Claudius

My turbo has got some heat spots (orange / rusty) on the exhaust housing which I suspect are due to a too restrictive exhaust. I have a MY 97 with standard downpipe (huge cat! ) and a small diamter aftermarket thing that was on it. Running 1.1 bars held on the TD04, running very rich.

I have a DP and Blitz muffler on order, as well as a better turbo and ECU, but I am curious what caused this. Too much backpressure? Any other ideas or experiences?

Pavlo

orange/rusty deposits on exhaust housing also sound the like the effects of running with some octane boosters, or some Super Unleaded (97ron and above) petrol.

I wouldn't be too concerned.

Paul

Claudius

I'm running on 98 RON SUL. That's what's recommended, right? Why would that cause this?

Pavlo

Ask Mycroft I think it's Manganese in the petrol.

I suspect the plugs will be a similar colour.

People running optimax have seen similar results.

Paul

Turbo_Steve

My entire exhaust housing looks like a massive lump of rust, with white deposits and black and a few pinky looking ones on it too.

Still works great, though

Mycroft

OK, so it has little to do with the fuel composition.
.. there will be no difference what ever Petrol you used.
.. the austenitic phase is where materials slightly adjust their boundaries within the alloy.
.. now although cast iron would appear to be a single material it is an alloy it has many 'ingredients'.
.. the best way to see the phase chase is as a sort of 'shuffle about' of the boundaries of the alloy.
.. what can happen is that some of the alloys coalesce to a single point, this might mean that in that area they are less able to cope with heat so well and may absorb more of the heat and glow brighter.
.. it can lead to fatigue but although it seems like a failure is imminent, only if you have a seriously large hot spot will it happen.
.. location is also a factor as well as the size, sadly the coalescing won't [usually] disperse.
.. cast iron, although a very basic metal, has some great properties, one is that as it heats it is tolerant of isolated hot spots unlike when it is below 350C when such differences will shatter it.
.. If it were my car I would get too stressed and would only worry if the Ex. temp. went over 1000 as this would start the next phase change process which has a different type of effect.
.. this can cause rupture as this change involves rapid micro-contractions of the material that would find the 'weakest' point and the thing would fail.

[Edited by Mycroft - 5/12/2003 1:17:35 AM]

Claudius

Thanks for that, that's reassuring ... I guess? LOL

I ma not measuring the EGT, but my tuner measured the air fuel ratios at 10.5:1 to 9.5:1 over the rev range. That sounds pretty rich even for a standard ECU if you ask me. I would guess EGT stays below say 850°C with 1.1 bars held boost on the standard TD04? Or could a restrictive downpipe (standard item with big cat) make things much hotter? My turbo holds 1.1 bars all the way to the limiter, when the standard setup would only see about 0.5-0.6 at that rpm...

Mycroft

###
I am not measuring the EGT, but my tuner measured the air fuel ratios at 10.5:1 to 9.5:1 over the rev range.
###
You should invest in a Temp gauge or swap an existing one into a combined unit.

###
...could a restrictive downpipe (standard item with big cat) make things much hotter?
###
Yes, if you go to the MLR there is a whole thread [huge] where Andy and I discuss this and I posted the Registerplate/flue length thing... follow that and use in its stead a high flow cat [if you HAVE to have one] in that location.

Claudius

Thanks for confirming my thoughts

As I said in my first post, I am junking the hairdryer and putting in a high volume 18 g IHI turbine kit. I'll be using a free flow 70mm downpipe with that as well as a Blitz NUR spec muffler (3 inches internal diameter). I was just wondering if the downpipe and related backpressure could actually increase EGT and cause heat sports like this even though the car is runnning rich. Thanks for the clarification ... I remember the thread now; probably why I asked in the first place. You have to see that standard downpipe: the cat is the size of a kitchen paper roll! LOL

Pavlo

exhaust housing is more like a high nickel steel alloy rather than a plain cast iron.

Mycroft,

Austentite in cast iron only over 700'C, and even then it's likely to be mixed with cementite.

I think the best way to see a phase change is as a change in phase, but I guess that's just me.

Paul

Pavlo

well done

cementite is Iron Carbibe basically, formed from the excess carbon that isn't solluble in the iron. You won't be reaching the ferrite phase with so much carbon, and 700'C is just the starting point of austentite formation, you need over 1100'C to get pure austentite with the amount of carbon present in cast iron. With a mix of aust and cememtite in between. but it doesn't really matter.

either way, the turbo gets fecking hot in some places and it makes it more prone to corrosion

Paul

Mycroft

Pavlo, the orange spots occur on surfaces that have been 'touched' by the exhaust gases... these sound like changes to the metal through its Austenitic phase...

So we need to clear that up first...

Claudius, are these spots visible on the outside of components or did you have to dismantle the whole thing?

[Edited by Mycroft - 5/11/2003 11:59:54 PM]

Claudius

Thanks for that, Iain

I saw it simply looking at the turbo with the bonnet open. In fact, a friend of mine spotted it while I was scraping some insulation material from the boot. Cheeky Jerome wanted to see if I did any modifications given that I go faster than him with his Impreza and opened the bonnet... Anyway, it's on the outside, yes. Is that bad? I would guess it is?

Oh, and what does Austenitic phase mean?

[Edited by Claudius - 5/12/2003 12:27:19 AM]

Mycroft

From memory.
.. so exact temps may be off a little.

''Austenite phase'' in carbon steel at high temperature. [900C?]

''Ferrite phase'' almost zero carbon phase.

Cementite is not a phase it is a compound formed from the phase change of ferrite and austenite, Cementite is a very hard material consisting of high levels carbon.

Cementite is very hard, but when mixed in this coalesced form the overall structural strength is less than the plain Cast.

Slow cooling produces that whitish stuff mentioned earlier by another contributor here the white stuff is Perlite, it is a soft material.





[Edited by Mycroft - 5/12/2003 12:55:02 PM]