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CC Displacement in Johno 140

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mr_johnson

Regular Contributor
How important is displacement

How important is displacement for a donk?

I understand the 150 Yami has 163cubic inches and the Johno 140 has 125cubic inches.

How is this marked difference in CC displacement a factor in performance - if at all?

What is CC displacement?

Is this differnce of 37 cubic inches of dislacement significant (despite the extra 10HP on the Yami)?

Is bigger better for a given HP?

Cam
 
"Cameron, you will get differe

"Cameron, you will get different opinions on that. Here is what I have "heard" for years.

Traditional thinking was that maximum horsepower should not be more than a 1:1, ie, a 140 cu in motor should not exceed 140 horsepower. In fact, if the ratio was closer to 2:1 - 140 cu = 70 horse, the motor wouldn't work anywhere near as hard, would last longer, run cooler etc.

Builders are now pushing the envelope way beyond that. BRP's rotax engines that they stick on Personal WaterCraft (PWC's) make gross amounts of horsepower out of very small displacement engines.

Personally I still like the 1:1 (rule)....."
 
"Thanks Graham,

I canno tse

"Thanks Graham,

I canno tsee any of todays' new engines with a 1:1 HP:CC - can you?

Why is 1:1 good for you?

Cam"
 
"Graham,

Am I correct in sa

"Graham,

Am I correct in saying that the 140 Johno may work harder, run hotter becuase the ratio is approx. 0.8:1 (125 cubic inches for 140HP)?

Cam"
 
"Cameron, alot of todays small

"Cameron, alot of todays smaller outboards (or the lowest hp for any given head size) are built on a (near) 1:1 ratio.

40-60hp are usually something around a 60 cu in head.

The larger models 150/175/200 horse, may use a 2.5L (153 cu in) head. In that case the (base) horsepower is 1:1, the others have their horsepower boosted through better porting, fuel injection, computerized ignition etc, etc, - many of technologies which didn't exist just a few years ago.

If you ask most marine techs that work on inboards/IO's, they will tell you that you are far better off with a "big block" (over 400 cu in) putting out say 250-300 horses, than a small block 305 cu, putting out the same. As you mentioned, they run cooler, less stress etc = last longer. I apply the same theories to outboards.

So for me, if I was shopping for a large (over 90 horse) engine, personally, I would probably pick one with a greater displacement/hp if I had a range of options. But that's just personal.

As to how it's related to torque....

In the 1980's, most mfgs switched to measuring ouboard horsepower in terms of Shaft horse power (SHP), compared to Brake horse power (BHP).

In the case of most (new) engines, their torque is actually what is measured on a dyno and then factored by the RPM to determine what their horsepower is.

So in a modern outboard, the Horsepower is a calculated result of that particular models "torque", not the other way around (if you get what I'm saying) - so when building/designing an engine, they use a displacement that will produce "x" torque so that the end result will be "y" horsepower....."
 
"THanks Graham.
Very intersti

"THanks Graham.
Very intersting post!

Am I correct in saying that the 140 Johno may work harder, run hotter becuase the ratio is approx. 0.8:1 (125 cubic inches for 140HP)?

Cam"
 
"Yes, that would be the genera

"Yes, that would be the general theory. And if you take a look over say the past 5 to 10 years at all the motors that have been real "problems" intially - they were almost always the "new" high horspower motors....."
 
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