Topic: Why does it run better when cooler temps?

[Tews19]

I just rode home tonight. The temp was 70 and its noticeble here in SC. When your bike doesnt run well in MI because its cold you should be on a sled! .

Man, us UPNORTH fell'rs would never be able to ride our bikes if we couldnt when it is cold.. We are lucky to have a a few good months to ride... How many have actually riden their bikes in the snow and I mean some nice fluffy or slushy snow?

[HondaMan]

I've often felt moisture (i.e. relative humidity) has something to do with it also. I too have noticed smoother running when cool, but even more so if cool and foggy. No science to back this up, just seems that way.

It's that way for another reason: the moisture in the air lets the valve faces seal a bit better, raising the compression slightly and letting the valves run cooler, which shrinks them a tiny bit, deshrouding the flow a tiny bit more. This is easy to see 'why' when you deshroud, say, the 550 inlet valves just the typical .010" on the cylinder side and .003" on the center side: the flow rises dramatically from this small change. This tiny bit of change almost removes the [in]famous 'flat spot' in low midrange RPM that these engines have always shown: this same 'flat spot' gets significantly better when riding in rain or foggy conditions (in my experience). The cooler air can drop a valve diameter as much as 0.002" for a 1" diameter valve (so said the engine teacher when I was in college), compared to an inlet temperature 50 degrees higher.

[RustyStuff]

I've noticed this quite pronouncedly with my truck. It has alot more ooomf when it's cool and damp outside than when it's hot and dry.

[Don R]

When we go drag racing we have an altronics weather station on the trailer that reads temp, humidity, air density, (altitude) water grains, wind, etc. We use a mathmatical formula to determine how much performance change there will be with a measured change in the corrected altitude. We are at 600 feet approx above sea level but the corrected density has been below sea level in the fall or as high as 4,000 feet when it is hot and humid. The car will change approx .01 with every 150 feet of corrected density altitude. This is in the quarter mile at approx 7.5 seconds at 179 mph. We had a best ever of 7.52 but last week the air was good and we ran 7.49. In the heat of summer it may slow to 7.70's.
 We can also determing the required jet change to correct for weather but we usually just allow for the air.
 Hondaman raises an interesting point about valve seal and humidity. We usually lose HP with increased humidity but our valve seal is very good. That (increased performance with humidity) might be an indicator of when the valve seal starts to fade a little.

[Goldbug]

Of course in drag racing the weather is also going to be a variable in traction.

[grumpy]

What about the fuel?
I seem to recall something about high temps causing a rich condition b/c fuel vaporizes more readily when it's hot...

[dave500]

having a few beers mostly results in cars and bikes running a lot better aswell,a bit too much better.

[Danno]

it is true my machine runs slightly lean on top end rich on bottom colder temps make this into a problem which I deal with by turning the air screws in a quarter turn which keeps the performance the same in the cold and protects the valves