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The following information is from the Impco training manual.

The temperature of the air entering an engine is very important for two reasons.
1. Hot air entering an engine can lead to detonation and preignition, which will injure or
destroy an engine in short order. The cooler the temperature of the incoming air the
healthier it is for the engine.
2. As the temperature of air entering an engine increases it expands becoming less
dense and lighter. This reduces the volumetric efficiency and therefore the
horsepower output of the engine. For every 10-degree increase of engine intake air
temperature the horsepower output drops 1%. Since underhood air temperature can
easily reach 200 degrees it is very important that the engine air intake be ducted
outside the engine compartment. As an example an engine that makes 100 HP
breathing air at 60 degrees will only make 86 horsepower breathing air at 200
degrees. This decrease in power can be explained by the fact that an engine
requires 7lbs. of air to make 1 horsepower for 1 hour. As air is heated it expands and
becomes less dense and lighter (as in a hot air balloon). A greater volume of air is
required to weight 7lbs. An engine running at rated full load RPM can only breath a
fixed volume of air. The number of available pounds of air is reduced by using hot air
(a 100 cubic inch displacement 4 stroke engine will only pass 100 cubic inches of air
and fuel for every 2 revolutions of the crankshaft. The displacement is fixed by the
bore and stroke. The displacement cannot increase to allow for the high temperature
and lower density of the incoming air).

Edited by SimpleOrange

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The altitude at which an engine operates has a dramatic effect on power output. Since
atmospheric pressure (14.7psi at sea level) drops as altitude increases air becomes
less dense and lighter. Therefore it has the same effect on horsepower output as air
temperature described in the previous section. The rate of horsepower decrease is 3%
for each 1000 feet increase in altitude. As an example an engine that makes 100
horsepower in Washington, D.C. (elevation 30 feet) will only make about 85 horsepower
in Denver Colorado (elevation 5280 feet).
An advantage to using gaseous fuels (LPG, CNG) over liquid fuel (gasoline) is that
when altitude increases the density of air and gaseous fuels change at about the same
rate, therefore the air fuel ratio remains unchanged. However with a liquid fuel, as
altitude increases, air becomes less dense but the liquid fuel does not change therefore
the air fuel mixture becomes richer (less pounds of air to a fixed amount of gasoline)
and engine horsepower output decreases more than the 3% reduction (per 1000-feet)
caused by the decrease in atmospheric pressure.

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Oh how I loved this time of year back in my days of impromptu street racing! I drove carburated cars back then, no computer to compensate for atmospheric conditions. It was amazing what kind of cars I could whip when the sun went down! Cool air, hot pavement, perfect conditions to hook and book with a well tuned engine! 

In more recent times I enjoyed mowing in the evening this time of year, a 16hp Briggs turned into an absolute beast with cool damp air flowing into it. 

Thanks for the reminder, I love science. 

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The last couple of Simplicity and Allis large frame tractors that had been listed on Craigs prompted me to post the above information.

What I've noticed from the tractors being pictured in those ads is that the heat shield is almost always missing, this is the shield that fits in between the muffler and the carburetor.

Another thing that I've noticed on my own Onans is that the air filter aka air intake is located some distance towards the outside of the engine and that the intake manifold on the CCKB is made so that the carburetor is well elevated above the heat zone allowing for cool air to flow over and under.

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I am sure there are a few "old" snowmobilers here, and they can confirm this.

Many snowmobile engines have "burn" up when the temperature was - 20 deg.   But man do they go!


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Without a doubt.

Free air machines with factory jetting would go like the wind when 0 degrees.  Take them out at 27 degrees & they run like a bag crap.  Some of it had to do with the 20-1 mix they recommended & that made it even worse.  Plug fouling after so many minutes of idle.  Kawaski came out with the Quad-Plug which I don't think helped but they looked cool.  

I never wanted to be "That Guy" that was changing jets & clutch weights every time we stopped because a cloud came over.  I'd rather be the guy inside warming up having some chicken wings, poppers & a PBR.

I used to & still say to people who clutch, jet & pipe those engines within an inch of there life..."Shut up & ride your snowmobile."  "You wanna build a race sled?"  "This is a Pro-Stock race sled."

It's not mine.  My best friend built it for a customer.




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