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RPMs and prop
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. |
RPMs and prop
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. |
RPMs and prop
That 4000 ft altitude may have something to do with decreased performance. A
bit less oxygen to support combustion. |
RPMs and prop
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 |
RPMs and prop
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 |
RPMs and prop
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 |
RPMs and prop
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. |
RPMs and prop
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 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. |
RPMs and prop
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. WinXP |
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!!! |
RPMs and prop
"Kevin Noble" Wrote in message om... 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!!! Just providing links to back up my original statement, dip****. Don't agree with it? |
RPMs and prop
"Joe" wrote in message .. .
"Kevin Noble" Wrote in message om... 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!!! Just providing links to back up my original statement, dip****. Don't agree with it? I love how you hide, and cow down every time I mention that I'll be in your area, and would like to meet you, but you are still stupid enough to call people names, even though you are too much of a sissy to back up anything you say! And, no, you don't simply provide links to back up "what you say", your total knowledge rests on the internet. Face it. You don't know ****, and never will. You are too narrow minded to learn. |
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