Well.... I've decided to post an interesting idea I had with regards to turning my daewoo engine into a modified 2 stroke engine.
Originally I was going to keep this to myself but I'm Lazy so I probably would never actually get around to trying it out anyways.
Perhaps one of you guys out there might find it interesting enought to try.
This is a fairly lengthy explaination so I will try to explain it in detail so that everyone can understand.
First off this is a Miller / 2 Stroke Otto hybrid.... most of you probably haven't the slightest clue what a Miller cycle is so I will explain it
High horsepower 4 stroke to 2 stroke engine conversion
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PrecisionBoost
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PrecisionBoost
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OK.... your standard Daewoo car uses a 4 stroke engine that runs on somthing called an "Otto Cycle"
Everyone calls it an internal combustion engine but there are actually a whole pile of different types of engines.... example the Diesel Cycle and the Miller cycle (which I will explain in the next post) and the list goes on and on.
All you really need to know is that your every day engine should really be called a 4 stroke Otto engine (just like a diesel engine uses a diesel cycle)
Here is an explaination of a simple Otto cycle......
So with a 4 stroke Otto cycle the engine sucks in fuel and air together as the piston moves away from the cylinder head ( called Top Dead Center or TDC )
During this phase the intake valve is open and the exhaust valve is just about fully closed ( there is a small overlap... the intake valve opens before the exhaust valve is completely shut)
So at the piston travels towards the crank shaft (which is bottom dead center or BDC ) it creates a vacume and sucks in the air/fuel.
Then just past BDC the intake valve closes (all valves closed) and the piston starts to compress the air/fuel mixture
The just as the piston squeezes the air/fuel against the cylinder head (TDC) the spark plug fires and ignites the air/fuel
Now if the spark plug fires when the piston is at exactly it's highest point ( which is Top dead center) then that is called 0 degrees of ignition timing.
If you advance the timing that means that the spark plug will fire just a fraction of a second before the piston is all the way up.
If you retard the timing that means that the spark plug will fire just after the piston passes TDC and starts moving towards the crankshaft.
Typically there is a slight delay from when the spark plug fires and when the air/fuel burn starts so usually the timing is advanced so that it starts burning before TDC... this way by the time the piston passes TDC the power in the cylinder will have just started to increase.
If the ignition is advanced too much the air/fuel will fully ignite before the piston passes TDC which is really bad and can damage pistons and connecting rods.
If the ignition is retarded you will loose power because the air/fuel will burn too late and you won't get the maximum "push" on the piston.
Think of it this way.... you are pushing someone on a swing..... push too early while they are still coming at you and they either fall off or knock you backwards..... push too late and you hardly do anything.... they move away quicker than you can push.
But push at exactly the right time when they are just past stopping and barely starting to go down and you will transfer the maximum energy to the person.
Ok enough about ignition.....
The piston pushes away from TDC towards the crank and just as it's about to get to BDC the exhaust valve opens up to relieve some of the pressure in the cylinder.
The exhaust valve stays open and the piston pushes the exhaust out the valve as it moves back towards TDC.
Then as it passes TDC the whole cycle starts again as the intake valve opens.
Well.... there it is.... the 4 stroke Otto cycle in a nut shell.
Sorry for those of you who know how it works .... I just want to be sure everyone can understand this new engine design.
Everyone calls it an internal combustion engine but there are actually a whole pile of different types of engines.... example the Diesel Cycle and the Miller cycle (which I will explain in the next post) and the list goes on and on.
All you really need to know is that your every day engine should really be called a 4 stroke Otto engine (just like a diesel engine uses a diesel cycle)
Here is an explaination of a simple Otto cycle......
So with a 4 stroke Otto cycle the engine sucks in fuel and air together as the piston moves away from the cylinder head ( called Top Dead Center or TDC )
During this phase the intake valve is open and the exhaust valve is just about fully closed ( there is a small overlap... the intake valve opens before the exhaust valve is completely shut)
So at the piston travels towards the crank shaft (which is bottom dead center or BDC ) it creates a vacume and sucks in the air/fuel.
Then just past BDC the intake valve closes (all valves closed) and the piston starts to compress the air/fuel mixture
The just as the piston squeezes the air/fuel against the cylinder head (TDC) the spark plug fires and ignites the air/fuel
Now if the spark plug fires when the piston is at exactly it's highest point ( which is Top dead center) then that is called 0 degrees of ignition timing.
If you advance the timing that means that the spark plug will fire just a fraction of a second before the piston is all the way up.
If you retard the timing that means that the spark plug will fire just after the piston passes TDC and starts moving towards the crankshaft.
Typically there is a slight delay from when the spark plug fires and when the air/fuel burn starts so usually the timing is advanced so that it starts burning before TDC... this way by the time the piston passes TDC the power in the cylinder will have just started to increase.
If the ignition is advanced too much the air/fuel will fully ignite before the piston passes TDC which is really bad and can damage pistons and connecting rods.
If the ignition is retarded you will loose power because the air/fuel will burn too late and you won't get the maximum "push" on the piston.
Think of it this way.... you are pushing someone on a swing..... push too early while they are still coming at you and they either fall off or knock you backwards..... push too late and you hardly do anything.... they move away quicker than you can push.
But push at exactly the right time when they are just past stopping and barely starting to go down and you will transfer the maximum energy to the person.
Ok enough about ignition.....
The piston pushes away from TDC towards the crank and just as it's about to get to BDC the exhaust valve opens up to relieve some of the pressure in the cylinder.
The exhaust valve stays open and the piston pushes the exhaust out the valve as it moves back towards TDC.
Then as it passes TDC the whole cycle starts again as the intake valve opens.
Well.... there it is.... the 4 stroke Otto cycle in a nut shell.
Sorry for those of you who know how it works .... I just want to be sure everyone can understand this new engine design.
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PrecisionBoost
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Ok.... the Two stroke engine is just a modified Otto cycle engine where by which the fuel/air mixture is ignited every time the piston comes to TDC.
In the 4 stroke you have the intake stroke,compression stroke, power stroke and your exhaust stroke so the air/fuel only gets ignited every second time the piston comes to TDC.
This is why 2 strokes make way more power than a 4 stroke when they are the same size..... the 2 stroke fires twice as often as the 4 stroke.
So in theory if you had a 2 stroke and a 4 stroke of equal displacement and compression ration the 2 stroke should make exactly two times as much power.
In a 2 stroke the exhaust valve and intake valve overlap at BDC.
The best way to see it is to look it up on the internet.... usually the site "how things work" has good explainations.
The problem with a 2 stroke is that there is allways exhaust in the intake air that's going to be ignited and because both valves are open some of the fuel will escape out the exhaust valve. (which is bad for emissions so most car companies won't use a 2 stroke )
I don't want to get into detailed information on the 2 stroke because the standard design with a reed valve has very little to do with my 2 stroke engine.
In the 4 stroke you have the intake stroke,compression stroke, power stroke and your exhaust stroke so the air/fuel only gets ignited every second time the piston comes to TDC.
This is why 2 strokes make way more power than a 4 stroke when they are the same size..... the 2 stroke fires twice as often as the 4 stroke.
So in theory if you had a 2 stroke and a 4 stroke of equal displacement and compression ration the 2 stroke should make exactly two times as much power.
In a 2 stroke the exhaust valve and intake valve overlap at BDC.
The best way to see it is to look it up on the internet.... usually the site "how things work" has good explainations.
The problem with a 2 stroke is that there is allways exhaust in the intake air that's going to be ignited and because both valves are open some of the fuel will escape out the exhaust valve. (which is bad for emissions so most car companies won't use a 2 stroke )
I don't want to get into detailed information on the 2 stroke because the standard design with a reed valve has very little to do with my 2 stroke engine.
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PrecisionBoost
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Ok.... next is the miller cycle.
This is used on a Mazda Millenia (not sure if that's how it's spelled)
Basicly the Mazda engine uses a supercharger to force air into the cylinder but it's not like a standard supercharged Otto 4 stroke engine.
What happens is that the intake valve doesn't close until the piston gets fairly far past BDC.
In a normal engine you take full advantage of the extra air but the miller cycle is not about power... it's about fuel efficency.
So what happens usually in a standard engine is that when the compression cycle starts it robs the engine of power because it takes a great deal of force to compress the air.
In a miller cycle engine the intake stays open much longer into the compression stroke so the engine doesn't rob power from the crankshaft for until quite a bit later in the stroke. (which increases total power output and makes for better fuel efficency)
This is where the supercharger comes in.... if you didn't have it the engine would make almost no power because the air would escape back out the open intake valve.
The supercharger forces air into the cylinder very late and makes up for the smaller compression cycle.
So imagine this..... lets say it didn't start the compression cycle until half way down towards BDC....and you have 1 liter of displacement in the cylinder.
Natural pressure is 14.7 psi..... so when you say your car is running 8psi of boost the actual pressure in the cylinder is 14.7psi + 8psi = 22.7psi
A standard Otto cycle engine with a supercharger running at 14.7 psi would force 2.0L of air into a 1.0L cylinder volume.
But in a miller cycle there might only be 0.5 L of cylinder volume because the piston has allready moved half way down.
With a 14.7psi supercharger however you would cram 1.0L into that 0.5L space.
So what do you get????
You get exactly the same amount of power as a standard naturally aspirated engine ( otto cycle )
The advantage is that you didn't suck power off the crankshaft for half of the compression cycle.
The result is better fuel economy and slightly more power than the equivilently sized naturally aspirated Otto cycle engine.
Sorry if this is confusing.... it's hard to demonstrate without diagrams.
I don't really have time to find example diagrams for you guys but I'm sure that a simple search on Yahoo will bring up some demonstrations of the miller cycle.
This is used on a Mazda Millenia (not sure if that's how it's spelled)
Basicly the Mazda engine uses a supercharger to force air into the cylinder but it's not like a standard supercharged Otto 4 stroke engine.
What happens is that the intake valve doesn't close until the piston gets fairly far past BDC.
In a normal engine you take full advantage of the extra air but the miller cycle is not about power... it's about fuel efficency.
So what happens usually in a standard engine is that when the compression cycle starts it robs the engine of power because it takes a great deal of force to compress the air.
In a miller cycle engine the intake stays open much longer into the compression stroke so the engine doesn't rob power from the crankshaft for until quite a bit later in the stroke. (which increases total power output and makes for better fuel efficency)
This is where the supercharger comes in.... if you didn't have it the engine would make almost no power because the air would escape back out the open intake valve.
The supercharger forces air into the cylinder very late and makes up for the smaller compression cycle.
So imagine this..... lets say it didn't start the compression cycle until half way down towards BDC....and you have 1 liter of displacement in the cylinder.
Natural pressure is 14.7 psi..... so when you say your car is running 8psi of boost the actual pressure in the cylinder is 14.7psi + 8psi = 22.7psi
A standard Otto cycle engine with a supercharger running at 14.7 psi would force 2.0L of air into a 1.0L cylinder volume.
But in a miller cycle there might only be 0.5 L of cylinder volume because the piston has allready moved half way down.
With a 14.7psi supercharger however you would cram 1.0L into that 0.5L space.
So what do you get????
You get exactly the same amount of power as a standard naturally aspirated engine ( otto cycle )
The advantage is that you didn't suck power off the crankshaft for half of the compression cycle.
The result is better fuel economy and slightly more power than the equivilently sized naturally aspirated Otto cycle engine.
Sorry if this is confusing.... it's hard to demonstrate without diagrams.
I don't really have time to find example diagrams for you guys but I'm sure that a simple search on Yahoo will bring up some demonstrations of the miller cycle.
Last edited by PrecisionBoost on Thu Aug 12, 2004 5:26 am, edited 2 times in total.
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PrecisionBoost
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Ok.... here is where I talk about my engine cycle ( finally 
)
What happens in this engine is that it runs as a 2 stroke engine but it uses a supercharger to force air into the cylinder to make more power and stop any fuel from getting into the exhaust (like a normal 2 stroke)
Because it's a two stroke we will start from the begining of the power stroke at TDC.
The air/fuel is ignited and the piston moves up towards BDC.
As usual the exhaust valve opens a few degrees before the piston reaches BCD but here is the twist....
The intake valve opens up just 5 degrees after the exhaust valve opens.
All the real major pressure exits the exhaust valve during these 5 degrees when the intake valve is still closed so there isn't any major backpressure into the intake.
Part of the reason is because the supercharger is pushing air (without fuel) into the intake at 60 psi !!!!!!!
So the high pressure air rushes from the supercharger through the intake valve and out the exhaust valve which effectivly sweeps the exhaust out of there right away.... so there is pretty much only 60psi air in there at say 10 degrees after BDC.
Still there is no fuel entering so there is no emissions problem like there is in a normal 2 stroke.
Oh I forgot to mention.... while the intake valve is closed (during the power cycle) the supercharger is building up "surge pressure" way in excess of 60 psi... so instead of "dumping" this surge pressure it is directed into the cylinder to help push the exhaust out.
I don't know what the actual surge pressure could be but who knows it might reach 200psi.... and a fraction of a second after the intake valve opens this would "purge" the intake of the really high pressure and the supercharger would push it's normal 60psi.
So anyways I hope you guys are with me so far....
Next comes the miller cycle part of the engine..... the intake and exhaust valves stay open fairly far into the next stroke as the piston moves towards TDC (away from the crankshaft and towards the cylinder head)
The exhaust valve finally shuts at say 35 degrees past BDC.
At this point the fuel is finally introduced into the cylinder with the highly compressed 60psi air. (will explain the fuel introduction later)
The intake valve still stays open and finally closes about 35 degrees before TDC ( so the piston is gettin very close to the cylinder head)
But because we are doing the miller cycle thing that 60 psi pushes a whole pile of air into the remaining cylinder volume.
Lets say that the piston really gets into it's compression phase at about 22.5 degrees..... which is 3/4 of the way from BDC towards TDC.
Lets say the volume was originally 1L (usually much smaller than this but it makes for easy math)
So with the piston 3/4 of the way down there is only 0.25 liters of displacement between the piston and cylinder head ( 1/4 or 1 liter)
But since we are pushing 60psi into that small space which happens to be approximatly four times more pressure than regular atmospheric pressure everything comes close to equalling out.
So 14.7 PSI pressure at 1L is the same as 60psi at 0.25L
But wait.... the actual pressure inside the cylinder is 14.7psi + 60 psi of boost so the real internal pressure is 75psi
so 0.25 L X 75 psi = 18.75 which is more than 1L X 14.7psi
So pushing 60psi into a quarter of the cylinder will actually be like having a boost level of about 4psi on a normal Otto cycle engine.
This extra 4psi boost helps make up for the losses involved in running the supercharger
So the fuel/air compresses and bang.... here comes the power stroke again.
The cool thing is that each piston fires once per revolution of the crank vs once every second rotation in a standard engine.
So the result is more than double the power of an equivilent 4 stroke engine..... don't forget about the advantage of not compressing the air until 3/4 of the way down the cycle.... so there is less power pulled off the crankshaft during each cycle (compared to the normal 4 stroke)
Now the only thing remaining to explain is the fuel delivery.
If I simply injected the fuel with a standard fuel injector one of two things would happen..... fuel would exit out the exhaust valve (bad emissions) or the fuel would not have time to atomize and mix properly.
The result would be major losses in power.
So instead the fuel injector injects it's fuel into a separate cylinder which has a piston and valve that push the air/fuel into the intake chamber just as the exhaust valve closes.
This way the fuel has about 165 degrees of piston movement to atomize with the air in it's own little chamber.
The piston is mechanical so it's set to push out when the exhaust valve closes.... ideally this would be done at a higher pressure than 60psi so that it gets into the intake chamber without any backpressure problems.
So it's sort of like direct injection because its a very short burst of highly mixed fuel and air at just the right time ( only about 15 degrees of the cycle)
So that's basicly the design.... there are many more details and calculations to the design but I tried to make it as simple as possible for everyone to hopefully understand.
)What happens in this engine is that it runs as a 2 stroke engine but it uses a supercharger to force air into the cylinder to make more power and stop any fuel from getting into the exhaust (like a normal 2 stroke)
Because it's a two stroke we will start from the begining of the power stroke at TDC.
The air/fuel is ignited and the piston moves up towards BDC.
As usual the exhaust valve opens a few degrees before the piston reaches BCD but here is the twist....
The intake valve opens up just 5 degrees after the exhaust valve opens.
All the real major pressure exits the exhaust valve during these 5 degrees when the intake valve is still closed so there isn't any major backpressure into the intake.
Part of the reason is because the supercharger is pushing air (without fuel) into the intake at 60 psi !!!!!!!
So the high pressure air rushes from the supercharger through the intake valve and out the exhaust valve which effectivly sweeps the exhaust out of there right away.... so there is pretty much only 60psi air in there at say 10 degrees after BDC.
Still there is no fuel entering so there is no emissions problem like there is in a normal 2 stroke.
Oh I forgot to mention.... while the intake valve is closed (during the power cycle) the supercharger is building up "surge pressure" way in excess of 60 psi... so instead of "dumping" this surge pressure it is directed into the cylinder to help push the exhaust out.
I don't know what the actual surge pressure could be but who knows it might reach 200psi.... and a fraction of a second after the intake valve opens this would "purge" the intake of the really high pressure and the supercharger would push it's normal 60psi.
So anyways I hope you guys are with me so far....
Next comes the miller cycle part of the engine..... the intake and exhaust valves stay open fairly far into the next stroke as the piston moves towards TDC (away from the crankshaft and towards the cylinder head)
The exhaust valve finally shuts at say 35 degrees past BDC.
At this point the fuel is finally introduced into the cylinder with the highly compressed 60psi air. (will explain the fuel introduction later)
The intake valve still stays open and finally closes about 35 degrees before TDC ( so the piston is gettin very close to the cylinder head)
But because we are doing the miller cycle thing that 60 psi pushes a whole pile of air into the remaining cylinder volume.
Lets say that the piston really gets into it's compression phase at about 22.5 degrees..... which is 3/4 of the way from BDC towards TDC.
Lets say the volume was originally 1L (usually much smaller than this but it makes for easy math)
So with the piston 3/4 of the way down there is only 0.25 liters of displacement between the piston and cylinder head ( 1/4 or 1 liter)
But since we are pushing 60psi into that small space which happens to be approximatly four times more pressure than regular atmospheric pressure everything comes close to equalling out.
So 14.7 PSI pressure at 1L is the same as 60psi at 0.25L
But wait.... the actual pressure inside the cylinder is 14.7psi + 60 psi of boost so the real internal pressure is 75psi
so 0.25 L X 75 psi = 18.75 which is more than 1L X 14.7psi
So pushing 60psi into a quarter of the cylinder will actually be like having a boost level of about 4psi on a normal Otto cycle engine.
This extra 4psi boost helps make up for the losses involved in running the supercharger
So the fuel/air compresses and bang.... here comes the power stroke again.
The cool thing is that each piston fires once per revolution of the crank vs once every second rotation in a standard engine.
So the result is more than double the power of an equivilent 4 stroke engine..... don't forget about the advantage of not compressing the air until 3/4 of the way down the cycle.... so there is less power pulled off the crankshaft during each cycle (compared to the normal 4 stroke)
Now the only thing remaining to explain is the fuel delivery.
If I simply injected the fuel with a standard fuel injector one of two things would happen..... fuel would exit out the exhaust valve (bad emissions) or the fuel would not have time to atomize and mix properly.
The result would be major losses in power.
So instead the fuel injector injects it's fuel into a separate cylinder which has a piston and valve that push the air/fuel into the intake chamber just as the exhaust valve closes.
This way the fuel has about 165 degrees of piston movement to atomize with the air in it's own little chamber.
The piston is mechanical so it's set to push out when the exhaust valve closes.... ideally this would be done at a higher pressure than 60psi so that it gets into the intake chamber without any backpressure problems.
So it's sort of like direct injection because its a very short burst of highly mixed fuel and air at just the right time ( only about 15 degrees of the cycle)
So that's basicly the design.... there are many more details and calculations to the design but I tried to make it as simple as possible for everyone to hopefully understand.
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PrecisionBoost
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Now here is the problem...... getting it started..... you need the 60psi of supercharger pressure to get enough power for it to run.... no pressure will result in no power.
So you would need a second starter motor....one to crank over the engine and one to spool up the supercharger and create a high pressure air pulse to get things rolling.
This would be the time where it would be questionable how easy it would be to start..... much like a Diesel engine you would probably have to have some kind of pre-heater that would bring the fuel mixture chamber up to temperature before you try to start the engine. ( otherwise it wouldn't burn very well and would foul the spark plugs right away )
So there are a number of things that would be needed to get it to run inside of the first few seconds.....but it should be possible to satisfy all the needs with over the counter parts from the local supplier.
Well.... that's about it..... I hope you guys learned something out of this hugely long post.
Feel free to tell me how you think it will or won't work.
Basicly the main thing that would need to be done to an engine is to make the camshafts rotate at exactly the same speed as the crankshaft (normally rotate at exactly 1/2 the speed thanks to a 2:1 pulley ratio)
The camshafts would also need to be reground to a slightly different specification and the valvetrain would need to be beefed up because of the fact that it's pushing the valves down every rotation instead of every second rotation..... so 4000 RPM on one of these 2 stroke engines would be like 8000 RPM in a standard 4 stroke engine.
But I think it would be possible to make a valvetrain fast enough to keep up with the demand of a 2 stroke engine.
There are all kinds of new valvetrain designs out there including a rotary valve system that can run at very high RPM without breaking down. ( 30,000 RPM ???? or was it 100,000 RPM..... can't remember)
So you would need a second starter motor....one to crank over the engine and one to spool up the supercharger and create a high pressure air pulse to get things rolling.
This would be the time where it would be questionable how easy it would be to start..... much like a Diesel engine you would probably have to have some kind of pre-heater that would bring the fuel mixture chamber up to temperature before you try to start the engine. ( otherwise it wouldn't burn very well and would foul the spark plugs right away )
So there are a number of things that would be needed to get it to run inside of the first few seconds.....but it should be possible to satisfy all the needs with over the counter parts from the local supplier.
Well.... that's about it..... I hope you guys learned something out of this hugely long post.
Feel free to tell me how you think it will or won't work.
Basicly the main thing that would need to be done to an engine is to make the camshafts rotate at exactly the same speed as the crankshaft (normally rotate at exactly 1/2 the speed thanks to a 2:1 pulley ratio)
The camshafts would also need to be reground to a slightly different specification and the valvetrain would need to be beefed up because of the fact that it's pushing the valves down every rotation instead of every second rotation..... so 4000 RPM on one of these 2 stroke engines would be like 8000 RPM in a standard 4 stroke engine.
But I think it would be possible to make a valvetrain fast enough to keep up with the demand of a 2 stroke engine.
There are all kinds of new valvetrain designs out there including a rotary valve system that can run at very high RPM without breaking down. ( 30,000 RPM ???? or was it 100,000 RPM..... can't remember)
Last edited by PrecisionBoost on Thu Aug 12, 2004 5:23 am, edited 2 times in total.
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PrecisionBoost
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Chris:
Now i understand your post of Cam shaft grinding... you nutter
I loved the idea...
Do you really think the mayor problem will be to start it?
I dont know a thing about engine desings or manufacturing.... but i think the hard par it would be the desing of the valve train... not the stregth... its just how perfect it can be and how fast it can go without the valves metting mr piston!
Also i think it wont be a high reving engine! maybe 6000-7000 rpm...
Oh wait... since its a 2 stroke engine that means that it would get the same power of one 4 stroke engine reving to 12000-14000 rpms!!
Now i understand your post of Cam shaft grinding... you nutter
I loved the idea...
Do you really think the mayor problem will be to start it?
I dont know a thing about engine desings or manufacturing.... but i think the hard par it would be the desing of the valve train... not the stregth... its just how perfect it can be and how fast it can go without the valves metting mr piston!
Also i think it wont be a high reving engine! maybe 6000-7000 rpm...
Oh wait... since its a 2 stroke engine that means that it would get the same power of one 4 stroke engine reving to 12000-14000 rpms!!
'88 Pontiac Lemans GTE - 2.0 16v XE - fully programable ECU, Custom made intake manifold and other bits.
146.6WHP/135lb.ft - 14.81@94mph
146.6WHP/135lb.ft - 14.81@94mph
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KnightWalace
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PrecisionBoost
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In a normal engine the valves are opening and closing while the piston is right up near the head (TDC) but with this other design the valves open and close when the piston is close to the crankshaft (BDC).Efrain A. wrote:the hard par it would be the desing of the valve train... not the stregth... its just how perfect it can be and how fast it can go without the valves metting mr piston!
So you would never have to worry about the pistons contacting the valves.
The problem is with the cams running twice as fast the valve train ( valves,springs, retainers ) will be accelerating twice as fast as they would in a 4 stroke.
4000 RPM in this engine would be roughly the equivilent of running your engine at about 8,000 RPM which is where things like lifters tend to snap due to excessive force.
Force = mass X acceleration
So doubling the speed doubles the force on the components.
Less duration will make the cam lobe more like a triangle than a nice round elipse... basicly with two cams rotating at the exact same speed the valve that is pushed by the rounder lobe with more duration will see less acceleration and therefore less force.