2.7 liter 4 stroke vs. 2.7 liter 2 stroke help me understand?

[SEASWIRL PAUL]

How can a 2.7 liter 4 stroke make as much power as a 2.7 liter 2 stroke without overtaxing the engine.

If you compare say a honda xr80 dirtbike to a honda cr80 dirtbike the cr 80 2 stroke would flat blow away the 4 stroke. It puts out 2.5 times the power and is lighter as well.

When they rate a 4 stroke motor as 150hp whats the torque? Thats the important part when moving heavy loads.

A 2.7 liter 2 stroke HAS to put out more torque.

Gee, I wonder why the manufacturers keep us all in the dark about torque or "pulling power" of their motors? Do they think that were that stupid?

[Fletch]

I'm no engineer so I won't go there.

I will say however, that comparisons between marine engines and land based engines need to be taken with a giant grain of salt especially when thinking of 4 stroke engines.

Most land based engines have wheels to reduce friction and transmissions to ease the load on the engine. Four stroke technology makes a lot of sense here.

Marine outboard engines have no transmission and a hull dragging through the water creates a lot more friction than wheels rolling over ground. And a marine engine has to bear the full load from idle up to cruising speed and maintain it without the benefit of transmissions or wheels. Given this environment, torque is a huge consideration IMHO. Two stroke technology makes a lot of sense here.

A bit of a caveman analysis but all other things being equal, I'll take the two stroke in your comparison any day.

[grin]

But remember, the CR80 will need a new set of rings about every 3 months(or every race if your serious) and the XR80 will run for about 10 years with nothing. I actually had a XR80 way, way way back in the late 70s. I ran it every day as a punk in mud, sand and rain and snow. I think I changed the spark plug once and the oil twice in about 5 years. I then graduated to two stroke YZ125, RM125, CR125, CR250 Banshee 350, XR600 and I forget what else. I can attest I had the cylinder off the 2 strokes continually on every one of my 2 strokes. Never removed the head on a 4 stroke once. I agree with what your trying to get at, but I also am being swayed to 4 strokes myself. Look at the Motocross machines, they are all 4 strokes now. Yes the 250 class has 350(or whatever they are) 4 strokers to barely be faster than a 250 2 stroke. I would have put money on it that this would never have happened, but it did! I still like the 2 stroke race engijnes better, but for a offshore fishing machine where time is spent cruising and idling 99.9% of the time I have to admit I think a 4 stroker is better suited. I still might get me another set of 2 strokers(when I repower) just because I can't stand it though. If I had a bassboat or something that just gets run hard for short miles or wanted to race around, I would buy 2 strokes, but fishing machines are a good sell for 4 strokers. Especially offshore machines that are going 25-50+ miles one way, then troll all day at low rpms, then back, all the time.

[Rider1]

2 stroke outboards do have more torque than 4 stroke outboards, look at the time it takes to plane with similiar horsepower 4s vs 2s. Second, 2 stroke outboards are not at all optimized for horsepower. if they were they would wear expansion chambers which would increase horsepower greatly. as it is, they still outperform 4strokes.
regarding your comparison between an XR80 and a CR80 - apples to oranges. one's good for putting around the barn, the others a screaming race machine. a better comparison would be a modern YZ-250F 4 stroke vs a YZ-250 2stroke. the 4stroke is maintenance intensive and will get blown away by the 2stroke. they don't even allow the 2stroke to run in the same class as the 4stroke, that's how bad the difference is.

[blah97]

4 strokes are more efficient compared to 2 strokes. The air a 2 stroke gets is not as oxygen-rich because of the scavenging cycle.

I'm pretty sure (in outboards, anyway) 4 strokes run at higher compression ratios as well. More compression = more air to mix with fuel for burn (this is more or less how turbo/superchargers work).

Still, I think the 4 stroke would have to be run "hotter" to make the same power as the same displacement 2 stroke, and be prone to accelerated wear over a 2 stroke.

[Fletch]

blah97 - 9/13/2005 9:02 AM

4 strokes are more efficient compared to most 2 strokes - Direct Injection 2 strokes excluded of course.

fixed.