Wrapping Exhaust Pipes 3

[S2Knut]

thread71-162827

Re: the above thread

Concensus seems to be that wrapping exhaust header pipes is a bad idea but that coatings may be beneficial. I saw no clear concensus on wrapping exhaust pipe downstream of header outlet except that corrosion may increase.

My question is: Is it possible that keeping the gas hot and the velocity high is beneficial because flow in the pipe is pulsating, not steady state, and that a column of gas flowing at higher velocity has more momentum and at the end of the pulse a rarifaction is created at the outlet of the header and the entrance to the exhaust pipe and that said rarifaction improves scavenging.

 

[btrueblood]

Oh yeah, keeping velocity high in your exhaust system is a great idea. Here's a neater one: use a smaller diameter exhaust pipe, it will have the same effect.

 

[ivymike]

not sure that's true.
- speed of sound changes with temperature, and the smaller diameter pipe would make temperature lower not higher.
- would pressure drop (restriction) w/higher density flow in a smaller diameter pipe be the same as w/lower density flow in a larger diameter pipe at the same average velocity (and mass flow)? Offhand it seems like no, but I've not looked at the equations.

I was going to ignore this thread because I think the question is too general to answer in any definitive sense (like always, "it depends")... but I just had to pick a bone.

 

[btrueblood]

How would a change in pipe diameter change (lower) the temperature of the exhaust? Ok, I know, heat transfer. But that's what the wrap is supposed to prevent. I'm just pointing out that for less material and fuss, some smaller o.d. pipe will create the same velocities as adding wrap. And smaller pipe is an anathema to the people who think about wrapping exhaust pipes. Higher velocities mean more pressure drop, oscillating or not. The idea of tuning for scavenging is better addressed by tuning pipe lengths and diameters, not randomly changing heat transfer.

Shielding nearby components is a reasonable reason for adding insulation, but should be approached with caution. FWIW, I used to specify exhaust pipe/manifold insulation, for a machine that had a heat exchanger on the exhaust for cogeneration. Getting an insulation of any kind to work effectively for 1000 hrs. was a real chore, due to corrosion/erosion of the manifold caused by vibrating abrasive oxides next to hot metal. And, the fiber insulations do make lovely wicks for oil lamps, changing small oil leaks, spills and drips into lovely, bright flames.

 

[Tmoose]

If the tuning effect of interest is pressure/rarefied waves bouncing along the pipe, hotter gasses will make the pipe seem shorter. a tuning effect reportedly used to broaden power bands on 2 stroke road racers is to dynamically mess with the timing, to raise exhaust temp, make the pipe seem shorter, thus more powerful at hi rpm. I guess the power lost to less than optimum ign timing was more than made up for by being on the pipe.

 

[patprimmer]

Isaac

Was that a bone or a nit or maybe even a nits bone. ;-)

OP

Pipe insulation certainly helps upstream of a turbo, however if to well insulated, the structural integrity can be reduced both short and long term by stress cracks from thermal cycling while bolted to more stable parts and from increased corrosion or oxidation at higher .

downstream or with no turbo, like Isaac says, it all depends. Protecting surrounding parts or keeping intake charge cooler might be valid reasons.

Changes to air speed and speed of sound and tuned length and scavenging effect and camshaft overlap and back pressure, ambient conditions, engine speed etc etc etc all are variables to be considered.

Regards
Pat
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[AviatorJim]

We (low budget race cars) do it to hide rusty headers and keep all that heat away from the firewall, fuel lines, brake lines, and air intake. I have never heard anyone credit keeping the exhaust warm to a performance improvement. There is always a performance improvement when you can reduce the under hood temp by any means.

 

[evelrod]

A star for the budget racers of the world. I wrap our steel tubing headers to keep the under hood heat manageable. Agreed that performance is enhanced by simply keeping the engine area heat to a reasonable range. Engine setup is done from the "get-go" with wrapped headers.

However...I have insulated a couple of turbo installations that saw a distinct performance increase. These were for street applications and not race specific. The insulation typically lasted several years before needing touchup/replacement. Race specific applications have no such limitations. Race headers don't normally last more than a few seasons due to other problems...crash...demounting/mounting damage, etc.

Rod