According to my local service men I should be able to turn about 4600
RPMs with my 5.0 liter mercruiser. I am only turning 4200 at full
throttle, on plane and empty except for drive and one passenger. My
service guys say I can burn the valves at this rate.

What say you all?

Elevation of test, about 4000 feet above sea level, 5.0 liter Cobalt
21 foot open bow, 450 hours on engine.

Thanks in advance for any helpful info.

On Mon, 07 Jun 2004 07:28:56 -0700, James Gemmill
gemmilljim@hotmail,com wrote:
According to my local service men I should be able to turn about 4600
RPMs with my 5.0 liter mercruiser. I am only turning 4200 at full
throttle, on plane and empty except for drive and one passenger. My
service guys say I can burn the valves at this rate.
===========================================

4200 is generally regarded as being at the low end of "acceptable".

Burning valves shouldn't be an issue if you keep your cruising RPMs
below 3400 and you're running at least 89 octane gas.

If you are still concerned, switch to a prop with slightly less pitch.

That 4000 ft altitude may have something to do with decreased performance. A
bit less oxygen to support combustion.

James Gemmill wrote:

Elevation of test, about 4000 feet above sea level ...

At that altitude you are producing considerably less than the rated sea
level power output to begin with. If the temperature of the intake air
is much above 45 degrees F you will lose even more.

The chances of burning exhaust valves at reduced power are pretty slim,
especially if the engine was not adjusted for altitude and is running rich.

Does the boat live at this altitude or did you just take it up to a
mountain lake for the weekend and found the performance less than you
expected? Is your mechanic based at sea level or a local guy on the lake
who is familiar with local conditions and boats and knows your setup?

Rick

On Mon, 07 Jun 2004 15:32:29 GMT, Rick
wrote:

James Gemmill wrote:

Elevation of test, about 4000 feet above sea level ...

At that altitude you are producing considerably less than the rated sea
level power output to begin with. If the temperature of the intake air
is much above 45 degrees F you will lose even more.

The chances of burning exhaust valves at reduced power are pretty slim,
especially if the engine was not adjusted for altitude and is running rich.

Does the boat live at this altitude or did you just take it up to a
mountain lake for the weekend and found the performance less than you
expected?

The boat lives at 3545 but there is no water here, Arizona has water
in only a few places. I took it to lake Powell where elevation is a
little greater.

The mechanics are here at 3545 and one in Flagstaff at 7200.

Is your mechanic based at sea level or a local guy on the lake
who is familiar with local conditions and boats and knows your setup?

This might be a good tip, I'll checd with local mechanics at Lake
Powell next time I'm there.

Rick

James Gemmill wrote:
This might be a good tip, I'll checd with local mechanics at Lake
Powell next time I'm there.

If they are familiar with your boat/engine combination they should be
able to tell you if you are getting anywhere close to the performance
expected or possible under the conditions existing at the lake.

It sounds like the mechanic you mentioned my have just been stating the
published figures based on sea level performance. At high elevations and
temperatures your engine simply will not produce as much power as it
will at sea level conditions and may be effectively "overpropped."

There is a combination of prop size and rpm (maybe higher than 4600)
that will allow the engine to produce as much power as possible at the
lake but the boat will still not perform as well as the best prop/rpm
combination at sea level.

Good luck.

Rick

On Monday 07 June 2004 3:28 pm in rec.boats James Gemmill wrote:

According to my local service men I should be able to turn about 4600
RPMs with my 5.0 liter mercruiser. I am only turning 4200 at full
throttle, on plane and empty except for drive and one passenger. My
service guys say I can burn the valves at this rate.

Hmmm --- Transvaal disease.
Your engine will produce about 20% less power at 4000 feet unless
it is turbocharged. The air is thinner so there is less oxygen.

Firstly get a finer pitch prop to get your RPM back up.
You should probably use different carburetter jets or different
fuel injection settings for high altitude operation, ask the
manufacturer for advice on this point, it is possible to burn valves
if the mixture is too lean. Some engines run rich enough at sea level
to be OK at 4000 feet, others will run too lean and therefore hot.


--
My real address is crn (at) netunix (dot) com
WARNING all messages containing attachments or html will be silently
deleted. Send only plain text.

On Mon, 07 Jun 2004 11:59:18 -0400, "Gene Kearns"
wrote:

On Mon, 07 Jun 2004 07:28:56 -0700, James Gemmill
gemmilljim@hotmail,com wrote:

According to my local service men I should be able to turn about 4600
RPMs with my 5.0 liter mercruiser. I am only turning 4200 at full
throttle, on plane and empty except for drive and one passenger. My
service guys say I can burn the valves at this rate.

What say you all?

Elevation of test, about 4000 feet above sea level, 5.0 liter Cobalt
21 foot open bow, 450 hours on engine.

Thanks in advance for any helpful info.

I guess the first question to ask is..... "has it always run like
this, or has anything changed?"

It was worse until I changed to a less pitch prop. I think it is a
16 now.

If the boat does not stay at that altitude you may want two different
props to allow for similar performance at differing altitudes.

On Monday 07 June 2004 7:38 pm in rec.boats Gene Kearns wrote:


Firstly get a finer pitch prop to get your RPM back up.
You should probably use different carburetter jets or different
fuel injection settings for high altitude operation, ask the
manufacturer for advice on this point, it is possible to burn valves
if the mixture is too lean. Some engines run rich enough at sea level
to be OK at 4000 feet, others will run too lean and therefore hot.

An engine at 4000 feet needs less fuel to air.... since the air is
thinner. It will burn less fuel and produce less horsepower.

Unless accounted for, an engine that runs properly at sea level will
likely be too rich at "altitude."

Sorry but you are wrong. For a given throttle opening the same volume
of less dense air passes through the carburettor venturi. The partial
vacuum created in the venturi is smaller, sucking less fuel into the
airstream and thus giving a weak mixture. Thus larger jets are
required at altitude to enrich the mixture.

Fuel injected engines will behave differently, the result will depend
on the amount of intelligence in the control system, the type of
environmental sensors used, and the firmware logic in the controller.
Some will get it right, others will screw up rather badly at altitude.

--
My real address is crn (at) netunix (dot) com
WARNING all messages containing attachments or html will be silently
deleted. Send only plain text.

On Mon, 07 Jun 2004 07:28:56 -0700, James Gemmill gemmilljim@hotmail,com wrote:

According to my local service men I should be able to turn about 4600
RPMs with my 5.0 liter mercruiser. I am only turning 4200 at full
throttle, on plane and empty except for drive and one passenger. My
service guys say I can burn the valves at this rate.

What say you all?

Elevation of test, about 4000 feet above sea level, 5.0 liter Cobalt
21 foot open bow, 450 hours on engine.

Thanks in advance for any helpful info.

Hi.

All is OK.

Consult your engine performance table or specifications and you will find out that EVERY combustion engine LOSSES 1 % of
it Power every 100 meter above sea level. (about 1% every 330 feet asl.

So you power lost will be around 12 % equivalent to 450-550 RPM at 4,000 feet asl.

Consider else "Tacho error" varying from 8-to 10 % more or less.

There is nothing to do to increase power back to Sea level task.

You could increase performance to original increasing of the same ratio air intake (Turbo) or Gasoline Ottans (or N2O)
or Carburetor Yet (+ 10% bore) or increasing Exhaust pipelines and Silencers...

Else a less "fine" Air Filter" (or no filter of all) could increase performance (no dust there... I suppose...).

Else Temperature decreases Power 1 % every 4°C less than 20 °C.

Suggestion is to put intake over the muffler just to have hot air intake.

Not sure but else the weight of the Floater inside Carb could be varying the Gasoline supply.

A more fluid Oil has to be used both inside Engine than inside Gearbox, or your power will be wasted to turn "glued"
gears.

Else a propeller with a 10 % less pitch could be useful.

....and this is only "some" things to verify for...

You could else remove the Alternator Belt and spare some HP in this way...

Good Luck.


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