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 Physics and the theoretical
 Hotter air, colder air, which makes an engine run better?
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Niebor
Ex-Member

Posted - 03/08/2006 :  2:12 PM                       Like
I'm curious, I hear support for both making an engine run better. Cold air, by virtue of containing "more oxygen", hot, because it burns better?

Mine seems to favor cold weather, (unlike my body), its just crisper, more responsive. In hot weather, after a couple red lights, it becomes sluggish, coughs, sputters. If I throttle up too quickly, may even cough and die. Hate when that happens, rather embarrassing. I perceive this has more to do with fuel mixtures than jetting. But at the moment, am too cheap to fork over the $250, for rejet/dyno service. Nor am I ambitious enough to buy a dozen jets, and experiment to seek the best setup. The dyno calls this information out on a basis possible to quantify. Therefore, setup is, theoretically, quite positive. Anyhow, just curious as to which should run better overall, cold or hot.

I have been monitoring the plugs, and the burn falls into the "perfect" category. So I perceive its more of an annoyance, than destructive. In the near future, I will give in, and have the service performed. As this should be done for a given setup, I will have it done, after I finish my inner debate over loud pipes

scottrnelson
Advanced Member
6950 Posts
[Mentor]


Meridian, ID
USA

Honda

XR650L, 790 Adv R

Posted - 03/08/2006 :  2:27 PM
The last street bike I had that was not fuel injected was a 1997 Ducati M900 Monster. I bought a Factory Pro jet kit for the bike and rejetted it to get it to run better in the conditions in which I normally rode, and to account for airbox and exhaust changes that were made to the bike. After rejetting, I rode it for a few months to make sure I understood exactly how it behaved in any given circumstances, then made one set of adjustments to get it close to perfect. On that particular bike, it takes nearly an hour to get to the carburetors, then almost that much more time to put everything back. Actual jet changes take about five minutes.

That bike ran well in all conditions except hot at higher altitudes (above 2000 feet). Hot weather at lower altitudes (I live at 350 feet) it was fine and cold weather was great anywhere.

What I noticed, however, is that the bike had more power in cold weather. Normally the bike was rated at about 75 hp. On a cold day with the air temperature below 45 degrees or so, it felt like it had another 5-10 horsepower. My method of verifying additional power is based on (dare I say this here?) how easily the bike would wheelie and whether or not it would do a power wheelie (no clutch) in second gear.

I can't say it's true for all bikes, but at least one that I've owned definitely made more power with cold air. I can't really say for sure with my current fuel injected bikes.
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Indiana Randy
Moderator
2118 Posts
[Mentor]


Fort Wayne, Indiana
USA

Honda

2000 Magna V4 750

Posted - 03/08/2006 :  2:32 PM
neibor- Also keep in mind you're a mile up!
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Niebor
Ex-Member

Posted - 03/08/2006 :  3:40 PM
quote:
Originally posted by Indiana Randy

neibor- Also keep in mind you're a mile up!


Oh yea, I did fail to mention that. Suburb of Denver is our base elevation, about 5800 feet. From here, the mountains are our obvious magnet. Thumper IS gasping for air at the top of some of them thar' hills
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subvetSSN606
Senior Member
418 Posts
[Mentor]


Ellettsville, IN
USA

Suzuki

800 Intruder

Posted - 03/08/2006 :  4:57 PM
I would say it depends on which it was tuned up for. If the bike is tuned perfect for a given temperature, any variation from that temperature (up or down) would decrease performance. Once you have that ideal fuel/air mix, going up in temperature you get a richer mix, going down you get a leaner mix.

Tom
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rolog
Standard Member
107 Posts


city of smiles
Philippines

Ducati

2004monster620

Posted - 03/08/2006 :  7:26 PM
My bike, and all bikes i assume run better in cooler weather.
I notice an increase in engine power.
Cold air has more mass and allows the engine to produce more power.
I think the "intercooler" on a turbo-intercooled engine does the same thing?
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Eric
Junior Member
56 Posts


Ottawa, Ontario
Canada

Kawasaki

1984 ZX 900 Ninja

Posted - 03/09/2006 :  8:20 AM
Cooler air is denser so the engine can pack more in during the intake stroke. Most bikes(non FI) run a bit sluggish in hot humid conditions.
I would say that my bike pulls a bit better in the spring and fall but I also feel this is only a result of outside air temp. and moisture %.

PS. Your HD carb may require a minor tweek. How is the float height these days? A small adjustment may cure your problem while pulling away from a stop or during the 1-2 shift. If the float is set a bit low then you can experince minor fuel starvation in warmer weather causing the "flame out".
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Eric
Junior Member
56 Posts


Ottawa, Ontario
Canada

Kawasaki

1984 ZX 900 Ninja

Posted - 03/09/2006 :  8:29 AM
Gas grade may also factor in during warm moist days. While you can not get more power from hi test you do get a better burn during the ignition point of the cycle.
I run regular unleaded in my bike with no issues whatsoever. I had my wife with me last summer and with the extra mass and the hot weather I topped up with hi test just to be safe. Old bikes...whatta ya gonna do?
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timbo
Advanced Member
594 Posts


Uxbridge
United Kingdom

BMW

R1100S

Posted - 03/09/2006 :  11:56 AM
Thoeretically colder air is better. From 3 viewpoints -
1) its colder therefore more dense, therefore more oxygen per unit volume. This allows the engine to burn more fuel and generate more power.
2) Thermodynamically, the engine efficiency is partly a function of the temperature differences between the hot and the cold. The greater the difference the more efficient the engine can be
3) Better heat rejection/cooling with colder air. (this one is a bit marginal as it only applies really at maximum power output for long periods)

In practice, as you have found, this is in fact the case.

I would say that you probably ought to get it rejetted to suit the normal environmental conditions at 5800feet up.

Tm
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Mydlyfkryzis
Senior Member
274 Posts
[Mentor]


West Milford, NJ
USA

Honda

1991 Nighthawk 750

Posted - 03/09/2006 :  7:20 PM
I agree with Timbo completely. Having said that, define "Better".

My NH750 got the best mileage (52MPG) on a 102 degree day in stop and go traffic. I average 47MPG. It also ran very smooth that day. Smoother than usual. I was surprised. I can't comment on the power aspect, as it is hard to determine it when you average 5MPH.

So colder, denser air is thermodynamically conducive to more power, there are other factors that may make your particular motor run better whne cooler or hotter that mat have a bigger effect than the increase from denser air alone.
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Niebor
Ex-Member

Posted - 03/09/2006 :  8:53 PM
I found this article very informative:

http://www.factorypro.com/tech/carb...engines.html

Covers CV type carbs, low RPM motors.

It covers hot vs cold symptoms, what they mean to fuel mixtures. Its time to cough up the dough. I know it would be a happier ride
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petercwelch
New Member
18 Posts


Carmel, NY
USA

Suzuki

vs800

Posted - 03/11/2006 :  4:30 PM
Cold air is denser and has more oxygen per unit of volume and therefor more gasoline can be injected per power stroke and more power results.
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gnorv
Male Junior Member
44 Posts


Overland Park, KS
USA

Kawasaki

Z750S

Posted - 06/02/2006 :  11:47 PM
petercwelch hit it on the head. The cooler the air the denser the air which enables more fuel to used which results in more power. You have to tune the bike for optimal power in cool or hot air. Two other important factors are barometric pressure and humidity. Humidity makes the air less dense (takes up room literally). Barometric pressure is huge; the higher the better. When I drag race my car, it is a two tenth difference between 29.5 and 30.5. The next time your bike feels crisp (or like a dog), check out the barometric pressure it will explain why.

Edited by - gnorv on 06/02/2006 11:48 PM
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Kentucky
Male Starting Member
1 Posts


Kentucky, New South Wales
Australia

Kawasaki

1600 cc Nomad

Posted - 06/08/2006 :  6:59 AM
G'Day thought you might be interested in checking out this site from Australia for a cold air induction unit for Kawasaki's. Cold air induction is a means of cooling air and ramming it into the combustion chamber.
Regards
Steve
www.aus-air.com.au.

Edited by - Kentucky on 06/08/2006 7:01 AM
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Mydlyfkryzis
Senior Member
274 Posts
[Mentor]


West Milford, NJ
USA

Honda

1991 Nighthawk 750

Posted - 06/08/2006 :  10:45 AM
quote:
Originally posted by petercwelch

Cold air is denser and has more oxygen per unit of volume and therefor more gasoline can be injected per power stroke and more power results.


While this is true, more power may not be "better".

As I stated before, my motor ran smoother with better mileage in HOT conditions.

The engine tune, clearances, are other factors that affect engine efficiency and power output. So while cold dense air provides the maximum air charge for the engine, with the greatest potential for power, a cold engine with loose tolerances may put out less horsepower despite the denser air. The engine has to be optimized for those conditions. I suspect mt air-cool nighthawk engine has large clearances, and in hot weather, the engine clearances are reduced to the point of maximum efficiency on the hard parts of the engine. What is lost in hot air density is gained in better engine sealing and compression.

So depending on wether you have a computer controlled engine that can adjust timing and fuel mixture to take advantage of the cooler air, and maybe a liquid cooled engine of tighter tolerances, a blanket statement that cold air produces more power is not entirely accurate.
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Nicolas27
Male Senior Member
364 Posts
[Mentor]


Atlanta, Georgia
USA

Honda

'06 Aero

Posted - 06/08/2006 :  12:41 PM
Another thing to consider is how clean the air is where you are.

In warmer climate zones near the ocean (Florida), there's a certain amount of salt (as well as other particulates) in the air, which can definately affect engine performance. But, there is a lot of oxygen in the air at lower altitude, especially with a lot of green vegetation around (oxygen producers).

In colder mountainous climate zones (Colorado), the air is super clean, but there is less oxygen in the air due to the higher altitude.


Between those two climates there are more factors that determine engine performance than temperature and oxygen content. To notice the difference you'd have to ride in a cold beach environment and a warm beach environment, or likewise, a cold mountainous environment (same altitude) and a warm mountainous environment.

Of course to be for sure you'd have to creat an indoor environment with all relative factors under control. Which I'm sure has been done before. Anyone seen any research like this?
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Mikey
Male Senior Member
287 Posts


Benton, Kentucky
USA

Harley-Davidson

FLHTCI/XL/FXDL

Posted - 06/08/2006 :  2:21 PM
quote:
Originally posted by Niebor

I'm curious, I hear support for both making an engine run better. Cold air, by virtue of containing "more oxygen", hot, because it burns better?

Mine seems to favor cold weather, (unlike my body), its just crisper, more responsive. In hot weather, after a couple red lights, it becomes sluggish, coughs, sputters. If I throttle up too quickly, may even cough and die. Hate when that happens, rather embarrassing. I perceive this has more to do with fuel mixtures than jetting. But at the moment, am too cheap to fork over the $250, for rejet/dyno service. Nor am I ambitious enough to buy a dozen jets, and experiment to seek the best setup. The dyno calls this information out on a basis possible to quantify. Therefore, setup is, theoretically, quite positive. Anyhow, just curious as to which should run better overall, cold or hot.

I have been monitoring the plugs, and the burn falls into the "perfect" category. So I perceive its more of an annoyance, than destructive. In the near future, I will give in, and have the service performed. As this should be done for a given setup, I will have it done, after I finish my inner debate over loud pipes


Neibor,
I take it that you have not done anything yet to adjust your fuel mixture. You can help the low end coughs and sputters some by richening your idle mixture, just don't get too carried away or you will start fouling your plugs & your your fuel mileage will suffer. The idle mixture screw is on the underside of the carburetor,next to the manifold connection. THere is a small lead plug that you will have to remove to gain access to the idle screw. If the dealer has already worked on your carb, chances are this plug is already removed. TO get rid of low speed backfires, you will have to change out the carburetor low speed jet to a size or two larger. I am attaching a link to a good carb illustration for your use.
mikey


http://www.nightrider.com/biketech/carbadjust.htm



Edited by - Mikey on 06/08/2006 2:22 PM
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tmonroe
Male Advanced Member
752 Posts
[Mentor]


Seattle, WA
USA

Kawasaki

ZX-10R

Posted - 06/08/2006 :  5:57 PM
I was towing a trailer (during the summer) with an '88 ford escort from Washington to California, and I noticed that the top speed on level pavement was 10mph higher at night than during the day.

That car had Fuel Injection, and an O2 sensor, so I would assume that all of the mixture adjustments were being made for me.

Someone mentioned engine tolerances affected by temp, and I'm a little skeptical - once the engine is up to temp, I doubt that outside temperature has much effect on the engine temp - especially on a water-cooled bike (small, hard working engine). There is a thermostat that will basically stop the flow of water if the engine is not at temp, because they are designed to work in a specific temperature range (air cooled bike might be different - but I'm not sure).

However, on any vehicle that has an electic fan, the fan will probably run more in hot temps (especially if the vehicle isn't moving). Fan running more = more load on the engine from the Alternator (or generator depending on the bike).



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HeavyMetal
Male Junior Member
94 Posts


Kimberly, Idaho
USA

Honda

Valkyrie

Posted - 06/08/2006 :  7:33 PM
It's been 30 years since my diesel mechanics course so am a little rusty on the exact theory on this but as I remember the cooler, denser air contains more oxygen so will result in more power because more air will fit in the cylinder. This assumes the proper amount of fuel being introduced. This is why most modern diesel engines have some type of aftercooler to cool the intake air after it has gone through the turbocharger or blower. There is more power per cu. in. this way. On a gasoline engine this is true also to some extent. Denser air = more power. However, this will not necessarily mean the best fuel mileage. That is why so many modern gasoline engines run at a higher coolant temperature. To maximize the fuel economy. There is a trade off between power and economy.
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Mydlyfkryzis
Senior Member
274 Posts
[Mentor]


West Milford, NJ
USA

Honda

1991 Nighthawk 750

Posted - 06/10/2006 :  12:19 PM
Well, being a mechanical engineer in the energy business, I try to keep it simple.

Cold air os more dense than warm air and will provide the opportunity for more power.

The question remains, can you take advantage of the colder air?

The ideal situation for maximum power is the coldest, Densest air and the hottest engine. The BTUs (energy) available is the difference between the lower energy stated (Cold) and the higher energy state (Hot). (You can substitute pressure for temperature or even combine the 2). The problem arises when the engine is operated outside it's most efficient area. The engine is designed for a specific temperature range. In addition, other factors are working against you. For instance, you would expet your engine to develop the most power with -40 degree air (I know, this is only in the far north). The problem at -40 degrees is the fuel does not vaporize and therefor, you have dense air yet practic ally no fuel. Unvaporized fuel does NOT burn.

So with -40 degree air, you would develop less power than at 70 DEg.

A warmer engine vaporizes fuel better. So there is an optimum temperature for and engine that will allow it to get the most power from a given amount of fuel/air mixture.

More to follow.

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subvetSSN606
Senior Member
418 Posts
[Mentor]


Ellettsville, IN
USA

Suzuki

800 Intruder

Posted - 06/10/2006 :  5:14 PM
Thought I tried to express this earlier.
Given carbs...
You tune a bike for optimum performance at a given set of conditions, and that's where the bike will perform the best. Vary from those conditions, in any direction, and performance goes down, even if the amount of air/oxygen goes up, you don't get any more power because the bike was NOT tuned to take advantage of the increased air.
Does increased Oxygen increase your potential for power? You bet! But not without re-tweaking the carbs.

Tom
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