Downpipe flow question

["Mad Max"]

Folks, I'm trying to get smarter on downpipes and perfromance. I'm planning to upgrade the exhaust on my '93 to a 4" system, and I'll make it all myself, but I have a question on downpipe flow.



I'm planning an upgrade to either a 16 or 14 cm housing behind an HX-35, and I can make any custom downpipe, but I wanted to know if it matters how the exhaust exits the turbo, meaning is a tapered downpipe (from the collar to the full 4" pipe) better for flow, or just go directly from the collar diameter directly (non-tapered) to the 4"?



Here's an illustration:



Direct to 4": #ad




Tapered to 4": #ad




I can make either, and both are as easy as the other, but from a performance standpoint would the tapered make less turbulence than the direct method? Thoughts?



Many thanks,



- M2

 

[Greenleaf]

Just go to 4" @ the elbow. The connection will not matter, it's the short radious 90° elbow that is the restriction if your pushing a great deal of exhaust/ flow.

 

[JLEONARD]

I'd bet a case could be made for tapered, but in practicality I doubt you'd notice any difference. Since it's easier, go straight.

Jay

 

[JRussell3176]

Anyone have a guess as to what kind of air speeds we're talking about in the DP at WOT? If it's as high as I think, a tapered design could be very important. Here's a good closeup of Stan's 4" DP...



#ad

 

[Greenleaf]

I think I still have a picture of mine in the reader rigs. I simply cannot recall if my DP is tapered or not... ... ... ... ... my truck is a lawn ornament now.

 

[Idually]

Sam. We could pull the DP off of "Red" for the weekend. I want to build one for the Dually too, so... I have pics too.



Red's DP is like the first pic. Just built a "donught" from a piece of old /4" framerail. Used about 1-1/2" of the original 3" DP. Built the "donut" to fit inside the 4" and made the center big enough to accpet the original DP piece. I originally did the taper, but you can run out of room pretty quick. The 4" takes some space.



Let me know!

Thanks!

 

[Greenleaf]

I forgot to mention that all the buses have a 3" DP and then the 3" to 4" adapter. I obtain my adapters from a number of sources. Some are a blunt 3 to 4 and others are a gradual taper.



I build my own school bus exhaust systems and use either/or. Whatever I have in stock @ the time.

 

[dpuckett]

The picture above looks similar in shape to the one on my 91. Looks were a different story. But I did notice a definite drop in EGT and gained a few psi boost. Cooldown was quicker to boot.



I vote tapered.



Daniel