RPMs and prop
 

Chris Newport wrote:

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.

You been running engines on JaxWorld or something?

Rick

RPMs and prop
 

"Chris Newport" wrote in message
news:2363140.IDQUMr8mnb@callisto...

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.


Smaller jets are required as the altitude is increased.
Typically 1 size down for each 2000' in altitude.

RPMs and prop
 

16 pitch is not much of a prop. Are you sure about the pitch? With an open
bow 21 foot boat I would expect you to be able to easily turn that prop even
at altitude with a 5.0. Unless the boat is particularly heavy. Are you
sure the engine is running well? Have you had this boat all it's life and
what's the past performance history?

First step in propping a boat is to make sure the engine is running at
optimum performance.

"James Gemmill" gemmilljim@hotmail,com wrote in message
...
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.

RPMs and prop
 

On Monday 07 June 2004 9:40 pm in rec.boats Rick wrote:

Chris Newport wrote:

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.

You been running engines on JaxWorld or something?

Hey - stop baiting the wrong guy.
I spent many years tweaking engines (mostly rally cars but some boats)
in South Africa for high altitude operation. Unlike Jax I actually know
my facts. Living at 6000 feet can be interesting.

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

RPMs and prop
 

"Chris Newport" wrote in message
news:1805950.6eUoHazK65@callisto...

Hey - stop baiting the wrong guy.
I spent many years tweaking engines (mostly rally cars but some boats)
in South Africa for high altitude operation. Unlike Jax I actually know
my facts. Living at 6000 feet can be interesting.



http://www.saabclub.com/242/altitude.htm#physics
http://www.4strokes.com/tech/howtojet.asp
http://www.mortec.com/carbtip1.htm
http://www.redriverdirtriders.org/Te...%20Details.htm

RPMs and prop
 

Chris Newport wrote:

I spent many years tweaking engines (mostly rally cars but some boats)
in South Africa for high altitude operation. Unlike Jax I actually know
my facts. Living at 6000 feet can be interesting.

Didn't you ever wonder why they ran so rough and sooted up the plugs and
exhaust so badly?

Rick

RPMs and prop
 

On Monday 07 June 2004 11:37 pm in rec.boats Gene Kearns wrote:

On Mon, 07 Jun 2004 20:21:13 +0100, Chris Newport
wrote:

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.

Fuel injected engines will behave differently...

Nope. Not wrong. Trust me, Chris.... not wrong. I am a aircraft
pilot and mechanic....

This is getting interesting, if slightly off topic.
I can say with certainty that we always needed larger jets in the
Ford Lotus Cortinas (1640 twin cam) in the Transvaal compared to
Durban or CapeTown. The issue is complicated by different fuels, 83
octane in the Transvaal and 98 at the coast.

Every aircraft piston engine, carbureted, fuel injected, naturally
aspirated or supercharged(above critical altitude) has some
contrivance to lean the mixture at altitude.

Yes but it is not that simple - for example you need a richer setting
for best power in the climb and leaner for economy in the cruise.
I was taught to adjust for best RPM before takeoff and then adjust
for EGT. Aircraft carburettors are a completely different animal
to the automotive variety which do not have to deal with extememes
of both pressure and temperature as well as such delights as ram air
and carb heating. I never saw a car with a priming pump and dual mags.

And you *need* to use it or you'll foul plugs and soot-up the
engine.... I've fixed far too many fouled plugs from pilots that were
afraid to lean the mixture enough for fear of burning valves.

Having driven a pair of Wasps for many years I appreciate your point
but I assure you that I always managed to keep them running sweet.
Thankfully they were replaced by PT6s which are a lot less tempremental.
[ You will probably be able to figure out which airframe this was, and
thus my probable age, but dont tell the group B-). ]


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

RPMs and prop
 

Chris Newport wrote:

Aircraft carburettors are a completely different animal
to the automotive variety which do not have to deal with extememes
of both pressure and temperature as well as such delights as ram air
and carb heating. I never saw a car with a priming pump and dual mags.

They are no different in the principle of operation. There is no magic
in an aircraft carb, most of them used on small engines very much less
complex than automotive units.

The priming pump is not a part of the carb, automotive carb heat could
be said to be supplied automatically by the manifold heat riser, and the
carb doesn't know ram air exists, it only knows flow. The carb could
care less what the atmospheric pressure is, only the pressure difference
between the throat and the bowl (if there is one) and temperature only
effects the mass of the air passing through and thus the mixture
requirement.


Having driven a pair of Wasps for many years I appreciate your point
but I assure you that I always managed to keep them running sweet.
Thankfully they were replaced by PT6s which are a lot less tempremental.
[ You will probably be able to figure out which airframe this was, and
thus my probable age, but dont tell the group B-). ]

Beech 18 or maybe a Goose ... spent couple of years hauling mail in an
18 across the mountains of Montana. Will never forget the winters, never
care to repeat them either.

Rick

RPMs and prop
 

Gene Kearns wrote:

Having driven a pair of Wasps for many years I appreciate your point
but I assure you that I always managed to keep them running sweet.
Thankfully they were replaced by PT6s


Arggggggghhhhhh..... heresy......

Not when you're is loaded with 3000 pounds of mailbags, it's 0230 and
you have 100 miles to go across the Rockies against a 100 knot headwind
and icing is fluctuating between heavy and "oh f..k" and Missoula is
bouncing around minimums it's not!

And that is no sea story G

Rick

RPMs and prop
 

"Joe" wrote in message ...
"Chris Newport" wrote in message
news:1805950.6eUoHazK65@callisto...

Hey - stop baiting the wrong guy.
I spent many years tweaking engines (mostly rally cars but some boats)
in South Africa for high altitude operation. Unlike Jax I actually know
my facts. Living at 6000 feet can be interesting.



http://www.saabclub.com/242/altitude.htm#physics
http://www.4strokes.com/tech/howtojet.asp
http://www.mortec.com/carbtip1.htm
http://www.redriverdirtriders.org/Te...%20Details.htm

Unlike having real knowledge, JoeTechnician just argues about what
he's learned on the internet!!!! In his eyes, if it's not on the
internet, then it's either not true, or not debatable, because that's
all the knowledge he has! Hell, I'm still waiting for that reciprocity
for a GA engineer, that only requires a contractors license from
another state!!!