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High Elevation max mph problem

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Doshlaw

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I have a 2000 Maxum 2400 scr. Bought at 200 ft. elevation. Stock main jets 1.65, power valve jet .90. Gear ratio is 1.62:1. Mercruiser 305 cid 220hp.
Now run at 6200 ft elevation. Changed main jets to 1.45 and power valve jet to .74 per Mercruiser tech. Prop was a 15/16 4 blade, now 15/15 3 blade.
200 ft elevation pre jet and prop change I got 4400 rmp and 40 mph. At 6200 ft. got 3800 rpm and 29mph. After jet and prop change I now get 4000rpm and still have only 29mph. I want more top end speed.

Any suggestions other than motor and or gear ratio change????
 
The air at a 6K feet elevation is less dense, therefor your cylinders are not able to compress the air/fuel mixture enough to create the same hp that you had at a 200 feet elevation.
Since you are not open to a drive ratio change, short of turbo-charging this engine you'll just have to live with it.


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Doshlaw said:
I have a 2000 Maxum 2400 scr. Bought at 200 ft. elevation. Stock main jets 1.65, power valve jet .90. Gear ratio is 1.62:1. Mercruiser 305 cid 220hp.
Now run at 6200 ft elevation. Changed main jets to 1.45 and power valve jet to .74 per Mercruiser tech. Prop was a 15/16 4 blade, now 15/15 3 blade.
200 ft elevation pre jet and prop change I got 4400 rmp and 40 mph. At 6200 ft. got 3800 rpm and 29mph. After jet and prop change I now get 4000rpm and still have only 29mph. I want more top end speed.

Any suggestions other than motor and or gear ratio change????

Thank you for the responses. It looks like an engine change is the way to go. Is it possible to go to a 300 hp fuel injected engine from my 220 hp carb engine and would I need to re-gear as well with a 300 hp?

Again, thank you for your replies. It helps alot.
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I recall driving a rental car at 8,000 foot altitude--felt like half the lug wires fell off!

Other than supercharging, there's little one can do to improve the power with a 305. A 350 is not any larger externally, weighs about the same, and would allow that speed to come on back.


Jefff
 
What outdrive are you using? You would be near the top end of of horsepower rating on an Alpha 1 Gen 2 at 300hp.

Doesn't your engine have a rating up to 5000 rpm? If so then you have some room to change the pitch some more. Are you running a cupped blade on the prop?

Is it the model with the 2bbl carburetor? If it is then you have some room for improvement on the intake side of the engine.
 
Top rated rpm is 4800. The prop blades are not cupped. 2 bbl carb. Talked with Merc techs today. Looks like the solution is a gear ratio change from 1.62 to 1.81. They say an engine change will not solve my problem. They also stated that if the boat is rated for 4400-4800 rpm and I am running at 4000 max, I am going to damage the engine. Change of gear ratio will be best solution and give me more rpm and top mph. They indicated that I might want to go to a 4 blade prop.
 
Turning the prop takes torque and that is what the gear swap will give you. That may get your torque and engine RPM up but may not increase your top speed. The key to efficient engine performance is volumetric efficiency. That is the ability to get a cylinder full of fuel (air and gasoline). The 2bbl carb on the chevy 305 decreases the engines ability to get a cylinder full of fuel. You can see this by looking at the power rating of engines with 2bbl vs 4bbl carburetors with fuel injection giving an increase over carburation. A larger engine will make a difference and it is not even logical that they would say otherwise but they must own their claim. There is no replacement for displacement. The only way that running at 4000 RPM would be damaging is if you run often and long periods at wide open throttle (WOT).Clearly there are engines with enough torque to run the outdrive with 1.62 gears in them or they would not have that gear combination. You can figure a 2% derate in horsepower for every 1000ft above sea level increase. My boat was under powered with a 2bbl carburetor, even with the prop changed to allow the engine RPM to be near the top end of the engine rating. I had to run everywhere at WOT to stay on plane. Anything less and I would fall off plane. In my quest for power for my Mercruiser 5.7l I first swapped the 2bbl intake manifold and carb with a 4bbl that I found on-line off of an engine of the same year. It is not hard to do the swap. That improved the power but was not enough to satisfy me. I dug into the engine to find that it had bad heads and a couple of bad cylinders. I took it to a machine shop and had them bore and stroke the engine to 383 cu.in, also a possibility for you. The machine shop put a crankshaft designed for a 400Cu.in. small block into the 5.7l small block. The resulting increase in torque and power were incredible. The added torque permitted the boat to stay on plane at part throttle, seriously improved the hole shot and top end speed of the boat. A failed raw water pump forced me to pull the engine which was producing about 330 Hp at 315 ft.lb of torque. This is over what the Alpha 1 Gen 2 is rated to take I finally replaced that engine with a 6.2 MPI with a Bravo 3. No one would recommend pushing a recreational boat around at WOT. The engine should be able to easily put the boat on plane and have enough torque to maintain it there at a reasonable reduction in engine RPM so that it is not pushed to its maximum. That is the only way to expect a reasonable life span out of the components in my opinion.
 
You asked if it would be possible to change from your 305cu.in. engine to a 300hp EFI engine. The short answer is yes if you stick to the small block chevy engine! Chevy V-8 engines share the same bell housing bolt pattern (not sure about the 8.1). Both of these engines in the Mercruiser line are built on the small block Chevrolet engine.
 
Keep in mind that a 300 hp motor at sea level will be far less (say, 250 ?) than that at that altitude, so the outdrive will not be overworked.

Jeff
 
Doshlaw said:
Top rated rpm is 4800. The prop blades are not cupped. 2 bbl carb. Talked with Merc techs today. Looks like the solution is a gear ratio change from 1.62 to 1.81. They say an engine change will not solve my problem. They also stated that if the boat is rated for 4400-4800 rpm and I am running at 4000 max, I am going to damage the engine. Change of gear ratio will be best solution and give me more rpm and top mph. They indicated that I might want to go to a 4 blade prop.
Click to expand...

At sea level, I would agree with that statement.
However, at altitude (especially at 6,000 feet) I would disagree with that statement.

These guys are apparently not considering that the engine is being starved of dense air.
So in essence, the inability to acheive WOT RPM is not necessarily due to excessive prop pitch, it's more so due to lack of cylinder pressures.



fastjeff said:
Keep in mind that a 300 hp motor at sea level will be far less (say, 250 ?) than that at that altitude, so the outdrive will not be overworked.

Jeff
Click to expand...

Fully agree!

And these guys call themselves Merc Techs????? :eek:


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I'm doubtful about a change in prop pitch as a means to get more speed. Yes the engine will spin up slightly higher in revs but the loss of pitch makes every turn of the prop achieve less thrust so there is some gain but its not linear.
 
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