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#1
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Cruising RPM vs. WOT
All,
I have a 1971, 21' Fiberform that has a 165 Mercruiser (250 cid inline Chevy 6 cylinder). WOT on the tachometer registers @ 4200 RPM. I believe that the Rochester 2 barrel carburetor is governed for that RPM. I am running a 17 pitch aluminum prop which came with the boat, but have contemplated a more aggressive pitch as well as stainless steel. The outdrive is geared @ 1.68 to 1. This all equals to 32 mph according to the fish finder/gps unit @ WOT. The original owners manual states that "3900-4300 RPM lets the engine turn at the recommended RPM and gives best performance". It also states that I could go up to a 21 pitch prop and still be in this window. The boat currently has very good hole shot. I am more about getting to the fishing hole than pulling a skier. My question is, what is a good RPM for cruising without thrashing on the motor and leading to premature engine failure? The recommended RPM seems a little high compared to what my truck's RPM is cruising down the interstate (2800 RPM). Any suggestions? Tim |
#2
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Cruising RPM vs. WOT
You can't "govern" an engine with the carburator. The rpm at wide open
throttle is controlled by the pitch of the prop. You can't go to a 21 pitch since you're already at 4200 with a 17 pitch. You might be able to go to a 19 pitch but the 17 is probably good, particularly if you did not have the boat fully loaded when you tested the wot rpm. Boat rpm is high compared to cars. Best cruising is probably around 3400. "TJF" wrote in message . .. All, I have a 1971, 21' Fiberform that has a 165 Mercruiser (250 cid inline Chevy 6 cylinder). WOT on the tachometer registers @ 4200 RPM. I believe that the Rochester 2 barrel carburetor is governed for that RPM. I am running a 17 pitch aluminum prop which came with the boat, but have contemplated a more aggressive pitch as well as stainless steel. The outdrive is geared @ 1.68 to 1. This all equals to 32 mph according to the fish finder/gps unit @ WOT. The original owners manual states that "3900-4300 RPM lets the engine turn at the recommended RPM and gives best performance". It also states that I could go up to a 21 pitch prop and still be in this window. The boat currently has very good hole shot. I am more about getting to the fishing hole than pulling a skier. My question is, what is a good RPM for cruising without thrashing on the motor and leading to premature engine failure? The recommended RPM seems a little high compared to what my truck's RPM is cruising down the interstate (2800 RPM). Any suggestions? Tim |
#3
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Cruising RPM vs. WOT
d
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#4
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Cruising RPM vs. WOT
If anything I'd suggest that your truck needs an overdrive to get its
rpm down on the highway unless its towing or a 4 cyl. |
#5
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Cruising RPM vs. WOT
TJF wrote:
All, I have a 1971, 21' Fiberform that has a 165 Mercruiser (250 cid inline Chevy 6 cylinder). WOT on the tachometer registers @ 4200 RPM. I believe that the Rochester 2 barrel carburetor is governed for that RPM. I am running a 17 pitch aluminum prop which came with the boat, but have contemplated a more aggressive pitch as well as stainless steel. The outdrive is geared @ 1.68 to 1. This all equals to 32 mph according to the fish finder/gps unit @ WOT. The original owners manual states that "3900-4300 RPM lets the engine turn at the recommended RPM and gives best performance". It also states that I could go up to a 21 pitch prop and still be in this window. The boat currently has very good hole shot. I am more about getting to the fishing hole than pulling a skier. My question is, what is a good RPM for cruising without thrashing on the motor and leading to premature engine failure? The recommended RPM seems a little high compared to what my truck's RPM is cruising down the interstate (2800 RPM). Any suggestions? Tim That's a big boat for the 165. If your holeshot is good, you are already in the "sweet spot" prop wise. A good 17" SS cupped prop with adequate surface area will give a better bite and let you trim out more before blowing out, resulting in more boat out of the water & a slightly higher top end. It's a real common prop - you may be able to borrow one for a test run. A 19" or 21" will most likely cause a drop in holeshot & top speed, but stranger things have happened. The 165 straight six is a "torque-y" engine with good grunt, but it likes to run 4400 too. A very smooth running engine due to the inherent balance of the I-6 configuration. Best cruising range is almost always 3000-3500 rpm when propped right & on plane. The 165 is not too bad on gas w/ the 2bbl Rochester anyway. Rob |
#6
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Cruising RPM vs. WOT
On Mon, 14 Nov 2005 16:57:51 -0700, "TJF"
wrote: My question is, what is a good RPM for cruising without thrashing on the motor and leading to premature engine failure? The recommended RPM seems a little high compared to what my truck's RPM is cruising down the interstate (2800 RPM). Any suggestions? ================================= For a gas engine with WOT in the normal range (as yours is), recommended cruising RPM is about 70% of WOT, typically 3200 to 3400. At 70% you will be developing about 50% of the engines rated power since horsepower varies as the square of RPMs (.7 x .7 = .49). |
#7
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Cruising RPM vs. WOT
The Merk 165 has been a good engine over the years and may deliver the
torque you need to turn a 19 or even a 21 pitch prop. (Depending on a number of factors). If you can find a used or loaner prop form a friend or marina with the understanding that you'll purchase it from the dealer if it accomplishes your goal or if you damage it. Only an live test will answer the question. You COULD overload the engine with a higher pitch prop and loose speed, so a trial is in order in my opinion. Typically a thinner stainless steel prop pitch for pitch will provide a minor performance improvement. Don Dando "TJF" wrote in message . .. All, I have a 1971, 21' Fiberform that has a 165 Mercruiser (250 cid inline Chevy 6 cylinder). WOT on the tachometer registers @ 4200 RPM. I believe that the Rochester 2 barrel carburetor is governed for that RPM. I am running a 17 pitch aluminum prop which came with the boat, but have contemplated a more aggressive pitch as well as stainless steel. The outdrive is geared @ 1.68 to 1. This all equals to 32 mph according to the fish finder/gps unit @ WOT. The original owners manual states that "3900-4300 RPM lets the engine turn at the recommended RPM and gives best performance". It also states that I could go up to a 21 pitch prop and still be in this window. The boat currently has very good hole shot. I am more about getting to the fishing hole than pulling a skier. My question is, what is a good RPM for cruising without thrashing on the motor and leading to premature engine failure? The recommended RPM seems a little high compared to what my truck's RPM is cruising down the interstate (2800 RPM). Any suggestions? Tim |
#8
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Cruising RPM vs. WOT
On Mon, 14 Nov 2005 16:57:51 -0700, "TJF" wrote:
All, I have a 1971, 21' Fiberform that has a 165 Mercruiser (250 cid inline Chevy 6 cylinder). WOT on the tachometer registers @ 4200 RPM. I believe that the Rochester 2 barrel carburetor is governed for that RPM. I am running a 17 pitch aluminum prop which came with the boat, but have contemplated a more aggressive pitch as well as stainless steel. The outdrive is geared @ 1.68 to 1. This all equals to 32 mph according to the fish finder/gps unit @ WOT. The original owners manual states that "3900-4300 RPM lets the engine turn at the recommended RPM and gives best performance". It also states that I could go up to a 21 pitch prop and still be in this window. The boat currently has very good hole shot. I am more about getting to the fishing hole than pulling a skier. My question is, what is a good RPM for cruising without thrashing on the motor and leading to premature engine failure? The recommended RPM seems a little high compared to what my truck's RPM is cruising down the interstate (2800 RPM). Any suggestions? Tim About 3400 for me! 5.7L GMC -- John H. "Divide each difficulty into as many parts as is feasible and necessary to resolve it." Rene Descartes |
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