinions
Andy
Thread Starter
Joined: Nov 2003
Posts: 6,216
Likes: 236
From: Wichita KS / Rancho San Diego
Thanks, but I can't seem to get my question addressed directly. Let me try an analogy:
Say you have two 3" pipes flowing water at full capacity going into one 3" pipe. Obviously there is going to be turbulence and backpressure in the water at the point of merge. If the pipes were 2.5" into 3", it certainly wouldn't flow any worse, since the point of greatest restriction is the same. It might even flow better due to less turbulence and backpressure at the merge.
How does it work with hot exhaust gasses flowing into a Y-pipe? Would a dual 2.5" system going into a single 3" be as efficient as a dual 3" into a single 3"? If the point of greatest restriction is the same, does the size of the prior tubing make any difference - again, as long as the sum is greater than at the most restricted point?
Say you have two 3" pipes flowing water at full capacity going into one 3" pipe. Obviously there is going to be turbulence and backpressure in the water at the point of merge. If the pipes were 2.5" into 3", it certainly wouldn't flow any worse, since the point of greatest restriction is the same. It might even flow better due to less turbulence and backpressure at the merge.
How does it work with hot exhaust gasses flowing into a Y-pipe? Would a dual 2.5" system going into a single 3" be as efficient as a dual 3" into a single 3"? If the point of greatest restriction is the same, does the size of the prior tubing make any difference - again, as long as the sum is greater than at the most restricted point?
I think with the 3" to 3" exhaust, there would be more scavenging and less backpressure. Correct me if I am wrong but isn't the point of any exhaust system to flow as much exhaust as possible to the point of where there isnt enough backpressure to allow the motor to scavenge exhaust gases efficiently? If the exhaust is too big, it may hurt the system. Like if we put a 4" exhaust on a bone stock ls1, it wouldnt be the optimal setup. Now if we were talking about a 632 cubic inch big block, a 4" or 5" exhaust may be optimal.
In the 2.5" to 3" the velocity of gases would be greater at the ypipe but there may be a slight restriction at that point.
Another issue is the effectiveness of the ypipe like I stated above. If you have a 2.5" to 3" system with a crappy y-merge, it wont flow as efficient as a 2.5" to 3" exhaust with a true y-merge. Velocity should in theory increase slightly due to the efficiency and therefore power would increase slightly.
Andy
In the 2.5" to 3" the velocity of gases would be greater at the ypipe but there may be a slight restriction at that point.
Another issue is the effectiveness of the ypipe like I stated above. If you have a 2.5" to 3" system with a crappy y-merge, it wont flow as efficient as a 2.5" to 3" exhaust with a true y-merge. Velocity should in theory increase slightly due to the efficiency and therefore power would increase slightly.
Andy
