Torque curves for four stroke V two stroke

dylanwinter

dylanwinter

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www.keepturningleft.co.uk
Last week I was sailing in Scotland

Paul, who is going to use Katie L for this coming year is going to borrow the Yamaha 5 hp

The engine started fine and apart from destroying a shear pin ran okay.

I was amazed at the difference in power between the 6hp 4 stroke Tohatsu and the 5HP 2 stroke Yamaha. The Tohatsu will really bury the stern of Katie L and start the cockpit flooding while the Yamaha at full blatt got her up to hull speed but did not get the bow to rise

the most striking thing was that while the Tohatsui runs at dead even revs whatever the sea state the Yamaha was surging up and down the revs between waves as the stern went deeper and shallower

Now I know that one engine is only two years old while the Yamaha is lower HP and 25 years old but is the electronic ignition the reason or is there something about the torque characteristics of the two?

I am not after a two stroke V four stroke 24 page scrap - just curious and seeking enlightenment

D
 
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dylanwinter said:
they look comparable to me

but would the prop have an impact on the way the revs varied?

D
Click to expand...
Because the prop could be cavitating might be one explanation. The big difference between 2t and 4t is that the former relies on a tuned exhaust to scavenge the cylinder so can be affected seriously by the amount of water in the exhaust. one of the reasons you should not run a long shaft on a short shaft mounting is the increase in back pressure. Same effect as you are going up and down in the waves. The back pressure varies and the revs go up and down. Explains in part why outboards on brackets aft of the transom are not ideal for motoring in lumpy seas. 4t engines are better in this respect as they do not rely so much on exhaust pipe scavenging.
 
Tranona said:
Because the prop could be cavitating might be one explanation. The big difference between 2t and 4t is that the former relies on a tuned exhaust to scavenge the cylinder so can be affected seriously by the amount of water in the exhaust. one of the reasons you should not run a long shaft on a short shaft mounting is the increase in back pressure. Same effect as you are going up and down in the waves. The back pressure varies and the revs go up and down. Explains in part why outboards on brackets aft of the transom are not ideal for motoring in lumpy seas. 4t engines are better in this respect as they do not rely so much on exhaust pipe scavenging.
Click to expand...

thanks for that - makes sense
 
found this

dyno_results.jpg
 
dylanwinter said:
found this

dyno_results.jpg
Click to expand...

Not an outboard motor. My (and your) Tohatsu wouldn't rev to 10,000rpm. Plus there are X2 power strokes per 1000 revs for two strokes. Those torque curves look very steep. These are racing motors, aren't they?

A torque/power graph more normally looks like this (although this is for a big V8 motor)

horsepower-photo1.jpg
 
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Lazy Kipper said:
I just wish I knew where to get a lightweight 3hp 2 stroke without risking the clapped out 2nd hand market.
Click to expand...

Mainbrayce in Alderney, outside the EEC, VAT-free, sell Tohatsu 3.5 2 strokes new, plus larger ones. I have two Tohatsu 3.5 2-strokes and they are pretty bulletproof and you can get cheap spares, and download free manual on the web.
If you don't know Alderney it is gorgeous by the way, cheers Jerry
 
jerrytug said:
Mainbrayce in Alderney, outside the EEC, VAT-free, sell Tohatsu 3.5 2 strokes new, plus larger ones. I have two Tohatsu 3.5 2-strokes and they are pretty bulletproof and you can get cheap spares, and download free manual on the web.
If you don't know Alderney it is gorgeous by the way, cheers Jerry
Click to expand...

I am sure I saw something here saying that they would now only sell them to people with non-EU addresses, perhaps Alderney addresses.

He could always take his chances on the illegally imported market:

http://www.ebay.co.uk/itm/3-5HP-Inf...at_Engines_EngineParts_SM&hash=item2583145f41
 
JumbleDuck said:
I am sure I saw something here saying that they would now only sell them to people with non-EU addresses, perhaps Alderney addresses.

He could always take his chances on the illegally imported market:

http://www.ebay.co.uk/itm/3-5HP-Inf...at_Engines_EngineParts_SM&hash=item2583145f41
Click to expand...

if two strokes are as prone to rev changes with waves as the Yamaha then in my opinion they are not as good as the four strokes

I assume it is the back pressure on the exhaust that is causing the problem

D
 
dylanwinter said:
if two strokes are as prone to rev changes with waves as the Yamaha then in my opinion they are not as good as the four strokes

I assume it is the back pressure on the exhaust that is causing the problem

D
Click to expand...

Since it also happens on ships with exhausts going up the funnels..
You notice it more on 2 stroke outboards because they rev up more quickly when the load is removed.
 
Dylan, I think there could be too many variables for you to be able to draw any valid conclusions from those two engines - different props, perhaps, different gearing, maybe even different engine capacity? Size-for-size, I'd expect a 2-stroke to have a lighter flywheel (because it doesn't have to "carry" the engine as far between power strokes, but I could be wrong. If that is the case, then there might just be more "inertia" in the whole system on the 4 stroke, helping to damp-out the speed fluctuations?
 
dylanwinter said:
found this

dyno_results.jpg
Click to expand...

That is not a consistent graph.
The 4stroke power is linear from 3000 to about 8000rpm.
Power=torque x rpm x (a constant depending on what units we're in)
So a straight line power graph which extrapolates through the origin would have a constant torque in that region.

Outboards are quite low power per cc, they are not designed for peak power like other 2 strokes.
The power is taken at the prop, so you might add on 20% or something to getpower at the crank, but they are still very much lower power than say a motorbike engine of the same cc.
I think a Yamaha 2 is 100cc? where a 100cc motorbike would be 10hp even for an old fashioned commuter bike.
Outboards of course get run a full throttle a lot.
 
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