Better performance when cold?

I've been noticing recently when the engine is cold, it seems to have
better acceleration. I always thought cars performed better when they were
"at normal operating temperature" ... Am I wrong about that, or could
something be causing the performance to degrade when warm?

It's a '96 Civic EX (D16Y8 engine, if it helps) with about 33k miles on it.

 

Engines have the highest *power* when the fuel mixture is on the rich
side of stoichiometric. They get best "efficiency* when the mixture is
on the lean side of stoichiometric. As long as the engine is cold, the
ECU will enrich the mixture a tad, so it is very possible that on
partial throttle you have a little more power before the engine warms up
completely. However, most fuel injection systems will say "screw it" to
fuel efficiency and emissions on full throttle, and enrich the mixture
for max power. I would expect that at full throttle you would not see
any more power on cold engine than on fully warm.

 

The inlet air is more dense when it is colder, and volumetric
efficiency rises in the cylinders thereby increasing engine
performance.

Dick

 

I've heard this too. The theory I've heard is that when the air is
cold, it is more dense and therefor contains more oxygen per cubic inch.
This in turn would help the fuel burn better, giving you more power.

Not sure how true it is, but at least scientifically it make sense.

 

Well, that's why "cold air" intakes are so popular .. Come to think of
it, the only time I've got better acceleration is when the weather is
cool outside.. If it's sweltering outside, there isn't much of an
increase.

 

The humidity would also affect the level of O2 in the incoming air.
Don't modern FI systems adjust for this?

 

Kind of, the amount of fuel injected matches the amount of oxygen. If
for some reason less oxygen is pulled in (e.g. high temperatures, high
altitude etc.) the amount of fuel is reduced accordingly, and power goes
down. With a turbo or supercharger, you can compensate for the effects
of high altitude etc.