AQ140 power loss at high altitude

My 79' AQ140A powers my 19' Glasply at 40 mph at sea level, taching at 5200 rpms with ease. I operate it year round at 5,000' and cannot get to 4,000 rpms. Have changed everything we can think of, no luck. (twin side draft solex carbs) I run a fishing guide business, this is primary boat. Engine completely rebuilt, runs beautifully, just very underpowered at this altitude

Re: AQ140 power loss at high altitude

Every 1000' of altitude takes away about 3% of your engine's HP, so at 5000' your HP is down 15%. Have you tried a lower pitched prop? 2" less pitch would be my guess...

I'm running a 19P prop (recently switched to a 18P 4 blade) flat land and a 15P up at 6500-8800'.

Oh yeah, if you are running up at 5000' all the time, re-jetting the carb will cure the engine running too rich. I'm not sure of Volvo specs, but Mercruiser recommends re-jetting above 5000'. (0-5000' leave stock, 5000-9000 one size, 9000' and above another size.) DonS might be able to dig up the Volvo specs for you. I wouldn't do this unless you ALWAYS run at high elevation though, unless you like tearing carbs apart every time you go down in elevation......

Re: AQ140 power loss at high altitude

Fishrdan, Thanks for the info, I have re propped gradually down to a 13", re jetted numerous times, sizes, rebuilt the carbs, changed plugs, wires, timing, fuel, compression is good.. has good, working trip tabs. I run it exclusively now at 5,000' - 6,000' (depending on which lake I am on for the day) The boat is perfect for the harsh weather conditions up here, just no power. With 3-4 clients on board, I am lucky to make 14-15 mph..

Re: AQ140 power loss at high altitude

Sounds like the boat doesn't have enough power for that elevation and load. I assume the 79 AQ140A = 140HP, that would be 140HP at the crank, so it's about 125HP to the prop. Factor in the 3% elevation loss and the engine is only putting out a bit over 100HP to the prop. That isn't much power for a 19' glass hull and a load of 4-5 adults.

I run a 140HP Mercruiser in a 18' aluminum hull, 2000# boat. It hauls butt down at 1000', but at 6500-8800' elevation it's marginal at best with 2 adults and 2 kids, 600-800# of people and gear. That's with dropping 3-4" of prop pitch and re-jetting the carb.

Is there a lower gear ratio available for your outdrive? It won't be cheap, but might solve your problem and get you back up in a higher pitched prop.

What props have you tried, pitch and diameter?

Here's something I pulled from Mercruiser Manual 26, even though you have a Volvo, it's still good info for your problem.

Elevation has a very noticeable effect on the wide-open-throttle power of an engine. Since
air (containing oxygen) gets thinner as elevation increases, the engine begins to starve for
air. Humidity, barometric pressure and temperature do have a noticeable effect on the
density of air. Heat and humidity thin the air. This phenomenon can become particularly
apparent when an engine is propped out on a cool dry day in spring and later, on a hot, humid
day in August, does not have the same performance.
Although some performance can be regained by dropping to a lower pitch propeller, the
basic problem still exists. The propeller is too large in diameter for the reduced power output.
A Quicksilver Propeller Repair Station or experienced marine dealer can determine how
much diameter to remove from a lower-pitch propeller for specific high-elevation locations.
In some cases, installing high altitude gears in the drive unit is possible and very beneficial.
Weather conditions may effect the power output of internal combustion engines. Therefore,
established horsepower ratings refer to the power that the engine will produce at its rated
rpm under a specific combination of weather conditions.

Re: AQ140 power loss at high altitude

The AQ140A is only 125 hp at the flywheel, not the prop. It's only a 2.31L /140 cu in engine, and in a 19ft. boat fully loaded at that altitude, it's just not enough engine.

Re: AQ140 power loss at high altitude

Don S and Fishrdan, thanks for the info on this. You have given me several ideas but it sounds like I may be 'beating my head against a rock' if it just isn't possible with the existing powerplant. I would really like to put a different engine in it. The AQ140 is in great condition but just not for up here. I have been so frustrated with it for the last 5 years of experimentation of 'fixes' to the problem.... (I have considered the 'blower', and 'turbo' options) I am guessing it is just time to figure out how to upgrade the motor. Thanks again.