The rule of thumb for altitude induced horsepower loss is 3% per 1000’ of elevation gain but that assumes standard atmospheric conditions. What pilots are trained to use is density altitude, engine tuners also use this. For pilots this is crucial in terms of corrected power and flight performance, you’ve got to know how much runway you are going to need to take off and land as well as climbing performance. When the air is less dense air speed remains the same but ground speed dramatically increases, that’s the reason jets fly at such high altitudes.
Here’s the Pilot Friend density altitude calculator (
density altitude calculator ) for Lake Tahoe yesterday, I just went to the National Weather Service for Lake Tahoe airport (
Time Series Viewer ) to get the data to fill in the chart for yesterdays high of 63°, altimeter of 30.28, dew point of 35° which results in a density altitude of 7691’, or the air only being 79.36% of sea level at standard atmospheric conditions, or roughly a 20% power loss at rated rpm.
Just to confirm the HP loss I brought up an engine tuners calculator (
Engine Tuning Calculator - using relative humidity ) plugged in the same data except this calculator uses relative humidity instead of dew point and it shows HP at 80.4% of sea level.
Look what happens on a 95° at Lake Tahoe, almost 26% power loss.
With your oem impellers at high altitude there is a bit of a double whammy happening. As opposed to a car with a transmission your boat is direct drive, kinda like a velodrome bicycle, so it can’t use a low gear to compensate and get the rpm’s up to rated rpm where the horsepower is its greatest, so not only is the gear too high so to speak, but the engine can’t turn the rated rpm for its max power output, the less air it pumps the less fuel it can burn which is less power. To put it another way, the calculators show hp loss at a specific density altitude but that is at rated rpm, so if your engine can reach rated rpm the perceived hp loss is even greater.
I mainly boat at 5000’ and in the height of summer I’m seeing as high as 30% power loss. My boat had dismal performance especially with five people on board and pulling a tube. 6900 rpm is all I could get on those days. Here’s a link to the thread I did on my journey with high altitude hp loss.
High Altitude Impellers As you will see I got a lot of performance by going to high altitude impellers. My suggestion would be to buy a set of impellers and have them pitched so you can keep your oem impellers in case you want to run down to Sac or other low elevation places since Lake Tahoe is so high.
You now have two choices for impeller pitching and repair services, Impros (
Impros – The Impeller Professionals ) and Leading Edge (
Leading Edge Impellers | PWC - Jet Boat Impellers - Pump Parts - Repair ), Leading Edge was not in business yet when I started my journey with high altitude impellers. The owner of Leading Edge is the founder of Impros and sold that business to Glen the current owner of Impros. I did have to test several iterations to get the pitch just right and to find the best compromise, you will find as I did there is a noticeable difference in engine performance between cooler / cold days and hot days, the difference is stark to say the least. Having said that I ended up with impellers that will produce 7500 rpm on a hot day and close to 8000 rpm on a cold day at 5000’ above MSL (mean sea level). I boat on lakes between 5000’ and 1250’ so it took me several tests of different pitches to get the best compromise
I also picked up 200 rpm by going with a RIVA K&N style air filter, that was well worth the cost as I’m clawing for every rpm I can get.
Also, at a minimum run what octane fuel is specified in your manual, I’ll guess that it is 86 octane, but verify that. If I am correct about your octane requirement at your altitude you can easily run 85 octane. This is because the cylinder pressure is lower at high altitude than at sea level, this lower cylinder pressure is the main reason why your engine is lacking power. The rule of thumb for naturally aspirated engines is a decrease of one octane number per 1000’ of elevation gain if memory serves. Super charged engines is .5 octane decrease per 1000’ and turbocharged engines run the same octane at all elevations. Used to be up at high elevations high octane fuel was not available, just 85, it’s still that way in some places I’ve been.
There is a common belief that higher octane fuel has more power, it does not. Higher octane fuel has the same BTU’s that lower octane fuel has, the higher octane fuel is intended for higher compression engines (higher cylinder pressure), if higher octane fuel is used in an engine designed for lower octane fuel that fuel will burn slower, have less power and greater heat than the lower octane fuel.
Summary, your boats engines will still have less hp at Lake Tahoe than down in Sac, but you can compensate for this by getting the correctly pitched impellers (re propping an outboard, I/O, or stern drive is a common practice) for your altitude, running the specified octane fuel, getting a set of RIVA K&N style air filters, and using the correct engine oil for the temps you will be in, I’m going to guess that 10W-30 would be best for your area as opposed to 10W-40.
