9th Gen Civic Forum banner

41 - 60 of 65 Posts

·
Registered
Joined
·
11,032 Posts
Discussion Starter #41
Your measuring from the inside

Where it tapers down towards flange/exhaust?

All websites selling it....list it as 3"

What gives?
yes, it's 2.75"

the down pipe reduce from 2.75 to 2.5" to increase gas velocity..


like i said for many years... they are drop ship online store, most have no idea what they are selling... nor knows the pros & cons of a product.
 

·
Registered
Joined
·
11,032 Posts
Discussion Starter #43
http://www.golfmk6.com/forums/showthread.php?t=15690

Should I go 2.5" or 3"?


This question seems to be asked daily on GTI forums. I'm going to try to approach the topic from the fluid mechanics perspective.

First, we have to understand that flow velocity in the exhaust is as important as high volumetric flow rate, but for a different reason. Second, increasing pipe diameter reduces velocity (bad) and increases max volumetric flow rate (good). Last, high velocity equals low pressure. That's it - that's the physics.

But what does it mean to downpipe performance?

It's pretty simple:
  • For max power at WOT, we want high volumetric flow rate. This means a BIGGER pipe. -but-
  • For maximum throttle response (minimum turbo lag), we must keep flow velocity up. This creates the max pressure differential to drive the turbine. This means a SMALLER pipe.
Obviously, we want both. But we can't maximize both simultaneously, so we compromise. Experience has taught that if we go smaller than 2.5", power at WOT (AKA dyno runs) suffers. If we go bigger than 3", lag rears its ugly head.

So... When folks say they lose power down low with a 3" DP, they are right - low flow velocity creates low delta-p which means lag, even in our small turbo. The flow is slowest at part-throttle, and that's where the lag is greatest.

And when folks point to dyno runs that prove that no power is lost down low with a 3" DP, they are right too. At WOT and steady-state flow, the turbo loves that high flow potential.

Another velocity-related consideration is exhaust gas cooling. We don't want the gas to cool. Cool gas is dense (heavy), and trying to move it creates backpressure. Score another one for smaller pipes.

But, in truth, I doubt that the differences between 2.5" and 3" cause much of a perf diff for the stock turbo.

Finally, why does low-RPM lag appear to increase at Stage II? I'm not sure, but I suspect the issue is turbulence. The Stage II ECU requests lots of boost NOW, creating violence in the flow. Turbulence is another big velocity-killer, and low velocities cause lag.

It's been a long time since I studied Euler and Bernoulli, but hopefully I've remembered enough to make this useful. I have over-simplified a bit. This was a deliberate choice.

Bob

PS Credit to grambles423 for exposing some of the equations in an earlier thread.
 

·
Registered
Joined
·
304 Posts
Exhaust diameter sizing for turbos - VW GTI MKVI Forum / VW Golf R Forum / VW Golf MKVI Forum / VW GTI Forum - Golfmk6.com

Should I go 2.5" or 3"?


This question seems to be asked daily on GTI forums. I'm going to try to approach the topic from the fluid mechanics perspective.

First, we have to understand that flow velocity in the exhaust is as important as high volumetric flow rate, but for a different reason. Second, increasing pipe diameter reduces velocity (bad) and increases max volumetric flow rate (good). Last, high velocity equals low pressure. That's it - that's the physics.

But what does it mean to downpipe performance?

It's pretty simple:
  • For max power at WOT, we want high volumetric flow rate. This means a BIGGER pipe. -but-
  • For maximum throttle response (minimum turbo lag), we must keep flow velocity up. This creates the max pressure differential to drive the turbine. This means a SMALLER pipe.
Obviously, we want both. But we can't maximize both simultaneously, so we compromise. Experience has taught that if we go smaller than 2.5", power at WOT (AKA dyno runs) suffers. If we go bigger than 3", lag rears its ugly head.

So... When folks say they lose power down low with a 3" DP, they are right - low flow velocity creates low delta-p which means lag, even in our small turbo. The flow is slowest at part-throttle, and that's where the lag is greatest.

And when folks point to dyno runs that prove that no power is lost down low with a 3" DP, they are right too. At WOT and steady-state flow, the turbo loves that high flow potential.

Another velocity-related consideration is exhaust gas cooling. We don't want the gas to cool. Cool gas is dense (heavy), and trying to move it creates backpressure. Score another one for smaller pipes.

But, in truth, I doubt that the differences between 2.5" and 3" cause much of a perf diff for the stock turbo.

Finally, why does low-RPM lag appear to increase at Stage II? I'm not sure, but I suspect the issue is turbulence. The Stage II ECU requests lots of boost NOW, creating violence in the flow. Turbulence is another big velocity-killer, and low velocities cause lag.

It's been a long time since I studied Euler and Bernoulli, but hopefully I've remembered enough to make this useful. I have over-simplified a bit. This was a deliberate choice.

Bob

PS Credit to grambles423 for exposing some of the equations in an earlier thread.
The 3" catless dp dominates according to Vit....

Tested: Full Race Catless Downpipe for 2012+ Civic Si | VitTuned Blog
 

·
Registered
Joined
·
11,032 Posts
Discussion Starter #45
during dyno, it's always WOT... this is what the article is about...


So... When folks say they lose power down low with a 3" DP, they are right - low flow velocity creates low delta-p which means lag, even in our small turbo. The flow is slowest at part-throttle, and that's where the lag is greatest.


And when folks point to dyno runs that prove that no power is lost down low with a 3" DP, they are right too. At WOT and steady-state flow, the turbo loves that high flow potential.
 

·
Registered
Joined
·
48 Posts
I think Vit needs to get his hands on this thing and dyno test this and see how it compares with the full race, ultimate racing and the rv6! Time for a good old fashioned face off!
 

·
Registered
Joined
·
521 Posts
this downpipe seems to be the best of both worlds, larger and more free flowing toward the head and narrower for increased velocity where it meets the cat back. I think the difference between the BC DP and the uncatted RV6 DP is minimal, they both end with 2.5" piping. However i do feel there is a slightly larger difference when comparing to a full 3" DP. Originally I wanted a catted RV6 DP but the BC DP is more in my price range. Being catless sucks but I will have to work around that somehow. Maybe down the road I will have a cat welded on prior to inspection.
 

·
Registered
Joined
·
813 Posts
Wow...that would be the mother of all threads

Probably would cause a server blackout

More like vendors coming in and bitching and moaning to high hell trying to disprove what vit proves because their product wasn't at the top and mods shutting the thread down eventually....lulz
 

·
Registered
Joined
·
11,032 Posts
Discussion Starter #50
Originally Posted by slee0730
Was tuned by vit and I requested an edyno and that's what he sent back


Sent from AutoGuide.com Free App



slee0730, thank you for the vit edyno

now we know, bone stock civic si with buddy club 2.75 dp + vit tune = 191.29whp

 

·
Registered
Joined
·
376 Posts
We need someone to post out of the car drive-by sound clips
Heres outside sound of downpipe plus axleback.

Can anyone comment on long term reliability? Does the pipe feel like it can stand up to being heat wrapped?
Ive ran this downpipe for close to a year now about 25k miles at least and i havent had any issues and ive periodically checked under my car and everything seems to be ok behind the head. im sure it could be heat wrapped if you wanted to but i havent felt the need to.

there MAY be better DPs out there but for the price and performance ive quite satisfied. and lets be honest... it sounds good lol.
 

·
Registered
Joined
·
86 Posts
I'm really wanting to pull the trigger on this downpipe but i found this picture the other day and it worried me. Does it come with any kind of warranty?

 

·
Registered
Joined
·
11,032 Posts
Discussion Starter #57
I'm really wanting to pull the trigger on this downpipe but i found this picture the other day and it worried me. Does it come with any kind of warranty?


full race down pip, cracked & looks rusted...



skunk down pipe cracked and look rusted




crack in the welding point may happen if the car is running lean, you just need to make sure the vendor (you purchase from) & manufacturing will take care of it...
 

·
Registered
Joined
·
44 Posts
Just made a thread regarding this, but I bought a used BC downpipe and the bolt holes really don't line up with the OEM Catback. When two bolt holes are aligned, the other one does not line up. The only way to make it fit is by putting in one bolt at an angle.
 

·
Registered
Joined
·
11,032 Posts
Discussion Starter #60
Just made a thread regarding this, but I bought a used BC downpipe and the bolt holes really don't line up with the OEM Catback. When two bolt holes are aligned, the other one does not line up. The only way to make it fit is by putting in one bolt at an angle.
not sure about used, but i haven't heard brand new bc dp don't line up with stock exhaust
 
41 - 60 of 65 Posts
Top