OK a 2 stroke engine fires on every revolution so in theory it would be able to make twice as much HP as a four stroke of the same displacement. I know that it really doesn't work out that way because of frictional loses and all that stuff. But what I am trying to get at is the fact that it should be pasable for a cr500 engine with the right porting to make the same or better HP numbers compared to a 600cc sport bike engine. I have a 88 cr500 supermotard that I want to try to get more power out of. I have never done any perf. work on 2strokes other than playing with pipes and reeds so I don't know what kind of HP I can reallistcaly expect to get from this engine and have it still be ridable. Thanks for any input on the subject.
2stroke tuning questions
- Thread starter swingarm
- Start date
The keys to what you are looking for are BMEP (Brake Mean Effective Pressure) or the cylinder pressures in a running engine and the losses associated with high rpm (frictional and pumping losses). Two-strokes fire more often, but do so with much lower cylinder pressures in the running engine than a racing four-stroke. Averaged over time (RPM) the “fires twice as often” advantage of a big bore two-stroke is diminished to a large extent by the lower working pressures and the lower rpm it has to run at compared to a 600cc 4cyl.
I’ll try to explain this more completely but I was up till 5:00am (tending to a friend with a sick ~giraffe~) so my brain isn’t really firing on all cylinders. :yeehaw:
If you don’t like LONG boring explanations it’s best to bail out NOW and I’ll just tell you a well tuned CR500 will make between 65 and about 80 horsepower with reliability running inverse to the total power (i.e more power = less reliability) . A 600 cc 4cyl sportbike engine can easily make 120hp and run a LONG LONG time without breaking parts. The upside is a 600cc four-stroke single won't easily be able to touch a CR500 in absolute horsepower but it will tend to have a greater spread of power and for SuperMoto probably a bit of an advantage.
On to the long boring technical explanation part of our show. :scream:
In the simplest terms, torque and by association horsepower are a function of cylinder pressure during the expansion (power) cycle balanced against the internal losses encountered during the rest of the cycle events.
Cylinder pressure and Torque are to a large degree a function of cylinder filling (how much air and fuel are pulled in during the intake cycle). When you look at a torque/rpm graph you are essentially looking at a cylinder-filling (volumetric efficiency) graph. In relative terms two strokes have huge ports that open and close very quickly. Four strokes on the other hand have tiny valves (especially in the YZF series) that open and close very slowly. If you want to improve cylinder filling you have a number of choices:
- Tune the intake and exhaust waves (lengths and diameters) very sharply. This works exceptionally well but only over a very narrow rpm range (an area where two strokes are especially strong) . Outside of that rpm range the wave tuning disrupts cylinder filling and causes a torque trough (an area where two-strokes suffer greater losses).
- You can increase the TIME available to fill the cylinder by increasing the intake valve or port open duration. That works well at high rpm, but leaves the valves/ports open too long at lower rpm and again disrupts cylinder filling and causes a torque trough.
- We can increase the valve or port AREA with larger openings but you again push the torque peak rpm up with this approach as well as running into space and reliability issues.
The second major part of the cylinder pressure equation relates to how much air/fuel we can trap in the cylinder and compress before the ignition fires. Greater compression of the initial charge before ignition has the theoretical advantage of producing greater cylinder pressure after combustion, but as all things in life there are trade offs.
The Mechanical (or Static) compression ratio of an engine is fixed
and is based on the ratio of the total cylinder volume to the clearance
volume at TDC. While the Trapped (or Dynamic) compression ratio is based on the ratio of the cylinder volume remaining at Intake Valve Closing (IVC) on four-strokes to the clearance volume at TDC. Or in the case of a two-stroke the ratio of the cylinder volume remaining at Exhaust Port Closing (ECP) to the clearance volume at TDC.
So the way it works in a four-stroke goes like this, if you close the intake valve early you have a greater total cylinder volume and a higher ratio when compared to the clearance volume, but you compromise the cylinder filling (volumetric efficiency) at high rpm. This higher ratio will raise cylinder pressure and make it easier for an engine to detonate. Late intake valve closing has the opposite effect, lower ratios and lower pressures. By leaving the intake valve open longer you can fill the cylinder more completely. The obvious trade off
is the fact that at low speeds this late IVC will cause major flow reversals problems in the intake ultimately limiting the mass of air/fuel trapped in the engine when the valves close. Two-strokes by design have much shorter time periods to work with and can’t easily separate the individual events of cylinder filling and emptying. So even though two-stroke ports can be opened and closed very quickly and they can provide and incredibly efficient flow path, the lack of clean separation of the intake and exhaust events makes for very good filling but very POOR trapping of the fuel charge. With proper port and pipe design the two-stroke can be made to trap much more efficiently but only in a fairly narrow rpm range. So even though the two-stroke can fill, trap and fire a charge twice as often as the four-stroke the end result is much less than the 2 to 1 advantage that it looks like form the surface.
Even with all those words that was a MASSIVE oversimplification of a fairly complex subject. Once you start to factor in things like pressure versus piston area (bore size) and rpm things become more complicated, and when you factor in the frictional and pumping losses associated with rpm and compression ratios things it get more complicated still. This is a big part of the reason that most cheapo engine simulation software does such a poor job of estimating power and basic engine requirements.
There are TONS of great technical papers and books that go into great detail on this subject. If anyone is interested in persuing this further on their own I'd be happy to dig up some references for you.
I hope this helps. :thumb:
I’ll try to explain this more completely but I was up till 5:00am (tending to a friend with a sick ~giraffe~) so my brain isn’t really firing on all cylinders. :yeehaw:
If you don’t like LONG boring explanations it’s best to bail out NOW and I’ll just tell you a well tuned CR500 will make between 65 and about 80 horsepower with reliability running inverse to the total power (i.e more power = less reliability) . A 600 cc 4cyl sportbike engine can easily make 120hp and run a LONG LONG time without breaking parts. The upside is a 600cc four-stroke single won't easily be able to touch a CR500 in absolute horsepower but it will tend to have a greater spread of power and for SuperMoto probably a bit of an advantage.
On to the long boring technical explanation part of our show. :scream:
In the simplest terms, torque and by association horsepower are a function of cylinder pressure during the expansion (power) cycle balanced against the internal losses encountered during the rest of the cycle events.
Cylinder pressure and Torque are to a large degree a function of cylinder filling (how much air and fuel are pulled in during the intake cycle). When you look at a torque/rpm graph you are essentially looking at a cylinder-filling (volumetric efficiency) graph. In relative terms two strokes have huge ports that open and close very quickly. Four strokes on the other hand have tiny valves (especially in the YZF series) that open and close very slowly. If you want to improve cylinder filling you have a number of choices:
- Tune the intake and exhaust waves (lengths and diameters) very sharply. This works exceptionally well but only over a very narrow rpm range (an area where two strokes are especially strong) . Outside of that rpm range the wave tuning disrupts cylinder filling and causes a torque trough (an area where two-strokes suffer greater losses).
- You can increase the TIME available to fill the cylinder by increasing the intake valve or port open duration. That works well at high rpm, but leaves the valves/ports open too long at lower rpm and again disrupts cylinder filling and causes a torque trough.
- We can increase the valve or port AREA with larger openings but you again push the torque peak rpm up with this approach as well as running into space and reliability issues.
The second major part of the cylinder pressure equation relates to how much air/fuel we can trap in the cylinder and compress before the ignition fires. Greater compression of the initial charge before ignition has the theoretical advantage of producing greater cylinder pressure after combustion, but as all things in life there are trade offs.
The Mechanical (or Static) compression ratio of an engine is fixed
and is based on the ratio of the total cylinder volume to the clearance
volume at TDC. While the Trapped (or Dynamic) compression ratio is based on the ratio of the cylinder volume remaining at Intake Valve Closing (IVC) on four-strokes to the clearance volume at TDC. Or in the case of a two-stroke the ratio of the cylinder volume remaining at Exhaust Port Closing (ECP) to the clearance volume at TDC.
So the way it works in a four-stroke goes like this, if you close the intake valve early you have a greater total cylinder volume and a higher ratio when compared to the clearance volume, but you compromise the cylinder filling (volumetric efficiency) at high rpm. This higher ratio will raise cylinder pressure and make it easier for an engine to detonate. Late intake valve closing has the opposite effect, lower ratios and lower pressures. By leaving the intake valve open longer you can fill the cylinder more completely. The obvious trade off
is the fact that at low speeds this late IVC will cause major flow reversals problems in the intake ultimately limiting the mass of air/fuel trapped in the engine when the valves close. Two-strokes by design have much shorter time periods to work with and can’t easily separate the individual events of cylinder filling and emptying. So even though two-stroke ports can be opened and closed very quickly and they can provide and incredibly efficient flow path, the lack of clean separation of the intake and exhaust events makes for very good filling but very POOR trapping of the fuel charge. With proper port and pipe design the two-stroke can be made to trap much more efficiently but only in a fairly narrow rpm range. So even though the two-stroke can fill, trap and fire a charge twice as often as the four-stroke the end result is much less than the 2 to 1 advantage that it looks like form the surface.
Even with all those words that was a MASSIVE oversimplification of a fairly complex subject. Once you start to factor in things like pressure versus piston area (bore size) and rpm things become more complicated, and when you factor in the frictional and pumping losses associated with rpm and compression ratios things it get more complicated still. This is a big part of the reason that most cheapo engine simulation software does such a poor job of estimating power and basic engine requirements.
There are TONS of great technical papers and books that go into great detail on this subject. If anyone is interested in persuing this further on their own I'd be happy to dig up some references for you.
I hope this helps. :thumb:
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Swingarm, in recent years volumetric efficiency of certain 2 strokes has actually reached the level of normally aspirated high performance 4 strokes (slightly over 100%). Of course, HP is = to torque x RPM/5252. So higher RPM and better cylinder filling/pressure result in HP. These 2 stroke engines actually do make around twice the HP of a comparable 4 stroke.
The CR500 has a long stroke and therefore is mechanicaly limited in RPM. In addition port area vs. displacement is limited by simple mathematics. Ie: displacement increases faster than cylinder area and therefore port area. That is one reason why 125cc 2 strokes can make upwards of 45HP, yet a 500cc single would not make 180HP.
I ride a well modified RZ350 2 stroke and can keep up with stock 600cc 4 strokes most of the time. The RZ is a very low tech 2 stroke!
Chris
The CR500 has a long stroke and therefore is mechanicaly limited in RPM. In addition port area vs. displacement is limited by simple mathematics. Ie: displacement increases faster than cylinder area and therefore port area. That is one reason why 125cc 2 strokes can make upwards of 45HP, yet a 500cc single would not make 180HP.
I ride a well modified RZ350 2 stroke and can keep up with stock 600cc 4 strokes most of the time. The RZ is a very low tech 2 stroke!
Chris
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