Safe RPM range?

[dnyhuf]

I have a 141/2 x 22 Raker prop on a Cajun 1900 Tournament Series that gave me 5800 RPM, 58 MPH at WOT. Still felt like hole shot needed improvement, so installed a 141/2 x 20 Raker. WOW what a difference, hole shot is great, Wot speed is the same, and as you would imagine, RPM climbed to between 6200-6300. I know that RPM's exceed the 5000-6000 recommendation from manufacturer, but some reading on this forum leads me to think it's OK. What are some thoughts?

 

[Silvertip]

Re: Safe RPM range?

My thoughts are that propping for the upper end of the RPM band is perfectly ok. Even if you exceed the upper number by a tad (300 RPM in this case) you can always throttle back. Conversely, you can't throttle up to reach that number if the engine is overpropped. If hole shot is important to you, then leave things as they are.

 

[Frank Acampora]

Re: Safe RPM range?

What many people don't realize is that the crank stroke on a 2 cycle outboard is usually somewhere around 3 inches. The piston is basically only loaded (for comparison purposes, don't take me to task on specifics.) in one direction because it does not pull air through an intake valve like on an auto engine.

The loads on the crank and bearings are relatively light and I would not be surprised if these engines could happily turn 10,000 rpm. That is , if there are no voiding engine warranty issues. Hell! My 2007 sebring is redlined at 6 or 7 thou. and if I nail it, it spins up there real quick.

A couple of hundred over the manufacturers rated rpm won't do any harm.

Mind you: I am NOT advocating that you run your outboard 1-2000 over the rated rpm--just saying it probably could be done.

 

[dnyhuf]

Re: Safe RPM range?

Thanks guys, I sure didn't want to go back w/the 22p prop.

 

[kenmyfam]

Re: Safe RPM range?

My outfit (see signature at bottom of post) runs 6,200 RPM at WOT with a 14" X 17" aluminum prop. Hole shot is great. I use it for beginner skiers and 4 folk on the inflatable. 200rpm over max at WOT. I dont pull skiers or tubers at anywhere near WOT but it is great for initial pull. With boat alone I just throttle back to 6,000 rpm. It loses less than 2 mph and is still well fast enough for me. I am planning to port my 19" pitch prop over the winter for better hole shot and see what happens.

 

[dnyhuf]

Re: Safe RPM range?

Guess I could have mentioned the motor on that setup is a 1992 Johnson 200GT.

 

[Texasmark]

Re: Safe RPM range?

I run Mercs and have no problem running 6000 or so with an upper limit rating of 5500 (2 strokes). I think the 4's are rated for 6000. Can't comment of if it hurts them.

On Piston loading Frank, you gotta get the fuel/air in the crankcase somehow so the piston's gotta suck, but like you may have said, it only sucks for part of the stroke, not 100% like in 4 strokes, on the 2 stroke engine, which this is.

Might bump Lube Dude. He is running a 200 on his latest toy and I think his rpm's are way up there, I know his speed is....70+. I think Dhadley said they have raced somewhat stock 2 stroke V OMC's up to the 10,000 mentioned.

Mark

 

[Frank Acampora]

Re: Safe RPM range?

Don't forget that on a two cycle engine, the piston is pumping in air on the crancase side (suck) while it is being driven up against compression and on the power stroke while it is being driven down, it is compressing the crankcase--thus the load is always down against the bearings. And even at the top of the stroke where the piston momentarily stops dead before reversing its direction, fuel has already started burning and creating pressure to counter the inertial forces of the piston wanting to continue upward.

By the way, because the piston on any engine stops at top dead center and bottom dead center, it must be accelerated and de-accelerated through the various strokes. Acceleration of a mass takes horsepower. That is why lighter pistons allow a given engine to produce more available horsepower than the same engine with heavier pistons. That's one of the reasons that piston skirts on all 4 cycle engines are cut away except for the portion that keeps the piston from rocking in the bore. Of course, smaller skirts have less drag on the cylinder walls too.

 

[Texasmark]

Re: Safe RPM range?

Hear you Frank on the stop at the top then race to the bottom just to stop again, then race back to the top......and do it 6000 times a minute.....and last for 30 years. Modern (not so modern) miracle.

Agree on the light pistons also. As I recall, the 265/283 Chev small block had a very short stroke and I think they were cut away also to provide clearance from the crank.

Mark

 

[Dhadley]

Re: Safe RPM range?

6300 won't hurt that motor in the least. Some versions of that motor have 6700 rpm rev limiters. That stock ignition won't give up until the low 7000 range. If you tweak the ignition on that motor, with stock porting, will easily make it to almost 8000.

 

[moterboat]

Re: Safe RPM range?

Hear you Frank on the stop at the top then race to the bottom just to stop again, then race back to the top......and do it 6000 times a minute.....and last for 30 years. Modern (not so modern) miracle.

Agree on the light pistons also. As I recall, the 265/283 Chev small block had a very short stroke and I think they were cut away also to provide clearance from the crank.

Mark

3 inch stroke to be exact...

 

[dnyhuf]

Re: Safe RPM range?

Dhadley, what kind of motor life expectancy do you get from running at those speed's? Are those kind of numbers limited to the 200GT? If so, what makes the GT different from other models?

 

[Dhadley]

Re: Safe RPM range?

The 200 GT / XP's had different heads than the other V6 loopers. The fuel induction was different too, including the airbox. Obviously there are styling differences too.

We've never seen anything at 6700 that would shorten the life. That same motor maxed out at 5200 won't live as long as one propped aginst the limiter. Assuming the motor is healthy and running correctly. Plug a carb, put the wrong plugs in it, bad gas, wrong timing etc and any one of them will die prematurely.

 

[dnyhuf]

Re: Safe RPM range?

"That same motor maxed out at 5200 won't live as long as one propped aginst the limiter. "

Allright, thats great info. Brings up one more question. Why would Johnson recommend max RPM of 5000-6000? Power curve?

 

[Dhadley]

Re: Safe RPM range?

The hp will drop off after about 5800. However, on a performance boat if you spin the prop faster the boat will go faster. Once the boat is on the pad there is very little drag or resistance. It may take 200 hp to get it on the pad but it may only take 150 or less to keep it there. In the meantime you're spinning the prop faster and therefore you're going faster.

 

[Texasmark]

Re: Safe RPM range?

3 inch stroke to be exact...

I have the books in the shop, I'm in the house. I remembered that stroke at like 2.5 and the 3 came out with the 307 or 309 (forget which is Ford and which is Chevy) when emission controls kicked in......but it may have been 3 and 3.5.

Anyway it was short and fast. Had a friend with a '58 4 door Bel Air 283, 2 BBL with a 2 speed Powerglide........Mom's car. Another friend had the, just out, '59 Biscayne 348 with 3 deuces and a 3 speed stick. The latter was from California and had the typical attitude.

The Bel Air smoked him big time. The ribbing went on for weeks. Grin

Mark

 

[moterboat]

Re: Safe RPM range?

I have the books in the shop, I'm in the house. I remembered that stroke at like 2.5 and the 3 came out with the 307 or 309 (forget which is Ford and which is Chevy) when emission controls kicked in......but it may have been 3 and 3.5.

Anyway it was short and fast. Had a friend with a '58 4 door Bel Air 283, 2 BBL with a 2 speed Powerglide........Mom's car. Another friend had the, just out, '59 Biscayne 348 with 3 deuces and a 3 speed stick. The latter was from California and had the typical attitude.

The Bel Air smoked him big time. The ribbing went on for weeks. Grin

Mark

oh man.. mamories.. we were running a 57 chevy with a 1958 250 hp.stock 283ci. with a iskey cam out of a wrecked sprint car,had to run double headgaskets for the valves to clear the pistons,and a 4 barrel,it was quick,and could boil the tires right off it.. I have a complete set of chiltons manuels that go back to 1953, it shows the 283 & 265 at a 3' stroke,in 67 they took a 327 block & pistons and put in a 283 crank with fuelie heads to make the GM 302 ci.. what a screamer.. they used the 283 crank because of the 3' stroke..this motor was used for 2 years..those wer the days when you could tell the year of every ford and chevy's,every year was different..nowadays they all look like a toyota..

 

[moterboat]

Re: Safe RPM range?

Thanks guys for the feedback on the Rpm range,I know not to exceed the 5800 mark for a long period of time..now I just need to get the prop rehubbed..

 

[Texasmark]

Re: Safe RPM range?

Sorry dnyhuf for getting distracted, but us senior citizens have a senior moment periodically.

And yeah motorboat, that was a 302, forgot that too. So I guess the Ford chevy thing was 307/302.

Mark

 

[moterboat]

Re: Safe RPM range?

Sorry dnyhuf for getting distracted, but us senior citizens have a senior moment periodically.

And yeah motorboat, that was a 302, forgot that too. So I guess the Ford chevy thing was 307/302.

Mark

Your forgivin,I know all about those .. .... .. Waht wer we talkin about???