Johnson 9.9 and 15 HP at same RPM

[Jim Hawkins]

Just wondering...

IF a 9.9 and 15 HP Johnson are basically the same motor, save that the 9.9 is de-tuned,

THEN assuming that prop, gears and timing are all the same, is running a 9.9 at WOT no harder on the motor than running the 15 backed off proportionally (running at same RPM) from WOT?

 

[multimech]

Re: Johnson 9.9 and 15 HP at same RPM

Outboards are made to run at high RPM. They work hard. The specifications for the WOT is for the engine that it is assigned to. Sure the bearings are the same. The wrist pins. The rods. All those components are made to take more than the WOT operating range. Besides the difference in horsepower between these two is nominal.

In my experience, the only wear I usually see is in the cylinders and the piston and rings. That is to say unless something catastrophic happens, like the engine goes underwater while running. Bearings, blocks, rods are all tough and very durable. Pistons and cylinders wear and will wear out. Usually their life is shortened by being too lean, not enough oil or out of time.

Note: If the prop is the same and the conditions are the same (same boat, same load) then the 9.9 is going to have a tough time making it to WOT or the 15 is going to go over WOT.

 

[multimech]

Re: Johnson 9.9 and 15 HP at same RPM

I would add that the difference between a 4 stroke and a 2 stroke working at WOT is completely different. That is where you would see the difference. A 4 stroke only works half the time a 2 stroke works. More strain on the 2 stroke. They also (2 strokes) have ports instead of valves and the rings can get stuck in those when they get sharp (from wear).

Interesting question.

 

[Jim Hawkins]

Re: Johnson 9.9 and 15 HP at same RPM

I think your looking at it a little more in depth than what I thought. If memory serves me the 9.9 has a top RPM of 5500 and the 15 is 6500 RPM. This comparison would be with both running at 5500 , which, I think is WOT for the 9.9. Naturally in order for them to reach that RPM they would have to be on boats that wouldn't bog them down so much as to prevent it, so what boat they are on is immaterial so long as they both reach the top RPM for the 9.9

I've always heard you should run a motor backed off a little from WOT but if I understand you, you feel the motors are built to take it.

 

[Daviet]

Re: Johnson 9.9 and 15 HP at same RPM

A 2 cycle developes horse power by RPM, if you are not running it at the proper RPM, you are going to develope the rated horse power.

 

[F_R]

Re: Johnson 9.9 and 15 HP at same RPM

I think your looking at it a little more in depth than what I thought. If memory serves me the 9.9 has a top RPM of 5500 and the 15 is 6500 RPM. This comparison would be with both running at 5500 , which, I think is WOT for the 9.9. Naturally in order for them to reach that RPM they would have to be on boats that wouldn't bog them down so much as to prevent it, so what boat they are on is immaterial so long as they both reach the top RPM for the 9.9

I've always heard you should run a motor backed off a little from WOT but if I understand you, you feel the motors are built to take it.

They are built to take it, and designed with the notion that they are going to be run WOT. Everybody wants to go faster. As I understand your question, you feel that the 15 at WOT is under higher stress than a 9.9 also at WOT. Therefor, the 15 should wear out faster if both run at WOT. But back the 15 off to the 9.9's WOT RPM and the stresses should be lessened to what the 9.9 is experiencing. That probably is a fairly valid assumption. But a lot is being assumed or ignored, such as combustion temperatures, distance travelled, and stuff I'd have to think about to come up with. When it comes right down to the bottom line, I doubt there is going to be much difference in longevity. Just my opinion, which along with $1.65 will get you a cup of coffee at most gas stations.

 

[Jim Hawkins]

Re: Johnson 9.9 and 15 HP at same RPM

No F_R, I meant it the other way around.

GIVEN - 2 Johnson motors of the same year, say 1985 for example, both 2 stroke and one a 9.9HP and the other a 15HP. Both motors identical save for the changes done at the factory for the difference in HP.

IF you can run the 15HP at 5500 (the top RPM for the 9.9 but 1000 RPM under WOT for the 15) without causing excessive wear

THEN you should be able to run the 9.9 at 5500 (or wide open throttle) and not expect any more wear on the motor than the 15 running at 5500.

Or another way, A 9.9 running at WOT and a 15 running at the same RPM can be expected to wear similar.

The bottom line point I'm trying to make is that I think running the 9.9 motor at wide open throttle for extended periods should not be especially hard on the motor.

 

[steelespike]

Re: Johnson 9.9 and 15 HP at same RPM

I would add that the difference between a 4 stroke and a 2 stroke working at WOT is completely different. That is where you would see the difference. A 4 stroke only works half the time a 2 stroke works. More strain on the 2 stroke. They also (2 strokes) have ports instead of valves and the rings can get stuck in those when they get sharp (from wear).

Interesting question.

"4 stroke works 1/2 the time of a 2 stroke" Actually the 4 stroke experiences twice the power impulse on each power stroke compared to a 2 stroke. While the 2 stroke experiences twice as many power impulses they are each approximately 1/2 that of the 4 stroke.
A 2 cylinder 2 stroke has 6 moving parts pistons,rods, and crank.The 2 cylinder 4 stroke has Has at least 18 moving parts if you include the valve springs.A lot of reciprocating weight.A lot of parts to wear out.
Any motor would be better off running at a slightly reduced rpm. But you might mix in running slower the motor will be operating longer point to point. The 9.9 and 15 come equipped with the same base prop.

 

[Patate]

Re: Johnson 9.9 and 15 HP at same RPM

If your 9.9 at wot run the same rpm as the 15 at wot, the 9.9 will be less lubricated, it will may be wear faster?

 

[Jim Hawkins]

Re: Johnson 9.9 and 15 HP at same RPM

If your 9.9 at wot run the same rpm as the 15 at wot, the 9.9 will be less lubricated, it will may be wear faster?

Patate - The 15 is not running WOT, it's running at the same RPM as the 9.9

SteelSpike - I'm talking about identical motors, 2 strokes, not 4 strokes. I edited my post prior to this one with a GIVEN statement for clarity.