Discuss
Discuss
Heat, melting and weight are the main detractors to make a cast alloy or tube, you need very thick alloy have a look at old air cooled motorbikes early ones have alloy end cans which can be lite weight and thin as the heat is desipated from the the down pipe and flange before it gets to the back of the bike.
I had a still chainsaw that had an alloy exhaust and was forever blowing holes and fracturing it was finaly replaced by an all steel exhaust.
So to make a tuff duriable zorst manifold it has to be quite thick which would make it heavy, it would also need machineing two detractions, costly against the cost of cheap thin wall steel pipe so not a good idea.
I'd guess it's cost? They're popular amongst boat tuners i think.
you'd never make a set of headers strong enough if they were made of aluminium. It's not uncommon for exhaust gases leaving the cyclinder head to be over 800 degrees C, with aluminium melting somewhere around 550-650 degrees C, I think it's obvious why you wouldn't want to use it ;)
Further down the exhaust things might be cool enough, but then you still have to worry about strength. With aluminium's typically poor fatigue strength, if you had a heavy muffler attached to a vibrating aluminium tube, it would only be a matter of time before it failed. IMO you'd be much better off pushing the boundaries of wall thickness with a steel exhaust - you could easily get away with 0.9mm wall thickness, which would weigh less than a 3mm wall thickness aluminium tube, and would be a lot cheaper and easier to fabricate.
At the manifold end, heat is a big issue, the EGTs are often above melting point for the aluminium. Further down the system, you can make silencer end plates out of it, but not anything structural. Aluminium does not do so well with heat cycling at above 170 deg C and fatigue can be an issue too.
Chris
inconel is the way to go if youve got lots of money. or as has been said, just push the boundaries with thin steel.
I'd say it was largely (but not completely) down to melting points. Iron melts at around 1500 degrees C and aluminium melts at a positively chilly 660 degrees C, which means that with the EGTs of a petrol engine an aluminium exhaust manifold would melt. But there may be fuels that produce much lower EGTs, where aluminium would be more useable.
There are engines using aluminium pistons out there though. It's all in the cooling and heat transfer, which is easier to achieve in-engine than on an external component.
Another possible reason would be the reactivity. Aluminium is dangerous as hell at times (which is why you do NOT want to have aluminium dust collected around a grinding wheel in a workshop for example), and hot aluminium can have some interesting reactions with things like steel and iron.
I had some 'temporary' aluminium hangers on the end section of my mk1's Janspeed exhaust for a good year or so.
I think the only reason they held up were because they were not directly subjected to an awful lot of heat.
(When I say hangers, I mean a selection of random aluminium sheeting and bolts that were the closest thing to hand when the exhaust started to fall off!)
If you want lightweight, Titanium is the only way to go, though with Titanium thickness is often a factor, as is cost.
Inconel provides the ultimate in durability and corrosion resistance, but it's pricey, and heavy. Typically around 8g/cm^3 (on a par with cast Iron).
Fantastic stuff for turbo manifolds and hangers which support weight whilst subjected to extreme heat (e.g. Turbo hanger to take lweight off manifold) but as Titanium is cheaper and lighter than Inconel-type alloys, I'd still go with a Titanium exhaust to go with the Inconel manifold.
Titanium is often obscenely priced when you consider the actual cost of raw titanium though.
In the coming months I may start investigating Nimonic and/or Inconel parts, as I may be able to purchase some tubing at near enough intrinsic cost (anywhere between £10 and £25 per kilo depending on exact alloy).
You're not joking there. The raw price of titanium is only about £10 per kilo at the moment (and you can get some nice sized chunks), or if you're buying sheet you can buy sufficient to flat-bottom a mk1 for around the £400 mark. Not sure what tools would be able to cut it though :icon_lol:Quote:
Originally Posted by Frogger
Inconel is heavy, but you can make it a lot thinner, so the end result is significantly lighter than steel. its is expensive though.Quote:
Originally Posted by Frogger