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Retired Air Force
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My jet boat ran 110 @ 5900 RPM back in March but has now lost 200 RPMs and 5mph. Will a cooler vs hotter days make this much difference in hp?
 

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"Need For Speed"
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My jet boat ran 110 @ 5900 RPM back in March but has now lost 200 RPMs and 5mph. Will a cooler vs hotter days make this much difference in hp?
Sarge,

Yes, very possible. I ran my boat on a cool day back in April. The barometer was high and humidity low with a density altitude close to "0" feet. Then comes a race day in May where the temperature was in the high 90's, barometer quite a bit lower and humidity high with a calculated Density Altitude of over 3000'. I lost 200 RPM on the same impeller, same boat setup, etc and close to 4 mph.

Same thing about lakes in this area. We can run a local lake which is about 300' above sea level on a Spring day, then go to a popular lake in Oklahoma that is 600' above Sea Level on a Summer day and loose a hundred ot two
RPM.

Gear

PS- My deal is normally aspirated - carbs on gas.
 

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Definitely! You can't expect to make the same power on a hot day vs a cold. Those numbers sound right to me, when it 90 degrees I run about 4mph faster than when it's 110 out.
 

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Back when I ran NHRA Super Comp/Gas I had a 540 powered Lumina, and we ran off an 8.90 or 9.90 index depending on the class, for each 160' change in corrected altitude my et would change .01, one hundredth of a second. That ratio was pretty consistent any where from Boise ID., to Tucson AZ. to the Pacific ocean.
 
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