I was thinking about the durability of 2 strokes versus that of 4 strokes and the bad rap 2 strokes get for wearing out so quickly. I then began to figure that since 2 strokes tend to be run at much higher rpm's than a 4 stroke a 2 stroke has got to last at least as long as a 4 stroke when comparing rpm's.
I don't really have accurate numbers but figuring that a 2 stroke develops its best power at lets say 6000 rpm's and a 4 stroke at 4000 rpm's then the 2 stroke should break down 33% sooner than the 4 stroke.
Since I think that 2 strokes last longer than that I actually think the 2 stroke is the better choice even though you may wind up "top ending" it sooner than a 4 stroke.
The other benefit to the 2 stroke is that all you have to do is hone it to the next size and drop new pistons in. You don't have to worry about valves and timing chain etc.
What do you think?

 

You have to keep in mind that the two-stroke must work harder since it has only a single stroke in which to perform intake and combustion. The four-stroke performs each of these as a separate and complete stroke resulting in a greater fuel-air mixture and compression while requiring less rpm to achieve full power. The laws of friction do not lie in that the engine that produces equal torque with less movement will last longer without necessary maintenance.

 

Well, I have been saying much the same thing for a long time know. In the past, our four poppers were very simple, air cooled, two-valve deals that were tuned for reliability and good torque at low RPM's. We still operate under the belief that four strokes are less maintainance and more reliable. I think that as more of these "racier" four stroke motors are around for a while we will find that they need a top end almost as often as a comprably tunes two stroke. Holyman, you nailed it when you talked about friction and RPM's. Obviously a motocross bike sees many more revs during use than a four banger does, hence more wear on the piston, rings, and cylinder. Now with the cannondale, yz426, and the rest of these 'new' engines turning respectable RPM's, we will be doing top ends on them frequently.
As far as two strokes having more load on them, I would respectively disagree. Any motor is tuned for efficiency in a certain RPM range. This is true of your weedeater as well as a hill-shooting Banshee. Two strokes rev very quickly because of the fact that they have little "parasitic" drag on them. They turn a water pump and the transmission. Also, the disadvantage they face at low RPM's by not having valves, is greatly offset at higher Rev's where they become much more efficient due to many factors. (Go to MacDizzy's site, it is a good read) Anyhow, your four stroke is turning a water pump, oil pump, cam chain, cam, opening valves, etc. That is all power that could go towards moving your machine.
I think it boils down to what kind of power delivery you want, and do you want to spend some time putting a new top end in once or twice a year, or do you want to spend a lot of time rebuilding the top end of your four stroke when it is needed.
I am not a engine guru, but I have built a few hot rod small blocks and I have done countless top ends on two-strokes. Just my opinion.

 

EXACTLY! what I'm saying is that if you add up the revs in the life of a 2 stroke and add up the revs of a 4 stroke by the time they need a top end I firmly believe that the 2 stroke lasts longer AND costs less to rebuild when it does need a new top end. I think the injected 2 strokes might even top that.

 

Here's a counter point to what you all are saying...

Barring apples to oranges comparisons like a Banshee to a gutless 4 stroke utility quad......

I think we can all agree that the 400EX/XR400 engine is typically thought of as a low to mid range 4 stroke powerplant with a mild top end...

Take a typical piped, ported, carb'd, etc... 40hp 250R - It typically develops peak HP at 7000~7500 RPM and drops rapidly after that point. Surprised?

Take a stock 400EX at just under 30hp. It develops peak HP between 6500~7000 RPM. Surprised again?

Now, take a typical piped, carb'd, cammed, etc... 40hp 400EX engine, guess where peak HP is developed..... Yep, 8500 to almost 9000 RPM after which, it begins to drop rapidly.

That said, my 400 spends a lot of it's life in the top, just like my 250R does. In fact, it probably spends more time in the extreme top than the R does because it has a little better over-rev (power doesn't fall off quite as quickly).

I think everyone assumes that a 2 stroke turns a lot more RPM's than it really does because of the exhaust note - remember that the exhaust note is high because it's firing twice for every one time the thumper does.

Also keep in mind that a 4 stroke's rings don't have to deal with ports in the cylinder, remember too that a 4 stroke has a constant, dedicated oil supply that in theory doesn't get combusted with the rest of the mixture. These both play big roles in how fast the piston, cylinder wall, and rings wear.

Modern 2 strokes are usually pretty square engines, most modern 4 stroke engines are oversquare (bore being larger than the stroke length). This means that the piston speed of the 4 stroke will be lower, effectively raising the safe operating RPM limit.

Both engine types have their good & bad. No need in trying to dispute that. Both engines need a lot of work to keep at peak performance, especially under race conditions. 2 Strokes just need more frequent top ends.

Later all...

 

Good points. Especially about the fact that the four banger doesn't have to deal with the ports. I think that on a properly ported two stroke the ports are correctly beveled and it reduces the amount of wear on the rings. I have seen some stock barrels that were pretty sad right from the factory. I think your RPM estimates for the R are a little low. But, it depends on what you have it set up for. I like midrange and top-end. That's what I have a two stroke for anyhow. I disagree that the EX is a low to mid performance motor, also. The only thing it does not have is liquid cooling and it doesn't seem like it really affects the power of it much, if any. In fact, it has the advantage of not having the additional weight of a radiator and coolant.

 

<< In fact, it has the advantage of not having the additional weight of a radiator and coolant. >>



The problem with that statement is that the power made by liquid cooling would be more than enough to help lug around the few extra pounds. If the 400ex was the same exact thing except liquid cooled, you'd probably be making 4-5 more hp. heat=power loss. By liquid cooling it, you can keep it running at a much lower temp thereby increasing efficiency and reliabilty.

A very poor example would be junkyard wars. Some of the machines they build(like the drag racers)used big v-8 small blocks, without the cooling system. They left it off to save weight. Their dragsters worked poorly. By adding the few pounds of a cooling system, they would have made better times because the engine and transmission would be running better.

That tiny bit of wieght saved wasn't to reduce wieght on out air-cooled machines, it was to make it easier for the owner to work on them. Look at the 250r, they could have made it air cooled. It would have been much simpler and cheaper to produce, but those are race bikes and the factory designed it to perform with more power. The extra power produced was better than loosing a few pounds.

 

Ridzhard -
I pulled my data directly from dyno sheets I've seen. Those are typical numbers, of course an engine can be built to turn harder (MacDizzy's site has a run of a 53hp 250R engine that peaks at 8300), but I figure those numbers are a realistic average for an engine that turns approx 40hp.

Keep in mind that a 250R has a longer stroke than even a 400EX (which has a longer stroke than any other modern 400cc bike that comes to mind). This increases piston speed, which reduces the RPM that peak HP is developed at &amp; reduces the safe peak operating RPM.

If you look at the modern crop of 4 stroke 400 class engines, the 400EX/XR400 engine IS a very low RPM, low/mid style engine due to it's bore vs stroke &amp; it's head/camshaft/rocker design. Maybe not if you compare it to a typical automotive 4 stroke engine, but when compared to it's peers it is certainly not a high RPM 400 class engine.

dirt817 -

Actually, heat in the combustion chamber MAKES horsepower (to a point). This is why all the modern vehicles run much higher thermostat temperatures than the &quot;old school&quot; hotrods of the past. Now, if you take that LS1 Firebird &amp; stick a 160 degree 'stat in it, you'll lose HP.

However, heating the mixture before it enters the combustion chamber reduces density, which equals reduced HP.

Heat in an engine is a VERY complex issue, you have to deal with detonation, combustion temperature, incoming air temp, mechanical limits, etc... Liquid cooling doesn't necessarily = HP, unless the advantages of liquid cooling are, well, taken advantage of.

Liquid cooling typically makes better power, because a more consistant temperature within the engine can be attained (fewer hot spots), they can be tuned to a higher level, the engine remains more compact which reduces design limitations, and tighter tolerances can be run. If this is not taken advantage of, a liquid cooled engine won't make any more HP than an air cooled engine.