What impact does the weather have on engine performance



We are having some real hot and humid days here and I was wondering, what impact does days like that or very cold bitter days have on how an engine, especially and older one performs?


Ford engines seem to like it better when it is colder and the air is denser.


Cars built after 1990 were pretty much immune to weather related problems… Before that, it could be a nightmare, depending on the technology employed…The dividing line is pretty much Fuel injection / carburetors… Carburetors with their chokes and choke pull-offs and vapor lock required some operator skill to operate…


By “older car,” do you mean

  1. “Dumb” (non-feedback) carb, or
  2. “Smart” (feedback) carb, or
  3. EFI?

Both 2) and 3) compensate, to one extent or another, to atmospheric conditions, so effects will be lesser than with 1). However…

Warmer-than-standard, or lower atmospheric pressure

  1. Reduces the density of air, so that a (normally-aspirated) car has less power.
  2. Slightly increases the fuel efficiency of 3)…and possibly 2).
  3. Decreases the aero drag (due to less-dense air).
  4. Will cause 1) to run rich, completely canceling out (and then some) any FE gain.
  5. Increases vaporization of fuel, resulting in easier starting with 1) and 2) (unless you wind up with vapor lock).

Colder than standard (or higher atmospheric pressure)
Essentially the opposite effect of all 5 above.


The 91, 94, 99, and 2000 that I had all had a little more pep when it was cold out. I haven’t noticed that with the 04 or the 06 that I have now.


I assume the OP is still talking about his 1998 with possible head gasket issues.


All engines are like that. The denser the air, the more oxygen content in a given volume (like, say, your cylinders). The more oxygen, the more fuel gets put into the mix. The more fuel and oxygen in the mix, the more powerful the explosion, and therefore, the engine puts out more power as well.


A internal combustion engine is limited by the mass of air it can ingest per time period. The mechanics of the engine determines the swept volume and the volumetric efficiency, i.e. the larger cubic inches or liters and high volumetic efficiency at a set RPM determines the torque available and the power produced.

If the barometric pressure is low, air temperature high, and the relative humidity high the less mass of oxygen is available per volume air to burn with added fuel. On a computerized feedback fuel delivery system only the mass of fuel appropriate to the mass of air is mixed. If the mass of air and proportion of oxygen in the volume of air is low the amount of heat generated at each burn will be lower reducing the engine power across the torque band. On a carburated engine, the mixture will be richer than it should be so some of the excess fuel cools the burn reducing the power production further. If there is supercharging, turbocharging, or turbocharged with intercooling, the power can be brought back up. Example the Merlin V12 had two soperchargers with intercooling between each. In military emergency it could maintain ~1700 horsepower using 74 inches of mercury manifold pressure at 20k feet.

Most civilian engines lose power at high altitudes, when the humidity is high, and when temperaure is above standard lapse rate. The worst airplane engine performance occurs when the air strip is at high altitude, the air temperature is high, and in high humidity conditions. Yes, the mixture has to be leaned to the best power point but you still are going to use a lot more runway length getting the airplane up to lift off air speed. Many summer flight into terrain accidents are the result of hot, high, and heavy miscalculations.