Exhaust Wrap

[Stevie S 49]

I am wanting to wrap my 335i aftermarket downpipes. Not for performance but to see if it will help reduce some rattle they amplify over stock pipes. Googling gives me a lot of conflicting advice, lots of people say the wrap will hold moisture and corrode the metal. Surely as a daily driver any moisture will be vapourised?

[handgrenade 189]

What is rattling??

[Stevie S 49]

What is rattling??

Wastegate /actuator arm

[handgrenade 189]

are you able to secure the wastegate actuator arm .. some sort of bushing or a hard mount from the vacuum actuator to the wastegate flap?

Seems to be logical to address that first if the noise you're trying to remedy is the rattle... no rattle = no amplification of rattle from aftermarket pipes.

[Stevie S 49]

are you able to secure the wastegate actuator arm .. some sort of bushing or a hard mount from the vacuum actuator to the wastegate flap?

Seems to be logical to address that first if the noise you're trying to remedy is the rattle... no rattle = no amplification of rattle from aftermarket pipes.

Agree , but easier said than done. The usual bmw mechanics can't do it.

[handgrenade 189]

hrmm looks like the front turbo would be an issue due to accessibility. I'm sure it can be done, but not without dropping both manifolds and associated plumbing.

Seems to be a common problem .. google has lots of info.

[Stevie S 49]

hrmm looks like the front turbo would be an issue due to accessibility. I'm sure it can be done, but not without dropping both manifolds and associated plumbing.

Seems to be a common problem .. google has lots of info.

That it does, I have read nearly all of them. I have replaced both turbos to remedy with no success.

[handgrenade 189]

has anyone in NZ modified the factory turbos to run an external gate (or two?) haha!

But back on your original topic, what material are your aftermarket dump pipes made from? also what grade? There are plenty of ways to prevent corrosion .. one which I don't see being used as often is using the galvanic table and dissimilar metals approach (by adding to the assembly a sacrificial metal lower on the galvanic scale which will be the anode which will corrode and save the cathode - your downpipes).

[Stevie S 49]

has anyone in NZ modified the factory turbos to run an external gate (or two?) haha!

But back on your original topic, what material are your aftermarket dump pipes made from? also what grade? There are plenty of ways to prevent corrosion .. one which I don't see being used as often is using the galvanic table and dissimilar metals approach (by adding to the assembly a sacrificial metal lower on the galvanic scale which will be the anode which will corrode and save the cathode - your downpipes).

304 Stainless

[handgrenade 189]

Is it bent pipe or cut and welded sections?

304 should be fine unless your wrap is very abrasive. If you are really worried bolt on a sacrificial zinc plate.

If you have cut and weld sections then did they use 316? Also look at your flanges and joints .. Make sure you've used plated bolts etc.

[E30 325i Rag-Top 2915]

I bought a heap of wrap to do the stainless headers on my race car, and was warned away from using it as it can make the headers very brittle due to the heat build up under the wrap.

Still in the courier envelope sitting under the bench, doh.

[handgrenade 189]

Stainless 304 is fine for standard exhaust manifold (na) applications .. Which can be up to 750c but not sure how hot it would get with wrap. 321 grade is used for higher temp uses such as turbo manifolds. However will your car really be seeing such high sustained temps?

[jake1829 119]

Stainless 304 is fine for standard exhaust manifold (na) applications .. Which can be up to 750c but not sure how hot it would get with wrap. 321 grade is used for higher temp uses such as turbo manifolds. However will your car really be seeing such high sustained temps?

I think Steve has the same downpipes as me, AR ones?

I got the ceramic coating on mine : http://www.ardesign.info/store/ar-design-bmw-135i-335i-3-hi-flo-catless-downpipes.html

[handgrenade 189]

downpipes don't get AS hot as pre turbo manifolds. The point of the turbo is to convert heat energy into kinetic energy, so if you have an efficient turbo exhaust side, then a lot of the heat/energy should have been converted to kinetic energy to spool the turbo.

that being said, once the desired boost pressure has been reached, the wastegates open and you get a tonne of very hot, highly energetic gases released into the downpipes. I suppose the temperatures you'd see in the downpipes would depend on how much time spent at full boost and gates open.

race cars will build up a lot of heat being on boost (likely full boost for extended periods of time), but street cars are likely just to see spikes in temperature which the mass of the pipes and rest of the system will be able to absorb and distribute before reaching any sort of critical temp... even if you wrap your downpipes, most/all metals are fairly good conductors of heat and the heat will dissipate into the rest of the exhaust system which would have a lot of air running past to cool it.

tbh, heat wrapping is only performance productive on pre-turbo pipes. everything else after .. doesnt matter. the only reason to wrap pipes after the turbo is if post-turbo pipes are able to conduct heat back into the engine bay etc. proper aero development should help find the low pressure zones and high pressure zones etc and set it up to bring cold air in to the ehxaust side manifold area and out the bottom of the car.

[jake1829 119]

downpipes don't get AS hot as pre turbo manifolds. The point of the turbo is to convert heat energy into kinetic energy, so if you have an efficient turbo exhaust side, then a lot of the heat/energy should have been converted to kinetic energy to spool the turbo.

that being said, once the desired boost pressure has been reached, the wastegates open and you get a tonne of very hot, highly energetic gases released into the downpipes. I suppose the temperatures you'd see in the downpipes would depend on how much time spent at full boost and gates open.

race cars will build up a lot of heat being on boost (likely full boost for extended periods of time), but street cars are likely just to see spikes in temperature which the mass of the pipes and rest of the system will be able to absorb and distribute before reaching any sort of critical temp... even if you wrap your downpipes, most/all metals are fairly good conductors of heat and the heat will dissipate into the rest of the exhaust system which would have a lot of air running past to cool it.

tbh, heat wrapping is only performance productive on pre-turbo pipes. everything else after .. doesnt matter. the only reason to wrap pipes after the turbo is if post-turbo pipes are able to conduct heat back into the engine bay etc. proper aero development should help find the low pressure zones and high pressure zones etc and set it up to bring cold air in to the ehxaust side manifold area and out the bottom of the car.

Sounds logical , guess won't be much of a issue for a street car, but if you take the car on the track that's a different story....

those down pipes look pretty hot!

[E30 325i Rag-Top 2915]

From what I understand (please bear in mind this is what I have taken from others) is that it is also the heat cycling created within the wrap, the rate of heating and cooling of the manifold is greatly affected, not just the overall temperature it gets to in between times. This then acts as a form of "heat treating" of the manifold changing the physical properties of the stainless steel.

Yes, it is a very good idea on a turbo application, especially where there is limited space under the bonnet, to keep the engine bay temps down and possibly improve the performance of the turbo-charging. My main reason for thinking of wrapping was to keep the exhaust heat away from the steering column, as it passes right through the middle of the headers.

[handgrenade 189]

on a street car application (or even a moderately run track car) the material properties of a decent AISI 304 stainless should not come into play.

You would have to be running a dedicated race car, high out put, constant high heat cycling etc before you really have to even cast a thought towards stainless 304.

all that being said is under the proviso that the parts you are using are of the proper good material grade, and well made components, especially when it comes to welds and joints (hoping that any welded joints are done with 314) and also dependent on whether low or high content stainless 304 (L/H) is used.

Fatigue is something which comes into mind, but cyclic loading (fatigue) is measured with very high numbers - hundreds of thousands to hundreds of million cycles. when it comes to exhausts .. you're looking at relatively low loads, but potentially high cycles due to vibration being counted as a cycle. even still, it is something which is to be only worried about if you have sub par welds and joints (those would be your critical failure points first).

Heat treatment or changes to the material strength and properties due to thermal cyclic loads are also mentioned by some .. but with 304 being an austenitic steel, it can not be hardened by heat treatment. Now there is more to be said about martensitic transformation of austenitic steel but that requires high temperature rapid quenching which you probably wont see unless you decide to drive your race car after 500 laps into a lake.

Now corrosion of 304 .. at sustained temperatures of 850c-950c, for 50+ hours, you're looking at oxidation rates in the ball park of 0.2mg^2/cm^2.

Not many people are going to be running those temps for those times.

The transition colours (glowing red) of stainless and other metals during heat are well below the temperatures required to induce any sort of material property change. You would have to hit a much higher spectrum into the high 10,000k range before your metal is in a stage where its physical properties become unstable.

excuse me if I got any of this wrong.. been a while since I looked into this area lol

[handgrenade 189]

errr .. for those TLDR

if its good quality, wrap the damn thing. if you're wrapping welds, make sure the welds are good and done right (full penetration, proper materials) and just watch out for your critical failure points, flanges, welds etc. periodic checks once in a while is a good idea.