Topic: Why does it run better when cooler temps?

[Duanob]

I've always been curious about why carburated engines run better when it's cooler out then when warm or hot?

I've been riding to work lately and i the morning it's been in the 40's. My 550 has never run better! But in the evening when it's around 70+ it doesn't seem to run as smooth. Of course most people wouldn't notice the difference but since it's my bike I notice it right away. It's not that it runs bad when warm but just not as smooth as in the morning jaunt to work. I've always noticed this about every vehicle I've owned with a carb. Any explanation?

[Duke McDukiedook]

Air is more dense when the ambient temperature is cooler as opposed to hot- you get more air in the engine per combustion cycle which means your carbs are tuned a little on the rich side.

[70CB750]

What Dukie says,  that's why cars often have air intake all the way front - to get coldest air possible.

[TwoTired]

Cooler air is more dense, meaning each gulp of air contains more oxygen.

[vames]

It's even more prounounced for some reason with two strokes. Many years ago I had a vespa that seemed to gain 5 hp when the temp dropped toward freezing.

[CycleRanger]

You get more spark too since the colder wires conduct with less resistance.....

 

[TwoTired]

You get more spark too since the colder wires conduct with less resistance.....

Perhaps in theory.  Measurably, not so much. 

It's the spark gap that determines the voltage at the plugs.  The air density may lower the voltage requirement for spark there somewhat, though.

Cheers,

[CycleRanger]

You get more spark too since the colder wires conduct with less resistance.....

Perhaps in theory.  Measurably, not so much. 

I know, I was trying to be funny.
I'll try harder....

[Duanob]

Sounds good to me. I just need to remember to dress a bit warmer on those cold mornings. The faster you go the colder the windchill temps.

Dukie it's a 76 550K model, they naturally run richer. Maybe that's why I notice it more.

[DH]

Fuel injected engines act this way as well. My cbr 600's engine management reacts in a good way when its cooler outside. Driveability is improved a bit. My old '92 5.0 mustang acted that way too. Guess this is why racers are so keen to enviromental changes when tuning thier engines.

[Danno]

that is funny bud my 74 550 requires me to turn the air screws in a quarter turn when it gets cold out to keep performance the same as in the summer my bike loves the heat hates the cold

[xsmooth69x]

my subaru is the same way. i have the air coming in from my hood scoop and when it gets cold outside she pulls a lot harder 

[Duanob]

that is funny bud my 74 550 requires me to turn the air screws in a quarter turn when it gets cold out to keep performance the same as in the summer my bike loves the heat hates the cold

*LIGHTBULB*

So if the air temp is affecting the oxygen intake mixture wouldn't things be running on the lean side? It seems to me if it were running too rich the air part of the mixture would have less affect, possibly? Especially if Danno has to richen is mixture which to me makes sense. I know my bike isn't running too rich probably more lean. I've torn into the carbs many times and made many adjustments over the past year to get the right mix. Currently I would not mess with them any more, I got the bike to strat easily, idle smoothly, pull strongly in all RPM ranges and the plugs have a nice tan color to them. Everything else is by Honda design at this point.

The reason I bring this up is I just had a *voila* moment about my boat. When I first start it and cruise around it runs beautiful. But after an hour it doesn't want to idle as smooth and sometimes I get a slight hesitation when pulling skiers out of the water. Imagine how hot a boat engine gets inside a dog house! there is no cooling in there at all. And the hesitation is usually an indication of a lean mixture correct? Not to get off topic too much but I think I just figured out how to fix an annoynace on my boat that's been bugging me for the past 4 years. My idle screws are too lean! Funny but nobody on hte boat forum I belong to has ever brought this theory up but it plagues a lot of boat stern drive boat owners. I may be on to something. they will think I'm a freakin genious!

As far as EFI cars I've really never noticed it all on any of mine and I've owned lots of them. Mostly slow pokey SUVs and trucks.

[Duke McDukiedook]

Danno is probably running slightly lean, you are running slightly rich.
If your performance improves with cooler weather you are probably running slightly rich.
If your performance improves with hotter weather you are running slightly lean probably.

Physics and thermodynamics, very hard to beat.

 

[TwoTired]

A stumble on power application can be from too lean or too rich.  You can tell during power recovery if it was stumbling from to lean, in that it recovers cleanly, like someone had turned it off and then on again.  If it is recovering from too rich a mixture it will burble back to life and miss random power cycles as it returns to power.

Cheers,

[Duanob]

Well I think the boat is running a bit lean.

The bike is really fine it just seems to be smoother running cold but it really doesn't run bad at all when it's warm. It's something that only I notice from knowing every little thing about this bike. If someone else rode it on a cool morning and then on a hot after noon they probably wouldn't even notice a difference. But it's a common phenomenon with all carbed vehicles I've ever had and now I know why.

[Bob Wessner]

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.

[70CB750]

same experience with cabureted cars - seem to run better when it rains.

[RustyJC]

For the technophiles - see http://www.engineeringtoolbox.com/density-air-d_680.html.

Rusty

[Bob Wessner]

For the technophiles - see http://www.engineeringtoolbox.com/density-air-d_680.html.

Rusty

Jeez, you could have sent us directly to #6! 

[Goldbug]

I had a turbocharged car running about 25psi of boost. It would usually pull 36lb/min of air according to the airflow sensor. Depending on the weather it might pull 38lb/min and a few times when it was about 32F it pulled 40lb/min.

Every lb/min is about 10hp. So on a typical Summer day it made 360hp. In the Winter it made 400hp. Quite a significant difference. Of course do not expect this sort of disparity on a naturally aspirated engine.

[paulages]

No air cooling, but lots of water cooling. Unless you have a closed cooling system, you are constantly bringing fresh water into the cooling system.

[nccb]

i concur with everyones statements.  Mine runs better when it is about 60-75 outside, same as my old golf when i lived in the mountains.  I think they love the cooler weather just as much as we do

[CycleRanger]

Modern fuel-injected vehicles have MAF (Mass Air Flow) sensors that allow the air/fuel ratio to adjust dynamically.

[bender01]

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! .

[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