Proof of what we say, if all manufacturers made 2 strokes.

[xX JonthE Xx]

Can you imagine an open class 2 stroke that revs to the moon?!

Hell yeah!

[2T Institute]

that highly oversquare piston is what put 4 strokes back in motocross, as it allowed them to rev high to find their horsepower. Totally fair in my opinion - a 250cc engine is 250cc no matter how you slice it.

And THAT is the beauty of direct injection. Because now, 2 strokes can be oversquare!! They no longer need to depend on port timing like conventional 2 strokes. Check this out: one of the DI 2 stroke outboards (evinrude etec) is running 433cc cylinders at 91mm bore X 66mm stroke. Not as oversquare as the 450F's, but oversquare nonetheless. Can you imagine an open class 2 stroke that revs to the moon?!   

Unfortunately a 2T engine is always going to rely on the ports and the pipe to do the work.They are the heart an soul of a Two Stroke.Over square dimensions have heavier pistons which severely limit RPM.That's why a 250/450F needs a minimalist slipper piston to survive the RPM the stroke is capable of. Sooner punters get over the DI dream the better because it isn't going to happen, well probably not until Ficht or Orbitals patents expire.
Over square dimensions didn't put the 4T back on the map it was moving away from plain bearing bottom ends to roller bearings in the big end.  

[VintageBlueSmoke]

OK, now we are getting into semantics. The roller bearings allowed for the higher rotations capable with the shorter stroke. Improvements in heads and the overall metallurgy also played a role in all this, but the point is that the advancement of 4$ technology to produce more horsepower came at a price and that price is high revs. The tax placed upon that cost of higher revs, regardless of engine design, is reduced reliability.

Back in the day, the 4$ was dead reliable because it was long stroke (compared to a 2T) and turned slowly. Today, the tables are turned with the 4$ having a short stroke and revving to the moon and the 2T being considered reliable precisely because of the lower revs per HP produced.

@2T Institute - Disagree with you completely. DI may not be the answer of all our ills but we will see it and soon. It is already available in outboards and snowmobiles. It is only a matter of time. And the reason may surprise you. They will not produce it for America though we may see it here. It will be produced for the developing markets (India, China, Brazil) where the 2T is more common and emissions more of a concern.

[miedosoracing]

I think you guys all got off track here.  Who cares about all that stuff you just chatted about. In MX it comes down to on the track.  The videos all over Youtube show that cc vs cc is very fair racing.  The 4 strokes still are winning all over, but there are more and more 2 strokes coming back.  So from the beginning of this post, when it was stated cc vs cc is equal. I agree.  Maybe not on your charts or calculators, but in the real world.  Neither one of them are whooping up on eachother.  But what my point was, that for cost and other reasons, when someone has to go out and purchase their own bike and pay for parts etc, they chose 2 strokes if they could.  They weren't any faster than they were if they chose the 4 stroke, but they probably have a lot more money in their pockets.  

[teriks]

<snip>
And THAT is the beauty of direct injection. Because now, 2 strokes can be oversquare!! They no longer need to depend on port timing like conventional 2 strokes.
<snip>

Could you please explain how DI could accomplish such a thing?
-It is, after all only another way of delivering fuel, not air, into the combustion chamber.

[streaks383]

Can someone please explain undersquare and oversquare?  I am having a hard time keeping up.

[SachsGS]

Undersquare - stroke exceeds bore.Example:72mm stroke by 64mm bore 250 2T engine.
Oversquare - bore exceeds stroke.Example:any modern 4T.
Square - bore equals stroke.Example:typical 125 2T.

As 2T Institute has stated a DI 2T would still be governed by port timing,the advantage of a DI 2T is that at lower rpms due to the fuel/air charge being injected into the combustion chamber after the exhaust port is closed you would not experience charge losses out the exhaust port (at low rpm).

 This would result in an increase in low rpm torque,and coupled with other efficiencies related to DI,would create a new generation of 2T capable of beating the 4Ts under the present rules.There,I've said it,I think a DI 125 2T could beat a 250 4T (for example) - delusional mensch that I am.

[TMKIWI]

As 2T Institute has stated a DI 2T would still be governed by port timing,the advantage of a DI 2T is that at lower rpms due to the fuel/air charge being injected into the combustion chamber after the exhaust port is closed you would not experience charge losses out the exhaust port (at low rpm).
This would result in an increase in low rpm torque,

Correct.

<snip>
And THAT is the beauty of direct injection. Because now, 2 strokes can be oversquare!! They no longer need to depend on port timing like conventional 2 strokes.
<snip>

Wrong sorry.

- delusional mensch that I am.

Correct 

[2T Institute]

OK, now we are getting into semantics. The roller bearings allowed for the higher rotations capable with the shorter stroke. Improvements in heads and the overall metallurgy also played a role in all this, but the point is that the advancement of 4$ technology to produce more horsepower came at a price and that price is high revs. The tax placed upon that cost of higher revs, regardless of engine design, is reduced reliability.

Back in the day, the 4$ was dead reliable because it was long stroke (compared to a 2T) and turned slowly. Today, the tables are turned with the 4$ having a short stroke and revving to the moon and the 2T being considered reliable precisely because of the lower revs per HP produced.

@2T Institute - Disagree with you completely. DI may not be the answer of all our ills but we will see it and soon. It is already available in outboards and snowmobiles. It is only a matter of time. And the reason may surprise you. They will not produce it for America though we may see it here. It will be produced for the developing markets (India, China, Brazil) where the 2T is more common and emissions more of a concern.

Yeah sure whatever you reckon. 4T engines don't rev to the moon they make peak powerat 11,500 or so 250F

[Chris2T]

TMKIWI - that's why Evinrude didnt consult you when they were designing their oversquare DI engine. But i admit, i should have said DI's are no longer "fully" dependant on port timing. When the injector releases its spray into the combustion chamber is now at the discretion of the engineer, not the piston

[ACMX]

Not in America, its gone to point of being political.

I am in America. Most racers here don't give a hell about the politics of motocross, they just like to ride, race, have fun, and compete! And most of them are coming back to smokers. the top three to five racers in every class here are on smokers.

[factoryX]

Its besides the point, what I stated was the fact that there are only a few new 2 strokes being sold in the united states today, this goes for ATV's as well. As for political, 2 strokes are considered dirty while 4 strokes are media biased.  Local racing is one thing, Nationally coverage races are another.

[VintageBlueSmoke]

Yeah sure whatever you reckon. 4T engines don't rev to the moon they make peak powerat 11,500 or so 250F

Sorry, I guess saying the "rev to the moon" is an exaturation. However, many 2T 250's are making peak at about 7.5K. Much lower than the 11.5k revs of the 250F you reference. Rev the 2T that high and you'll think you are going to the moon! A motor making peak hp with less revolutions will generally be more reliable than one spinning faster.

About dirty 2 strokes: I am not going to look it up again so I won't quote, but the 4$ has cleaner emissions in all but 1 measurement, the worst one. However, referencing the engine tested in the report I was reading, the same 2T outfitted with DI made more HP, better fuel economy and MUCH lower emission numbers across the board than the equivalent 4$'s they were comparing it to.

[miedosoracing]

I think rev to the moon was more referring to what we know of on older 4 strokes or even current mx 2 strokes.  Take a XR250 and compare the RPM to a CRF250.  You will see it matches approximately with increase in RPM's to power.  Same can be said with 2 strokes. They can be set up to rev much higher, take a look at GP 125's.  BUT, they also have 0 bottom end.  So on an MX track, it would outright be horrible.  But 2 or 4 strokes, to increase peak HP, you can increase RPM's. 

[teriks]

TMKIWI - that's why Evinrude didnt consult you when they were designing their oversquare DI engine. But i admit, i should have said DI's are no longer "fully" dependant on port timing. When the injector releases its spray into the combustion chamber is now at the discretion of the engineer, not the piston

I'll try again...

That injector injects fuel (and depending on injector type, a tiny bit of air to help atomize the fuel) into the combustion chamber, but in order to burn that fuel you need a lot of air, and that air is delivered the same way as it is in a normal carbed two-stroke.  Any improvement in the power spread (due to DI) will be a result of improved engine management.

Now, drastically reduced emissions and reduced fuel consumption aint bad either...