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Exhaust wrap

Discussion in 'General Discussion' started by Chuckmeister, Mar 18, 2020.

  1. Chuckmeister

    Chuckmeister Member

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    I've recently had some engine work done on my car and noticed that my bonnet is getting freakin' hot since. I've been considering wrapping my headers but I'm unsure of products on the market and what is best.
    Some people say that heat wrapping will destroy the header...others say that it works...
    Please if you have personal opinion on this then please share !! Wondering if anything works?
     
  2. mechanic

    mechanic Active Member

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    It does both.
    It keeps the heat in the exhaust, just like an esky keeps the cold inside the box, but... in certain circumstances it can cause the metal in the manifold to reach the limits of its design parameters, or super-heat. This will lead to metal fatigue.

    If you race regularly and the extra 1-2kw gained from the slight exhaust scavenging advantages makes the difference between winning and losing, and can afford to replace the manifold regularly... go for it.

    For the average street car it will do nothing for performance and keep your engine bay a bit cooler.
     
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  3. Skylarking

    Skylarking Well-Known Member

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    We've all got personal opinions but not always based on personal experience :p

    I’ve heard that wrapping exhausts can cause rust if they get wet and the car isn’t drive for a while. I’ve also read that ceramic coatings can reduce engine bay heat as well... no experience with either so I’ll go back to my corner.
     
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  4. Skylarking

    Skylarking Well-Known Member

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    Energy always flows from hot to cold while insulation meanly slows the rate of travel. Exhaust wrapping or Esky foam is simply insulation and as such slows the heat transfer (from inside exhaust to engine bay or from outside esky to your cold drinks).

    So wrapping an exhaust may help during short runs but drive for an hour and it will do nothing to reduce under bonnet temps. Managing under bonnet airflow and ensuring the bonnet insulator is in place may yield better results ;)
     
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  5. mechanic

    mechanic Active Member

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    There are three ways that heat transfers - convection, conduction and radiation

    Convection is the heat that rises from a hot object. Think steam from a pot, smoke from a cigarette etc
    Conduction is the heat from direct contact... hand on hot iron etc
    Radiation is the heat projected in all directions, as heat has similar properties to visible light on the electromagnetic spectrum.

    Yes, wrap would slow the rate of heat that comes from the metal manifold. In a contained environment the heat would slowly build up until a temperature equilibrium was reached averaging all contact surfaces of that contained environment.

    An engine bay isn't a contained environment though - it has air flow. Think of two scenarios.
    1. unwrapped metal exhaust with surface temp of 500 deg. Convection means that 500 deg heat is rising into the engine bay (if no surrounding airflow). 500 deg of heat is conducting to the film of air adjacent to the manifold, 500deg of heat is radiating into all other components, which also convects, conducts and radiates that heat back into the next object... and so on.

    2. wrapped exhaust with wrap surface temp of 150deg. Actual metal exhaust temp might be 700deg, but is not exposed.
    Convection, conduction and radiation of the specific heat is only 150 deg.

    Introduce airflow and most of the convected and conducted heat is blown away. Radiant heat becomes the only heat affecting surrounding engine bay components, and only at 150deg.

    I'm not an engineer, so happy to be corrected.
     
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  6. Skylarking

    Skylarking Well-Known Member

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    ^ the primary reason for posting was you said "esky keeps the cold inside" but it's more correct to say an esky keeps the hot outside ;) The rest was just supportive the the thread :p

    Insulation slows heat movement whether it be conductive, convective or radiated... its the nature of an insulator. And any heat transfer is proportional to surface area, heat coefficient and temperature gradient. I haven't played with such math for hat sems eyons :rolleyes:

    Normally, headers aren't insulated and engine bay heat is indeed carried away by airflow, which is why the manufacturer installs heat shields, those aluminium laminated things that often protect brake lines and other components radiated heat :cool: Such shields are indeed helpful as i've had turbo exhaust headers glowing red hot after a mountain run :eek: Suffice to say a cool down drive was needed o_O
     
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  7. VS 5.0

    VS 5.0 Well-Known Member

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    Venting ?

    Low lying reverse cowl ?
     
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  8. mechanic

    mechanic Active Member

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    Ahh, yes... now I understand. You are absolutely right.
     
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  9. 07GTS

    07GTS Well-Known Member

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  10. gtrboyy

    gtrboyy Well-Known Member

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    Exhaust wrap works really well but yeah it will eventually send the pipes brittle/rust although it will take longer than people think...had a toasty engine bay in lc gtr with wild 202,extractors were tad old & restrictive anyway so tried heatwrap....5-6 years later when took them out decent whack with spanner put hole in pipe lol.

    If pipes coming off & plan to keep car may as well get them coated.

    Was told if still plan on wrapping pipes like on turbo or cars with supertight engine bay best get them coated first then wrap for better longetivity.
     
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  11. Chuckmeister

    Chuckmeister Member

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    Thanks for the feedback guys. I have a basically new pacemaker 1 7/8 headers and I'm pretty sure they are ceramic coated but really a coating is not going to solve the heat being radiated atm. Engine is running at temp...nothing abnormal so I guess its the price I have to pay for adding a couple of extra HP.

    I think I might start with adjusting the back of my bonnet up...like a 1/4, 1/2cm or so. Hopefully it will blow a little more heat out.
     
  12. Skylarking

    Skylarking Well-Known Member

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    Thats very old school but is is appropriate on a modern car which has carefully controlled engine bay airflow considering all the seals around the bonnet, radiator and such? May cause more problems than it solves...

    Have you checked the tune isn’t running too rich which must increase exhaust gas temps if it is... An exhaust gas temp sensor could be an interesting upgrade and may be quite revealing... as would a wideband O2 sensor which should give you an idea how well the tune is working... and fuel consumption numbers also say lots :p
     
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  13. Chuckmeister

    Chuckmeister Member

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    I think your right on the money there. I just took my car down to the mechanics this morning for a fine tune on the dyno. It was definately running rich. I could smell it when I pulled the car up and the exhaust was definately scorching hot.
    I just got my car back just then and have immediately noticed the difference. Its sort of driving like a different car to be honest. Seems alot smoother and the bonnet heat which was like a frying pan yesterday is almost non existent today (though it was a short run)
     
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  14. mechanic

    mechanic Active Member

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    Don't do this.
    The cabin air intake draws air from the scuttle panel (just below the windscreen). If you lift the rear of the bonnet and have an exhaust leak in the engine you risk sending carbon monoxide into the cabin (really bad).

    Most cars have a positive pressure in this scuttle area as the air flows over this scuttle panel at speed. This helps to force air into the cabin when the vents are open. Lifting the rear of the bonnet may allow this pressure to force air back into the under-bonnet area - the reverse of what you'd expect.

    In any case the aerodynamics of the car will change unpredictably... possibly with dangerous results.
     
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