Any value in keeping exhaust pipe hotter?

[spirilis]

I've had a mod floating around in my head for a while, and a rather minor experiment (on my maxi-scooter--a 2006 Yamaha Majesty) to report.

The question is whether it is worth adding some kind of header wrap or metal heat shield (even an aluminum foil wrap might work, as a low-emissivity arrester to slow/reduce radiant heat loss) to the exhaust system AFTER the catalytic converter.

I say AFTER the catalyst because while it might help wrapping the manifold and pipes before the catalyst, it's probably not a good idea to do anything that could run the catalyst hotter than it was designed for (risk of overheating).

On thinking about this, what came to mind is that if the pipe is hotter, the exhaust gasses may stay hotter further downstream from the engine and with hotter gasses come lower density, thus while the engine is still pushing the same amount of exhaust in both the control instance and the pipe-wrapped instance, the gas should move faster through the pipe-wrapped example thus reducing backpressure from cooling/condensing exhaust (I think?)

Any thoughts? Crazy idea or worth experimenting? (Or has it been done before...?)

Actually before closing I should mention the anecdote I do have--my Majesty scooter, I wrapped some foil around the exhaust pipe from the head to the furthest section I could get without fiddling with the pipe's mounting hardware, and while it's all VERY subjective I could swear I noticed more "punch" from the engine after it was warmed up. That could very well *all* be in my head though, and I wasn't keeping good fuel economy records with that thing before & after the experiment.

[old jupiter]

Reducing the heat-loss and keeping more energy in the exhaust gases is generally said to improve scavenging (gets the exhaust out of the pipe better) and maybe reduce back-pressure (reduces dilution of intake charge, even may assist cylinder filling for more volumetric efficiency). In a closed engine compartment, wrapping the exhaust can mean the intake manifold and therefore the intake charge aren't heated as much, which is likely to help power, and could be either bad, good, or indifferent for fuel economy.

I'm merely guessing now, but I doubt that wrapping a header pipe from engine to cat will cause the cat to overheat, and it should cause the cat to light sooner, which is good for emissions. Someone else here might know this.

[slowmover]

I think of heat and pressure as being roughly equivalent in re exhaust systems. A Corvette with a typical small block Chevy will benefit from a larger diameter dual exhaust system where a long pickup truck with the same motor will be just fine with a single decent diameter single, all due to length.

Have seen Japanese cars with outer shell exhaust pipes: inner pipe runs exhaust, outer shell insulates. Problems with rust.

There are formulas out there about this subject, generally for racing, but it's been years since I read them. All I recall for purposes of economy (reason I read them) is that there is a fairly quick decrease in benefit past a certain point (depends on vehicle, use, etc).

SWAIN TECH and others will coat pieces to help retain heat (expensive; not just cosmetic), and there are companies that specialize in turbocharger "blankets" and the like (also not cheap).

Might be worthwhile to make marks on the current exhaust system and take a series of temp readings under normal conditions. Records. See if some of that info above can be found. David Vizard would be my first guess (a gifted writer on autp race tech subjects).

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