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2004 Forester XT Premium 4EAT
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Discussion Starter #1
So I recently ripped out the insulation from the underside of Subed's hood after having had issues with waterlogging all winter long. I found that rain water kept leaking into the engine compartment around the hoodscoop seal, soaking the underhood insulation blanket and subsequently the intake filter cone and MAF, which made the car run like crap.

Things are much improved since the removal of the insulation, but I was wondering if I'm compromising the longevity of the paint. Does the insulation serve any other purpose than keeping the hood from getting too hot? Has anyone ever had paint issues after removing the stuff? These are the things that keep me up at night...
 

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2015 OutBack Ltd
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2,937 Posts
Hi Silke:

Not to forget that many cars still don't have under hood insulation without any paint issues even if the engine is turbo'ed.

I think the under hood pad is there to make the hood thunk closed with a satisfying sound as well as keep the running engine sounding a bit less tinny......

It's hard to imagine that a 4 cylinder engine would put out enough heat in an otherwise open bay to cause paint problems. Perhaps if the bay was really packed with stuff, enough to block airflow, heat might burble upwards against the hood but I really doubt you have anything to worry about.......as far as the car.

Now, the state of the world? That's a different thing entirely.

Steve
 

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2011 Toyota Tacoma DCSB Auto
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took that blanket out on the first week i had the car, still no ill effects just more places to put stickers ß
 

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2004 Forester XT Premium 4EAT
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Discussion Starter #4
That's good to know... it didn't even occur to me that it may be there for sound-proofing rather than heat retention.
 

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2004 Forester XT 5MT
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292 Posts
Some people just replace it with Dynamat or similar.
 

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02 Forester L (sold) Manual
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Has nothing to do with heat...it's simply for sound deadening. :)
 

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2004 Forester XT
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190 Posts
Mine gets all water logged when it rains so it may be heading to the dumpster also!!! :biggrin:
 

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2004 Forester XT Premium 4EAT
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Discussion Starter #10
^^ Hmm, so the consensus on lgt.com is that it is for paint protection and also fire protection? That's a little worrisome that one of the guys said he took his out and his paint went to crap afterwards.
 

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02 Forester L (sold) Manual
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Bullcrap.

Unless it's a turbo car and the hood insulation contains a metal based product to deflect the heat, it has no purpose to manage heat, period. I've owned literally dozens of vintage cars that never had any padding under the hood, cars that frequently ran much hotter than modern vehicles and it never damaged the paint. Additionally, the only car that I ever did have paint damage to a hood was when the engine caught fire(!).

Auto body shops will typically bake the paint to speed drying anywhere from 140-180 degrees. The coolant temperature of your engine (which is inside the engine, remember) typically would not get hotter than 220 unless something is wrong. Outside temperature would of course be less.

I'd like to know what specifically Mr Legacy know-it-all in that thread had happen to his paint. If it got cloudy, that's the clearcoat failing and is a result of inferior application or poor maintenance. Heat damage will blister the paint, causing discoloration but also bubbling and adhesion damage.

I had to LOL at the idea the hood insulation would act as some kind of fire suppressant. I've seen so many engine fires and that stuff is the first to burn...it frequently makes the underhood fire worse.

Now, if you have a turbocharger you're going to be generating some serious heat, especially if you've modified it to run more boost/fuel. Managing heat in a turbo car is important, and I would probably advise adding some heat shielding to the hood directly over the turbo if you plan on getting that sucker cherry red. Still, my few experiences dinking with turbo cars including my brother-in-law's Miata (with a fairly close hood proximity to the snail) has yet to result in any damaged paint.

I had this same discussion on the BMW message board, arguing with kids who just knew the paint would spontaneously combust the minute they removed the insulator panel from under the hood. My race car proves otherwise. :D
 

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2004 FXT 5MT
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Mine has been off for half a year or something, and it's been fine. I don't even have a turbo heat shield.
 

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2001 Forester Slushbox
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Just looked under the hood of my 2010.
Little strip of it that doesn't even cover half the hood area.

Sound deadening,period.
 

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Discussion Starter #15
Bullcrap.

Unless it's a turbo car and the hood insulation contains a metal based product to deflect the heat, it has no purpose to manage heat, period. I've owned literally dozens of vintage cars that never had any padding under the hood, cars that frequently ran much hotter than modern vehicles and it never damaged the paint. Additionally, the only car that I ever did have paint damage to a hood was when the engine caught fire(!).

Auto body shops will typically bake the paint to speed drying anywhere from 140-180 degrees. The coolant temperature of your engine (which is inside the engine, remember) typically would not get hotter than 220 unless something is wrong. Outside temperature would of course be less.

I'd like to know what specifically Mr Legacy know-it-all in that thread had happen to his paint. If it got cloudy, that's the clearcoat failing and is a result of inferior application or poor maintenance. Heat damage will blister the paint, causing discoloration but also bubbling and adhesion damage.

I had to LOL at the idea the hood insulation would act as some kind of fire suppressant. I've seen so many engine fires and that stuff is the first to burn...it frequently makes the underhood fire worse.

Now, if you have a turbocharger you're going to be generating some serious heat, especially if you've modified it to run more boost/fuel. Managing heat in a turbo car is important, and I would probably advise adding some heat shielding to the hood directly over the turbo if you plan on getting that sucker cherry red. Still, my few experiences dinking with turbo cars including my brother-in-law's Miata (with a fairly close hood proximity to the snail) has yet to result in any damaged paint.

I had this same discussion on the BMW message board, arguing with kids who just knew the paint would spontaneously combust the minute they removed the insulator panel from under the hood. My race car proves otherwise. :D
You're probably right... the heat generated by the turbo was my main concern, but seeing as it sits pretty low in the engine bay compared to the Legacy or Impreza it may not present so much of an issue. I guess this is where one of those Subtle chimney setups would really make sense, though!
 

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2009 Outback XT-B 5MT
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I remember hearing of a few instances where underhood fires were quenched by the blanket under hood. Apparently, the plastic tabs melt, and the pad drops to smother any flames. The instances I heard about were A/C leaks flashing on exposed header pipes, catching various bits of plastic on fire underhood.

I do think the primary purpose is sound insulation, with a secondary "oh, that'd be nice" function of helping to smother out fire if the conditions are perfect.
 

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I remember hearing of a few instances where underhood fires were quenched by the blanket under hood. Apparently, the plastic tabs melt, and the pad drops to smother any flames. The instances I heard about were A/C leaks flashing on exposed header pipes, catching various bits of plastic on fire underhood.

I do think the primary purpose is sound insulation, with a secondary "oh, that'd be nice" function of helping to smother out fire if the conditions are perfect.
On our 1996 Mercury Mystique, the primary function of the pad was for fire, so I was told. It was formed to fit snugly around the bits and pieces of the engine and smother any fire. I never had a chance to verify if it in fact would work :icon_biggrin:

It was a pretty tight fit for the V6 in the Mystique, so heat build up may have been an issue, hence a bigger chance of an engine fire (sure wore out hoses in a hurry). In comparison, the Subaru 2.5 is very open, so I can not see that the pad has anything to do with heat unless it has some sort of heat reflecting properties (anything inside it that would act as a heat shield?). As its black it will tend to absorb heat and transfer it to the hood rather than reflect it. So maybe your paint will last longer without the pad as there will be less heat transfer to the hood?
 

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Ok, since there is so much speculation on the fire snuffing ability of these things, I did some research since I believe I'm right that they serve no such purpose. The following quote is from an SAE paper and independent lab study on flammability of vehicle components:
Research was conducted by Biokinetics and Associates to investigate the use of fire safety
technologies in 2003 model year vehicles. That survey identified the use of underhood insulation
as a potential fire preventative feature [Fournier 2005; Fournier 2006]. It was speculated that
heat from an underhood fire would melt the mounting hardware supporting the underhood
insulation, allowing it to descend onto the engine and reduce the oxygen supplied to the fire.
Another scenario was that these insulation materials might act to delay the time that the fire burns
through the upper surface of the engine compartment. Such a delay could act to extend the
survivability time of the occupant compartment.
In this project, the flammability of the underhood liner materials was tested to determine the
degree to which they may be able to retard underhood fires [Fournier 2005]. An SAE paper also
summarized this research [Fournier 2006].
The research included a visual inspection of 89 vehicles from the North American market. The
presence of underhood insulation was found in 74 instances. In those instances, however, the fire
retardant properties of the insulating materials could not be ascertained by visual inspection
alone.
Twenty vehicles were selected for further testing of the fire properties of underhood materials in
the fleet. The 20 vehicles represented a convenience sample of various manufacturers and classes
of SUVs, passenger cars, minivans and pickup trucks. Coupons cut from the insulating liners
were tested to assess their fire retarding properties with a cone calorimeter according to the test
procedures of ASTM E 1354-03. The mounting hardware used to affix the liners to the under
side of the hood was also tested to determine if their materials would melt or distort sufficiently
to allow the insulating materials to fall.
The cone calorimeter test subjected a 10 cm x 10 cm material coupon to a constant radiant heat
flux of 35 kW/m2 for 20 minutes. From the materials behavior under the heat load the
ignitability, heat release rates, mass loss rates, effective heat of combustion and visible smoke
development of materials were determined and documented. Of the 20 insulating underhood
liners tested, 5 did not ignite. An additional 7 insulation samples that did ignite exhibited a short
Page 6 of 7
time to flameout with comparatively low peak heat release rates. These samples with the
inclusion of the non igniting samples show the most potential for retarding an engine fire. There
was a wide range of weight loss and heat release rates among the group of samples tested.
Several of the insulation materials would have contributed fuel to an underhood fire rather than
acting to retard it.
The cone calorimeter testing indicated that the application of a metal foil to the engine facing side
of an underhood insulting liner can significantly enhance the fire resistance of an insulating
material by preventing ignition.
If an underhood insulation liner is to smother a fire the mounting clips affixing it to the hood
must disengage from the hood under high heat conditions of an engine compartment fire. The
mounting clips for the underhood insulation from the twenty different vehicles tested disengage
from the supporting structure at temperatures ranging from 133 °C to 268 °C. The results seemed
to indicate that the design of the mounting clips may have an influence on the deformation pattern
and the temperature required for the insulating sample to disengage.
Given that fire resistant underhood insulating liners were identified amongst the small sample of
vehicles examined, the possibility of the liners acting to retard engine compartment fire is
plausible . However, the effectiveness of such a system can not be determined strictly from the
component tests that were performed. Several of the underhood insulation materials in current
use would contribute fuel to an underhood fire.
(Bolding mine)
So basically, it's inconclusive. Some may help, some may make a fire worse. Vehicles catch fire under hood for many different reasons, each having a different burning temperature (oil, fuel, electrical). The hood insulators were more fire resistant than the majority of under hood plastics, according to the study. A dropping hood liner at best would only buy time. Expecting it to snuff out a fire completely is unrealistic. If that were the case, I'd have far fewer burnt-to-a-crisp vehicle claims from engine fires (which, honestly, are pretty rare anyway).
 

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I'll change my opinion about the fire snuffing aspect as soon as someone can show me published manufacturer data that states it is engineered for that purpose.

Until then...complete bunk.
Agreed.
 
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