Very small diesel downpipe design

[Dust Buster]

I came upon this site while looking for exhaust designs as there are no off-the-shelf systems for my vehicle. I read the anti-reversion thread and then looked through the “Engine Tech” and “General Tech” titles, but couldn't find anything suitable. So, pardon my first post seeking your advice on an engine not usually found in this forum. I'd just like to test my theoretical assumptions with you.

The principle that I would like confirmed is that after the variable geometry turbo (no wastegate), just get rid of the exhaust. As the turbo “absorbs” most of the noise, a sufficiently large, straight pipe would suffice as providing the least back pressure, subject to length and bore of course. Stock exhaust size is 1 3/4” (44mm). Sufficiently large for my 91.5 CID (1.5 litres) now 150 HP (112 kW) with 122.37 g/sec at the MAF is 2 1/4” (57 mm). The larger pipe would reduce some of the frictional losses of the laminar flow along the edges, but as these are pulses, surely there is some turbulent flow as well, but I think most of the negative flow is negated by the turbo.

My only concern then is possible droning (resonance) of a free-flow exhaust system. Perhaps two boxes would be better than one in this case.

I wanted a downpipe (DP) (turbo to pre-CAT pipe) in 304 stainless (SS) as 304 softens in high heat while the less chromium and nickel 409 SS hardens, although it is protected somewhat depending upon the quantity of titanium. However, the nearest 304 fabricator is 882 miles (1,419 kms) away, requires my DP as a template and two days (overnight stay) as he doesn't ship. That ain't goin' to happen. So, that leaves me with a local fabricator who does 409 stainless only. However, he doesn't have a mandrel or wiper die. Although he promises no crinkles, it might go slightly ovoid. Will this cause a venturi effect?

While he worked on the rest of the system, I was going to degrease (lacquer thinner, then acetone), sand, silicone ceramic 2,000 degrees F (1,093 C) heat withstanding paint - VHT flame proof primer, which is metal etching, VHT flame proof colour epoxy, to protect the steel from rusting, apply an exhaust heat wrap, then paint the heat wrap (primary? and colour) to prevent moisture penetration.

The wrap is supposed to move 35% to 70% (depending what you read) of the heat from the engine bay. Also, wrapping should remove some of the diesel clatter noise – well, I'm hoping.

DP_Sketch.PNG

[Kevin Johnson]

I seem to recall that the viscosity of gases increases with temperature and so the insulation will have a constrictive effect on the effective inner cross-sectional area. There is also a California Department of Forestry study of stock exhaust system temperatures. Bends accentuate temperatures which would seem to exacerbate sentence one.

Ask Neels if his software will model this.

[Dust Buster]

Where viscosity or thickness expresses the internal friction of a fluid or resistance to flow, I would have thought that keeping the pipe hotter for longer would delay the inevitable hitting a wall of colder air mass further down the pipe. I guess one could argue that a larger, warmer section of pipe would allow an initial increased loading (burst of speed) until the increasingly compressed new gasses started moving the colder mass.

I totally agree about the bends, which is probably why diesel dragsters vent straight up through the hood / bonnet.

I will reach out to Neels. Thanks Kevin

[Kevin Johnson]

https://www.google.com/search?rlz=1C1CH ... scosity%22

I know the topic has come up a number of times. I believe the boundary layer on the interior wall of the pipe increases in depth and this contributes to the effective constriction. Compare to water cooled marine exhausts with an effective infinite heat sink.

[Dust Buster]

Hmmm - I've got some reading to do - good.

I can prolly skip the ones worried about pre-detonation. I don't have that problem any more.

[dannobee]

The material normally used in fabricating turbocharged exhaust is 321 stainless. Fabrication of the exhaust is usually accomplished using pre-bent 90 degree and 180 degree bends, carefully cut to the desired angles and welded back together. Few shops have mandrel bending equipment available to bend exhaust tubing, and even fewer have an assortment of dies of varying radii and diameters.

Applying a wrap usually results in faster burn through of the tubing.

VHT high temp paint is inferior to a ceramic coating such as Jet Hot or something similar. In either case, it might not be necessary in your application. Stainless conducts far less heat than carbon steel.

These guys offer 321 stainless tubing that should get you going on your project.

https://burnsstainless.com/

[Dust Buster]

Thanks @dannobee

I can only dream of 321. I've never seen it locally. Even the off-the-shelf DPs for hot hatch were mostly 304.

Interesting about the pre-bent. I have seen one locally for a size larger pipe in pre-bent 90 degrees of 304 – no idea about the gauge though.
OK – thanks, now I see. I would also need a 45 and 30 degree bend as well as the 90. So, I could “carefully cut” as required.

I haven't see the faster burn through relating to the wrap before. I have always wrapped. I sold the last off-the-shelf DP (304 again) to the guy that helped me take it off. He saw that I had wrapped it, but interestingly to me, he took off the wrap. Perhaps he knew this already.

Perhaps it's just my bad experience with ceramic coatings and their expensive nonsense. The paint was probably .002” in thickness, unlike the Swaintech's claim of .015”. Actually, I've never had much luck with so-called 2000 degrees F paint either. I thought I'd pay a bit more and try VHT. Apparently 409 stainless will show an oxide dusting and a wrap will hold moisture. I was trying to get 10 years out of this set-up.

One potential problem is that diesel burns in the DP can hit 1,000 to 1,200 degrees F vs 700 to 1,000 degrees F in normal petrol motors (I know you guys push much more out of our engines).

Many thanks for the link. I have some place to start now. Now I just need to locate a local 321 welder.

Oh! what about the flanges? Does someone sell these in 321 as well? I wouldn't want to introduce electrolysis by mixing grades.