2 Stroke tech question

just James
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just James
Edited Date/Time 4/12/2013 10:31am
What is it that limits the rpm that 2 strokes turn? If a 250F will turn 14, 000 or so, why wouldn't a 2 stroke? Do they just become inefficient at high rpm, or do they not function properly with the short stroke required for high rpm?
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zippy895
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zippy895 4/11/2013 5:47pm
4/11/2013 5:47pm
a rev limiter for one.
i think you kinda answered your own question . i would think ,if the 2 stroke could rev out higher rpm's,it would prob. be flat on the bottom end power even more.or on top,for that matter,depending on porting design .
Jakes Dad
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Jakes Dad 4/11/2013 5:50pm
4/11/2013 5:50pm
Saw a dyno sheet on an Aprilia 125, it was beyond that and 54hp to boot. Two stroke mx bikes need torque to get out of the corners, so I'd assume building them for this is what limits the motor.
blusmbl
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blusmbl 4/11/2013 6:02pm
4/11/2013 6:02pm
Different pipe designs change where a 2 stroke loses it's efficiency and falls on it's face. It's really a matter of how the cylinder is filled & scavenged, not so much an issue of the crank/piston being able to take it.
moto814
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moto814 4/11/2013 6:04pm
4/11/2013 6:04pm
Pumping efficiency limits the RPMs that a Two-Stroke engine can turn. Design a proper pipe/porting/intake/head combination, and the engine can and will rev until the crank turns blue from heat. Very few motocross/dirtbike engines have had actual electrical rev limiting devices incorporated into them.

But what do I know...

-Steve

The Shop

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SwapperMX
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SwapperMX 4/11/2013 6:05pm
4/11/2013 6:05pm
zippy895 wrote:
a rev limiter for one. i think you kinda answered your own question . i would think ,if the 2 stroke could rev out higher rpm's,it...
a rev limiter for one.
i think you kinda answered your own question . i would think ,if the 2 stroke could rev out higher rpm's,it would prob. be flat on the bottom end power even more.or on top,for that matter,depending on porting design .
Two strokes dont have limiters. The two stroke engines, once past a point of efficiency, can no longer create power due to the simple fact of their design. Which is why they sound like they do when you hear guys over revving them, especially in the air.
just James
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just James 4/11/2013 6:08pm
4/11/2013 6:08pm Edited Date/Time 4/11/2013 6:11pm
My theory is that if there is no danger to the bottom end, then there is no reason for a rev limiter.
I had also wondered if the bearing design/lubrication might have something to do with it.
And yes, I do know that at least some of them do have a rev limiter.
Bytor
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Bytor 4/11/2013 6:10pm
4/11/2013 6:10pm
They make twice the horse power at the same RPM. How high do they need to rev?
zippy895
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zippy895 4/11/2013 6:14pm
4/11/2013 6:14pm
zippy895 wrote:
a rev limiter for one. i think you kinda answered your own question . i would think ,if the 2 stroke could rev out higher rpm's,it...
a rev limiter for one.
i think you kinda answered your own question . i would think ,if the 2 stroke could rev out higher rpm's,it would prob. be flat on the bottom end power even more.or on top,for that matter,depending on porting design .
SwapperMX wrote:
Two strokes dont have limiters. The two stroke engines, once past a point of efficiency, can no longer create power due to the simple fact of...
Two strokes dont have limiters. The two stroke engines, once past a point of efficiency, can no longer create power due to the simple fact of their design. Which is why they sound like they do when you hear guys over revving them, especially in the air.
oh.i just remember people saying them kids are reving the piss out of those minis till they hit the rev limiter...back in the 80-90's.
no,wait.my jet ski had msd ignition chips w/ optional rpm max's.
just James
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just James 4/11/2013 6:35pm
4/11/2013 6:35pm
Many years ago I had a 1984 Honda CR125 which had the crankshaft literally come apart at high rpm. The mechanic at the Honda dealer couldn't figure out why it had happened, but the first thing he checked was the rev limiter wire to make sure it hadn't been disconnected. He fixed it with all new parts, and it did the same thing again a short time later.
BAMX
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BAMX 4/11/2013 8:52pm
4/11/2013 8:52pm
blusmbl wrote:
Different pipe designs change where a 2 stroke loses it's efficiency and falls on it's face. It's really a matter of how the cylinder is filled...
Different pipe designs change where a 2 stroke loses it's efficiency and falls on it's face. It's really a matter of how the cylinder is filled & scavenged, not so much an issue of the crank/piston being able to take it.
This... In moto you don't use those top RPM's so the bikes pipe is designed to maximize the power that you use. Karts on the other hand do and they rev to 14k no prob.
mynewcr250
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mynewcr250 4/11/2013 10:15pm
4/11/2013 10:15pm
moto814 wrote:
Pumping efficiency limits the RPMs that a Two-Stroke engine can turn. Design a proper pipe/porting/intake/head combination, and the engine can and will rev until the crank...
Pumping efficiency limits the RPMs that a Two-Stroke engine can turn. Design a proper pipe/porting/intake/head combination, and the engine can and will rev until the crank turns blue from heat. Very few motocross/dirtbike engines have had actual electrical rev limiting devices incorporated into them.

But what do I know...

-Steve
am i in the twilight zone?

are we ever going to see you do your magic again?
Jakes Dad
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Jakes Dad 4/12/2013 4:31am
4/12/2013 4:31am
moto814 wrote:
Pumping efficiency limits the RPMs that a Two-Stroke engine can turn. Design a proper pipe/porting/intake/head combination, and the engine can and will rev until the crank...
Pumping efficiency limits the RPMs that a Two-Stroke engine can turn. Design a proper pipe/porting/intake/head combination, and the engine can and will rev until the crank turns blue from heat. Very few motocross/dirtbike engines have had actual electrical rev limiting devices incorporated into them.

But what do I know...

-Steve
mynewcr250 wrote:
am i in the twilight zone?

are we ever going to see you do your magic again?
Steve's work earned my kid many a holeshot on his KX 65, many moons ago at Dade City MX.
Thomas Jefferson 4/12/2013 5:53am
4/12/2013 5:53am
If you have serious questions and desire knowledge about two-stroke engines, join a superkart or road bike forum.
E-man811
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E-man811 4/12/2013 5:58am
4/12/2013 5:58am
moto814 wrote:
Pumping efficiency limits the RPMs that a Two-Stroke engine can turn. Design a proper pipe/porting/intake/head combination, and the engine can and will rev until the crank...
Pumping efficiency limits the RPMs that a Two-Stroke engine can turn. Design a proper pipe/porting/intake/head combination, and the engine can and will rev until the crank turns blue from heat. Very few motocross/dirtbike engines have had actual electrical rev limiting devices incorporated into them.

But what do I know...

-Steve
WOW! Long time no see. Glad to see you chiming in Steve.
If ever you you decide to grace the insides of another 2 stroke engine, I'll be waiting in line.
Long time happy customer and testimonial in waiting right here. (not that you'd need it anyway)
I know there's no money in making engines scream, but there sure is joy.
Glad to see you getting a little taste of moto.

Ian
Ing
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Ing 4/12/2013 6:12am
4/12/2013 6:12am
2 cycle RC car engines rev to 35,000 and beyond.
Suns_PSD
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Suns_PSD 4/12/2013 8:35am
4/12/2013 8:35am Edited Date/Time 4/12/2013 8:40am
It's all about the distances traveled for the piston and the fuel mixture.

In short, because of the way a 2 stroke piston location controls the opening and closing of the ports, a fairly long stroke has to be used to control and separate those functions. This greatly increases piston speed over a short stroke engine at identical RPMs. But the 2 stroke doesn't have the option of a real short stroke like a 4 stroke has.

The other issue is it just takes a certain amount of time to move a mass of fuel from down in the crankcase in to the combustion chamber. The higher the revs, the larger the transfer ports have to become to flow the needed fuel in a shorter window. In turn making the engine a pig w/ no port velocity at low RPMS.

Lastly, it's just design for the application. If they make the motor rev too much, the torque will really suffer and that's just not useful for our application. That said, a 4 stroke by design will rev higher than an equivalent 2 stroke.

Can you make a 2 stroke rev? Sure, but w/ trade-offs.

I think when some people argue that a 2 stroke of equal displacement isn't a fair comparison to a 4 stroke because it fires 2x as often, they need to take in to consideration the fact that the 4 stroke can turn way more RPMs making up for the majority of that disadvantage.

As far as rev limiters go: My stock 250s I use to bang the choke point about 1x per lap. It sucks and will practically throw you over the bars. As a 300 I was hitting it multiple times per lap, I really didn't like that and I had to regear to deal w/ that. As a stroked 314cc however, the same engine revs insanely high and I have yet to hit the choke point. I guess my point is that they don't have a rev limiter but it's controlled by the flow of the engine and it sure feels like it's electronic, but it's not.
just James
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just James 4/12/2013 8:50am
4/12/2013 8:50am
Suns_PSD wrote:
It's all about the distances traveled for the piston and the fuel mixture. In short, because of the way a 2 stroke piston location controls the...
It's all about the distances traveled for the piston and the fuel mixture.

In short, because of the way a 2 stroke piston location controls the opening and closing of the ports, a fairly long stroke has to be used to control and separate those functions. This greatly increases piston speed over a short stroke engine at identical RPMs. But the 2 stroke doesn't have the option of a real short stroke like a 4 stroke has.

The other issue is it just takes a certain amount of time to move a mass of fuel from down in the crankcase in to the combustion chamber. The higher the revs, the larger the transfer ports have to become to flow the needed fuel in a shorter window. In turn making the engine a pig w/ no port velocity at low RPMS.

Lastly, it's just design for the application. If they make the motor rev too much, the torque will really suffer and that's just not useful for our application. That said, a 4 stroke by design will rev higher than an equivalent 2 stroke.

Can you make a 2 stroke rev? Sure, but w/ trade-offs.

I think when some people argue that a 2 stroke of equal displacement isn't a fair comparison to a 4 stroke because it fires 2x as often, they need to take in to consideration the fact that the 4 stroke can turn way more RPMs making up for the majority of that disadvantage.

As far as rev limiters go: My stock 250s I use to bang the choke point about 1x per lap. It sucks and will practically throw you over the bars. As a 300 I was hitting it multiple times per lap, I really didn't like that and I had to regear to deal w/ that. As a stroked 314cc however, the same engine revs insanely high and I have yet to hit the choke point. I guess my point is that they don't have a rev limiter but it's controlled by the flow of the engine and it sure feels like it's electronic, but it's not.
Thank you for a very good explanation of this topic. Pretty much confirmed what I had suspected. Do bearing design/lubrication also figure in here? Or is that not a factor?
mnomis484
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mnomis484 4/12/2013 10:31am
4/12/2013 10:31am
Ing wrote:
2 cycle RC car engines rev to 35,000 and beyond.
Thomas Jefferson wrote:
They're more or less diesel.
Right.
They dont have a spark plug, just a glow plug for cold starts.
they ignite due to the pressure in the cylinder.


A 2 stroke motocross engine simply doesnt need to rev that high. A 4 stroke only ignites once in 2 crankshaft turns. a 2 stroke once in one crankshaft turn.
So the 4 stroke basically needs to have twice the rpm of a 2 stroke.

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