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Author Topic:   Runs Faster In The Cold
Kenstar posted 11-26-2003 01:28 AM ET (US)   Profile for Kenstar  
My boat runs faster in the cold than it does in the warmer weather. I see that you all are comparing performance and speed info with out anyone takin' in considerance for where the boat is runnin'. Up in the north with less humidity and warm weather you all will see about 7% better performance than in the good ole swamp where thangs gobout a bit slower.

I don't have any ideas about the cold Rockie mountains , but there is not too much oxygen and that may slow down those fat boats too.

Whalerdan posted 11-26-2003 10:39 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
I was in Denver last week. I don't think you'll run too fast in the mountains now, all the lakes were frozen.


kglinz posted 11-26-2003 11:14 AM ET (US)     Profile for kglinz  Send Email to kglinz     
The standard number for horsepower loss at altitude is 4% per thousand feet of altitude. That applies to all naturaly aspirated engines. I was recently at Lake Powell and a guy in a slip across from me had to drop the pitch on his prop 4 inches to get on plane.
Moe posted 11-26-2003 11:29 AM ET (US)     Profile for Moe  Send Email to Moe     
Temperature, barometric pressure/altitude, and humidity all affect the engine's ability to make horsepower. The software we use on DynoJet dynos allows correction based on several standards. Here's a calculator for naturally-aspirated engines based on the SAE correction factor:


Bigshot posted 11-26-2003 01:43 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I run a few RPM faster in the winter, especially compared to 93-degree Florida heat. My engine is EFI and it still makes a difference. Water temp might also on an EFI being she is running colder so the engine might not retard timing as much.
Tom2697 posted 11-26-2003 02:06 PM ET (US)     Profile for Tom2697  Send Email to Tom2697     
Cooler air is more dense than warm air. Dense air means there are more oxygen molecules per cubic foot. More oxygen in an engine means more power. More power means higher RPM can be reached.
jokor3 posted 11-26-2003 02:34 PM ET (US)     Profile for jokor3  Send Email to jokor3     
The atomization of fuel is greater when cooled, therefore better burn rate and better efficiency.
Jerry Townsend posted 11-26-2003 03:48 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
The answer primarily stems from the fact that the density (pound/cu-ft) of gasoline increases as the temperature decreases. That is, a gallon of cold gasoline will weigh more than a gallon of hot gasoline--and therefore a gallon of cold gasoline will have more energy than a gallon of hot gasoline.

If you could set your throttle setting precisely the same for the two cases, you would see higher RPM, higher speed, et al. Maintaining the same RPM in the two cases simply means that you will use less gasoline.

Now, before everybody heads north - where there isn't any water because of the drought and/or the cold temperatures, the effect is pretty small--like 1 to 3 percent.

And as Tom mentions, cooler air means denser air--which aids combustion--BUT it is detrimental regarding resistance and drag. That is, there will be more form drag from the air resistance - which counteracts against the benefit from the better combustion. --- Jerry/Idaho

Sal DiMercurio posted 11-26-2003 04:33 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Jerry is right. The more moisture there is in that cold air, the better the engine will run compared to dry cold air. When I was racing professionally, we would run down in Fresno (hot and dry) and the best my rig could turn would be 10,800 RPM annd 188 to 191 MPH. When we would run in the Oakland esturary and there would be a cool breeze coming in from the ocean and fog, my best times would consistently be between 194 & 196.5 MPH and 11,200 RPM.
These speeds were on a 21-foot Hondo Hydro with a full race Kieth Black Chrysler Hemi putting out 2,600-HP on nitro methalane.
Tom2697 posted 11-26-2003 05:28 PM ET (US)     Profile for Tom2697  Send Email to Tom2697     

Combustion does not depend so much on the volume of fuel, but on the mass of the fuel. This is why gasoline is rated in MJ/kg. The minutely changing density of liquid gasoline does not greatly affect the performance. Performance depends more on the pressure inside the combustion chamber. The greater the pressure, the more power that is produced. Combustion is also not entirely dependent upon the amount of fuel but rather the fuel/air mixture. This is why adjusting a carburator or the fuel/air mixture on injectors is so critical. Too much or too little fuel with degrade performance. Only with the proper mix ratio 12.5:1 (by weight) does max power come from the engine. But, max thermal efficiency of an engine comes around 16:1. Manufacturers use the computer in EFI engines to alter the fuel flow to match the available oxygen to optimize the engine for the required performance (cruise vs WOT). Now, if more oxygen is available because of denser air, more fuel can be added and hence, more power will result. This is entirely the principle of injecting NO2 into an engine. The added oxygen allows for more fuel to burn at a given time.

As my Thermodynamics professor told us, an engine is only an air pump. Using gas, diesel, or kerosene only increases the temperature inside the engine and allows for more volumetric flow to leave an engine than that enters it (Boyle's Law). The raised temperature after combustion expands the gases inside the engine and creates what most refer to as power. The compression of the fuel before combustion only increases the final pressure of the system after combustion and since PV=nRT, raises the temp and thermal expansion even further.

As for humidity affecting an engines performance, higher humidity helps performance. It lowers the octane requirement of an engine and increases the gaseous expansion inside the combustion chambers.

Moe posted 11-26-2003 06:45 PM ET (US)     Profile for Moe  Send Email to Moe     
Hyperlink to relevant article.

Perry posted 11-26-2003 07:53 PM ET (US)     Profile for Perry  Send Email to Perry     
Good link Moe, it relates directly to this thread. It states, like I assumed, that the lower the humidity the more power your motor produces.
Moe posted 11-26-2003 08:05 PM ET (US)     Profile for Moe  Send Email to Moe     
Higher humidity doesn't reduce horsepower a lot, but reduce it it does. The SAE correction factor is higher when humidity is higher and the engine is producing less power.


Jerry Townsend posted 11-26-2003 08:39 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Moe - thanks for the link - it properly addresses the effect of humidity. The material is good information for those interested in engine performance.

It also mentions one thing I thought about - after I pressed the send button - when I was advocating our cooler temperatures, I should have also mentioned the altitude is a bit higher which doesn't do engine performance any good - as explained in the article. ---- Jerry/Idaho

PSW posted 11-27-2003 02:34 AM ET (US)     Profile for PSW  Send Email to PSW     
As good a question as this is I think the author just wanted to see if he could slip in his favorite hompage under his profile. I am on the west coast so I must have read his profile while jim was sleeping. Pretty amazing that jim caught the inappropriate link so fast. I must say though it was someplace I had never been.

Anyway way good job jim.


jimh posted 11-29-2003 09:00 AM ET (US)     Profile for jimh  Send Email to jimh     
[Buffed up the spelling and capitalization in this thread. When a thread contains good information, it is a shame to mar it with typographic errors. No--I didn't catch the homepage link; it seems to have cured itself--jimh.]
rsgwynn1 posted 12-02-2003 10:54 PM ET (US)     Profile for rsgwynn1  Send Email to rsgwynn1     
Wouldn't a boat run faster in saltwater than in fresh because of the relative difference in buoyancy/weight?
lhg posted 12-03-2003 03:14 PM ET (US)     Profile for lhg    
My Mercury engine manual states that an internal combustion engine, including your auto engine, can lose up to 15% of it's power in hot, humid weather as opposed to dense, cool, dry air, often requiring a 2" drop in prop pitch. I think that these weather conditions can definitely account for some of the performance variances reported here.

It also seems to be my experience that a given hull will run faster in salt water, all other conditions being equal. Salt water also seems to give a softer ride.

Whalerdan posted 12-04-2003 08:34 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
I know this sounds crazy, and I'm not sure if it's physically true or not, but it might work into this equation of boat performance as a whole.

Back when I use to surf everyday in Cali, when a big cold wave broke on your head it was really hard. Then we went to the North Shore and surfed in big waves in warm water. Those waves where HUGH and they didn't hurt half as bad as the cold-water waves back home.

Anyway, I believe that the temperature of the water must have some sort of affect on the ability of the hull to plane and the foot to cut through the water. Wonder it this affect would add or subtract from the benefits to engine performance?


Kenstar posted 12-05-2003 12:32 AM ET (US)     Profile for Kenstar    
I thought this topic would generate a bit mor dilogue than it has. i suspect that many on this site overlooked the temp. influences on performance and/or are unaware.Some of yoall came close to the reasons why a motor reacts in various enviroments, but I guess a real topic on this forum gets little play. maybe ishould have asked about foam .

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