Underhood temperatures

scotianscotian Member Posts: 1,064
This seems as good a place as any to ask this:

How hot can the air get in the typical engine compartment? I'm thinking around 200 F.

I want to make my own air box/hot engine air deflector and was wondering if I could use thick plexiglass. Anyone know how hot that stuff can get before it degrades? It will be fairly close to, but not touching, the engine, of course.


  • Mr_ShiftrightMr_Shiftright Member Posts: 64,481
    Quite a few cars have used it, but I really don't know how to find out if the plexi was of any special type. I'm thinking Ferrari & Alfa for sure used it over the engine, and the Jaguar XJ220 as well. I bet that twin turbo Jag got pretty hot.
  • edwardh5edwardh5 Member Posts: 130
    suggest do a web search, contact a plexi manufacturer.
  • scotianscotian Member Posts: 1,064
    Is Plexiglass the same as polycarbonate?


    "Polycarbonate has good heat resistance (rated at 240 degrees farenheit continuous)"

  • scotianscotian Member Posts: 1,064

    "Lexan, which is a polycarbonate, whereas Plexiglas is an acrylic."

    I need to find some Lexan/polycarbonate, it seems.

  • adc100adc100 Member Posts: 1,521
    The engine block would be around that temp or a little less. The average temp in the engine compartment could be maybe 25 degrees above outside temp. Naturally a short distance from the block you could approach that temp (say 1/4 inch away.) You would have to be careful what you are doing so as not to create hot spote in areas where wires/electronic equipment would be.
  • q45manq45man Member Posts: 416
    The radiator typically has an input temperature of 195F [210-175] and the block is operating at around 220F.....in summer heat and stalled traffic a 180-200F under hood temp is typical.
    As to air box [for intake??] every 11F reduction in temp increases air density [power increase] by 1% with a cool air intake .....easy to see a 7% change summer to winter [77F temp change]...but air gets reheated by throttle body, plenum and runners. [primarily a cold weather strategy that hurts in Summer].
  • scotianscotian Member Posts: 1,064
    The throttle body, plenum, and runners (I don't know what this last thing is) are specifically designed to heat air? Under what conditions would heated air be better than cold when going into the engine for combustion?
  • blackgtpblackgtp Member Posts: 19
    Don't forget about the exhaust. Even at idle exhaust manifold temperatures will be over 500 F. Run the car hard and you could easily get exhaust manifold temperatures over 1000 F.
    When testing engines I see 1700 F exhaust gas temps at the end of WOT RPM sweeps.
    No, average underhood air temp will never reach these numbers. Just remember that anything close to the exhaust must be manufactured to withstand or be shielded from extreme heat.
  • q45manq45man Member Posts: 416
    Metal [plastic] parts in the air intake system absorb heat from the surrounding area [+ the intake runners/plenum are bolted to the engine so heat is conducted from the heads [240F] to them they are cooled by transfering heat to the incoming air flowing thru them] plus most throttle bodies have coolant flowing around them to help with warmup response.
    Fuel is blended winter, spring, summer with different Reid Vapor Point to make sure it vaporizes correctly with different air temps [hold over from carbs].....so from cold startup at 32F to warm startup in summer soak at 200F this 170F range [15%]must be compensated for.
    But even in winter conductively Warmed Up [after 5-10 minutes] running air usually doesn't drop below 75F as it passes by the fuel injector.
    This 100F difference [winter/summer] causes the 10% difference in power output {WOT] because of total air density changes.
    Just going to low conductive plastic plenum and runners has helped narrow the spread [better summer performance] while reducing cost.
    With carbs the evaporating fuel used to cool the plenum and runners [at the same time keeping them clean inside.....with injection this is no longer true]!
    Ford has a patent on using the existing AC system to cool the intake charge in summer heat to improve stalled traffic idle and initial acceleration until the outside air flow removes enough heat from the induction system!
  • q45manq45man Member Posts: 416
    It is important to understand that passenger car are not race cars and generally have limted cooling reserves much more than 60 seconds of wide open throttle will cause problems as the heat output of the engine increases by up to 10 times.....luckily much more than 15 seconds is rare.
  • lmacmillmacmil Member Posts: 1,758
    Rule of thumb in the industry is 100F above ambient. However, worst case (descending a mountain into Death Valley) results in design specs for underhood brake components to typically be 250F continuous and sometimes as high as 300F intermittant.
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