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Heat management

Discussion in '348/355' started by Targatime, Dec 6, 2020.

  1. The Outcast

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    And it's the radiant heat that's causing problems with the soft bits, not the convective heat. Simple baffles are all you need.
     
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  3. 355rockit

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    I used oem headers that were ceramic coated in and out. I also added a larger heat shield barrier between the headers and boots. It helped allot. I have a few threads on this from around 2012.
     
  4. Targatime

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    I agree that radiant heat is the biggest problem for a handful of specific components, but I suspect that convective heat is also a serious problem that contributes to the degradation of wiring, connectors, rubber, plastics, etc, much more quickly than necessary. My DD is a 993 with 8X more miles on it than a typical 355, and the engine compartment wiring, vacuum solenoids, rubber boots, etc, are all soft and pliable and in nice shape and have not needed to be replaced. This is unthinkable on a 355, and the reason is heat.
     
  5. INTMD8

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    ^ Interesting. I needed to replace a few vacuum lines on my 993. Fan boot was petrified and blown apart. Maybe mine was just a heap :D
     
  6. Targatime

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    Naw, that's a standard problem. Happened to me as well. It weakens a little with age and then blows out from the air pressure the intake fan creates under the shroud.
     
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  8. The Outcast

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    #31 The Outcast, Dec 18, 2020
    Last edited: Dec 18, 2020
    If you look at the 355's engine bay from the bottom up, keeping in mind that un-forced convective heat rises (doesn't fall), and cut an imaginary horizontal plane through the engine at the level of the manifold, you can easily see that, under the plane you cut through the manifolds, there's nothing like the kind of heat that comes from a manifold being generated from anything underneath that plane. Yet the inboard CV boots, which are outside the convection column and just inboard of the manifold, suffer some of the worst outcomes from heat.

    Why is that? It's because radiant heat projects in 360 degrees from its source, which makes heat transfer from the manifolds due to radiation able to affect adjacent components in any direction. In the automotive industry, we use what's called a "view factor" assessment in some cases to determine which heat shields to deploy in the vehicle architecture. In many cases, heat shields that are deployed are placed vertically UNDER heat producing components in the case that any components below those could be affected by the radiant heat; if we were dealing with convective heat transfer only in these cases, we wouldn't need a shield at all because the convective heat is rising, not falling.

    And, radiant heat can "bounce", with very high efficiency, to a surface from any component that simply reflects the radiant energy, which includes any of the surfaces of the aluminum castings, which are fairly good reflectors. This heat transfer is instantaneous, unlike convective or conductive heat transfer. This is why the CV boot heat shields that were produced for awhile by "Gothspeed" (another member here) were probably the best protection my colleagues and myself looked at for the 355; they surrounded the entire boot, not just the top...the reflection of radiant energy off of the gearbox UNDER the boot also becomes a significant radiant heat input to the boot.

    You can see this yourself; using a small tape measure with a fairly stiff tape or maybe a telescoping pointer or magnet, construct a "vector" between a component of concern and the heat source in question. At any point of the affected component, verify whether or not it can "see" the radiant heat source (the manifold). If it can, even a relatively small amount, that component will accumulate a disproportionate amount of the radiant heat rejected by the heat source, and that could potentially elevate that component to a temperature outside its design range.

    No heat source on the 355 produces the kind of radiant heat that the manifolds produce; not even close, and once the manifolds are warmed up to operating temperature the convection column is fast-moving and relatively compact; I can definitely see a bit of potential for this to affect spark plug wires, coils, secondary air, oil tank, intake plenums and engine cover, but that's about it, and it would be a fraction of the energy input which is at play from radiation.

    Considering the 355's mid-engine architecture, and the heat management tools available to the design team at the time of its development, convective heat is extremely well-managed. Radiant heat is the car's enemy in this case, and it's likely that the developers of the 355 would have done a whole lot better if there were more money, time, and technology available to attack the problem.. Better design and modeling tools are available now which can make this easier when a low-volume vehicle is being developed.

    One of these days I'll dot my engine bay with an array of thermocouples, sniffers, and radiant targets, so that we can build a proper 3D heat map; that would be the best way to promote a better understanding of this. Now all I need is some time...
     
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  9. SoCal1

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    Heat makes HP :)
     
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  10. Targatime

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    @TheOutcast -- thanks, fascinating. One question, you don't think the cats produce as much radiant heat as the manifolds? They're in a less destructive place in the engine compartment but I would think they run at similar temps.
     
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  11. DaisyCutter

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    The 993 engine is nowhere near as high performing as the F355. 3.6litres producing 272-285hp vs a 3.5litre pumping out 375hp. More power, more heat.
     
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  13. Targatime

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    True, but mine is a 3.8 pumping out 350hp ;)

    But hp per liter doesn't account for the 355's heat problems, what accounts for it is the exhaust being entirely contained inside the engine compartment. A 911's exhaust manifolds sit under the engine where they are cooled in three ways: a little by natural airflow under the car, a little by the air being forced down through the cooling fins of the heads and cylinders, and a lot by the air pumped by the intake fan around the headers inside the surrounding tin. Not only is a 993 exhaust not inside the engine compartment, but the manifolds themselves are actively cooled.
     
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  14. The Outcast

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    Sorry...I just plain forgot about the Cats! Yes...these are equally radiant, and you can use your "view factor" analysis to determine if they're an issue, and pull a countermeasure together.
     
  15. The Outcast

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    #37 The Outcast, Dec 20, 2020
    Last edited: Dec 20, 2020
    Some of it, anyway; about 20% of the energy potential in a gallon of fuel will make usable HP. The other 80% is mostly unrecoverable heat and ends up heating our little sphere of rock, water, and gases.

    Is what it is. Uncle Carnot laid it out long before any of us came along.
     
  16. Targatime

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  17. The Outcast

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  18. The Outcast

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    Here's something I saw this morning. Alternator- and Compressor-specific radiant heat shields. I am not affiliated with this; looks interesting:
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  19. jjtjr

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    Nice post. One question I have is; if one were to completely wrap the headers and cats in a thermal blanket in an attempt to mitigate the radiant heat from escaping into the engine compartment, does that help or hurt the headers and cats? In other words, will the temperature of the headers increase due to the loss of "cooling" or will it remain the same? I, like many of us 355 owners would like to reduce engine compartment temps but also want to be cautious in my approach. Thanks.
     
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  20. Targatime

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    I would think the exhaust system temps will increase between "slightly" and "somewhat" with heat blankets...this could be a problem with factory equipment, which is already fragile. But doubt it would be a problem with quality aftermarket stuff. But I'm not an expert and would be curious what others think.
     
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  21. The Outcast

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    #43 The Outcast, Dec 21, 2020
    Last edited: Dec 21, 2020
    That's a good question; and I was thinking about it when I looked at the blanket which you presented, but then I got on to other things.

    You're correct...if the radiant shield is combined with a blanket, which is in contact with the component rejecting the heat, the component could overheat. The trick here is to mitigate only the radiant heat, while allowing convection cooling of the component to still take place. Best way to block radiant heat is to space the shield (rigid materials are best) away from the component so that convection can still take place within the gap between component and shield.

    I wonder if a similar radiant shield to the one you presented, without the insulation, might be a better fit for Cats...this would allow you to shape it, and then offset it (maybe 20mm to 40mm) away from contact with the surface...maybe a couple of holes placed to avoid radiant spill to any precious components. Or, if you're going to use the material which you presented, just make sure to leave 20mm-40mm space between the shield and the Cat.

    As far as header "blankets" are concerned, I do think that use of blankets will raise the average operating temperature of the headers; whether or not they'd be damaged would be related to the metallurgy of the headers and the durability of the headers in general (welds, thickness of tubes, tube material, etc.) as you imply.

    Looks like I better do that 3D heat map sooner than later...
     
  22. The Outcast

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    As far as the headers themselves are concerned, I think this is a valuable insight; better quality headers (tube thickness, material, welds, etc.) would probably perform better than cheap ones. With regard to the Cats, higher internal temperatures might cause internal damage. I'm not a catalyst expert, but I will talk to some colleagues who are, and come back with what I find.
     
  23. The Outcast

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    Sorry jjtjr...between the "liking" and "quoting" and "reply" functions, I got all messed up and didn't specifically respond to you!

    I posted this as a response to you, but did not "quote" or "reply"...sorry about that!

    You're correct...if the radiant shield is combined with a blanket, which is in contact with the component rejecting the heat, the component could overheat. The trick here is to mitigate only the radiant heat, while allowing convection cooling of the component to still take place. Best way to block radiant heat is to space the shield (rigid materials are best) away from the component so that convection can still take place within the gap between component and shield.
     
  24. Culprit

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    Oh perfect I think you're describing exactly what I was looking at! These are from Caterham. Should do a great job of cutting down radiant heat without having to worry about anything. They look super cool too.

    https://caterhamparts.co.uk/205-collectors-catalysts

    I ordered a couple of these (the polished ones as they'll be more effective), will report out when I get them in.

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  25. The Outcast

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    That looks nice!
     
  26. Targatime

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    I'd be really curious to hear any professional guidance on using heat blankets on cats. I haven't found any reliable info on this in several google searches. I could imagine them being a no-go because they would cause the cats to operate in excess of their proper temp range (whereas on a well-built header it doesn't matter if you run a little hotter). I could also imagine the cooling the cats experience on a 355, where they're enclosed in the engine bay, is insignificant, and so wrapping them wouldn't cause any problems.
     
  27. Culprit

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    I finally got to installing the parts.. as usual an hour job turned into a weekend o_O

    Had to run to the hardware store a couple times to create some stand offs to get some good clearance. I mucked up a couple hose clamps so I just installed the driver side for now, gave me a good chance to compare both sides.

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    Did some temp measurements after a run:
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    The cats are right below the airboxes and putting the shields here looks like it's good for about 20° F reduction in that area. I think the setup is successful and looks cool. This should be good for scoring an extra few HP and protecting the cat ecu etc. that are back there. Will measure again once I get the other side sorted.
     
  28. Culprit

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    Capristo offers heat blankets with their cats and they seem to make well engineered products. Not every cat core is the same of course and some might deteriorate faster if the whole thing is wrapped. Personally I prefer the shielding method as heat can still escape, and fiberglass products tend to degrade and usually don't look as clean/OEM-like as a shield.
     
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