Jarle
If your exhaust system is going to be the same length as your motorcycle exhaust system then you won't have a problem. Since this is not likely, you WILL have a problem. The reason is that the gases in the motorcyle system do not reach a suffiently low
dew point temperature before they pass out of the system. Therefore it's not an issue as you say.
In a car, the system is much longer and dewpoint is reached around the rear axle area typically. This is why traditionally the rear muffler usually corrodes out before the front one due to the acid attack being more aggressive there.
The reason why manufacturers chose to lag exhaust systems by cladding them is to maintain as high a temp as possible not only for the Cat. to work properly but to minimise the effect of dew point from temperature drops along the length of the system.
I would recommend you finish your excellent project with stainless instead of aluminium because the weight difference will be neglible but the you will have a lifetime system for your efforts.
At the end of the day, the value of this site is to teach each other what we know to spread the knowledge. What you do with that information, is up to you.
Hey Mr Zee
There are many things you can do with a goilet- just ask JK.
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